Regulation of renal medullary endothelin B receptor function by angiotensin II: evidence of sex differences

Abstract

The renin angiotensin system and endothelin (ET) systems play critical roles in regulating kidney function and blood pressure. Angiotensin (Ang) II exerts its prohypertensive effects through AT1 receptor activation. ET-1 has similar effects mediated by ETA receptor stimulation. In contrast, ET-1, via ETB receptors, mediates vasodilation, anti-inflammation, and natriuresis. In the clinical setting, hypertension is more common in men than in premenopausal women of the same age. Moreover, in a number of animal models of genetic or experimental hypertension, females are somewhat protected from high blood pressure compared to males. We previously found that hypertensive male rats, induced by chronic Ang II infusion, have impaired ETB receptor function. Because ETB receptors are highly expressed in the renal medulla, the overall aim of this dissertation is to determine the role of Ang II in mediating renal medullary ETB receptor function, and to determine if differences in renal medullary ETB receptor function contribute to the sex differences observed in Ang II hypertension. The first aim was to test the hypothesis that renal medullary ETB receptor function is impaired in male Ang II hypertensive rats. However, ET-mediated natriuresis is preserved in female rats in response to chronic Ang II infusion. We compared the diuretic and natriuretic responses to intramedullary infusion of the ETB receptor agonist, sarafotoxin 6c (S6c), in male and female rats treated with Ang II (260 ng/kg/min s.c.) or vehicle for 14 days. Male Ang II hypertensive rats had impaired ETB-dependent sodium and water excretion. In contrast, renal medullary ETB receptor function was preserved in female Ang II-treated rats. Moreover, ETA-mediated diuretic and natriuretic responses were maintained in female Ang II hypertensive rats. These data demonstrate that, in contrast to male Ang II hypertensive rats, ET receptor-induced diuretic and natriuretic responses are preserved in female rats during chronic Ang II infusion. The second aim was to determine if ETB receptors limit the hypertensive response and renal injury induced by chronic Ang II infusion in female rats compared to males. Male and female rats received Ang II infusion (150 ng/kg/min; sc.) along with a high salt diet (4% Na) for 4 weeks; blood pressure was measured by telemetry. After one week of Ang II infusion with a high salt diet, subsets of both male and female rats received the ETB antagonist, A-192621, at three doses on consecutive weeks (1, 3, and 10 mg/kg/d in food). Male rats had significantly higher blood pressure compared to females after 4 weeks of Ang II. A-192621 resulted in a dose-dependent increase in blood pressure in female Ang II hypertensive rat while there was no significant change in males. After 4 weeks of Ang II infusion, the levels of proteinuria and nephrinuria were higher in male rats compared to female. A-192621 did not further increase urinary excretion of protein or nephrin in either male or female Ang II hypertensive rats. In conclusion, ETB receptors provide more protection against hypertension during chronic Ang II infusion in female rats compared to male. The third aim was to determine the physiological role of Ang II in regulating renal ETB receptor function during salt deprivation, a model with high levels of endogenous Ang II. After 2 weeks of normal (0.4% Na) or low (0.01-0.02% Na) salt feeding, the activation of ETB receptors in the renal medulla increased urine flow and sodium excretion of rats on normal salt diet. While urinary ET-1 excretion was comparable between a normal and low salt diet, ETB-dependent diuresis and natriuresis in response to acute intramedullary infusion of S6c was reduced in the low salt treated rats. Chronic treatment with the AT1 receptor antagonist, candesartan, restored ETB-induced water and sodium excretion in rats fed low salt diet. These findings support the hypothesis that AT1 receptors regulate renal medullary ETB receptor function in a low salt diet model to conserve sodium. From these studies, we conclude that Ang II via the AT1 receptor attenuates renal medullary ETB receptor function resulting in sodium and water retention. During pathological situations, Ang II has a greater inhibitory effect on ETB receptor function in male rats compared to females, leading to a greater increase in blood pressure in response to chronic Ang II infusion.