Upsala J Med Sci 92: 19-35, 1987 

Some Effects of Metolazone on Electrolyte Transport 
Bo Odlind,’ Bjorn Beermann,’ Bjorn Lindstrom,3 and Orjan Eriksson’ 

‘Departments of Medicine, University Hospiial, U p p s a l a ,  ’S:t Eriks Hospital, Stockholm and 
’Department of Drugs, National Board of Health and W e y a r e ,  Uppsala, Sweden 

ABSTRACT 
Metolazone a c t i o n  was s t u d i e d  1) i n  v i t r o  on i s o l a t e d  operculum of F u n d u l u s  

h e t e r o c l i t u s  ( a c t i v e  c h l o r i d e  t r a n s p o r t )  using an Ussinq chamber ( m e t o l  a z o n e  
c o n c  500 pM) and i n  v i v o  2 )  u s i n g  t h e  modified Sperber t e c h n i q u e  i n  t h e  hen 
(metolazone i n f u s i o n  r a t e  0.75-1.2 p g / k g / m i n )  and 3 )  i n  h e a l t h y  v o l u n t e e r s  
u s i n g  c l e a r a n c e  t e c h n i q u e s  (metolazone i n f u s i o n  r a t e  10 mg/h). 1 )  Metolazone 
reduced ( p < 0 . 0 5 )  s h o r t  c i r c u i t  c u r r e n t  and p o t e n t i a l  d i f f e r e n c e s  w i t h  20% from 
a v e r a g e  c o n t r o l  v a l u e s  ( p < O . O 5 ) ,  w h i l e  d i r e c t  c u r r e n t  r e s i s t a n c e  was 
unchanged. This i s  comparable t o  t h i a z i d e  b u t  much l o w e r  t h a n  l o o p  d i u r e t i c  
e f f e c t s .  2 )  T r u e  t u b u l a r  e x c r e t i o n  f r a c t i o n  of metolazone before and a f t e r  
novobiocin ( 2 . 7  pnol/kq/rnin c o i n f u s i o n  averaqed 14.1 and 4.5 %, r e s p .  (o<O.Ol; 
n = 8 ) .  T h u s  m e t o l a z o n e  i s  p a r t l y  e l i m i n a t e d  by r e n a l  t u b u l a r  s e c r e t i o n .  
However, t h e  d i u r e t i c  e f f e c t  (sodium, c h l o r i d e  a n d  potassium e x c r e t i o n )  - a n d  
c l e a r a n c e s  of Cr5’-EDTA a n d  1125-Na-o-iodohippurate - were symmetrical , i . e .  
independent of metolazone u r i n a r y  e x c r e t i o n  r a t e ,  a s  p r e v i o u s l y  shown f o r  
t h i z i a d e s .  3 )  Renal c l e a r a n c e  o f  m e t o l a z o n e  i n  h e a l t h y  v o l u n t e e r s .  ( H P L C -  
method) a v e r a g e d  173+20 m l / m i n  ( n = 8 ) .  P r o b e n e c i d  (1 g i v . )  s i q n i f i c a n t l y  
r e d u c e d  t h e  renal c l e a r a n c e  of metolazone t o  3327 m l / m i n  and potassium excre- 
t i o n  w i t h  maximum 30%, while d i u r e t i c  a n d  s a l u r e t i c  e f f e c t s  were s i g n i f i c a n t l y  
i n c r e a s e d  w i t h  maximum 30%. T h u s ,  a l s o  i n  humans t h e  d i u r e t i c  e f f e c t  o f  
metolazone i s  not coupled t o  t h e  u r i n a r y  e x c r e t i o n  r a t e  of t h e  drug, b u t  s u o -  
g e s t s  t h a t  i t s  d i u r e t i c  e f f e c t  i s  e l i c i t e d  p r i m a r i l y  from t h e  p e r i t u b u l a r  s i d e  
of t h e  nephron. Probenecid a p p a r e n t l y  d i s s o c i a t e s  s o d i u m  from potassium excre- 
t i o n  e f f e c t s  of m e t o l a z o n e .  T h i s  i m p 1  i e s  a l u m i n a l  , s o d i u m - i n d e p e n d e n t  
k a l i u r e t i c  e f f e c t  of the druQ. 

INTRODUCTION. 
Metolazone ( 7  - c h l o r o  - 1 , 2 , 3 , 4 ,  t e t r a h y d r o  - 2 methyl - 4 0 x 0  - 3 - o - 

t o l y l  - 6 q u i n a z o l  inesulphonamide) i s  a sulphonamide d i u r e t i c  chemically r e -  
l a t e d  t o  quinethazone. The d i u r e t i c  a n d  metabolic e f f e c t s  o f  m e t o l a z o n e  seem 

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l a r g e l y  t o  be s i m i l a r  t o  t h o s e  of t h e  b e n z o t h i a d i a z i n e s .  There a r e ,  however, a 
number of r e p o r t s  about s t r i k i n g  d i u r e t i c  e f f e c t s  of m e t o l a z o n e  i n  p a t i e n t s  
w i t h  s e v e r e l y  r e d u c e d  r e n a l  f u n c t i o n  ( 7 ,  8, 17, 19, 36) a c o n d i t i o n  where 
" o r d i n a r y "  t h i a z i d e s  a r e  g e n e r a l l y  held t o  be i n e f f e c t i v e  ( 3 3 ) .  Some a u t h o r s  
h a v e  t h e r e f o r e  c l a s s i f i e d  m e t o l a z o n e  a s  b e i n g  more r e l a t e d  t o  t h e  l o o p  
d i u r e t i c s  than t o  t h e  t h i a z i d e s  ( 4 3 ) .  The aim of t h i s  s t u d y  was t o  f u t h e r  
c h a r a c t e r i z e  t h e  e f f e c t s  of m e t o l a z o n e  on e l e c t r o l y t e  t r a n s p o r t ,  i n  v i t r o ,  
using f i s h  operculum and toad u r i n a r y  b l a d d e r  (11, 1 8 ) .  S t u d i e s  were a l s o  per- 
formed i n  vivo,using the modified Sperber t e c h n i q u e  i n  t h e  hen (231 and u s i n g  
conventional c l e a r a n c e  techniques i n  h e a l t h y  v o l u n t e e r s .  The main p u r p o s e  o f  
t h e  l a t t e r  s t u d i e s  was t o  compare m e t o l a z o n e  u r i n a r y  e x c r e t i o n  - e f f e c t  
r e l a t i o n s h i p  w i t h  t h a t  o f  loop d i u r e t i c s  and t h i a z i d e s  a s  o b t a i n e d  i n  e a r l i e r  
s t u d i e s  (24-30). 

MATERIAL A N D  METHODS. 
E l e c t r o o h y s i o l o g i c a l  s t u d y .  

T h e  e l e c t r i c a l  t r a n s e p i t h e l  i a l  p o t e n t i a l  d i f f e r e n c e ,  s h o r t  c i r c u i t  c u r r e n t  
and d i r e c t  c u r r e n t  r e s i s t a n c e  were s t u d i e d  i n  o p e r c u l a r  e p i t h e l i u m  from 
Fundulus h e t e r o c l i t u s .  The f i s h  were k e p t  i n  2.5 % a r t i f i c i a l  sea water f o r  a t  
l e a s t  4 weeks a f t e r  d e l i v e r y .  A f t e r  k i l l i n g  t h e  animal, t h e  operculum was i m -  
mediately removed. The e p i t h e l i u m  was d i s s e c t e d  f r e e  from t h e  o p e r c u l u m  a n d  
was mounted a c r o s s  t h e  c e n t r a l  hole of a g a s k e t  and placed i n  a n  Ussing cham- 
b e r  (9, 11). Metolazone stock s o l u t i o n  (27.3 m M )  was made by d i s s o l v i n g  t h e  
compound i n  R i n g e r  s o l u t i o n  a l k a l i n i z e d  w i t h  NaOH (pH 8.9). Addition of drug 
i n c r e a s e d  the pH of t h e  b a t h i n g  s o l u t i o n  0.03 pH u n i t s  which i t s e l f  h a d  no e f -  
f e c t  on t h e  s t u d i e d  parameters. 

The same t e c h n i q u e  was used t o  s t u d y  t h e  e f f e c t  of m e t o l a z o n e  on i o n  
t r a n s p o r t  i n  i s o l a t e d  u r i n a r y  bladder e p i t h e l i u m  from Bufo v u l g a r i s .  ( 1 0 )  .The 
t o a d s  were kept i n  t a p  water f o r  a t  l e a s t  2 weeks a f t e r  d e l i v e r y .  The w a t e r  
t e m p e r a t u r e  was 8-10°C a n d  t h e  a n i m a l s  were fed manually w i t h  p i e c e s  o f  ox 
l i v e r  once a week. The u r i n a r y  b l a d d e r  e p i t h e l i u m  was d i s s e c t e d  f r e e  a n d  
placed i n  a n  Ussing chamber a s  d e s c r i b e d  above f o r  t h e  o p e r c u l a r  eDithelium. 

T h e  t r a n s p o r t  of sodium a n d  c h l o r i d e  was determined e l  e c t r o p h y s i  o l  o a i  c a l l  y 
by m e a s u r i n q  t h e  t r a n s e p i t h e l  i a l  p o t e n t i a l  d i f f e r e n c e  ( P D ;  m V ) ,  t h e  s h o r t -  
c i r c u i t  c u r r e n t  ( S C C ;  pA x cm-2), and t h e  D C  r e s i s t a n c e  ( R ; O h m  x cm 1. The 
methods used a r e  d e s c r i b e d  i n  d e t a i l  e a r l i e r  ( 1 1 ) .  

The following spontaneous a c t i v i t y  of t h e  e p i t h e l i a  was observed. The k i l -  
l i f i s h  o p e r c u l a r  e p i t h e l i a  d i s p l a y e d  a t r a n s e p i t h e l i a l  PD of 10.3+1.8 mV ( s e a -  
w a t e r  side n e g a t i v e ) ,  a SCC of 49.9+4.0 - pA x cm-', and a t r a n s e p i t h e l i a l  D C  
r e s i s t a n c e  o f  217+44 O h m  x cm (mean + SE;n=4). The corresponding values f o r  

2 

- 

2 
- - 

20 



i s o l a t e d  t o a d  b l a d d e r s  were 57.8+4.4 mV ( s e r o s a l  s i d e  p o s i t i v e ) ,  82.5+8.7 pA x 

cm-', and 722+67 - Ohm x cm (mean+SE;n=8). - 
- - 

2 

The m o d i f i e d  S p e r b e r  t e c h n i q u e  i n  t h e  hen. 

The same e x p e r i m e n t a l  t e c h n i q u e  was used as has been d e s c r i b e d  i n  d e t a i l  and 

used i n  s i m i l a r  s t u d i e s  e a r l i e r  (23, 24, 2 6 ) .  The t e c h n i q u e  e n a b l e s  s e p a r a t e  

u r i n e  c o l l e c t i o n s  f r o m  t h e  t w o  i n j e c t i o n  s i d e  ( I s )  and c o n t r o l  s i d e  ( C s l  
u r e t e r s  a n d  t h e  c a l c u l a t i o n  o f  t h e  t r u e  t u b u l a r  e x c r e t i o n  f r a c t i o n  o f  

m e t o l a z o n e  (TTEFM) as 

Is - Cs e x c r e t i o n  o f  M 
x 100 = TTEFM ( % )  

Amount o f  M i n f u s e d  

A TTEF v a l u e  e x c e e d i n g  5 % p r o v e s  a c t i v e  t u b u l a r  s e c r e t i o n  o f  m e t o l a z o n e  

( 2 3 ) .  CI4 m e t o l a z o n e  was i n f u s e d  i n t o  a l e g  v e i n  on t h e  r i g h t  s i d e  o f  8 h e n s  
a t  a r a t e  o f  0.75 - 1.2 vg/kg/min. A f t e r  a t  l e a s t  4-5 c o l l e c t i o n  p e r i o d s  o f  10 
m i n  a t  s t e a d y  s t a t e ,  s o d i u m  n o v o b i o c i n ,  an i n h i b i t o r  o f  o r g a n i c  a n i o n  

t r a n s p o r t  (24) w a s  c o i n f u s e d  a t  a r a t e  o f  2.7 p m o l / k g / m i n .  The r e n a l  
c l e a r a n c e s  o f  Cr51 EDTA ( C E D T A )  a n d  1 1 2 5 - N a - o - i o d o h i p p u r a t e  (CHipp) w e r e  

d e t e r m i n e d  t h o u g h o u t  t h e  e x p e r i m e n t s .  

M 

C l e a r a n c e  s t u d i e s  i n  h e a l t h y  v o l u n t e e r s .  

S e v e n  h e a l t h y  men, 24 t o  35 y e a r s  o l d ,  v o l u n t e e r e d  i n  t h e  s t u d y  w h i c h  was 

approved b y  t h e  l o c a l  E t h i c s  Committee. They a r r i v e d  a t  t h e  l a b o r a t o r y  i n  t h e  

m o r n i n g  a f t e r  f a s t i n g  o v e r  n i g h t  and were s u p p l i e d  w i t h  i n d w e l l i n g  c a n n u l a s  i n  

t h e  b r a c h i c a l  v e i n s  o f  b o t h  arms. 

A t  z e r o  t i m e ,  i . v .  b o l u s  doses o f  i n u l i n  ( 5 0  mg/kg) and PAH ( 1 0  mg/kg) were 

g i v e n  and f o l l o w e d  b y  a c o n t i n o u s  i n f u s i o n  o f  28 and 14 mg/min, r e s p e c t i v e l y .  

An e q u i l i b r i u m  p e r i o d  o f  a t  l e a s t  1 h o u r  was a l l o w e d  b e f o r e  s t a r t i n g  t h e  

c l e a r a n c e  d e t e r m i n a t i o n s .  M e t o l a z o n e  was g i v e n  as an i . v .  b o l u s  o f  1 0  mg f o l -  

l o w e d  b y  a c o n t i n u o u s  i . v .  i n f u s i o n  o f  10 mg/h 1.5 t o  2 . 5  h r s  a f t e r  t h e  s t a r t  
o f  t h e  i n u l i n - P A H  i n f u s i o n .  P r o b e n e c i d  was a d m i n i s t e r e d  as an i . v .  b o l u s  o f  1 

g i n  50 m l  s a l i n e  a t  2.5 h r s  a f t e r  s t a r t  o f  t h e  i n f u s i o n  o f  metolazone, w h i c h  

was c o n t i n u e d  f o r  a n o t h e r  3 h r s .  The s u b j e c t s  e m p t i e d  t h e i r  u r i n a r y  b l a d d e r s  
b e f o r e  t h e  e x p e r i m e n t .  U r i n e  was c o l l e c t e d  i n  p e r i o d s  o f  30 m i n  and b l o o d  

samples were drawn w i t h i n  3 m i n  o f  t h e  b e g i n n i n q  and t h e  end o f  each c o l l e c -  

t i o n  p e r i o d .  F r o m  t h e  s t a r t  o f  t h e  e x p e r i m e n t  t o  t h e  i n j e c t i o n  o f  m e t o l a z o n e  

t h e  s u b j e c t s  d r a n k  200 m l  t a p  w a t e r / h .  U r i n e  l o s s e s  o f  e a c h  p e r i o d  a f t e r  t h e  

i n j e c t i o n  o f  t h e  d i u r e t i c  i n  excess o f  t h e  o r a l  w a t e r  r e p l a c e m e n t  was r e p l a c e d  

volume f o r  volume by c o n t i n u o u s  i . v .  i n f u s i o n  o f  s a l i n e  d u r i n g  t h e  s u b s e q u e n t  

21 



p e r i o d .  Food i n t a k e  a n d  smoking were not allowed d u r i n g  t h e  experiment. Except 
when voiding, t h e  s u b j e c t s  were supine d u r i n q  t h e  experiment. 

Determination of metolazone. 
A l i q u i d  c h r o m a t o g r a p h i c  method was s e t  u p  f o r  t h e  d e t e r m i n a t i o n  o f  

m e t o l a z o n e  i n  plasma and u r i n e . T h i s  u t i l i z e d  a 6000A p u m p ,  a U6K i n j e c t o r  
(Waters Assoc, M i l f o r d ,  M A ,  U S A )  and a 970 f l u o r e m e t r i c  d e t e c t o r  ( S p e c t r a  
P h y s i c s ,  S a n t a  C l a r a ,  C A Y  USA) o p e r a t e d  a t  an e x i t a t i o n  wavelength o f  230 nm 
and equipped w i t h  a 420 nm emission f i l t e r .  The column ( s t a i n l e s s  s t e e l ,  1 5 0  
m m  x 4 , 6  m m  I.D.) w a s  s l u r r y  packed w i t h  Lichrosorb RP-18.5 pm p a r t i c l e  s i z e  
(Merck, Darmstadt, G R F )  a n d  e l u t e d  w i t h  40% a c e t o n i t r i l e  i n  0,Ol M pH 7 phos- 
phate b u f f e r  a t  a r a t e  of 1 . 2  r n l / m i n .  

The sample ( 0 . 5  and 0.1 ml were used f o r  plasma and u r i n e  r e s p e c t i v e l y )  was 
added t o  0 . 5  ml of s a t u r a t e d  sodium hydrogen-carbonate s o l u t i o n  and e x t r a c t e d  
w i t h  5 ml e t h y l a c e t a t e  i n  a s c r e w - c a p p e d  t u b e  ( s h a k e  b o a r d  10 m i n ) .  A f t e r  
c e n t r i f u g a t i o n  ( 5 0 0 g )  4 ml o f  t h e  o r g a n i c  phase was removed and evaporated 
w i t h  n i t r o g e n  g a s .  The r e s i d u e  was r e d i s s o l v e d  in 500 pl of t h e  e l u e n t  s y s t e m  
and u s u a l l y  a 20 p1 a l i q u o t  was i n j e c t e d  i n t o  t h e  l i q u i d  chromatograph. These 
volumes could be a d j u s t e d  t o  t h e  c o n c e n t r a t i o n  range analysed. 

T h e  c a l i b r a t i o n  graph was c o n s t r u c t e d  by adding known amounts of metolazone 
t o  plasma o r  u r i n e  and a n a l y z i n g  them according t o  t h e  method d e s c r i b e d .  The 
r e s u l t i n g  peak h i g h t s  were p l o t t e d  v e r s u s  t h e  c o n c e n t r a t i o n .  

The t o t a l  recovery when e x t r a c t i n g  metolazone from plasma ( u r i n e )  a v e r a g e d  
88%. The p r e c i s i o n  o f  t h e  method was 4.5 and 7% (n=10) a t  400 and 500 n g h l  
plasma, r e s p e c t i v e l y .  

Chemical d e t e r m i n a t i o n s .  
Concentrations of i n u l i n  and PAH were d e t e r m i n e d  a c c o r d i n g  t o  S c h r e i n e r  

( 3 7 )  and B r u n  ( 6 1 ,  r e s p e c t i v e l y .  I n  t h e  v o l u n t e e r  study c o n c e n t r a t i o n s  of 
sodi urn, p o t a s s i  urn and chl o r i  de were determined according t o  r o u t i n e  methods a t  
t h e  h o s p i t a l  and i n  t h e  experimental study a s  p r e v i o u s l y  d e s c r i b e d  ( 2 4 ) .  

I s o t o p e  d e t e r m i n a t i o n s .  

spectrometer and carbon-14 a c t i v i t i e s  by l i q u i d  s c i n t i l l a t i o n  counting. 
and Cr5' a c t i v i t i e s  were d e t e r m i n e d  i n  a two-channel s c i n t i l l a t i o n  

M a t e r i a l s .  
Metolazone and CI4 metolazone ( s p e c .  a c t .  1.45 mCi/mmol) was o b t a i n e d  from 

P h a r m a c i a  A B ,  Uppsal a Sweden. Probenecid ( I n j e c t i o n  "Benemid") 0 , l  q/ml was 

22 



d e l i v e r e d  by Merck F r o s t  Research L a b o r a t o r i e s ,  M o n t r e a l ,  Canada. P A H  ( p a r a -  

ami n o - h i p p u r a t e - s o d i u m )  and i n u l  i n  ( I n u t e s t )  were s u p p l i e d  by Merck Sharp and 

Dohme, Rahway, NJ, and Laevosan G e s e l l s c h a f t ,  L i n z ,  A u s t r i a ,  r e s p e c t i v e l y .  
S o d i u m  n o v o b i o c i n  was o b t a i n e d  f r o m  M e r c k  S h a r p  a n d  Dohme, Rahway, NJ. 

C r  EDTA was b o u g h t  f r o m  B e h r i n g w e r k e  A G ,  V a r b u r g ,  Germany a n d  1125-Na-0- 

i o d o h i p p u r a t e  f r o m  K A B I  D i a g n o s t i k a  AB, S t u d v i k ,  Sweden. 

5 1  

S t a t i s t i c a l  a n a l y s i s .  

R e s u l t s  a r e  g i v e n  a s  a r i t h m e t i c  mean - + SD. D i f f e r e n c e s  i n  c l e a r a n c e  o r  
e x c r e t i o n  v a l u e s  w e r e  a n a l y s e d  a c c o r d i n g  t o  t h e  c o n c e p t  o f  c o n f i d e n c e  

i n t e r v a l s .  P - v a l u e s  < 0.05 were c o n s i d e r e d  as s i g n i f i c a n t .  The d i f f e r e n c e  be- 
tween p r e - d r u g  and p o s t - d r u g  v a l u e s  f o r  f i s h  operculum a n d  t o a d  b l a d d e r  w e r e  

e v a l u a t e d  b y  S t u d e n t - s  t - t e s t  ( t w o - s i d e d  t e s t )  f o r  p a i r e d  v a r i a t e s  and con- 

s i d e r e d  s i g n i f i c a n t  a t  P < 0.05 ( * ) .  

RESULTS. 

E l  e c t r o p h y s i o l  o g i c a l  s t u d i e s .  

M e t o l a z o n e  ( 5 0 0  pM) r e d u c e d  t h e  s h o r t  c i r c u i t  c u r r e n t  ( S C C )  and t h e  p o t e n -  

t i a l  d i f f e r e n c e  (PD) i n  t h e  i s o l a t e d  e p i t h e l i u m  o f  F u n d u l u s  h e t e r o c l i t u s  w i t h  

a b o u t  2 0 %  ( p  < 0.051, w h i l e  t h e  d i r e c t  c u r r e n t  r e s i s t a n c e  ( R )  was unchanged 
( T a b l e  I ) .  M e t o l a z o n e  ( 5 0 0  pM) e x c e r t e d  o n l y  s m a l l  e f f e c t s  on PD, SCC and R ( -  

1 4  t o  + 1 8 % ;  P > 0 . 0 5 )  when a d m i n i s t e r e d  t o  t h e  s e r o s a l  a n d  mucosal s i d e ,  
r e s p e c t i v e l y ,  o f  t o a d  u r i n a r y  b l a d d e r  ( T a b l e  I ) .  

T a b l e  I. E f f e c t s  o f  m e t o l a z o n e  on t h e  p o t e n t i a l  d i f f e r e n c e  (PD), s h o r t  c i r -  
c u i t  c u r r e n t  (SCC) and d i r e c t  c u r r e n t  r e s i s t a n c e  ( R )  i n  i s o l a t e d  o p e r c u l a r  
e p i  t h e 1  i a o f  Fundul u s  h e t e r o c l  i t u s  and i n  i s o l  a t e d  t o a d  b l a d d e r  .The d r u g  ( 5 0 0  

pM) was added t o  t h e  s e r o s a l  ( S )  o r  mucosal (M) s i d e  o f  t h e  e p i t h e l i u m .  The 
e l e c t r i c a l  p a r a m e t e r s  w e r e  r e c o r d e d  b e f o r e  a n d  o n e  h o u r  a f t e r  m e t o l a z o n e  

exposure. R e s u l t s  a r e  g i v e n  a s  p e r  c e n t  change from c o n t r o l  v a l u e s .  The number 

o f  e p i t h e l i a  t e s t e d  a r e  g i v e n  w i t h i n  p a r e n t h e s e s  ( x  d e n o t e s  p<0.05). 

PD SCC R 

% change f r o m  c o n t r o l .  

O p e r c u l a r  e p i t h e l i u m  S ( 4 )  -20.6 - + 2.8' -19.5 - + 4.5' -0.1 - + 8.7 

S ( 4 )  -0.4 + 6 . 2  -14.2 + 7.1 +18.0 + 11.0 - - - U r i n a r y  b l  adder 

M ( 4 )  + 6 . 4  + 4.9 +15.8 + 5.5 -5.8 + 8.2 - - - U r i n a r y  b l a d d e r  

, 
23 



The modified Sperber technique. 
A t  s t e a d y  s t a t e  t h e  TTEFM value of CI4 metolazone averaqed 14.1 - + 4.2 % 

( c a l c u l a t e d  a s  means of 4-5 c o l l e c t i o n  p e r i o d s  of 1 0  min from e a c h  of 8 
a n i m a l s ) .  A f t e r  a new steady s t a t e  had been reached during novobiocin c o i n f u -  
s i o n ,  t h i s  v a l u e  was s i q n i f i c a n t l y  ( p  < 0 . 0 0 1 )  r e d u c e d  t o  4 . 5  2 2 . 0  % 
( c a l c u l a t e d  a s  means from 3 - ( 4 )  c o l l e c t i o n  p e r i o d s  of 10 min from each of 8 
a n i m a l s ) .  Corresponding r e s u l t s  f o r  t o t a l  ( i n j e c t i o n  s i d e  p l u s  c o n t r o l  s i d e )  
u r i n a r y  e x c r e t i o n  was 58.8 - + 6.0% and 37.7 - + 2.9% ( p  < 0.001), o f  t h e  i n f u s e d  
amount, r e s p e c t i v e l y .  The u r i n a r y  e x c r e t i o n  r a t e  of c h l o r i d e ,  sodium and 

w e r e  s y m m e t r i c a l  t h o u q h o u t  a l l  p o t a s s i u m  a s  w e l l  a s  C E D T A a n d  C 
experiments. ( c f .  Fig 1). 

Hi P P  

F:g 1. 
Typical r e s u l t s  from one experiment 
with t h e  modified Sperber technique. 
I n f u s i o n  r a t e  o f  ~ 1 4  metolazone was 
0 . 9  pg/kg/min. The t r u e  t u b u l a r  ex- 
c r e t i o n  f r a c t i o n  o f  metolazone ( T T E F M ) ,  
sodium, c h l o r i d e  and p o t a s s i u  excretion 

I"5-Na-0-iodohippurate from i n j e c t  ion 
and c o n t r o l  kidneys, r e s p e c t i v e l y ,  a r e  
shown. 

a t h e  renal c l e a r a n c e s  of Cr Y IEDTA a n d  

.. 
C - 0 - 3 0  - 2 O - - l O  0-10 20-30 LO-SO 

COLLECTION PERIOD (min) 

Clearance s t u d i e s  i n  heal thy v o l u n t e e r s .  
The plasma l e v e l s  of m e t o l a z o n e  were s l o w l y  i n c r e a s i n g  a t  t h e  t i m e  o f  

i n j e c t i o n  of p r o b e n e c i d  when they averaged 402 2 64 n q / m l  ( F i g  21. The renal 
c l e a r a n c e  of metolazone ( F i g  2 )  was almost i d e n t i c a l  d u r i n q  t h e  two p e r i o d s  
b e f o r e  probenecid a d m i n i s t r a t i o n  ( p e r i o d s  - 2  a n d  -1) when i t  averaged 184 - + 25 
a n d  173 - + 20 m l / m i n ,  r e s p e c t i v e l y .  Accordingly, period -1 was c h o s e n  as con- 
t r o l  p e r i o d .  The i n j e c t i o n  o f  probenecid caused a decrease i p  renal c l e a r a n c e  
o f  metolazone t o  33 - + 7 ml/min, which corresponds t o  18 - + 2.8 per c e n t  o f  t h e  

24 



c l e a r a n c e  d u r i n g  p e r i o d  -1. The d e c l i n e  i n  c l e a r a n c e  was followed by a con- 
t i n u o u s  r i s e  i n  plasma l e v e l s  of metolazone u p  t o  930 2 204 ng/ml a t  3 h a f t e r  
t h e  a d m i n i s t r a t i o n  o f  probenecid ( F i g  2 ) .  

1000 - 

E 2 500- 
1 

- * *  * * * * * *  

1 IJ' 
f ' 

J 
' - 2  ' - 1  ' 1 2 ' 3 ' 4 ' 5 ' 6 ' Perlod 

P Fig 2 .  

.' 150- E 
I 
E 100- 

50 - 

C M  

2501 200 

U M  

100, 

* * * I * *  

T 

* * * * * *  
T T  

Plasma l e v e l s  ( S I ~ ) ,  renal c l e a r -  
ance (CM) a n d  u r i n a r y  e x c r e t i o n  
( U M )  of metolazone during i n t r a -  
venous i n f u s i o n  of metolazone 
b e f o r e  and a f t e r  i n j e c t i o n  of 
probenecid ( P ) .  * p<0.05 in com- 
parison t o  c o n t r o l  period 
(hatched a r e a ) .  

25 



The u r i n a r y  f l o w  d u r i n g  t h e  p e r i o d  p r e c e e d i n q  t h e  i n j e c t i o n  o f  p r o b e n e c i d  

was 4 . 1  - + 0.6 m l / m i n  i n  mean ( F i g  3 ) .  The i n j e c t i o n  o f  p r o b e n e c i d  c a u s e d  a 
s l i g h t  b u t  i n s i g n i f i c a n t  decrease i n  d i u r e s i s  w h i c h  was f o l l o w e d  b y  a s i g -  

n i f i c a n t  mean i n c r e a s e  ( p  < 0.05) i n  e x c r e t i o n  o f  u r i n e ,  d u r i n g  p e r i o d  3 and 4 
when t h e  d i u r e s i s  averaged 5.3 1.3 (131.4 2 32.4 p e r  c e n t  o f  c o n t r o l )  a n d  
4.9 - + 1.6 ( 1 2 0 . 3  - + 1 1 . 0  p e r  c e n t )  m l / m i n ,  r e s p e c t i v e l y .  The response t o  t h e  
i n j e c t i o n  o f  p r o b e n e c i d  w i t h  r e g a r d s  t o  t i m e  f o r  maximal c h a n g e  d i f f e r e d  be- 

t w e e n  i n d i v i d u a l s .  Thus, t h e  maximal i n d i v i d u a l  i n c r e a s e  i n  d i u r e s i s  averaged 
155.9 + 15.4 Der c e n t  o f  c o n t r o l  d i u r e s i s  ( p  < 0.001). 

* *  

m - 1  1 2  3 4 5 
P 

* * 

1 6 Period F i g  3 a,b. U r i n a r y  o u t p u t  o f  w a t e r  ( U H ~ O )  c h l o r i d e  ( U c i - ) ,  sodium (Uva+) and 
p o t a s s i u m  ( U K + )  d u r i n g  i n t r a v e n o u s  
i n f u s i o n  o f  m e t o l a z o n e  b e f o r e  and 
a f t e r  i n j e c t i o n  o f  p r o b e n e c i d  ( P ) .  
* p < 0 . 0 5  i n  comparison t o  c o n t r o l  
p e r i o d  ( h a t c h e d  a r e a ) .  

* * *  m 
- 1 1 2 3 4 5  

8 

KE - 1 1 2 3  
m +  

1 4 5  

6 Period 

1 6 Period 
1 6 Period 

26 



GFR was 1 1 2  - + 1 4  ml/min d u r i n g  t h e  period b e f o r e  t h e  i n j e c t i o n  of 
metolazone. I t  did not d i f f e r  from t h a t  during t h e  c o n t r o l  p e r i o d  when i t  
averaged 1 0 9  - + 13 rnl/min. Probenecid caused a s i g n i f i c a n t  increase in GFR 
during periods 3 ( 1 2 3  5 1 6  ml/min) and 4 (114 - + 9 ml/min). This increase could 
a t  l e a s t  i n  part explain the increased d i u r e s i s  during those periods, as cor- 
responding mean fractional excretion of water ( E F  ) ( 4 . 5  2 1.4 a n d  4.3 - + 1.3 
per cent, respectively) did n o t  d i f f e r  s i g n i f i c a n t l y  from t h a t  during the con- 
t r o l  period (3.8 + 0.5 per c e n t ) .  However, the maximal individual EF ( 6 . 1  - + 

0.6 per cent; period 4.7 - + 1) corresponded t o  163.3 - + 25.7 per cent of control 
( p  < 0,001). ( F i g  4 ) .  

"2O 

H2° 
- 

EF 
per cent 

H20 K '  
Na' 
CI- I" * * * 

* * I * 

Na+ CI - 
H P  

Fig 4. 
Maximal change ( u n f i l l e d  
a r e a )  in fractional ex- 
c r e t i o n  ( E F )  o f  water, 
sodium, chloride and potas- 
sium during intravenous 
infusion of metolazone 
before and a f t e r  injection 
of probenecid. 
** p<O.Ol 
*** p<O.OOI in comparison 
t o  control period (hatched 
a r e a ) .  

K' 

N a t r i u r e s i s  averaged 546 - + 7 3  pmol/min during the control period ( F i g  3 ) .  
After probenecid i t  was s i g n i f i c a n t l y  increased ( p  < 0.05) during p e r i o d s  3 - 5  
with maximal n a t r i u r e s i s  averaging 696 - + 117 pmolhin ( 1 2 7 . 8  - + 13.2 per c e n t ) .  
Again, increased G F R  could in p a r t  explain the change during p e r i o d s  3 a n d  4 
a s  t h e  f r a c t i o n a l  e x c r e t i o n  of sodium ( E F N a )  d i d  n o t  d i f f e r  from control 
value, which amounted t o  3 . 6  - + 0 . 5  per c e n t .  However, mean E F N a  was s i g -  
n i f i c a n t l y  g r e a t e r  ( p  < 0 . 0 5 )  t h a n  control value during period 5 ( 5 . 0  - + 1.3 
per c e n t ) .  Also, maximal individual E F N a ;  5.4 1.1 per c e n t  was seen d u r i n g  
period 5.0 - + 1.1 (150.3 - + 23.4 per cent of control value; p < 0.01; Fig 4 ) .  

Chloride excretion was s i g n i f i c a n t l y  higher during p e r i o d s  3 - 5  t h a n  d u r i n g  
t h e  c o n t r o l  p e r i o d  when i t  averaged 5 6 0  - + 77 pmol/min. ( F i g  3 ) .  Maximal 
chloruresis was 719 2 103 pmol/min ( 1 2 9 . 4  - + 16.7 per cent of control) in mean. 
The fractional excretion of chloride was 5.0 - + 0.4 per cent during the control 
period. I t  increased t o  5.4 - + 0.5 ( p  < 0.05) a n d  5.9 - + 0.6 ( p  < 0.01) per cent 

21 



d u r i n g  p e r i o d s  4 a n d  5 ,  r e s p e c t i v e l y .  Mean value f o r  h i g h e s t  i n d i v i d u a l  r i s e  
was 6.9 0.6 per c e n t  ( p e r i o d  4.9 - + 1 . 2 ) ;  ( F i g  5). 

Fig 5. 
D i s s o c i a t i o n  of t u b u l a r  e f f e c t s  of 
metolazone on sodium and potassium 
t r a n s p o r t .  F r a c t i o n a l  e x c r e t i o n  of 
sodium ( E F  ) and potassium ( E F  ) 
b e f o r e  ( 0 ) N g f t e r  ( 0 )  probenecid &e 
shown. 

EFK+ per cent 

10 

E F ~ ~ .  per cent 

T h e  e x c r e t i o n  o f  p o t a s s i u m  a v e r a g e d  99 - + 36 pmol/min during t h e  c o n t r o l  
p e r i o d .  ( F i g  3 ) .  The i n j e c t i o n  of probenecid caused a f a l l  i n  k a l i u r e s i s  which 
was s i g n i f i c a n t  ( p  < 0 . 0 5 )  d u r i n g  p e r i o d s  2 and 6 ,  when i t  averaqed 71 - + 22 
( 7 6 . 1  - + 1 4 . 3  p e r  c e n t )  a n d  77 - + 3 3  ( 7 0 . 3  - + 2 8 . 4  p e r  c e n t )  p m o l / m i n ,  
r e s p e c t i v e l y .  The f r a c t i o n a l  e x c r e t i o n  of potassium was 23.8 - + 9.0 per c e n t  
during t h e  c o n t r o l  p e r i o d .  I t  decreased t o  19.6 - + 7.0 ( p  < 0 . 0 5 ) ,  1 7 . 4  - + 5.1 
( p  < 0.01) a n d  19.0 - + 6.0 ( p  < 0.05) during p e r i o d s  1, 2 and 3 ,  r e s p e c t i v e l y .  
The most pronounced i n d i v i d u a l  decrease was down t o  14.6 - + 6 . 8  p e r  c e n t  ( p  < 
0.001) d u r i n g  period 4.4 - + 1.7. ( F i g  4 ) .  
Renal c l e a r a n c e  of P A H  decreased from c o n t r o l  value of 509 - + 89 t o  an a v e r a g e  
o f  357 - + 86 m l / m i n  d u r i n g  t h e  s i x  p e r i o d s  f o l l o w i n g  probenecid which cor- 
responds t o  70.0 - + 14.8 per c e n t  of c o n t r o l  value ( p  < 0.01). 

DISCUSSION. 
Metolazone (500ph1) caused a 20% r e d u c t i o n  o f  t h e  t r a n s e p i t h e l i a l  p o t e n t i a l  

d i f f e r e n c e  and s h o r t  c i r c u i t  c u r r e n t  without a f f e c t i n g  t h e  D C  r e s i s t a n c e  i n  
i s o l a t e d  o p e r c u l a r  e p i t h e l i a  o f  F u n d u l u s  h e t e r o c l i t u s  ( T a b l e  I ) ,  a n  
e p i t h e l i u m  t h a t  t r a n s p o r t s  c h l o r i d e  e l e c t r o g e n i c a l l y  (9). T h i s  m o d e r a t e  
d e c r e a s e  p r o b a b l y  r e f l e c t s  u n s p e c i f i c  a c t i o n s  o f  m e t o l a z o n e .  " C l a s s i c a l "  
t h i a z i d e s  e.g. h y d r o c h l o r o t h i a z i d e  have no e f f e c t  on the o p e r c u l a r  e p i t h e l i u m  
while loop d i u r e t i c s  cause a profound i n h i b i t i o n  of c h l o r i d e  t r a n s p o r t  i n  t h i s  
e p i t h e l i u m  (11). 

Moreover, we f o u n d  no e f f e c t s  of m e t o l a z o n e  ( 5 0 0 p M )  on e l e c t r i c a l  
p a r a m e t e r s  i n  t h e  i s o l a t e d  b l a d d e r  of Bufo v u l g a r i s  ( T a b l e  I ) .  T h i s  i s  con- 
t r a r y  t o  t h e  i n h i b i t i o n  o f  SCC and P O  r e p o r t e d  by R i v e c c a  e t  a 1  ( 3 4 )  

28 



s u g g e s t i n g  an i n h i b i t i o n  b y  m e t o l a z o n e  o f  a c t i v e  sodium t r a n s p o r t .  The d i f -  

f e r e n c e  c o u l d  r e f l e c t  a s p e c i e s  d i f f e r e n c e .  R i v e c c a  e t  a1 (34) t h u s  u s e d  
b l a d d e r s  f r o m  B u f o  m a r i n u s ,  a s a l t  w a t e r  t o a d  w h i l e  o u r  o b s e r v a t i o n s  were made 

on B u f o  v u l g a r i s ,  a f r e s h  w a t e r  t o a d .  R e s u l t s  f r o m  s i m i l a r  s t u d i e s  w i t h  o t h e r  

b e n z o t h i a z i d e s  a d d s  t o  t h e  c o m p l e x i t y .  F o r  i n s t a n c e ,  P e n d l e t o n  e t  a1 (311 
f o u n d  b e n z o f l u m e t h i a z i d e  t o  decrease SCC when added t o  s e r o s a l  s o l u t i o n  and t o  

s l i g h t l y  i n c r e a s e  S C C  when a d d e d  t o  t h e  mucosal s i d e  o f  t h e  b l a d d e r  o f  Bufo 
m a r i n u s .  Marumo e t  a1 ( 2 0 )  o n  t h e  o t h e r  h a n d  f o u n d  h y d r o c h l o r o t h i a z i d e  t o  

cause a p a r t i a l  i n h i b i t i o n  o f  SCC a f t e r  mucosal a d d i t i o n  and no e f f e c t s  on t h e  

s e r o s a l  s i d e  o f  t h e  f r o g  Rana c a t e s b e i a n a  b l a d d e r .  S c h n i e d e r s  and L u d e n s  ( 3 5 )  
o b s e r v e d  a 50% d e c r e a s e  o f  t h e  SCC by 640pM h y d r o c h l o r o t h i a z i d e ,  added t o  b o t h  
s i d e s  o f  t h e  chamber. Thus, i t  a p p e a r s  t h a t  t h e  e p i t h e l i a  s t u d i e d  d i f f e r  much 

i n  t h e i r  s e n s i t i v i t y  t o  t h i a z i d e s .  A l s o ,  i t  seems t h a t  i f  t h e  e f f e c t s  o f  d i f -  

f e r e n t  t h i a z i d e s  a r e  t o  b e  c o m p a r e d  t h e  same e p i t h e l i u m  s h o u l d  b e  u s e d .  

I n t e r e s t i n g l y ,  i n  a r e c e n t  s t u d y  b y  S t o k e s  e t  a1 ( 4 0 )  a t h i a z i d e - s e n s i t i v e  

e l e c t r i c a l l y  n e u t r a l  t r a n s p o r t  svstem has been d e s c r i b e d  i n  t h e  b l a d d e r  o f  t h e  

w i n t e r  f l o u n d e r .  I n  t h i s  system, h y d r o c h l o r o t h i a z i d e  and m e t o l a z o n e  ( b u t  n o t  

f u r o s e m i d e )  had q u a l i t a t i v e l y  s i m i l a r  a c t i o n s .  

U s i n g  t h e  m o d i f i e d  Sperher t e c h n i q u e  i t  was found t h a t  C14 m e t o l a z o n e  was 
s u b j e c t  t o  a c t i v e  t u b u l a r  s e c r e t i o n  i n  t h e  k i d n e y  ( F i g  1 ) .  The TTEF v a l u e  o f  

14% shows a t u b u l a r  s e c r e t i o n  t h a t  i s  s m a l l e r  t h a n  f o r  h y d r o c h l o r o t h i a z i d e  
(TTEF 59%) i n  t h i s  s p e c i e s  ( 2 8 ) .  N o v o b i o c i n  c o i n f u s i o n  r e d u c e d  t h e  s t e a d y  
s t a t e  TTEF v a l u e  o f  m e t o l a z o n e  t o  1/3 o f  p r e - i n h i b i t i o n  v a l u e s  w h i c h  i s  
e v i d e n c e  f o r  a c t i v e  s e c r e t i o n  o f  m e t o l a z o n e  by an o r g a n i c  a n i o n  t r a n s p o r t  sys- 

t e m  ( 2 4 ) .  T h r o u g h o u t  a l l  e x p e r i m e n t s ,  t h e  u r i n a r y  sodium c h l o r i d e  e x c r e t i o n  

was s y m m e t r i c a l  ( F i g  1). Thus, t h e  s a l u r e t i c  e f f e c t  was n o t  c o u p l e d  t o  t h e  
t u b u l a r  s e c r e t i o n  o f  t h e  d i u r e t i c .  T h i s  s u g g e s t s  t h a t  t h e  s a l u r e t i c  e f f e c t  o f  

m e t o l a z o n e  i s  n o t  p r i m a r e l y  l i n k e d  t o  t h e  u r i n a r y  e x c r e t i o n  o f  t h e  d r u g  

( l u m i n a l  e f f e c t ) .  A p r e d o m i n a n t  a c t i o n  f r o m  t h e  b l o o d  ( p e r i t u b u l a r )  s i d e  o f  

t h e  nephron i s  t h e r e f o r e  proposed, i n  a n a l o g y  w i t h  p r e v i o u s  s u g g e s t i o n s  f o r  

h y d r o c h l o r o t h i a z i d e  and c h l o r o t h i a z i d e  based on s i m i l a r  e x p e r i m e n t s  (28). T h i s  
t h e n  i s  d i f f e r e n t  f r o m  t h e  p r e d o m i n a n t e l y  l u m i n a l  e f f e c t s  o f  l o o p  d i u r e t i c s ,  

s u c h  a s  f u r o s e m i d e ,  p i r e t a n i d e  o r  e t h a c r y n i c  a c i d  ( 2 4 ) .  A l s o ,  t h e  p o t a s s i u m  

e x c r e t i o n  was s y m m e t r i c a l  d u r i n g  m e t o l a z o n e  i n f u s i o n  ( F i g  1 )  a s  p r e v i o u s l y  

s h o w n  f o r  h y d r o c h l o r o t h i a z i d e  w h i l e  c h l o r o t h i a z i d e  showed a s e c r e t i o n -  

d e p e n d e n t  k a l i u r e s i s  ( 2 8 ) .  It h a s  b e e n  p r o p o s e d  t h a t  t h i s  r e f l e c t s  a n  

i n h i b i t i o n  o f  c a r b o n i c  anhydrase b y  c h l o r o t h i a z i d e  i n  t h e  a v i a n  nephron. The 

l a c k  o f  a s i m i l a r  e f f e c t  o f  m e t o l a z o n e  s u p p o r t s  t h i s  i n t e r p r e t a t i o n  s i n c e  

m e t o l a z o n e  i s  a p o o r  i n h i b i t o r  o f  c a r b o n i c  anhydrase ( 1 2 )  

29 



I n  n o n - h y d r a t e d  b u t  u r i n e - l o s s  s u b s t i t u t e d  h e a l t h y  v o l u n t e e r s ,  m e t o l  a z o n e  

i n f u s i o n  ( 1 0  mg b o l u s  a n d  1 0  m g / h r )  g a v e  c l e a r  d i u r e t i c  b u t  e s p e c i a l l y  

s a l u r e t i c  e f f e c t s  w i t h  f r a c t i o n a l  e x c r e t i o n  o f  w a t e r  a n d  s o d i u m  c h l o r i d e  i n  

s t e a d y  s t a t e  o f  3.5 - 5% ( F i g  2 and 3 ) .  I n  p r e v i o u s  s i n g l e  dose e x p e r i m e n t s ,  
maximal f r a c t i o n a l  sodium e x c r e t i o n  o f  4 - 7% has been r e p o r t e d  f o l l o w i n g  5 - 
1 0  mg metolazone i n t r a v e n o u s l y  ( 8 ,  32, 39, 4 1 ) .  Thus, t h e  maximal d i u r e t i c  and 

s a l u r e t i c  e f f i c i e n c y  o f  m e t o l a z o n e  i s  much s m a l l e r  t h a n  t h o s e  o f  l o o p  

d i u r e t i c s  as r e v i e l e d  i n  s i m i l a r  s t u d i e s  ( 2 5 ,  29, 3 0 ) .  It i s  o f  i n t e r e s t  t h a t  
i n  h y d r a t e d  s u b j e c t s  metolazone had no ( a d d i t i v e )  d i u r e t i c  e f f e c t  (21, 32, 38, 

3 9 ) .  

Use o f  a s t e a d y  s t a t e  e x p e r i m e n t a l  t e c h n i q u e  w i t h  c o n t i n u o u s  i n f u s i o n  o f  

t h e  d r u g  a n d  f u l l  s u b s t i t u t i o n  o f  d i u r e t i c - i n d u c e d  volume l o s s e s  has p r o v e n  

s u c c e s s f u l  i n  p r e v i o u s  s t u d i e s  o f  d i u r e t i c  e x c r e t i o n - e f f e c t  r e l a t i o n s h i p  i n  

h e a l t y  v o l u n t e e r s  ( 2 5 ,  29, 3 0 ) .  T h i s  t e c h n i q u e  r e d u c e s  i n t e r p r e t a t i o n  d i f -  

f i c u l t i e s  t h a t  may a r i s e  i n  s i n g l e  dose e x p e r i m e n t s  due t o  t h e  t i m e  c o u r s e  o f  

d i u r e t i c  e x c r e t i o n  ( c f .  1 6 )  o r  due t o  a c u t e  t o l e r a n c e  d e v e l o p e m e n t . ( l 5 )  

We f o u n d  t h e  r e n a l  c l e a r a n c e  o f  m e t o l a z o n e  i n  h e a l t h y  v o l u n t e e r s  t o  a v e r a g e  

1 7 3  t o  1 8 4  m l / m i n  i n  s t e a d y  s t a t e  ( F i g  4 ) .  I n  v i e w  o f  t h i s  a n d  p r e v i o u s  

r e p o r t s  o f  a plasma p r o t e i n  b i n d i n g  o f  m e t o l a z o n e  o f  95% ( 4 2 )  i t  i s  c l e a r  t h a t  

t u b u l a r  s e c r e t i o n  c o n t r i b u t e s  s u b s t a n t i a l l y  t o  t h e  r e n a l  e l i m i n a t i o n  o f  t h e  

d r u g  i n  humans. I n  c o n f i r m a t i o n  o f  t h i s ,  p r o b e n e c i d  r e d u c e d  t h e  u r i n a r y  e x c r e -  

t i o n  r a t e  o f  m e t o l a z o n e  a n d  i t s  c l e a r a n c e  a s  much a s  down t o  18% o f  p r e -  

i n h i b i t i o n  v a l u e s .  ( F i g  4 ) .  I n  s p i t e  o f  t h i s ,  p r o b e n e c i d  d i d  n o t  r e d u c e  t h e  

d i u r e t i c  o r  s a l u r e t i c  e f f e c t s  o f  m e t o l a z o n e ;  on t h e  c o n t r a r y  t h e r e  was an i n -  

c r e a s e  i n  t h e s e  e f f e c t s  ( F i g  2 a n d  3 ) .  T h e r e f o r e ,  i t  a p p e a r s  t h a t  a l s o  i n  

humans m e t o l  a z o n e  e x c e r t s  i t s  d i u r e t i c / s a l u r e t i c  e f f e c t s  p r i m a r i l y  f r o m  t h e  

p e r i t u b u l a r  ( b l o o d )  s i d e  o f  t h e  nephron and n o t  f r o m  t h e  l u m i n a l  ( u r i n e )  s i d e .  

T h i s  i s  c o n t r a r y  t o  t h e  c a s e  f o r  s e v e r a l  l o o p  d i u r e t i c s  ( 2 5 ,  29, 3 0 ) .  A 

p r o b e n e c i d  i n d u c e d  i n c r e a s e  i n  t h e  d i u r e s i s / s a l u r e s i s ,  s i m i l a r  t o  o u r  f i n d i n q  

f o r  metolazone, has, however, been d e s c r i b e d  by B r a t e r  ( 5 )  f o r  c h l o r o t h i a z i d e ,  

t h u s  a d d i n g  a n o t h e r  t h i a z i d e  - l i k e  p r o p e r t y  t o  metolazone. 
I n t e r e s t i n g l y ,  p r o b e n e c i d  d e c r e a s e d  m e t o l a z o n e  i n d u c e d  a b s o l u t e  and f r a c -  

t i o n a l  p o t a s s i u m  e x c r e t i o n  ( F i g  3 and 4 ) .  The i n h i b i t o r  t h e r e f o r e  d i s s o c i a t e d  

t h e  e f f e c t s  o f  m e t o l a z o n e  on t u b u l a r  sodium t r a n s p o r t  f r o m  t h a t  on p o t a s s i u m  

t r a n s p o r t  ( c f . F i g  5 ) ,  w h i c h  i s  e s p e c i a l l y  n o t e w o r t h y  s i n c e  i t  i s  g e n e r a l l y  

h e l d  t h a t  t h e  g r e a t e r  sodium l o a d  t h a t  appears a t  t h e  d i s t a l  t u b u l a r  s i t e s  o f  

sodium-potassium exchange, t h e  g r e a t e r  t h e  o u t p u t  o f  p o t a s s i u m  ( c f .  14). Thus, 
i t  appears t h a t  m e t o l a z o n e  c o u l d  have s e p a r a t e  e f f e c t s  on sodium and p o t a s s i u m  

e x c r e t i o n  i n  h e a l t y  s u b j e c t s  ( F i g  5 ) .  Moreover, t h e s e  r e s u l t s  a r e  c o m p a t i b l e  

w i t h  a d i r e c t  l u m i n a l  k a l i u r e t i c  e f f e c t  o f  m e t o l a z o n e .  M e t o l a z o n e  has a 

30 



" p o t a s s i u m - s p a r i n g "  e f f e c t ,  w h i c h  i s  d i f f e r e n t  f r o m  t h e  w e l l  d e f i n e d  

p o t a s s i  u m - s p a r i  n g  e f f e c t s  o f  b o t h  a m i l  o r i d e  and a1 d o s t e r o n e - a n t a g o n i s t s .  ( 3 2  

However, a1 t h o u g h  an a n t i k a l  i u r e t i c  e f f e c t  o f  metolazone h a s  b e e n  d o c u m e n t e d  

( 2 1 ,  o t h e r  a c u t e  c l e a r a n c e  s t u d i e s  i n  h e a l t h y  s u b j e c t s  have shown i n c o n s i s t a n t  
e f f e c t s  on p o t a s s i u m  e x c r e t i o n  ( 2 1 ,  3 9 )  o r  i n c r e a s e d  p o t a s s i u m  e x c r e t i o n  ( 2 2 ,  
38, 4 1 ) .  i n  p a t i e n t s  w i t h  o e d e m a t o u s  d i s o r d e r s ,  m e t o l a z o n e  i n c o n s i s t e n t l y  
cause h y p e r k a l i u r i a  a s s o c i a t e d  w i t h  h y p o k a l e m i a .  ( 1 9 ) .  T h i s  m i q h t  be r e l a t e d  

t o  t h e  p r e s e n c e  o f  h y p e r a l d o s t e r o n i s m ,  s i n c e  D O C A  p r e t r e a t m e n t  r e v e r s e d  

m e t o l a z o n e  a n t i - k a l i u r e s i s  i n  h e a l  t h y  s u b j e c t s .  M o r e o v e r ,  i n  c o m b i n a t i o n  - 
t h e r a p y  o f  s e v e r e  oedematous d i s o d e r s  w i t h  l o o p  d i u r e t i c s  and metolazone, ex- 

c e s s i v e  p o t a s s i u m  e x c r e t i o n  and s e v e r e  h y D o k a l e m i a  c o u p l e d  t o  t h e  d i u r e t i c  

e f f e c t  h a s  b e e n  d e s c r i b e d  ( 4 ) .  T h e r e f o r e ,  f r o m  a c l i n i c a l  p o i n t  o f  v i e w  
m e t o l a z o n e  does n o t  seem t o  o f f e r  any advantage o v e r  " o r d i n a r y "  t h i a z i d e s  w i t h  

r e g a r d  t o  p o t a s s i u m  l o s s e s .  

M e t o l a z o n e  i n f u s i o n  c a u s e d  n o  c h a n g e s  i n  GFR a s  m e a s u r e d  b y  t h e  r e n a l  

c l e a r a n c e  o f  i n u l i n .  T h i s  i s  p e r h a p s  n o t  s u r p r i s i n g  c o n s i d e r i n g  t h a t  we sub- 

s t i t u t e d  f o r  u r i n e  volume l o s s e s  f o r  each p e r i o d  and i t  i s  i n  a q r e e m e n t  w i t h  

o t h e r  r e p o r t s  i n  h y d r a t e d  s u b j e c t s  ( 2 2 ,  32, 38-40). B e n n e t t  and P o r t e r  ( 2 )  

even f o u n d  an i n c r e a s e d  GFR a f t e r  m e t o l a z o n e  w h i l e  GFR was u n c h a n q e d  a f t e r  

c h l o r o t h i a z i d e .  T h i s  c o u l d  perhaps be due t o  d i f f e r e n t  a d m i n i s t r a t i o n s  o f  t h e  

t w o  d r u g s  - m e t o l a z o n e  was g i v e n  as a b o l u s  w h i l e  c h l o r o t h i a z i d e  was g i v e n  a s  
a b o l u s  a n d  a s u s t a i n i n g  i n f u s i o n .  We f o u n d  t h a t  t h e  GFR i n c r e a s e d  
( s i g n i f i c a n t l y  d u r i n g  p e r i o d s  3 a n d  4 )  a f t e r  t h e  p r o b e n e c i d  i n j e c t i o n .  The 

r e a s o n  f o r  t h i s  i s  u n c l e a r .  I n  o u r  e x p e r i e n c e  f r o m  s i m i l a r  s t u d i e s  w i t h  o t h e r  

d i u r e t i c ,  p r o b e n e c i d  i n j e c t i o n  has e i t h e r  l e f t  GFR unchanged ( 2 5 )  o r  decreased 

( 2 9 ,  3 0 ) .  Renal plasma f l o w ,  as d e t e r m i n e d  by t h e  r e n a l  c l e a r a n c e  o f  PAH, was 

as e x p e c t e d  s i g n i f i c a n t l y  r e d u c e d  by probenec d; an e f f e c t  o f  i n h i b i t i o n  o f  

PAH s e c r e t i o n .  

On t h e  w h o l e ,  m e t o l a z o n e  e f f e c t s  o n  e l e c t r o l y t e  t r a n s p o r t  seem t o  b e  

q u a 1  i t a t i v e l y  s i m i l a r  t o  t h o s e  o f  " c l a s s i c a l "  t h i a z i d e s .  The f a c t  t h a t  

m e t o l a z o n e  ( a l o n e  o r  i n  c o m b i n a t i o n  w i t h  a l o o p  d i u r e t i c )  can be an e f f e c t i v e  

d i u r e t i c  a g e n t  i n  p a t i e n t s  w i t h  a p p a r e n t  r e s i s t a n c e  t o  d i u r e t i c s  e.g. i n  r e n a l  

f a i l u r e ,  has been t a k e n  as a c l a i m  f o r  some u n i q u e  p r o p e r t i e s  o f  t h i s  d r u g  a s  

c o m p a i r e d  t o  t h e  t h i a z i d e s .  However, some o b s e r v a t i o n s  w i t h  b e n z o t h i a z i d e s  (1, 

3 3 )  i n d i c a t e  t h a t  i f  t h e  dose i s  i n c r e a s e d ,  t h e s e  d i u r e t i c s  may a l s o  be e f f e c -  

t i v e  i n  s i m i l a r  s i t u a t i o n s .  A c l e a r  answer t o  t h i s  q u e s t i o n  a w a i t s  p r o p e r l y  

d e s i g n e d  c l i n i c a l  t r i a l s .  

31 



Acknowledgments. 
T h e  a u t h o r s  a r e  i n d e p t e d  t o  Mrs S i g r i d  S a n d b e r g ,  f o r  e x c e l l e n t  t e c h n i c a l  
a s s i s t a n c e .  We want t o  thank Pharmacia A B ,  Uppsala, Sweden f o r  providing t h e  
metolazone s o l u t i o n s .  

1. 

2. 

3. 

4 .  

5. 

6 .  

7 .  

8. 

9. 

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4 3 .  Webb-Peploe, M . M . :  New approaches t o  t h e  t r e a t m e n t  o f  h e a r t  f a i l u r e .  I n  
T o p i c s  i n  t h e r a p e u t i c s ,  Vol 5 .  Royal C o l l e g e  o f  P h y s i c i a n s  o f  London. 
E d i t o r s :  Davies DM and Rawlins M . D ;  Pitman Medical, Publ.; 137-161, 1979. 

Send r e p r i n t  r e q u e s t s  t o :  
60 Odlind, M D ,  P h . D .  
Department o f  Medicine 
U n i v e r s i t y  Hospital 
751 85 Uppsala 
Sweden 

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