Archives of Academic Emergency Medicine. 2022; 10(1): e90 REV I EW ART I C L E Apelin as a Candidate for Hypertension Management; a Systematic Review and Meta-Analysis on Animal Studies Mohammad Mohammadi1, Mobin Mohamadi1, Amirreza Moradi1, Hamzah Adel Ramawad2, Pantea Gharin1, Yaser Azizi1,3∗, Mahmoud Yousefifard1,4 † 1. Physiology Research Center, Iran University of Medical Sciences, Tehran, Iran. 2. Department of Emergency Medicine, NYC Health & Hospitals, Coney Island, New York. 3. Department of Physiology, School of Medicine, Iran University of Medical Sciences, Tehran, Iran. 4. Pediatric Chronic Kidney Disease Research Center, Tehran University of Medical Sciences, Tehran, Iran. Received: August 2022; Accepted: September 2022; Published online: 7 November 2022 Abstract: Introduction: Hypertension is a medical emergency that requires immediate medical attention. Recent studies have suggested that peripheral injection of Apelin may lower blood pressure. However, there is no compre- hensive conclusion on the role of Apelin in treatment of hypertension. The aim of this systematic review was to evaluate the effects of Apelin on blood pressure in animal studies. Methods: Extensive search and data gathering were conducted using keywords related to blood pressure and Apelin on Medline, Embase, Scopus, and Web of Science databases at the end of July 2022. Two researchers screened and summarized the articles independently. Analysis was then conducted based on Apelin dose, route of administration, and follow-up. The findings were reported as standardized mean difference (SMD) with a 95% confidence interval (95% CI). Results: Data from 10 animal studies were included in the present systematic review. Time interval between Apelin administration and blood pressure assessment was 1 to 21 minutes. Findings showed that administration of Apelin immediately reduces mean arterial pressure (MAP) (SMD=-3.13; 95% CI: -4.43 to -1.82; p<0.001), systolic blood pressure (SBP) (SMD= -1.62; 95% CI: -2.22 to -1.02; p<0.001), and diastolic blood pressure (DBP) (SMD= -1.10; 95% CI: -1.59 to -0.62; p<0.001). On follow-up, the effects of Apelin on MAP (meta-regression coefficient=-2.46; p=0.002) and DBP (meta-regression coefficient= -0.16; p=0.012) decreased over time, while the blood pressure lowering ef- fects of Apelin on SBP did not change during follow-up (meta-regression coefficient=-0.17; p=0.063). It was also found that by increasing the dose of Apelin, DBP and SBP further reduced. These findings suggest that the ef- fect of Apelin on SBP (meta-regression coefficient=0.08; p=0.001) and DBP (meta-regression coefficient=0.059; p=0.007) is dose-dependent, and their correlation is significant. Conclusion: The present systematic review showed that peripheral administration of Apelin immediately reduces MAP, SBP and DBP in hypertensive ani- mals. In contrast, central administration of Apelin increases these parameters. Keywords: Hypertension; Apelin; Arterial Pressure; Blood Pressure Cite this article as: Mohammadi M, Mohamadi M, Moradi A, Ramawad HA, Gharin P, Azizi Y, Yousefifard M.Apelin as a Candidate for Hypertension Management; a Systematic Review and Meta-Analysis on Animal Studies. Arch Acad Emerg Med. 2022; 10(1): e90. https://doi.org/10.22037/aaem.v10i1.1704. ∗Corresponding Author: Yaser Azizi; Physiology Research Center, Iran Uni- versity of Medical Sciences, Tehran, Iran. Tel: +982188622709, Email address: azizi.y@iums.ac.ir, ORCID: https://orcid.org/0000-0002-0452-7542. † Corresponding Author: : Mahmoud Yousefifard; Physiology Research Cen- ter, School of Medicine, Iran University of Medical Sciences, Shahid Hemmat Highway, Tehran 14496-14535, Iran. Tel: +98 (21) 86704771; Email: yousefi- fard.m@iums.ac.ir, ORCID: https://orcid.org/0000-0001-5181-4985. 1. Introduction Hypertension is a major risk factor for cardiovascular dis- eases and stroke, two leading causes of death worldwide. It is currently estimated that 1.4 billion (31.1%) people are af- fected by high blood pressure (1, 2). Based on data-driven projections, more than 50 percent of the world’s population will have hypertension within the next 30 years (1-5). This highlights the importance of blood pressure management as one of the most important priorities in health care. Risk fac- tors such as obesity, psychological stress, alcohol consump- This open-access article distributed under the terms of the Creative Commons Attribution NonCommercial 3.0 License (CC BY-NC 3.0). Downloaded from: http://journals.sbmu.ac.ir/aaem M. Mohammadi et al. 2 tion, high salt intake, sedentary lifestyle, and genetic predis- position are the leading causes of hypertension (5). Because of the multifactorial etiology, alternative modalities should be considered for the management of hypertension. Chronic mild inflammation during obesity is a potential risk factor of hypertension (3). Inflammation results in the release of adipokines from adipose tissues, which might be associated with hypertension. High blood pressure leads to vascular oxidative stress, which subsequently results in vascular damage, remodeling, fibro- sis, and decreased elasticity. As a result, many clinical trials and studies have shown that antioxidant therapies play an important role in the management of hypertension. How- ever, antioxidants such as vitamin C, α-tocopherol (Vit E), and flavonoids did not have significant effects of lowering blood pressures (1, 6). Discovery of the vasodilatory effects of endogenous peptides such as Apelin presents a potential direction of blood pressure management in patients with es- sential hypertension (7). Apelin is a vasoactive endogenous peptide produced from C- terminal of a 77-amino acid pre-proApelin. It is cleaved by enzymes to form different Apelin fragments (Apelin 13, 16, 17, 19, 36). The most active fragment is Apelin-13 and its receptor, APJ, is a member of the G-protein coupled recep- tors (8). Apelin and APJ are widely distributed in the car- diovascular system (9). The Apelin-APJ signaling is also im- portant for proper development of the cardiovascular system and formation of blood vessels. Apelin- and APJ-knockout mice displayed abnormalities with cardiac contractility, pres- sure overload, and aging (10). Similarly, animals with Apelin and APJ abnormalities developed spontaneous hypertension (11). In pregnant preeclamptic women, Apelin and APJ ex- pression in syncytiotrophoblasts and cytotrophoblasts were decreased compared with normotensive control. In ani- mal models with preeclamptic hypertension, treatment with Apelin was shown to decrease blood pressure (12-15). It has also been shown that in patients with pulmonary hyperten- sion, Apelin and APJ expression were decreased. In animal models of hypertension, treatment with Apelin was shown to reduce blood pressure (11, 16). Although Apelin’s abil- ity to temporarily lower blood pressure has been well doc- umented, there are studies that have reported no changes or an increase in blood pressure (9, 15, 17, 18). Despite many efforts to study endogenous peptides such as Apelin, there is no comprehensive conclusion on Apelin’s ef- fects on blood pressure. Thus, performing a meta-analysis and systematic review can be helpful for reaching an agree- ment. The aim of this study was to provide a more precise resource about the effects of Apelin on blood pressure. 2. Methods 2.1. Study design This meta-analysis was designed to evaluate the effects of Apelin injection on blood pressure in hypertensive rats and mice. PICO was defined as: P; Animals (rats and mice) with hypertension through different models, I; Apelin injection, C; Comparison of Apelin-treated hypertensive animals with non-treated hypertensive animals, and O; The outcomes re- lated to blood pressure based on changes in systolic blood pressure (SBP), diastolic blood pressure (DBP), and mean ar- terial pressure (MAP). 2.2. Selection criteria In this systematic review and meta-analysis, all experimental studies that evaluated the effects of Apelin on hypertension and blood pressure were included. Since most studies were conducted on rats and mice, we included the same popula- tion in our study – without any sex or race/strain limitations. Exclusion criteria included studies without a control group, and studies that did not report desired data, including blood pressure level, type, or time of the Apelin injection. Also ex- cluded were duplicate studies, review studies, human stud- ies, and in-vitro studies. 2.3. Search strategy Two reviewers separately conducted extensive search on Medline (via PubMed), ISI Web of Science, Embase, and Sco- pus in July 2022. The keywords used in searches were words related to blood pressure and Apelin. Keywords were se- lected as widely as possible so that no suitable study would be missed (Appendix 1). Although only animal studies were included in the present meta-analysis, the animal studies filter (from the online databases) was not used in the search strategy. This was done in order not to miss any related studies since the online databases cannot always correctly differentiate between an- imal and human studies. Keywords used in the search strat- egy were obtained using MeSH section of PubMed database, Emtree network of Embase database, and search in related article titles. Consulting with specialists in the field of hypertension was another method used to finalize the keywords. To find addi- tional articles or unpublished data, manual search was per- formed in the bibliography of relevant studies and related ar- ticles. On the other hand, for searching in Gray Literature, three strategies have been followed. First, searching Pro- Quest database for dissertations; second, contacting the au- thors of related articles to access unpublished or forthcom- ing data; and third, using Google and Google Scholar search engines to find more literature. This open-access article distributed under the terms of the Creative Commons Attribution NonCommercial 3.0 License (CC BY-NC 3.0). Downloaded from: http://journals.sbmu.ac.ir/aaem 3 Archives of Academic Emergency Medicine. 2022; 10(1): e90 2.4. Data gathering Search in the mentioned databases was conducted and du- plicate articles were removed, then two researchers started screening the articles independently. In the first step, the selection of articles was based on the title and abstract ob- tained from the databases, and in the second step, full text of possibly related articles was studied to select all related articles. All steps were accomplished separately by two re- searchers and in cases of disagreement the dispute was re- solved through consultation with a third researcher. Data extracted from articles included in this systematic search were recorded in a checklist designed based on Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement guidelines. Extracted data included in- formation related to the study design, the characteristics of all the samples (age, weight, sex, model of inducing hy- pertension), administration protocol, administration route (peripheral or central), number of samples studied, investi- gated outcomes (mean arterial pressure, systolic blood pres- sure, diastolic blood pressure), and follow-up duration. Since most experimental studies use graphs to report their find- ings; whenever necessary, data were extracted using Plot dig- itizer. 2.5. Risk of bias assessment Quality control of the articles was performed using a check- list designed based on Systematic Review Centre for Labora- tory Animal Experimentation (SYRCLE)’s risk of bias tool for animal studies (19). In cases of disagreement the dispute was resolved through consultation with a third researcher. 2.6. Statistical analysis To conduct analysis, mean standard deviation of recorded data and number of samples in each group was recorded in STATS 14.0 statistical software. Then, using the ‘Metan’ com- mand in this software, standardized mean difference (SMD) with a 95% confidence interval (CI) was calculated for each group. And finally, a pooled effect size was reported. Het- erogeneity was evaluated based on I2. Either Random effect model’ or ‘Fix effect model’ was used based on Heterogene- ity. Since Apelin dose, follow-up duration, and administra- tion protocol (peripheral or central) varied between different studies, studies were divided based on these variables and then analysis was performed. It is worth mentioning that ‘Funnel plot’ was used to identify publication bias using the Egger’s test (20). 3. Results The systematic search resulted in 1338 non-duplicate arti- cles. After screening titles and abstracts, 28 articles were re- viewed in detail. Ten articles (18, 21-29) were included in the present meta-analysis (Figure 1). All 10 of these studies were conducted on rats. Apelin-13 was used in 8 studies, Apelin- 12 in 1 study, and Apelin-36 in 1 study. The route of Apelin administration in 8 studies was peripheral, 7 of them intra- venous and 1 of them intraperitoneal. In the two remaining studies, the administration route was central. Model of hy- pertension induction was as follows: L-NAME injection in 3 articles, two-kidney-one-clip (2K1C) model in 3 articles, an- giotensin II injection in 2 articles, and DOCA-salt injection in 1 article. In 1 article, spontaneously hypertensive rats were used. Follow-up duration varied from 0 minute to 21 days. Two articles measured blood pressure noninvasively, using tail cuff. The remaining 8 articles measured arterial pressure invasively by placing a catheter in the carotid artery. Induced hypertension in 3 articles was acute, while in the other 7 it was chronic (Table 1). 3.1. Meta-analysis Effect of Apelin administration on mean arterial pressure (MAP) In our analysis, studies were divided into two groups based on Apelin administration route: 1) peripheral injection and 2) central injection. Results of statistical analysis showed that peripheral Apelin injection decreases MAP (SMD=-3.13; 95% CI: -4.43 to -1.82; p<0.001), while central Apelin injec- tion results in MAP elevation (SMD=3.55; 95% CI: 2.43 to 4.67; p<0.001) (Fig. 2). In the evaluation of the effect of total in- jected dose of Apelin on MAP, it was found that MAP does not alter with change in dose of Apelin, and there is no significant relationship between dose and beneficial effects of Apelin on MAP (meta-regression coefficient=0.11; p=0.454) (Fig. 5). On the other hand, according to the results of statistical analysis, by continuing the follow-up process, the effect of Apelin on MAP decreases over time (meta-regression coefficient=-2.46; p=0.002). Effect of Apelin administration on systolic blood pressure (SBP) As mentioned, the studies were divided into two groups based on Apelin administration route: 1) peripheral injection and 2) central injection. Results of statistical analysis showed that peripheral Apelin injection decreases SBP (SMD=-1.62; 95% CI: -2.22 to -1.02; p<0.001), while central Apelin injec- tion results in SBP elevation (SMD=5.13; 95% CI: 1.86 to 8.41; p=0.008) (Fig. 3). In the evaluation of the effect of total in- jected dose of Apelin on SBP, it was found that with increase in dose of Apelin, SBP is further reduced, which suggests that its effect on SBP is dose-dependent, and their relation is sig- nificant (meta-regression coefficient=0.08; p=0.001) (Fig. 5). On the other hand, based on our statistical analysis, by con- tinuing the follow-up process -and recording the changes in SBP- the effect of Apelin does not significantly change over time (meta-regression coefficient=-0.17; p=0.063). This open-access article distributed under the terms of the Creative Commons Attribution NonCommercial 3.0 License (CC BY-NC 3.0). Downloaded from: http://journals.sbmu.ac.ir/aaem M. Mohammadi et al. 4 Effect of Apelin administration on diastolic blood pressure (DBP) Results of statistical analysis also showed that peripheral Apelin injection decreases DBP (SMD=-1.10; 95% CI: -1.59 to -0.62; p<0.001), while central Apelin injection results in DBP elevation (SMD=4.09; 95% CI: 2.11 to 6.07; p= 0.089) (Fig. 4). In the evaluation of the effect of total injected dose of Apelin on DBP, it was found that with increase in dose of Apelin, DBP is further reduced, which suggests that its effect on DBP is dose-dependent, and their relation is significant (meta-regression coefficient=0.059; p=0.007) (Fig. 5). Based on the results of statistical analysis, by continuing the follow- up process, the effect of Apelin on DBP decreases over time (meta-regression coefficient=-0.16; p=0.012). Publication bias assessment In publication bias assessment of the present study, it was revealed that there was no evidence of publication bias in any of the studied parameters including: the relation be- tween Apelin administration and 1) mean arterial pressure (p<0.124), 2) systolic blood pressure (p=0.458) and 3) dias- tolic blood pressure (p=0.181). 4. Discussion Results of our analyses indicate that peripheral Apelin ad- ministration decreases MAP, SBP and DBP, while central ad- ministration increases these parameters. Statistical analysis also shows that increasing dose of administration causes fur- ther reduction in SBP and DBP, but no significant change is seen in its effect on MAP. When Apelin administration was continuous, its effects persisted during follow-up. The mechanisms by which Apelin changes blood pressure is not fully understood. Studies have shown that Apelin has a transient effect on blood pressure, which starts less than one minute after intravenous injection and can last up to 3-4 minutes before the blood pressure returns to its prior level (17, 22, 30-34). In contrast, it has been shown that Apelin micro-injection in the rostral ventrolateral medulla (RVLM) increases neural activity in this region, which in turn increases sympathetic activity and causes vasoconstriction that results in blood pressure elevation (35). Studies on the mechanism of vasodilatory effects of Apelin, when adminis- tered peripherally, have shown that Apelin increases the pro- duction of NO through increasing eNOS expression. As a re- sult, Apelin dilates blood vessels through the endothelium- dependent pathway (36). As previously mentioned, the Apelin administration was divided into two groups: 1) pe- ripheral administration, and 2) central administration. Re- sults show that peripheral administration decreases MAP, while central administration increases MAP. Our findings show that the effect of Apelin on MAP did not change with changing the dose of Apelin and no significant relationship was observed between dose and its effects. Based on sta- tistical findings, with continuing the follow-up process and recording the changes in blood pressure after the last admin- istration, effect of Apelin diminishes over time. It appears that Apelin exerts short-term effects by increasing the pro- duction of NO, and in the case of intraventricular injection, it increases blood pressure by increasing vasopressin produc- tion and sympathetic vasoconstrictor activity. It was shown that the administration of Apelin reduces SBP in the peripheral administration, while it increases SBP in the central route of administration. Studying the effect of total prescribed dose of Apelin on SBP revealed that by in- creasing the dose of Apelin, SBP is further reduced, and a significant relationship was observed between the dose and its effects. Additionally, our results indicate that by continu- ing the follow-up process and recording SBP after treatment with Apelin, the effect of Apelin does not change over time. From these findings, it can be inferred that the effectiveness of Apelin on SBP is not affected by time, but is more dose- dependent. This means that increasing the dose of Apelin further reduces SBP. Since p-value was close to the signifi- cance level, it is feasible that future studies can prove its ef- fectiveness. Since Apelin increases myocardial contractility, it may elevate SBP. However, the activation of baroreceptors in aortic arch and carotid sinus subsequently decreases SBP. Apelin increases production of NO, which decreases SBP and thus, decreases cardiac afterload. Most likely, the duration of administration should be longer to exert long-term effects. Since Apelin decreases MAP, it makes sense that Apelin also reduces SBP by improving vasodilation. Another indicator discussed in most of the studies was DBP. Peripheral Apelin administration increases DBP and its cen- tral administration decreases it. The effect of Apelin on DBP was dose-dependent, which means higher doses cause fur- ther reduction in DBP and it was statistically significant. Ac- cording to the obtained results, continuation of the follow-up process and recording changes in DBP shows that the effec- tiveness of Apelin continues over time. In the present research, none of the studied indicators, in- cluding the relationship between Apelin administration with MAP, SBP and DBP, showed evidence of publication bias so it can be concluded that Apelin is effective for controlling blood pressure. Experts believe that a suitable medication to control blood pressure crisis should reduce blood pressure by 10 to 15% in the first hour and lead to an extra 10 to 15% decrease in the subsequent 4 hours to prevent hypoperfusion complications. Although, Apelin, as a rapid-onset medication, can decrease BP in a few minutes, its optimum dose for treating hyperten- sion is not known. Therefore, we strongly recommend that future studies identify a dose of Apelin that can reduce blood pressure by 10-15% during the first hour. This open-access article distributed under the terms of the Creative Commons Attribution NonCommercial 3.0 License (CC BY-NC 3.0). Downloaded from: http://journals.sbmu.ac.ir/aaem 5 Archives of Academic Emergency Medicine. 2022; 10(1): e90 5. Limitation One of the most important limitations of the current study is presence of risk of bias in the included studies. Lack of re- porting required data in the eligible studies for passing judg- ment about random sequence generation, allocation con- cealment, random housing, random outcome assessment, blinding of observers, incomplete outcome data, and selec- tive outcome reporting are the main sources of high risk of bias. These items are generally not reported in experimen- tal studies, and shows one of the basic problems in report- ing of preclinical evidence. Another limitation of the cur- rent meta-analysis was the wide variation in the doses of Apelin prescribed in the studies. Although the variation in the prescribed dose was not a source of heterogeneity, sev- eral doses of this drug should be compared in future stud- ies (dose-response gradient) and finally, the optimum dose should be suggested. 6. Conclusion The present systematic review of preclinical evidence showed that peripheral administration of Apelin reduces MAP, SBP and DBP in hypertensive animals. On the other hand, central administration of Apelin increases these pa- rameters. 7. Declarations 7.1. Acknowledgments Not applicable. 7.2. Ethics approval Not applicable. 7.3. Patient consent Not applicable. 7.4. Informed consent Not applicable. 7.5. Availability of data and materials All data generated or analyzed during this study are included in this published article and its supplementary information file. 7.6. Permission to reproduce material from other sources Not applicable. 7.7. Conflicting interests The authors declare that they have no competing interests. 7.8. Funding This research was supported by Iran University of Medical Sciences (Grant NO: 16713). 7.9. Author contributions Ideation and design: MY, and YA. Data collection: MM, MM, AM, PG, HAR. Analysis: MY. Drafting the work: MY, YA, MM. Revising draft critically for important intellectual con- tent: All authors. The authors read and approved the final manuscript. References 1. Ahmad KA, Yuan Yuan D, Nawaz W, Ze H, Zhuo CX, Talal B, et al. Antioxidant therapy for management of oxidative stress induced hypertension. Free Radic Res. 2017;51(4):428-38. 2. Egan BM, Kjeldsen SE, Grassi G, Esler M, Mancia G. The global burden of hypertension exceeds 1.4 billion people: should a systolic blood pressure target below 130 become the universal standard? J Hypertens. 2019;37(6):1148-53. 3. Brito R, Castillo G, González J, Valls N, Rodrigo R. Ox- idative stress in hypertension: mechanisms and ther- apeutic opportunities. Exp Clin Endocrinol Diabetes. 2015;123(6):325-35. 4. Korsager Larsen M, Matchkov VV. Hypertension and physical exercise: The role of oxidative stress. Medicina (Kaunas). 2016;52(1):19-27. 5. Mills KT, Stefanescu A, He J. The global epidemiology of hypertension. Nat Rev Nephrol. 2020;16(4):223-37. 6. Massaro M, Scoditti E, Carluccio MA, De Caterina R. Ox- idative stress and vascular stiffness in hypertension: A renewed interest for antioxidant therapies? Vascul Phar- macol. 2019;116:45-50. 7. Horbal SR, Seffens W, Davis AR, Silvestrov N, Gibbons GH, Quarells RC, et al. Associations of Apelin, Visfatin, and Urinary 8-Isoprostane With Severe Hypertension in African Americans: The MH-GRID Study. Am J Hyper- tens. 2016;29(7):814-20. 8. Ma Y, Yue Y, Ma Y, Zhang Q, Zhou Q, Song Y, et al. Struc- tural basis for apelin control of the human apelin recep- tor. Structure. 2017;25(6):858-66. e4. 9. Mughal A, O’Rourke ST. Vascular effects of apelin: Mechanisms and therapeutic potential. Pharmacol Ther. 2018;190:139-47. 10. Kuba K, Zhang L, Imai Y, Arab S, Chen M, Maekawa Y, et al. Impaired heart contractility in Apelin gene-deficient mice associated with aging and pressure overload. Circ Res. 2007;101(4):e32-42. 11. Andersen CU, Hilberg O, Mellemkjær S, Nielsen-Kudsk JE, Simonsen U. Apelin and pulmonary hypertension. Pulm Circ. 2011;1(3):334-46. This open-access article distributed under the terms of the Creative Commons Attribution NonCommercial 3.0 License (CC BY-NC 3.0). Downloaded from: http://journals.sbmu.ac.ir/aaem M. Mohammadi et al. 6 12. Yamaleyeva LM, Brosnihan KB, Elsangeedy E, McGee C, Shi S, Caudell D, et al. Systemic Outcomes of (Pyr1)- Apelin-13 Infusion at Mid-Late Pregnancy in a Rat Model with Preeclamptic Features. Sci Rep. 2019;9(1):8579. 13. Yamaleyeva LM, Chappell MC, Brosnihan KB, An- ton L, Caudell DL, Shi S, et al. Downregulation of apelin in the human placental chorionic villi from preeclamptic pregnancies. Am J Physiol Endocrinol Metab. 2015;309(10):E852-60. 14. Inuzuka H, Nishizawa H, Inagaki A, Suzuki M, Ota S, Miyamura H, et al. Decreased expression of apelin in placentas from severe pre-eclampsia patients. Hypertens Pregnancy. 2013;32(4):410-21. 15. Wang C, Liu X, Kong D, Qin X, Li Y, Teng X, et al. Apelin as a novel drug for treating preeclampsia. Exp Ther Med. 2017;14(6):5917-23. 16. Andersen CU, Markvardsen LH, Hilberg O, Simon- sen U. Pulmonary apelin levels and effects in rats with hypoxic pulmonary hypertension. Respir Med. 2009;103(11):1663-71. 17. Yamaleyeva LM, Shaltout HA, Varagic J. Apelin-13 in blood pressure regulation and cardiovascular disease. Curr Opin Nephrol Hypertens. 2016;25(5):396-403. 18. Tatemoto K, Takayama K, Zou MX, Kumaki I, Zhang W, Kumano K, et al. The novel peptide apelin lowers blood pressure via a nitric oxide-dependent mechanism. Regul Pept. 2001;99(2-3):87-92. 19. Hooijmans CR, Rovers MM, de Vries RBM, Leenaars M, Ritskes-Hoitinga M, Langendam MW. SYRCLE’s risk of bias tool for animal studies. BMC Med Res Methodol. 2014;14(1):43. 20. Egger M, Davey Smith G, Schneider M, Minder C. Bias in meta-analysis detected by a simple, graphical test. BMJ. 1997;315(7109):629-34. 21. Akcilar R, Turgut S, Caner V, Akcilar A, Ayada C, Elmas L, et al. Apelin effects on blood pressure and RAS in DOCA-salt-induced hypertensive rats. Clin Exp Hyper- tens. 2013;35(7):550-7. 22. Ishida J, Hashimoto T, Hashimoto Y, Nishiwaki S, Iguchi T, Harada S, et al. Regulatory roles for APJ, a seven- transmembrane receptor related to angiotensin-type 1 receptor in blood pressure in vivo. J Biol Chem. 2004;279(25):26274-9. 23. Lee DK, Saldivia VR, Nguyen T, Cheng R, George SR, O’Dowd BF. Modification of the terminal residue of apelin-13 antagonizes its hypotensive action. En- docrinology. 2005;146(1):231-6. 24. Rostamzadeh F, Najafipour H, Yeganeh-Hajahmadi M, Joukar S. Opioid receptors mediate inotropic and depres- sor effects of apelin in rats with 2K1C-induced chronic renovascular hypertension. Clin Exp Pharmacol Physiol. 2018;45(2):187-97. 25. Siddiquee K, Hampton J, Khan S, Zadory D, Gleaves L, Vaughan DE, et al. Apelin protects against angiotensin II- induced cardiovascular fibrosis and decreases plasmino- gen activator inhibitor type-1 production. J Hypertens. 2011;29(4):724-31. 26. Soltani Hekmat A, Najafipour H, Nekooian AA, Esmaeli- Mahani S, Javanmardi K. Cardiovascular responses to apelin in two-kidney-one-clip hypertensive rats and its receptor expression in ischemic and non-ischemic kid- neys. Regul Pept. 2011;172(1-3):62-8. 27. Yeganeh-Hajahmadi M, Najafipour H, Rostamzadeh F. The differential effects of low and high doses of apelin through opioid receptors on the blood pressure of rats with renovascular hypertension. Hypertens Res. 2017;40(8):732-7. 28. Zhao Y, Li Y, Li Z, Xu B, Chen P, Yang X. Superoxide anions modulate the performance of apelin in the par- aventricular nucleus on sympathetic activity and blood pressure in spontaneously hypertensive rats. Peptides. 2019;121:170051. 29. Zhang F, Sun HJ, Xiong XQ, Chen Q, Li YH, Kang YM, et al. Apelin-13 and APJ in paraventricular nucleus con- tribute to hypertension via sympathetic activation and vasopressin release in spontaneously hypertensive rats. Acta Physiol (Oxf ). 2014;212(1):17-27. 30. Japp AG, Newby DE. The apelin-APJ system in heart fail- ure: pathophysiologic relevance and therapeutic poten- tial. Biochem Pharmacol. 2008;75(10):1882-92. 31. Kleinz MJ, Davenport AP. Emerging roles of apelin in biol- ogy and medicine. Pharmacol Ther. 2005;107(2):198-211. 32. Hashimoto T, Kihara M, Ishida J, Imai N, Yoshida S, Toya Y, et al. Apelin stimulates myosin light chain phospho- rylation in vascular smooth muscle cells. Arterioscler Thromb Vasc Biol. 2006;26(6):1267-72. 33. Reaux-Le Goazigo A, Morinville A, Burlet A, Llorens- Cortes C, Beaudet A. Dehydration-induced cross- regulation of apelin and vasopressin immunoreactivity levels in magnocellular hypothalamic neurons. En- docrinology. 2004;145(9):4392-400. 34. Reaux A, Gallatz K, Palkovits M, Llorens-Cortes C. Dis- tribution of apelin-synthesizing neurons in the adult rat brain. Neuroscience. 2002;113(3):653-62. 35. Yao F, Modgil A, Zhang Q, Pingili A, Singh N, O’Rourke ST, et al. Pressor Effect of Apelin-13 in the Rostral Ventrolat- eral Medulla: Role of NAD(P)H Oxidase-Derived Super- oxide. J Pharmacol Exp Ther. 2011;336(2):372-80. 36. Salcedo A, Garijo J, Monge L, Fernandez N, Luis Garcia- Villalon A, Sanchez Turrion V, et al. Apelin effects in human splanchnic arteries. Role of nitric oxide and prostanoids. Regul Pept. 2007;144(1-3):50-5. This open-access article distributed under the terms of the Creative Commons Attribution NonCommercial 3.0 License (CC BY-NC 3.0). Downloaded from: http://journals.sbmu.ac.ir/aaem 7 Archives of Academic Emergency Medicine. 2022; 10(1): e90 Table 1: Characteristics of included studies Author; year Gender; species; strain; age Dose (nmol/kg) / total number of administration / number administration/day / route Interval time between induction of BP to treatment (minutes) Model of HTN induction / dose of inducer / type of HTN Method of BP assessment Follow- up Akcilar; 2013 Male; Rat; Wistar Albino; 8–10-wk 200 / 17 / 1 / IP 24 DOCA-salt treatment / 25 mg/kg / Chronic Tail cuff BP 17 days Ishida; 2004 Male; Rat; WKY SHR; 12-wk 3, 6, 15 / 1 / 1 / IV 0.25 L-NAME / 10 mg/kg / Acute Intra-arterial catheter 5 minutes Lee; 2005 Male; Rat; SHR Wistar; approximately 15-wk 15 / 1 / 1 / IV 0 Angiotensin II / 30 ng/kg / Acute Intra-arterial catheter 1, 5, 10 minutes Rostamzadeh; 2018 NR; Rat; Wistar; NR 40, 60 / 1 / 1 / IV 2688 2K1C / NA / Chronic Intra-arterial catheter 1, 5, 10 minutes Siddiquee; 2011 Male; Mouse; C57Bl/6j; 8-wk 15 / 21 / 1 / IV 504 L-NAME, Angiotensin II / 1.0 mg/mL drinking water, 1.0 µg/kg/day / Chronic Tail cuff BP 21 days Soltanihekmat; 2011 Male; Rat; Sprague–Dawley; NR 10, 20, 40/ 1 / 1 / IV 672 2k1c / NA / Chronic Intra-arterial catheter 1, 5, 10 minutes Tatemoto; 2001 Male; Rat; Wistar; 8- 9-wk 7, 8, 20 / 1 / 1 / IV 0.16 L-NAME / 30 mgr per kg / Acute Intra-arterial catheter 1, 4 minutes Yeganeh- Hajahmadi; 2017 Male; Rat; Wistar; NR 20, 40 / 1 / 1 / IV 672 2k1c / NA / Chronic Intra-arterial catheter 1, 4, 10 minutes Zhang; 2014 Male; Rat; WKY SHR; 13-wk 4, 45, 450 / 15 / 1 / in PVN 0 SHR rats / NA / Essential Intra-arterial catheter 1 minute Zhao; 2018 Male; Rat; WKY SHR; 13-wk 4, 450 / 1 / 1 / in PVN 0 SHR rats / NA / Essential Intra-arterial catheter 1 minute 2K1C: 2 kidneys one clip; BP: Blood pressure; HTN: Hypertension; IP: Intraperitoneal; IV: Intravenous; DOCA: Deoxycorticosterone acetate; L-NAME: L-NG-Nitro arginine methyl ester; NA: Not applicable; NR: Not reported; PVN: Paraventricular nucleus; SHR: Spontaneous hypertensive rat; WKY: Wistar Kyoto; wk: Weeks. This open-access article distributed under the terms of the Creative Commons Attribution NonCommercial 3.0 License (CC BY-NC 3.0). Downloaded from: http://journals.sbmu.ac.ir/aaem M. Mohammadi et al. 8 Table 2: Risk of bias assessment among the studies based on SYRCLE’s tool Domain Sequence genera- tion Baseline charac- teristics Allocation conceal- ment Random housing Blinding of caregivers Random outcome assessment Blinding of observers Incomplete outcome data Selective outcome reporting Other sources Akcilar; 2013 Low risk Low risk High risk High risk Not reported High risk Not reported Not reported Cannot be determined Low risk Ishida; 2004 High risk Low risk High risk High risk Not reported High risk Not reported Not reported Cannot be determined Low risk Lee; 2005 High risk Low risk High risk High risk Not reported High risk Not reported Not reported Cannot be determined Low risk Rostamzadeh; 2018 Low risk Low risk High risk High risk Not reported High risk Not reported Not reported Cannot be determined Low risk Siddiquee; 2011 High risk Low risk High risk High risk Not reported High risk Not reported Not reported Cannot be determined Low risk Soltanihekmat; 2011 Low risk Low risk High risk High risk Not reported High risk Not reported Not reported Cannot be determined Low risk Tatemoto; 2001 High risk Low risk High risk High risk Not reported High risk Not reported Not reported Cannot be determined Low risk Yeganeh- Hajahmadi; 2017 Low risk Low risk High risk High risk Not reported High risk Not reported Not reported Cannot be determined Low risk Zhang; 2014 Low risk Low risk High risk High risk Not reported High risk Not reported Not reported Cannot be determined Low risk Zhao; 2018 High risk Low risk High risk High risk Not reported High risk Not reported Low risk Cannot be determined Low risk SYRCLE: Systematic Review Centre for Laboratory Animal Experimentation. Figure 1: Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) flow diagram of the present meta-analysis This open-access article distributed under the terms of the Creative Commons Attribution NonCommercial 3.0 License (CC BY-NC 3.0). Downloaded from: http://journals.sbmu.ac.ir/aaem 9 Archives of Academic Emergency Medicine. 2022; 10(1): e90 Figure 2: Forest plot of effect of Apelin on mean arterial pressure based on central and peripheral routes for administration of Apelin. SMD: standard mean difference; CI: confidence interval. This open-access article distributed under the terms of the Creative Commons Attribution NonCommercial 3.0 License (CC BY-NC 3.0). Downloaded from: http://journals.sbmu.ac.ir/aaem M. Mohammadi et al. 10 Figure 3: Forest plot of effect of Apelin on systolic blood pressure based on central and peripheral routes for administration of Apelin. SMD: standard mean difference; CI: confidence interval. This open-access article distributed under the terms of the Creative Commons Attribution NonCommercial 3.0 License (CC BY-NC 3.0). Downloaded from: http://journals.sbmu.ac.ir/aaem 11 Archives of Academic Emergency Medicine. 2022; 10(1): e90 Figure 4: Forest plot of effect of Apelin on diastolic blood pressure based on central and peripheral routes for administration of Apelin. SMD: standard mean difference; CI: confidence interval. This open-access article distributed under the terms of the Creative Commons Attribution NonCommercial 3.0 License (CC BY-NC 3.0). Downloaded from: http://journals.sbmu.ac.ir/aaem M. Mohammadi et al. 12 Figure 5: Meta regression analysis of effect of peripheral Apelin administration on blood pressure based on dose of Apelin and follow-up duration. SMD: standard mean difference. This open-access article distributed under the terms of the Creative Commons Attribution NonCommercial 3.0 License (CC BY-NC 3.0). Downloaded from: http://journals.sbmu.ac.ir/aaem 13 Archives of Academic Emergency Medicine. 2022; 10(1): e90 Figure 6: Publication bias in assessment of the effect of Apelin administration on blood pressure. SMD: standard mean difference. This open-access article distributed under the terms of the Creative Commons Attribution NonCommercial 3.0 License (CC BY-NC 3.0). Downloaded from: http://journals.sbmu.ac.ir/aaem Introduction Methods Results Discussion Limitation Conclusion Declarations References