Effects of Androgens in the Onset and Magnitude of Salt- Induced Hypertension in Male Sprague-Dawley Rats

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Oloyo, A.K
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Blood pressure is consistently higher in males compared with females from puberty onwards and men show an increased risk for hypertension compared to women. The gender disparity in cardiovascular functions and diseases has been linked to the effect of sex hormones on vascular reactivity. Although previous studies have suggested that the effect of chronic exposure to testosterone is an increase in vascular tone, it therefore implies that lack of testosterone should elicit vasorelaxation. Salt-sensitive hypertension in humans is associated with higher morbidity and mortality which may also be gender related. The role of gender in salt-sensitivity is not clear and the long-term effects of androgens on vascular reactivity especially in salt-induced hypertension have not been studied experimentally. Therefore experiments were designed to assess the effects of androgens on the development of salt-induced hypertension in male Sprague-Dawley rats by assessing vascular relaxation response to agonist as well as vascular smooth muscle histomorphology in orchidectomy-induced androgen deficient rats placed on a normal or high salt diet with or without testosterone supplementation. Weanling male Sprague-Dawley rats aged 8 weeks were divided into 8 groups of 16 rats each. They were either bilaterally orchidectomised or sham-operated under anaesthesia, with or without testosterone replacement (10mg/kg sustanon 250® i.m once in 3 weeks). They were placed on normal (0.3%) or high (8%) NaCl diet for 6 weeks. Arterial blood pressure was determined in conscious rats before and weekly throughout the experimental period using non-invasive tail cuff method. Terminal arterial blood pressure was also determined via femoral artery cannulation. Serum concentration of testosterone was determined using enzyme immunoassay (EIA) technique at the end of the feeding period. Thoracic aorta was isolated and 3mm aortic rings were suspended in organ baths and relaxation responses to acetylcholine (ACh), sodium nitroprusside (SNP) forskolin, diazoxide, testosterone and DHEA in the presence or absence of L- nitro-Arginine methyl ester (L-NAME), indomethacin, barium chloride, flutamide and Aminogluthetemide in noradrenaline pre-contracted rings were studied. Lipid peroxidation was studied in the heart and kidney homogenates using the TBARs method. The serum activity of super oxide dismutase (SOD) was also determined. Histological examination of thoracic aorta and mesenteric artery were carried out with specific dyes using haematoxylin and eosin stain for the cytoplasm and nucleus and Verhoeff – Van Geison and Picro-sirius red stains for elastin and collagen content estimation respectively. Histomorphometric analysis was determined using a programmed software IMAGE-PRO 3DS 6.1. The results indicate a significant increase (P < 0.001) in the mean arterial blood pressure (MABP) of rats placed on high salt diet (HSD), when compared with control or orchidectomised rats. Orchidectomy elicited a reduction in MABP while testosterone replacement normalized MABP to values seen in intact rats placed on high salt diet. High salt diet reduced relaxation response to ACh both in the presence and absence of eNOS inhibition with L-NAME. There was a significant decrease (P < 0.05) in the relaxation response to forskolin in rats placed on high salt diet when compared with controls. High salt diet reduced the relaxation response to diazoxide but not in orchidectomised rats while testosterone supplementation re-established the blunted diazoxide relaxation. The results also show a significant increase (P < 0.001) in lipid peroxidation of HSD groups compared with controls. Orchidectomy elicited a reduction in lipid peroxidation while testosterone supplementation returned it to the level observed in the intact groups. On the other hand there was a significant decrease (P < 0.01) in the serum level of SOD of the HSD groups when compared with controls, while orchidectomy increased the SOD activity, testosterone supplementation restored it to the level observed in the intact groups. Tunica media thickness and cross sectional area, elastin and collagen contents were all significantly elevated in the rats placed on high salt diet while orchidectomy significantly reduced the values of the parameters in high salt group but concomitant administration of testosterone restored them to the levels observed in intact rats. Both aromatase inhibition and androgen receptor blockade did not prevent the relaxing effect of testosterone on rings from rat aorta. There was also no significant difference between testosterone relaxation response in the presence or absence of L-NAME and indomethacin. However, 3µM BaCl2 almost completely abolished the aortic ring relaxation response to testosterone while 1 µM nifedipine potentiated the vasorelaxing effect of testosterone. The results indicate that in male Sprague-Dawley rats, endogenous testosterone promotes blood pressure-elevating effect of a HSD such that bilateral orchidectomy reduced the blood pressure and attenuated the impaired endothelial function induced by HSD. However ,this was reversed by concomitant administration of testosterone, suggesting a role for androgens in enhancing long term vascular smooth muscle tone and hence the maintenance of arterial blood pressure. Endogenous testosterone enhanced ROS-generating and promoted vascular hypertrophic effect of a HSD in the rats. On the other hand, exogenous testosterone relaxes rat aorta directly via a non-genomic pathway which is independent of endothelial derived vasoactive substances.
A Thesis Submitted to the School of Postgraduate Studies, University of Lagos
Blood Pressure , Cardiovascular , Sex hormones , Sprague-Dawley rats , Research Subject Categories::MEDICINE
Oloyo, A (2010). Effects of Androgens in the Onset and Magnitude of Salt- Induced Hypertension in Male Sprague-Dawley Rats. A Thesis Submitted to University of Lagos School of Postgraduate Studies Phd Thesis and Dissertation, 293pp.