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Recovery of Erectile Function in Aging Hypertensive and Normotensive Rats Using Exercise and Caloric Restriction Johanna L. Hannan, BSCH,*‡ Jeremy P. W. Heaton, MD, PhD, MBA,*†‡ and Michael A. Adams, PhD*†‡ *Department of Pharmacology & Toxicology, Queen’s University, Kingston, Ontario, Canada; †Department of Urology, Queen’s University, Kingston, Ontario, Canada; ‡Queen’s Human Sexuality Group, Queen’s University, Kingston, Ontario, Canada DOI: 10.1111/j.1743-6109.2007.00517.x

ABSTRACT

Introduction. Using aging spontaneously hypertensive rats (SHR), we established that antihypertensive drugs can improve erections and penile vascular structure, and lower arterial pressure. Using kidney cross-transplantations, our findings revealed that the benefit of this treatment resulted from drug-induced changes specific to the penile circulation, and not to the kidney-mediated lowering of pressure. Aim. The objective of the present study was to determine whether increased exercise and/or caloric restriction (CR) can reverse the decline in sexual responses in aging hypertensive and normotensive rats. Methods. From 30 to 40 weeks, food intake was restricted (10–40%), and SHR, Wistar, and Sprague-Dawley rats ran on treadmills (30 minutes/day, 5 days/week). Exercise was withdrawn at 40 weeks, and CR was stopped at 50 weeks. Using a separate group of older Wistars (56 weeks) and Sprague-Dawley rats (67 weeks), the effects of 10% CR or exercise plus 10–40% CR on erectile function were determined. Main Outcome Measure. Apomorphine-induced erectile responses and body weight were monitored weekly. Results. An age-related decline in erections was seen from 15 to 29 weeks of age in all strains. This decline paralleled increases in body weight, particularly in the normotensive strains. Exercise and CR induced a 10% weight loss in normotensive rats and improved erections in all animals. In SHR, increased erections occurred without decreasing body weight. Body weight and erectile responses were maintained by CR alone after exercise was withdrawn, but erectile function rapidly declined soon after CR was stopped and paralleled increases in body weight. In aged Wistar and Sprague-Dawley rats treated with exercise and CR, erectile function was also significantly improved. Conclusions. Similar to previous studies, erectile function progressively decreased with age in both hypertensive and normotensive rats. Erectile responses were found to be substantially improved by an intervention involving exercise and CR, but not necessarily involving weight loss. Hannan JL, Heaton JPW, and Adams MA. Recovery of erectile function in aging hypertensive and normotensive rats using exercise and caloric restriction. J Sex Med 2007;4:886–897. Key Words. Aging; Caloric Restriction; Erectile Function; Exercise; Hypertension; Weight Loss

Introduction

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rectile dysfunction (ED) is a pathophysiological condition that is based on multifactorial etiologies. This is not surprising, because the generation of an erection requires the integrated contribution of multiple regulatory systems, including psychological, hormonal, neurological, and vascular components [1]. The progression to ED is linked to a long list of risk factors, such as aging,

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smoking, alcohol, obesity, abnormal cholesterol, and altered endocrine function, and is commonly linked with comorbidities such as hypertension, coronary artery disease, stroke, renal failure, diabetes, neurogenic disorders, and mental illness [2,3]. In particular, cardiovascular conditions, such as hypertension and coronary artery disease, appear to share common underlying pathophysiological processes with ED, including endothelial dysfunction, vasculopathies, and abnormal lipid © 2007 International Society for Sexual Medicine

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metabolism. Of particular interest regarding the progression of these conditions is the evidence emphasizing that ED may be a harbinger for underlying cardiovascular disease [4–7]. Findings from studies such as the Massachusetts Male Aging Study have revealed that aging is an independent risk factor in the development of ED, with the prevalence and severity increasing threefold between 40 and 70 years of age [8]. Finally, obesity has become a comorbidity of epidemic proportions across North America, with the incidence of ED in overweight or obese men reaching nearly 80% [9–11]. Mechanistically, the initial onset and the subsequent progression of ED is recognized to be a consequence of the composite of insults to neurological, hormonal, and vascular pathways involved in the physiology of an erection. For example, events, procedures, or conditions that disrupt central neural networks involving dopaminergic or oxytocinergic pathways, particularly in the paraventricular nucleus (PVN) and medial preoptic area (MPOA) of the hypothalamus, or the cascade of signaling in peripheral autonomic pathways involved in sexual responses, all can result in ED [12]. Similarly, an appropriate androgen milieu appears to be necessary for normal sexual desire, erectile responses, and ejaculation [13,14]. Based on this understanding, both clinically and experimentally, these aspects of sexual function have been shown to improve with androgen supplementation, in conditions in which there is depressed testosterone bioavailability [15–17]. Finally, vascular insufficiency, specifically in vessels supplying blood to the penis and in the intrapenile erectile tissue, has also been strongly linked with decrements in erectile function [18,19]. As previously stated, men suffering from hypertension are twice as likely to experience ED compared with age-matched normotensive men [20]. Experimental studies using spontaneously hypertensive rats (SHR) have corroborated this finding and have further pointed to pathological structural changes in the penile vasculature as a potential mechanism of the dysfunction [21–23]. Further, reversal of these structural abnormalities in SHR using aggressive antihypertensive treatments has been linked with improvements in erectile function even after withdrawal of treatment [24]. In the various animal studies, the occurrence of diminished erectile status has been characterized by fewer centrally initiated erections, blunted nervestimulated cavernous responses, decreased smooth muscle contractility, elevated adrenergic sensitiv-

ity, and altered morphology of the corpus cavernosum [25–27]. A number of studies have compared the age-related decline in erectile function in SHR vs. normotensive Wistar-Kyoto (WKY), although the pathophysiological mechanisms remain substantially unresolved [28]. In addition, although there have been several agingrelated studies of outbred, normotensive strains, the focus has not been on the impact of caloric intake or changes in body composition, but more on effects involving conditions such as castration, experimental diabetes, secondary hypertension, hormone supplementation, or pharmacologic treatments [29–34]. Clinically, obesity and physical inactivity have been established as risk factors for cardiovascular disease and are further linked to an increased prevalence of ED, just as exercise and weight reduction have been shown to be beneficial [35]. One recent study found that increased exercise and caloric restriction (CR) improved erectile function in a third of obese patients [36]. Experimentally, obesity has usually been modeled using genetically modified animals or by giving animals abnormal diets. Interestingly, normal Sprague-Dawleys and Wistars, but not SHR, who are allowed to eat regular chow ad libitum will progressively gain weight such that their body composition reflects the accumulation of abdominal adipose tissue to a much greater extent than in young adult normotensive animals or age-matched SHR. In fact, CR in normotensive rodents, as well as primates, has previously been found to extend the maximum life span [37]. In the present study, one objective was to determine whether the time course of the age-related decline in erectile function in hypertensive (SHR) and normotensive (Wistar and Sprague-Dawley) strains was different. Once a significant age- and obesity-associated decline in erectile function was found, an additional objective was to assess whether a regimen of combined CR and exercise, followed by CR alone, could recover and then maintain erectile function. Finally, short-term testosterone supplementation was given to assess whether the deficits in erectile responses in the aging animals were, at least in part, due to decreased levels of androgens. Methods

Animals Male Wistar rats (N = 16), Sprague-Dawley rats (N = 18), and SHR (N = 6) were purchased at J Sex Med 2007;4:886–897

888 12 weeks of age (Charles River Laboratories, Montreal, Quebec, Canada) and were housed individually at a temperature of 22–24°C with a 12-h light/dark cycle. All rats were provided with free access to regular rodent chow (LabDiet® 5001, Ren’s Feed and Supply Ltd, Oakville, Ontario, Canada) and tap water. All procedures were performed in accordance with the guidelines of the Canadian Council on Animal Care as approved by the Queen’s University Animal Care Committee.

Assessment of Erectile Response From 15 weeks of age onward, erectile function and body weight were measured weekly. Erectile responses were monitored via the well-established rat bioassay of centrally induced erections using the dopaminergic agonist, apomorphine [38]. Prior to (20-minute acclimatization period) and during testing (30 minutes), rats were placed in one of four individual, hanging cages (with clear plastic inserts used to replace the wire floor) in an isolated, quiet, dimly lit room adjacent to the holding room. Each rat received subcutaneous injection of apomorphine in saline (80 mg/kg with 100 mg/mL ascorbic acid, 1 mL/kg) in the loose skin of the neck or back. Erections and yawns were counted over a 30-minute period via videomonitoring connected to monitors in an adjacent room. Erectile responses were recorded following identification of characteristic physical and behavioral responses, including concave arching of the back, pelvic thrusts followed by the emergence of the engorged glans penis and shaft, and immediate oral grooming of the genital area. Apomorphineinduced yawns were recorded as an index of delivery of the apomorphine and to confirm bioavailability of the drug within the central nervous system [39]. CR and Exercise Based on the marked decline in erectile responses, when all animals had reached 30 weeks of age, daily food intake was then progressively restricted in all rats (decrement of 10% per week from intake at 29–30 weeks) up to a maximum of 40%, or until they had lost 10% of their body weight, according to a previously established protocol [37]. Upon losing 10% of their body weight, food intake was then reduced by only 10–20% from initial intake levels in order to maintain body weight. In addition, throughout this period (30–40 weeks of age) rats were exercised for 30 minutes, 5 days per week, using a modified, full-size treadmill fitted to rotate rodent exercise balls at a speed of 24 m/ J Sex Med 2007;4:886–897

Hannan et al. minute. At 40 weeks of age, exercise was stopped, but the food intake remained restricted by 10–20% to maintain their body weight. Ad libitum access to food was reinstated when the animals reached 50 weeks of age. Erectile responses were always assessed prior to daily exercise.

Testosterone Treatment At 44 weeks of age, each rat from the three initial groups (Wistar, Sprague-Dawley, and SHR) received a single dose of testosterone (480 mg/kg, subcutaneous), administered 36 hours prior to erectile testing, according to a previously established protocol for normalizing erections in previously castrated rats [16]. Testosterone was prepared from a stock solution of testosterone propionate (Taro Pharmaceuticals, Bramalea, Ontario, Canada) diluted in peanut oil. Assessment of Heart Weights Rats were anesthetized with pentobarbital (65 mg/ kg, intraperitoneally), and hearts were removed and placed in cold saline. The right and left ventricle plus septum were then separated, blotted dry, and weighed. Analysis of the left ventricle-tobody weight (LV/BW) and right ventricle-to-body weight ratios was used as an index of changes in cardiac structure [40]. CR and Exercise in Old Rats In two separate groups of older rats, the impact of exercise and CR on erectile function was assessed at a later age. Aged male Wistars (56 weeks, N = 10) and Sprague-Dawley (67 weeks, N = 12) were separated into two treatment groups, which underwent a 10% CR or a 10% incremental weekly decrease to 40% and low-impact exercise on a treadmill (7 m/minute, 30 minutes per day, 5 days a week). Erectile responses and body weight were recorded weekly as previously described. Statistical Analysis Erectile response data are expressed as the number of erections per test period ⫾ SD. Body weight is expressed as mean ⫾ SD. Statistical analysis of erectile responses between and within treatment groups was performed using the Mann–Whitney rank sum test and one-way analysis of variance, with a comparison of means using a Newman– Keuls posthoc test (P < 0.05). Differences in erectile responses during testosterone treatment were determined using Student’s t-tests (P < 0.05). Heart weight data are presented as the mean ⫾ SD

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and were assessed by analysis of variance, followed by a Newman–Keuls posthoc test (P < 0.05).

increase over 20 weeks), rather than to produce progressive reductions in body weight [42]. Regardless of the approach, in all three strains, after ad libitum feeding was reintroduced at 50 weeks, body weight again progressively increased over the next 10 weeks (Wistar and Sprague-Dawley 16%, SHR 8%), an effect which was particularly evident in the first two after stopping CR. At the end of the study, there was no significant left ventricular hypertrophy in the heart weights (LV/BW) of Wistar or Sprague-Dawley rats (1.7 ⫾ 0.3 and 1.7 ⫾ 0.1 mg/g body weight). Conversely, as expected, in SHR after the various treatments, the left ventricle remained significantly enlarged (3.3 ⫾ 0.1 mg/g body weight).

Results

Overall Effects of Exercise and CR on Body Weight Despite being adult, there was a substantial increase in body weight between 15 and 30 weeks of age in the normotensive (34% in SpragueDawleys, 45% in Wistars) and hypertensive (25% in SHR) rats fed ad libitum (Figure 1). Following the initiation of the 10 weeks of combined exercise and CR and 10 weeks of CR alone, overall the weight of the Wistar and Sprague-Dawley rats was decreased by 10% and 8.5% respectively, compared with 30 weeks of age. In the SHR, a different approach had to be taken because this strain of rat is a much leaner strain, with a higher metabolic rate, and cannot be subjected to prolonged or severe CR [41]. That is, to maintain a healthy status of this strain, the protocol was adjusted to attenuate body weight increases in SHR (to 3%

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Figure 1 Overall profile of body weight gain throughout the study in Sprague-Dawley rats (N = 6), Wistar (N = 6), and SHR (N = 6). Periods in which rats underwent exercise and caloric restriction (CR + EX) and CR alone (CR) are outlined. Data are shown as mean. SHR = spontaneously hypertensive rats.

Effect of Aging and/or Associated Weight Gain on Sexual Function As seen in previous studies, there was a significant decline in erectile responses with age in all three strains of rats (Figure 2 left), such that by 29 weeks of age, the normotensive rats averaged only 0.2 ⫾ 0.4 erections and the SHR were at 0.6 ⫾ 0.8 erections per test period. In general, the increase in body weight paralleled the decline in erectile function in all rats. Paralleling the decrease in erectile responses was a progressive increase in the proportion of animals suffering from severe ED (Figure 2 right). Specifically, by 29 weeks of age, all animals had at least some level of ED according to the predefined severity scale (i.e., no dysfunction ⱖ2.0 erections; mild ⱖ1.0 erection; moderate 0.5 to