Sex hormone regulation of systemic endothelial and renal microvascular reactivity in type-2 diabetes: Studies in gonadectomized and sham-operated Zucker diabetic rats

Background: Male Zucker diabetic rats exhibit a more severe endotheliopathy in comparison with their female diabetic litter mates. The plasma concentrations of both thromboxanes and endothelins are elevated in diabetes, and the receptor cross-talk between TXA₂ and ET-1 receptors may be enhanced in type-2 diabetic Zucker rats. Aims: To determine the role of the endogenous sex steroid hormones, testosterone and estradiol on the systemic and renal microvascular reactivity to ET-1, thromboxane-mimetic U46619, ET-TXA ₂ receptor interaction, and the nitric oxide vasodilator system in Zucker hypertensive-diabetic rats. Methods: Male and female Zucker rats aged 8-10 weeks were each divided into two groups. The male rats were castrated or underwent a sham operation. The female rats were spayed (bilateral ovariectomy and hysterectomy) or had a sham operation. All rats were studied 4-6 weeks after the gonadectomy or sham operations. Blood glucose and insulin as well as plasma concentrations of testosterone and estradiol were determined. Haemodynamic studies were undertaken with determination of the dose-response curve for mean arterial pressure (MAP), renal cortical flow (RCF) and renal medullary blood flow (MBF) in response to ET-1 and U46619, and the effect of interdiction of the ET-TXA₂ interaction with ET-antagonists BQ610 and BQ788. The role of endogenous NO was assessed by its response to graded acetylcholine doses and to a L-NG-nitro-arginine methyl ester (L-NAME) infusion. Results: Castrated male rats had a significantly lower blood glucose concentration (295 ± 33 mg dL^-1) compared with their sham-controls (481 ± 40 mg dL ^-1), P = 0.008. Mean arterial pressure tended to be lower in the castrated rats. Gonadectomy reduced the plasma testosterone and estradiol concentrations. Castration abolished the hypotensive action of U46619 compared with sham-operated male rats (P < 0.0001, ANOVA). Conversely, the presser action of U46619 seen in the sham-operated female rats was reversed to a profound hypotensive action in the spayed rats (P < 0.001, ANOVA).The change in MAP after U46619 was inversely correlated to the plasma testosterone concentration (r = -0.73, P = 0.027). The paradoxical hypotensive response elicited by ET-1 in the Zucker diabetic rats of both sexes was abolished by castration only (P < 0.005, ANOVA). Castration caused a significant (P = 0.011) augmentation of the vasodilator response to acetylcholine, while spaying caused a slight attenuation. Castration, but not spaying, resulted in significant increases in MBF after U46619 (P = 0.003, ANOVA), ET-1 (P = 0.005, ANOVA) and acetylcholine (P = 0.053, ANOVA). The ET-B antagonist BQ788 augmented the U46619-induced rise in MAP in castrated male rats, and also abolished the U46619-induced increase in MBF (P < 0.01 ANOVA). L-NAME (25 mg kg ^-1) increased MAP and decreased MBF in the gonadectomized and sham-operated rats, except for the castrated male Zucker rats, where it significantly increased MBF (+90 ± 31 PU) (P = 0.0004, ANOVA) despite the increase in MAP. Conclusions: Testosterone and estradiol regulate systemic and microvascular reactivity to TXA₂ receptor stimulation in type-2 diabetic Zucker rats. The impact of testosterone on blood glucose concentration, blood pressure, and the systemic and renal microcirculatory response to ET-1 and NO, as well as the endothelin-thromboxane receptor cross talk, is greater, and opposite to that of estradiol. The effects of testosterone withdrawal may at least in part be mediated by the ET-B receptor subtype and NO generation. Androgen blockade should be investigated further for the reversal or delay of hypertensive-diabetic endotheliopathy.