Upregulation of Renal ENaC Activity in Spontaneously Hypertensive Rats Is Independent of Aldosterone

Abstract

Spontaneously hypertensive rat (SHR) has been widely used as a model to interrogate the contribution of the kidney to the pathogenesis of essential hypertension in humans. Elevation of blood pressure in SHR has been associated with altered renal function, since predisposition to hypertension can be transferred with a transplanted kidney into a normotensive Wistar‐Kyoto (WKY) strain. Renal defects result in excessive water and sodium retention leading to increased circulating volume and blood pressure. Abnormally elevated Na+ reabsorption in SHR is associated with the overactive renin‐angiotensin‐aldosterone system (RAAS) and with the increased abundance of the renal epithelial Na+ channel (ENaC) responsible for final regulation of sodium reuptake in the kidney and pivotal for long‐term blood pressure control. The existing data on regulation of ENaC by RAAS components in SHR is inconsistent and molecular determinants of excessive ENaC activation remain largely obscure. In the present study, we tested the efficacy of aldosterone antagonism to inhibit ENaC activity, decrease renal Na+ reabsorption and reduce blood pressure in SHR of both sexes. Where necessary, age‐matched (12–15 week‐old) WKY rats were used as controls. Consistent with previously published data, enzyme immunoassay analysis revealed elevated circulating aldosterone levels and semi‐quantitative immunoblotting – higher expression of renal ENaC protein in SHR when compared to WKY controls. Patch‐clamp experiments in split‐opened collecting ducts showed that ENaC activity was moderately increased in SHR when compared to WKY. Systemic administration of aldosterone antagonist, spironolactone (30 mg/kgBW·day) did not affect renal ENaC activity or protein abundance in SHR. Aldosterone antagonism did not alter the expression of sodium‐chloride co‐transporter (NCC) in the kidneys of SHR. Tail‐cuff plethysmography showed that treatment with spironolactone at doses 30–200 mg/kgBW·day for 2 weeks failed to decrease blood pressure in SHR and did not result in any detectable changes in urinary sodium excretion. We did not observe any sex‐specific differences pertaining to ENaC activity or responsiveness to spironolactone in SHR males and females. Thus, we conclude that aldosterone is not a primary driver of excessive ENaC‐dependent renal Na+ reabsorption in SHR.