Stimulation of renin secretion by alpha-adrenoceptor agonists

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Abstract
It was determined whether stimulation of intrarenal .alpha.-adrenoceptors can increase renin secretion rate (RSR) in the absence of increased renal vascular resistance and the accompanying changes in renal function were identified. Experiments were performed in pentobarbital-anesthetized dogs in which renal perfusion pressure was maintained at .apprx. 90 mm Hg and the infused kidney was acutely denervated. Renal artery infusion of the .alpha.-adrenoceptor agonist methoxamine (0.5 .mu.g .cntdot. kg-1 .cntdot. min-1 for 30 min) increased RSR from 160 .+-. 95 to 1376 .+-. 385 ng ANG I[angiotensin I]/min (P = 0.01) but did not decrease renal blood flow (RBF); the same dose infused i.v. had no effect on RSR or RBR. Intraarterial phenylephrine infusion (0.5 .mu.g .cntdot. kg-1 .cntdot. min-1 for 9 min) increased RSR by 500 .+-. 157 ng ANG I/min (P < 0.01) and decreased both inulin clearance (Cin) and urinary Na excretion (UNa.ovrhdot.V) by 25% but did not affect RBF. At a lower concentration of phenylephrine (0.2 .mu.g .cntdot. kg-1 .cntdot. min-1 for 9 min), RSR increased by 318 .+-. 103 ng ANG I/min (P < 0.01) and RBF, Cin and UNa.ovrhdot.V did not change. The increase in RSR was completely blocked by prazosin but was unaffected by propranolol. Renin secretion can be stimulated by activation of intrarenal .alpha.-adrenoceptors even in the absence of increased renal vascular resistance.