Elsevier

Experimental Gerontology

Volume 42, Issue 8, August 2007, Pages 807-814
Experimental Gerontology

Ischemic preconditioning is lost in aging hypertensive rat heart: Independent effects of aging and longstanding hypertension

https://doi.org/10.1016/j.exger.2007.04.005Get rights and content

Abstract

Although in experimental hypertension the cardioprotective effects of ischemic preconditioning (PC) appear to be maintained, most studies have examined the short-term hypertension in juvenile animals. However, aging may be an additional factor that influences the effectiveness of PC. The aim of this study was to characterise the effects on PC of LVH and aging simultaneously. Hearts from spontaneously hypertensive rats (SHR) and age-matched normotensive Wistar–Kyoto rats (WKY) were studied. Excised hearts were Langendorff-perfused to give equivalent coronary flow per gram heart weight. The left main coronary artery was occluded for 35 min followed by 120 min reperfusion. Infarct size was determined by tetrazolium staining. Heart size was assessed as left ventricle/body weight ratio (LV/BW). PC was effected with 2 × 5 min periods of global ischemia prior to coronary occlusion. Hearts were studied at 3–4 months (juvenile), 7–8 months (mature) or 12–13 months (aging). LV/BW ratio in SHR increased relative to WKY controls by 20%, 32% and 40% in juvenile, mature and aging hearts, respectively, but ischemic risk zone size was similar in all groups (52–59% of LV). PC was equally effective at limiting infarct size in juvenile and mature SHR and WKY hearts but was ineffective in aging hearts from both WKY and SHR. Since angiotensin-converting enzyme inhibitors enhance sub-threshold PC in normal myocardium, we also examined the action of captopril (Cap) in aging hearts. Additional aging hearts received treatment with Cap 200 μM as an adjunct to PC. Although Cap + PC was able to induce modest protection in aging WKY hearts, this was not seen in aging SHR hearts. We conclude that PC is lost in longstanding hypertension through independent contributions of both hypertension and aging. These findings may have implications for the clinical development of preconditioning-based therapies since elderly patients with longstanding hypertension are at high risk of developing ischemic heart disease.

Introduction

Numerous studies in experimental species and humans have shown that brief periods of ischemia can render the myocardium more resistant to a subsequent ischemic episode, a phenomenon termed ischemic preconditioning (PC) (Murry et al., 1986, Cohen et al., 2000, Yellon and Downey, 2003). In recent years, considerable understanding of the molecular mechanisms underlying this fundamental adaptive response to ischemia has accumulated, leading to some suggestions that the preconditioning mechanism could be therapeutically exploitable. However, there is a growing awareness that the experimental use of healthy juvenile animals may not be accurately predictive of mechanisms and potential applications in human patients with coronary artery disease who may be elderly and have other co-morbidities. Despite extensive research on PC, most studies have focused on the “healthy”, non-diseased myocardium (Ferdinandy et al., 1998, Kloner et al., 2002). So far, a limited number of studies have examined PC in aging or diseased myocardium, which would ultimately provide greatest clinical benefit.

Hypertension is a major risk factor for the development of hypertensive heart disease, including epicardial and microvascular coronary disease, left ventricular hypertrophy (LVH), heart failure, ventricular arrhythmias and sudden death (Diamond and Phillips, 2005, Gradman and Alfayoumi, 2006). Although there is experimental evidence that hypertrophied myocardium is more susceptible to the injurious effects of ischemia than normal myocardium (Gaasch et al., 1990, Snoeckx et al., 1993), several animal studies have reported that in hypertensive LVH the PC response is fully preserved (Speechly-Dick et al., 1994, Pantos et al., 1996, Butler et al., 1999, Randall et al., 1997, Boutros and Wang, 1995, Lu et al., 1999, Rajesh et al., 2004, Ebrahim et al., 2007). For example, we have shown that in DOCA-salt rats rendered hypertensive for 4 weeks, hypertrophied myocardium can be protected against ischemia-reperfusion injury with a reduction in infarct size using classical PC protocols (Speechly-Dick et al., 1994, Ebrahim et al., 2007). However, the hypertensive animals used in all such studies to date have been juvenile. An additional factor that may influence the responsiveness of myocardium to PC is age (Mbai and Knowlton, 2005). In experimental studies the effects of age on PC has been controversial, with some studies reporting attenuation or abrogation of PC in aging animals (Fenton et al., 2000, Tani et al., 2001, Schulman et al., 2001, Abete et al., 1996) while other studies report preservation of the PC response (Przyklenk et al., 2001, Bartling et al., 2003). In view of the uncertainty surrounding influences of age on myocardial PC, and the potential for longstanding, progressive hypertension to induce severe LVH in elderly subjects, the first aim of this study was to characterise the effects LVH and aging simultaneously on PC. We therefore investigated the ability of PC to protect against ischemia-reperfusion insult in normotensive Wistar–Kyoto (WKY) and spontaneously hypertensive rats (SHR) and various stages of post-natal development.

ACE inhibitors are widely used for the treatment of hypertension and in secondary prevention in ischemic heart disease. Through their ability to inhibit the enzymatic degradation of bradykinin, which is an important autacoid mediator of PC (Cohen et al., 2000, Yellon and Downey, 2003), ACE inhibitors have been demonstrated experimentally to potentiate sub-threshold preconditioning responses sufficiently to induce a full PC response (Baxter and Ebrahim, 2002, Miki et al., 1996, Morris and Yellon, 1997). Accordingly, the second aim of this study was to examine if ACE-inhibition augmented the PC response in aging normotensive and hypertensive hearts.

Section snippets

Animals

Male SHR and age-matched WKY rats were obtained from Harlan (Bicester, UK). The animals were housed in groups of 3–4 in the institutional animal house for up to 13 months with unlimited access to water and standard chow. The maintenance, handling and conduct of procedures were in accordance with the Guidelines on the Operation of the Animals (Scientific Procedures) Act, 1986, published by The Stationery Office (London, UK).

Measurement of blood pressure

Prior to the excision of hearts for Langendorff-perfusion, randomly

Exclusions

One hundred and eighteen rats were used in this study. Three hearts were excluded; two due to inadequate delineation of the risk zone; one due to severe bradycardia throughout the course of the experiment. Five SHRs died spontaneously: four at approximately 10 months of age and one at 5 months of age. Therefore, we report data for 110 successfully completed experiments. Final numbers included in the analyses are shown in Table 1.

Blood pressure and LVH index

Arterial blood pressures determined in anesthetised WKY and SHR

Discussion

The principal findings of this study are that PC induces protection against infarction in hypertrophied myocardium from juvenile and mature SHR with identical efficacy to that seen in normotensive rat hearts. However, protection was not afforded by PC in either SHR or WKY hearts at 12–13 months of age, suggesting that aging per se, independently of the duration of systemic hypertension or the degree of LVH, abrogates the normal preconditioning mechanism. A notable difference between the aging

Acknowledgement

This work was supported by a British Heart Foundation studentship awarded to Zaileen Ebrahim (FS/98075).

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