Role of renal alpha-adrenoceptors mediating renin secretion.

The increase in renin secretion resulting from low-frequency renal nerve stimulation (0.5 Hz) occurs in the absence of changes in urinary sodium excretion or renal blood flow and is apparently derived from a direct effect of renal sympathetic nerves on juxtaglomerular granular cells. We sought to determine the role of renal alpha-adrenoceptors in this neurally evoked renin secretion. The neurally evoked renin secretion was unaffected by renal alpha-adrenoceptor blockade with phentolamine or prazosin; however, two dose levels of phenoxybenzamine equally inhibited the renin secretion. The renal vasoconstrictor response to graded renal nerve stimulation was similarly diminished by phentolamine, prazosin, and the higher phenoxybenzamine dose, whereas the lower phenoxybenzamine dose was significantly less effective. Renal alpha-adrenoceptor stimulation with methoxamine infusion at doses that were just subthreshold for altering renal blood flow and urinary sodium excretion or at doses that just reduced urinary sodium excretion also did not change renin secretion. Higher doses of methoxamine that decreased both renal blood flow and sodium excretion increased renin secretion. Based on the inability of phentolamine and prazosin to prevent neurally mediated renin secretion and on the dose-response relationship between methoxamine and changes in renin secretion, renal blood flow, and urinary sodium excretion, we conclude that renal alpha-adrenoceptors do not mediate renin secretion elicited by direct neural activation of the juxtaglomerular granular cells. The data suggest that phenoxybenzamine inhibits neurally mediated renin secretion by a mechanism other than renal alpha-adrenoceptor blockade.