T

New Aspects on Training Bradycardia 
Eva Nylander and Nils-Holger Areskog 

From the Department of Clinical Physiology, University of Linkoping, 
Region Hospital, Linkoping, Sweden 

Dedicated to Professor Torsten Teorell 

ABSTRACT 

R a t s  were t r a i n e d  by t r e a d m i l l  running a f t e r  chemical sympathectomy with 

6-hydroxy-dopamine or d u r i n g  c h r o n i c  b e t a  r e c e p t o r  blockade. Contrary t o  un- 
t r e a t e d  t r a i n e d  a n i m a l s ,  sympathectomizCd r a t s  d i d  n o t  g e t  a r e d u c t i o n  of t h e  

i n t r i n s i c  h e a r t  r a t e  a f t e r  t r a i n i n g  d e s p i t e  an i n c r e a s e d  h e a r t  weight. I n  

c o n t r a s t ,  no c a r d i a c  hypertrophy o c c u r r e d  a f t e r  t r a i n i n g  d u r i n g  b e t a  a d r e n e r -  

g i c  blockade b u t  t h e  h e a r t  r a t e  d u r i n g  e x e r c i s e  was reduced i n  t h e s e  animals. 

I t ' i s  concluded t h a t  t h e  t r a i n i n g - i n d u c e d  b r a d y c a r d i a  c o n t a i n s  a lowering o f  

t h e  i n t r i n s i c  h e a r t  r a t e  and t h a t  t h i s  i s  n o t  dependent on t h e  s t i m u l a t i o n  

of c a r d i a c  b e t a  r e c e p t o r s  or t h e  magnitude of h e a r t  r a t e  i n c r e a s e  d u r i n g  

e x e r c i s e .  The r e s u l t s  a l s o  i n d i c a t e  t h a t  t h e r e  i s  not a c a u s a l  r e l a t i o n s h i p  

between t h e  t r a i n i n g - i n d u c e d  b r a d y c a r d i a  and c a r d i a c  h y p e r t r o p h y .  The l a t t e r  

c o n c l u s i o n  i s  s u p p o r t e d  by an e c h o c a r d i o g r a p h i c  study i n  humans where no 

c o r r e l a t i o n  was found between I H R  and c a r d i a c  dimensions. 

INTRODUCTION 

A low h e a r t  r a t e  ( H R )  a t  r e s t  and d u r i n g  e x e r c i s e  a t  submaximal work l o a d s  

i s  an e f f e c t  of endurance t r a i n i n g  t h a t  h a s  been r e c o g n i z e d  s i n c e  l o n g  ago 

and s i n c e  t h e  beginning of t h i s  c e n t u r y  a t t r i b u t e d  t o  an a l t e r e d  autonomic 

nervous a c t i v i t y ,  mainly an i n c r e a s e d  v a g a l  t o n e  ( 1 7 ) .  Recent i n v e s t i g a t i o n s  

have shown t h a t  a lowered r a t e  of t h e  c a r d i a c  pacemaker ( i n t r i n s i c  h e a r t  

r a t e ,  I H R )  i s  p a r t  of t h e  e x p l a n a t i o n  t o  t h i s  b r a d y c a r d i a  seen i n  p h y s i c a l l y  

t r a i n e d  i n d i v i d u a l s  (18,4,9). 

S t i l l ,  however, t h e  mechanisms for t h e  lowering o f  IHR through t r a i n i n g  a r e  
known, and t h e  i n v e s t i g a t i o n s  t o  be r e p o r t e d  were undertaken t o  i d e n t i f y  some 

f a c t o r s  of importance f o r  t h i s  a d a p t a t i o n .  We p r e f e r  t o  look upon p h y s i c a l  

t r a i n i n g  as an a d a p t a t i o n  t o  an i n c r e a s e d  p h y s i c a l  a c t i v i t y  r e s u l t i n g  i n  an 

i n c r e a s e d  c a p a c i t y  t o  perform t h i s  a c t i v i t y .  Hereby f o l l o w s  t h a t  t h e  c i r c u l a -  

t o r y  and o t h e r  a d j u s t m e n t s  a t  each e x e r c i s e  o c c a s i o n ,  r e p e a t e d  a t  c e r t a i n  

1-822855 1 



i n t e r v a l s  and f o r  a s u f f i c i e n t l y  l o n g  d u r a t i o n  induce t h e  " t r a i n i n g  e f f e c t s " .  

By manipulation of t h e  c a r d i o v a s c u l a r  a d a p t a t i o n  a t  each t r a i n i n g  s e s s i o n  

( e . g .  t h e  i n c r e a s e  of oxygen consumption, s t r o k e  volume or H R )  it would be 
p o s s i b l e  t o  draw c o n c l u s i o n s  about t h e  importance o f  t h e s e  d i f f e r e n t  f a c t o r s  

f o r  t h e  development of t r a i n i n g  e f f e c t s ,  i n  t h i s  c o n t e x t  t h e  t r a i n i n g  brady- 

c a r d i a  and r e d u c t i o n  of I H R .  

I n  t h i s  paper a r e  summarized such l o n g i t u d i n a l  t r a i n i n g  s t u d i e s  t h a t  have 

been c a r r i e d  o u t  i n  r a t s .  I n  a p r e v i o u s  c r o s s - s e c t i o n a l  human s t u d y  w e  have 

found evidence f o r  a t r a i n i n g - i n d u c e d  d e c r e a s e  of IHR a l s o  i n  man ( 9 ) .  A s  i n  
many o t h e r  s t u d i e s  we a l s o  found g r e a t e r  r o e n t g e n o l o g i c a l  h e a r t  volumes i n  

t r a i n e d  t h a n  i n  u n t r a i n e d  p e r s o n s .  We have now performed an e c h o c a r d i o g r a p h i c  

s t u d y  of endurance a t h l e t e s  t o  i n v e s t i g a t e  whether t h e r e  i s  any c a u s a l  r e -  

l a t i o n s h i p  between I H R  and c a r d i a c  dimensions. 

MATERIAL AND METHODS 

R a t s  were t r a i n e d  by treadmill running. A method f o r  r e c o r d i n g  of ECG 

d u r i n g  e x e r c i s e  and an e x e r c i s e  t e s t  f o r  r a t s  w i t h  s t e p w i s e  i n c r e a s e d  t r e a d -  

m i l l  speed t o  t e s t  t h e  e f f i c i e n c y  of t h e  t r a i n i n g  program were developed 

(5). One group of r a t s  w a s  t r a i n e d  a f t e r  chemical sympathectomy w i t h  6-hydroxy- 
dopamine (6-OH-DA)(10). L i t t e r m a t e  u n t r e a t e d  r a t s  were t r a i n e d  s i m u l t a n e o u s l y .  

Both t r e a t e d  and u n t r e a t e d  r a t s ,  l i t t e r m a t e s  t o  t h e  t r a i n i n g  o n e s ,  were k e p t  

as s e d e n t a r y  c o n t r o l s  ( 1 6 ) .  

I n  a s i m i l a r  s t u d y  one group of t r a i n i n g  r a t s  and one s e d e n t a r y  group were 

i n s t e a d  t r e a t e d  w i t h  m e t o p r o l o l ,  a c a r d i o s e l e c t i v e  b e t a  r e c e p t o r  a n t a g o n i s t  

( 1 ) o r a l l y  d u r i n g  t h e  t r a i n i n g  p e r i o d  ( 1 1  ) . 

A f t e r  a t r a i n i n g  p e r i o d  a l l  r a t s  were e x e r c i s e - t e s t e d .  I n  o r d e r  t o  measure 

t h e  I H R  a d e n e r v a t i o n  w a s  performed i n c l u d i n g  6-OH-DA t r e a t m e n t ,  c u t t i n g  of 

t h e  vagus n e r v e s  and s p i n a l  c o r d  d e s t r u c t i o n  ( p i t h i n g ) (  1 5 ) .  A t  t h e  end of t h e  

experiment t h e  h e a r t s  were weighed. 

I n  a s e p a r a t e  s t u d y  h e a r t s  from t r a i n e d  and s e d e n t a r y  r a t s  were i n v e s t i g a t e d  

a l s o  i n  a modified Langendorff p r e p a r a t i o n ,  t o  e n s u r e  t h a t  t h e  h e a r t  r a t e s  

measured i n  p i t h e d  p r e p a r a t i o n s  r e p r e s e n t  t h e  t r u e  i n t r i n s i c  HR ( 1 2 ) .  

I n  t h e  human study seven c o m p e t i t i v e  c y c l i s t s  of n a t i o n a l  e l i t e  c l a s s  were 

i n v e s t i g a t e d .  They had been t r a i n i n g  r e g u l a r l y  f o r  7-11 y e a r s  and t r a i n e d  

600-1000 km/week a t  t h e  t i m e  of t h e  s t u d y .  P h y s i c a l  c h a r a c t e r i s t i c s  a r e  given 

i n  Table 1. They performed a maximal e x e r c i s e  t e s t  with d e t e r m i n a t i o n  of maxi- 

2 



ma1 oxygen consumption with t h e  Douglas bag method and with r e c o r d i n g  of h e a r t  

r a t e  and b l o o d  p r e s s u r e  d u r i n g  e x e r c i s e .  On a s e p a r a t e  day t h e  t e s t  w a s  re- 

p e a t e d  a f t e r  t h e  a d m i n i s t r a t i o n  of a t r o p i n e  s u l p h a t e  0.04 mg/kg body weight 

and p r o p r a n o l o l  h y d r o c h l o r i d e  0.25 mg/kg body weight as d e s c r i b e d  p r e v i o u s l y  

(9). 

TABLE 1 .  P h y s i c a l  c h a r a c t e r i s t i c s  and maximal oxygen u p t a k e  (vo2 max) i n  
seven c o m p e t i t i v e  c y c l i s t s .  

GO max 
2 

$0 max 2 Age Height Weight 

Y cm kg l / m i n  ml/kg/min 

Mean 24 179 68 4 . 9  72 
+ + + -+ 
-1 SD -3 -7 -5 t 0 . 6  +5 

The e c h o c a r d i o g r a p h i c  examination was performed by one i n v e s t i g a t o r  ( E  . N .  ) 

b e f o r e  t h e  e x e r c i s e  t e s t .  A 3 mHz ATL (Advanced Technology L a b o r a t o r i e s )  

mechanical s e c t o r  scanner was u s e d  f o r  both 2-dimensional imaging and M-mode. 

A l l  r e c o r d i n g s  were performed w i t h  t h e  s u b j e c t  i n  t h e  l e f t  l a t e r a l  recumbent 

p o s i t i o n ,  e x c e p t  f o r  r i g h t  v e n t r i c u l a r  dimension which w a s  s t u d i e d  w i t h  t h e  

s u b j e c t  s u p i n e .  The measurements on t h e  M-mode r e c o r d i n g s  were performed 

f o l l o w i n g  t h e  recommendations of t h e  American S o c i e t y  of Echocardiography 

( 1 4 ) .  Echocardiographic s t r o k e  volume was e s t i m a t e d  w i t h  t h e  cube method. 
L e f t  v e n t r i c u l a r  mass was c a l c u l a t e d  a c c o r d i n g  t o  Bennett & Evans ( 2 ) .  

S t a n d a r d  s t a t i s t i c a l  methods were used. 

RESULTS 

E x e r c i s e  h e a r t  r a t e  and i n t r i n s i c  h e a r t  r a t e  i n  r a t s .  

A t  t h e  e x e r c i s e  t e s t  a f t e r  t h e  t r a i n i n g  p e r i o d  t r a i n e d  u n t r e a t e d  animals had 

s i g n i f i c a n t l y  lower HR t h a n  t h e  s e d e n t a r y  ones. Also a f t e r  d e n e r v a t i o n  and i n  
i s o l a t e d  h e a r t s  t h e r e  remained a s i g n i f i c a n t  HR d i f f e r e n c e  between t r a i n e d  

and s e d e n t a r y  r a t s  ( F i g  1 ) .  

There was an e x e r c i s e  HR d i f f e r e n c e  a l s o  between sympathectomized t r a i n e d  

and u n t r a i n e d  r a t s .  However, t h e  t r e a t m e n t  with 6-OH-DA r e s u l t e d  i n  h i g h e r  HR 

t h a n  were found i n  u n t r e a t e d  animals so t h a t  t h e  HR of t r a i n e d  t r e a t e d  animals 
were s i m i l a r  t o  t h o s e  of t h e - u n t r e a t e d  s e d e n t a r y  ones ( F i g  2 ) .  I n  t h e s e  a n i -  

mals t h e r e  w a s  no s i g n i f i c a n t  HR d i f f e r e n c e  between t r a i n e d  and u n t r a i n e d  

r a t s  a f t e r  c a r d i a c  d e n e r v a t i o n  o r  i s o l a t i o n .  

3 



heart rate 
beatslmin 

4oc 

300 

200 

100. 

T 

++ 

- - - _ _ _  
untraine 
trained 

I 
I 
I 
I 
I 
I 
I 
I 
I 
I 
I 
I 
I 
I 
I 
I 
I 
I 
I 
I 

I 

I 
I 

A 
++ 

I I I I I 1 I 

10 15 isoi!td 35 4 0  45 
time after 
pithing,rnin 

t 
pithing 5 

heart 

F i g  1 .  Heart r a t e s  i n  u n t r e a t e d  t r a i n e d  and u n t r a i n e d  r a t s  a f t e r  denervFtion 
(6-OH-DA, vagotomy and p i t h i n g )  and i n  t h e  i s o l a t e d  h e a r t ,  mean v a l u e s  -SEM. 
* p<0.05, ** ~ ( 0 . 0 1 .  From ( 1 2 ) .  

The metoprolol t r e a t e d  r a t s  performed t h e  e x e r c i s e  t e s t t h r e e  days a f t e r  

withdrawal of m e t o p r o l o l ,  when plasma c o n c e n t r a t i o n s  of t h e  drug were shown 

t o  be n e g l i g i b l e .  U n t r a i n e d  r a t s  had similar h e a r t  r a t e s  t o  t h e  u n t r e a t e d  

s e d e n t a r y  ones. The t r a i n e d  metoprolol t r e a t e d  r a t s  had s i g n i f i c a n t l y  lower 

HR t h a n  b o t h  t h e  s e d e n t a r y  animals and t h e  t r a i n e d  u n t r e a t e d  ones ( F i g  3 ) .  
I n  t h i s  s t u d y  t h e r e  was a s l i g h t  b u t  n o t  s i g n i f i c a n t  I H R  d i f f e r e n c e  between 

t r a i n e d  and u n t r a i n e d  r a t s ,  both i n  t h e  u n t r e a t e d  and t h e  m e t o p r o l o l  t r e a t e d  

group. 

Heart weights of r a t s  a f t e r  t r a i n i n g  

I n  u n t r e a t e d  a n i m a l s ,  t h e  a v e r a g e  h e a r t  weight was s i g n i f i c a n t l y  g r e a t e r  i n  

t h e  t r a i n e d  t h a n  i n  t h e  u n t r a i n e d  group (1.079 g 0.020 (S.E.) vs. 0.935 

O . O l L ,  p<O.OOl). Also t h e  h e a r t  weight / body weight r a t i o  (hw/bw) w a s  
g r e a t e r  i n  t h e  t r a i n e d  a n i m a l s ,  3.811 0100 t o . 0 6 2  vs. 3.293 0100 t 0 . 0 6 9 ,  
p<O.OOl). 

4 



heart rate 
b a t s  p i n  

6OC 

550 

I 

500 

450 

L *** 

0 normal 
0 sympathectornired 

untrained 
trained 

----- - 

y 
1 I 

21 25.5 30 
trsudrnilt speed 

rnlmin 

Fig 2. Heart r a t e s  d u r i n g  e x e r c i s e  a t  3 d i f f e r e n t  work l o a d s + i n  t r a i n e d  and 
u n t r a i n e d ,  u n t r e a t e d  and sympathectomized r a t s ,  mean v a l u e s  -SEM. A s t e r i s k s  
denote s t a t i s t i c a l l y  s i g n i f i c a n t  d i f f e r e n c e s  compared t o  c o r r e s p o n d i n g  un- 
t r a i n e d  group. * ~ ( 0 . 0 5 ,  ** p<O.Ol, *** ~ ( 0 . 0 1 .  From ( 1 2 ) .  

6-OH-DA t r e a t e d  t r a i n e d  r a t s  a l s o  had a s i g n i f i c a n t l y  i n c r e a s e d  hw/bw 

r a t i o  i n  one i n v e s t i g a t i o n  ( 1 6 )  and both hw and hw/bw i n  t h e  o t h e r  s t u d y  ( 1 2 ) .  
R a t s  t r a i n e d  d u r i n g  c h r o n i c  b e t a  b l o c k e r  t r e a t m e n t  d i d  n o t  e x h i b i t  a c a r d i a c  

hypertrophy ( 1 1 ) .  

IHR and c a r d i a c  dimensions i n  endurance a t h l e t e s .  

Heart r a t e s  and e c h o c a r d i o g r a p h i c  dimensions a r e  l i s t e d  i n  Table 2. The r e l a -  

t i o n  between I H R  and l e f t  v e n t r i c u l a r  mass or l e f t  v e n t r i c u l a r  i n n e r  dimension 

i s  i l l u s t r a t e d  i n  F i g  4, showing t h a t  t h e r e  was n o  s t r i k i n g  c o r r e l a t i o n  b e t -  
ween t h e s e  c a r d i a c  dimensions and I H R .  

5 



heart f a t e  
b e a t d m i n  

500 

450 

** 

400 

T 

** ? 3 
** -i 

trained untrained 
rn 0 untreated 
0 0 metoprolol treated 

r 

+/ I I I 
rest before 15 
exercise 

20 25 
treadmill speed m/min 

F i g  3. Heart r a t e s  a t  r e s t  and d u r i n g  e x e r c i s e  a t  t h r e e  d i f f e r e n t  work l o a d s  
i n  u n t r e a t e d  rats an$ i n  m e t o p r o l o l  t r e a t e d  r a t s  a f t e r  withdrawal of meto- 
p r o l o l ,  mean v a l u e s  -SEM. 

* : s i g n i f i c a n t  d i f f e r e n c e  between t r a i n e d  and corresponding u n t r a i n e d  
group . 

*: s i g n i f i c a n t  d i f f e r e n c e  between t r a i n e d  metoprolol t r e a t e d  and t r a i n e d  
u n t r e a k e d  group. 

*,*: pCO.05 *****: p(O.01 

DISCUSSION 

The group of endurance a t h l e t e s  i s  homogenous as r e g a r d s  p h y s i c a l  c h a r a c t e r -  

i s t i c s  and work c a p a c i t y .  Thus it i s  n o t  r e p r e s e n t a t i v e  of t h e  g e n e r a l  popula- 

t i o n  b u t  w i t h i n  t h i s  group t h e r e  w a s  no c o r r e l a t i o n  between I H R  and LV mass o r  

LV i n n e r  dimension. T h i s  i s  i n  accordance with our animal experiments and 

f i n d i n g s  by o t h e r s  ( 2 1 )  of a poor c o r r e l a t i o n  between h e a r t  weight and h e a r t  

6 



TABLE 2. Heart r a t e s  and e c h o c a r d i o g r a p h i c  dimensions i n  seven c o m p e t i t i v e  
c y c l i s t s .  

beatslmin beatslmin 
1201 120. 

100- 100, 

80 - * *  * *  80, 
+ *  

60, 60- 

40, 40- 

20 - 20- 

HR r e s t  IHR LA R V I D  IVS L V I D  LVPW LV-mass SV 

b/min b/min mm mm mm mm mm g m l  

Mean 52 8 1  37 1 8  1 2  58 1 2  380 130 

-1 SD -5 + I 3  22 24 21 23 - 1  ?65 :15 
+ + + 

LA = l e f t  a t r i u m ;  R V I D  = r i g h t  v e n t r i c u l a r  i n n e r  d i a m e t e r ;  IVS = i n t e r v e n -  
t r i c u l a r  s e p t a 1  t h i c k n e s s ,  L V I D  = l e f t  v e n t r i c u l a r  i n n e r  d i a m e t e r ;  LVPW = 
l e f t  v e n t r i c u l a r  p o s t e r i o r  w a l l  t h i c k n e s s ,  SV = e c h o c a r d i o g r a p h i c  s t r o k e  
volume. 

r a t e .  T h i s  i n d i c a t e s  t h a t  IHR i s  n o t  determined by c a r d i a c  dimensions only. 
The p r e s e n t  animal experiments show t h a t  an i n c r e a s e  i n  h e a r t  weight may occur 

without a l o w e r i n g  of IHR, as i n  t h e  sympathectomized t r a i n e d  animals and t h a t  
a t r a i n i n g - i n d u c e d  b r a d y c a r d i a  can develop without an i n c r e a s e  of c a r d i a c  mass, 

a s  was seen i n  t h e  metoprolol group. All t h i s  speaks a g a i n s t  t h e  b r a d y c a r d i a  

and r e d u c t i o n  of I H R  as d i r e c t  e f f e c t s  of an i n c r e a s e d  LV dimension of myo- 

c a r d i a l  mass. 

IHR IHR 

LV mass g LVID mm 

F i g  4 .  R e l a t i o n  between IHR and l e f t  v e n t r i c u l a r  (LV) mass e s t i m a t e d  by 
echocardiography ( l e f t )  and between IHR and l e f t  v e n t r i c u l a r  i n n e r  dimension 
( L V I D ) ( r i g h t )  i n  seven c o m p e t i t i v e  c y c l i s t s .  

7 



Which c a r d i o v a s c u l a r  a d j u s t m e n t s  d u r i n g  each e x e r c i s e  s e s s i o n  may t h e n  a c t  

a s  s t i m u l i  f o r  a r e d u c t i o n  of e x e r c i s e  and i n t r i n s i c  HR? T h i s  can n o t  y e t  be 

completely answered b u t  t h e  h y p o t h e s i s  of t h e  r e p e a t e d  h e a r t  r a t e  i n c r e a s e  

above a c e r t a i n  l e v e l  can be r e j e c t e d  s i n c e  b e t a  r e c e p t o r  blockade d i d  not 

p r e v e n t  t h e  development o f  t r a i n i n g - i n d u c e d  b r a d y c a r d i a .  Likewise, 6-OH-DA 

t r e a t e d  animals t h a t  c o n t r a r y  t o  t h e  b e t a  blocked ones have an i n c r e a s e d  HR 

r e s p o n s e  t o  e x e r c i s e  (16) d i d  n o t  g e t  a r e d u c t i o n  of e x e r c i s e  HR o r  I H R .  
Chronic a t r o p i n e  t r e a t m e n t ,  r e s u l t i n g  i n  r e p e a t e d  p e r i o d s  of t a c h y c a r d i a ,  i s  

a l s o  r e p o r t e d  not t o  a l t e r  t h e  i n t r i n s i c  r a t e  of i s o l a t e d  a t r i a  i n  r a t s  ( 7 ) .  
A d i r e c t  e f f e c t  of catecholamines a t  t h e  c a r d i a c  b e t a  r e c e p t o r s  i s  a l s o  un- 

l i k e l y  s i n c e  t h e  development of t r a i n i n g  b r a d y c a r d i a  was enhanced d e s p i t e  a 

diminished b e t a  s t i m u l a t i o n  i n  t r a i n e d  r a t s  r e c e i v i n g  metoprolol. However, 

i n d i r e c t l y  t h e  a d r e n e r g i c  n e r v e s  a r e  e s s e n t i a l  f o r  t h e  r e d u c t i o n  of I H R  s i n c e  

i n  t h e  6-OH-DA t r e a t e d  r a t s  t r a i n i n g  d i d  n o t  have t h i s  e f f e c t .  I n  accordance 

with t h i s  a r e  t h e  f i n d i n g s  by P a y n t e r  e t  a 1  ( 1 3 )  t h a t  a t r a i n i n g - i n d u c e d  

r e s t i n g  b r a d y c a r d i a  d i d  n o t  occur i n  immuno sympathectomized r a t s .  

It i s  w e l l  known from many animal s p e c i e s  t h a t  t h e  HR i n c r e a s e s  i n  response 

t o  a c u t e  mechanical s t r e t c h  of t h e  a t r i a  ( 8 ) .  The h y p o t h e s i s  h a s  been s e t  

f o r t h  by F r i c k  e t  a 1  ( 6 )  and Blomqvist and Lewis ( 3 )  t h a t  t h i s  HFi r e s p o n s e  t o  
s t r e t c h i n g  may b e  reduced as an a d a p t a t i o n  t o  t h e  r e p e a t e d  a t r i a l  d i l a t a t i o n  

r e s u l t i n g  from t h e  i n c r e a s e d  venous r e t u r n  d u r i n g  e x e r c i s e  and t h a t  t h i s  

phenomenon e x p l a i n s  t h e  lowered s e t t i n g  of I H R  and a l s o  HR a t  r e s t  and sub- 

maximal e x e r c i s e  a f t e r  endurance t r a i n i n g .  

The r e s u l t s  from t r a i n i n g  a f t e r  p r e t r e a t m e n t  with 6-OH-DA or t r a i n i n g  

d u r i n g  b e t a  r e c e p t o r  blockade fit i n t o  t h i s  h y p o t h e s i s .  I n  6-OH-DA t r e a t e d  
animals t h e  enhanced t a c h y c a r d i a  d u r i n g  e x e r c i s e  s h o r t e n s  d i a s t o l e ,  t h e r e b y  

p r e v e n t i n g  an i n c r e a s e d  d i a s t o l i c  f i l l i n g  and t h e  r e d u c t i o n  of I H R .  The nega- 

t i v e  c h r o n o t r o p i c  e f f e c t  of t h e  beta-blockade on t h e  o t h e r  hand would augment 

t h e  a t r i a l  d i l a t a t i o n  because of i n c r e a s e d  s t r o k e  volumes d u r i n g  e x e r c i s e .  

Thanks t o  t h e  c o n t r i b u t i o n s  of T e o r e l l  ( 1 9 ,  2 0 )  it i s  known t h a t  t h e r e  
e x i s t s  i n  t h e  h e a r t  n o t  o n l y  an e l e c t r o m e c h a n i c a l  b u t  a l s o  a mechanoelectri- 

c a l  c o u p l i n g  of e v e n t s .  The r e p o r t e d  r e s u l t s  s u p p o r t  t h e  t h e o r y  t h a t  t h i s  

r e l a t i o n s h i p  can undergo a d a p t a t i o n  and imply t h a t  a diminished HR response 

t o  a t r i a l  s t r e t c h  c o u l d  be t h e  mechanism f o r  t h e  lower I H R  s e t t i n g  i n  t r a i n e d  

i n d i v i d u a l s .  

8 



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Address f o r  r e p r i n t s :  

D r  Eva Nylander 
Department of C l i n i c a l  P h y s i o l o g y  
U n i v e r s i t y  H o s p i t a l  
s-581 85 Linkaping 
Sweden 

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