The role of alpha-adrenoceptors in the regulation of renal tubular sodium reabsorption and renin secretion in the rabbit.

A study was undertaken in the anaesthetized rabbit to classify the alpha-adrenoceptor subtypes responsible for increasing renal tubular sodium reabsorption and renin secretion. Intrarenal administration of noradrenaline, at doses which did not change renal blood flow or glomerular filtration rate, significantly decreased urine flow, absolute and fractional sodium excretion by between 26% and 29%. These renal responses to noradrenaline were abolished by the selective alpha 1-adrenoceptor antagonist, prazosin, but not by the selective alpha 2-adrenoceptor antagonist, idazoxan. Noradrenaline, given intrarenally, increased renin secretion between two and three fold and this response was not modified by either prazosin or idazoxan. Intrarenal administration of the selective alpha-adrenoceptor agonists, phenylephrine and methoxamine, at dose rates which did not change renal haemodynamics, significantly reduced urine flow, absolute and fractional sodium excretion by 15% to 33%, but at doses which reduced blood flow and filtration rate, by between 11% and 26%, urine flow, absolute and fractional sodium excretion decreased between 42% and 49%. Infusion of guanabenz (selective alpha 2-adrenoceptor agonist), at doses with no renal haemodynamic action, increased urine flow, absolute and fractional sodium excretion by 11% to 15%, while at doses which decreased blood flow by 7%, these other variables did not change. Administration of UK 14304 (selective alpha 2-adrenoceptor agonist) reduced blood flow and filtration rate by 3% and 9% respectively but had no other measurable action. At higher doses, which decreased blood flow by 14% and filtration rate by 24%, urine flow, absolute and fractional sodium excretion fell by between 27% and 50%. Renin secretion was significantly increased by the high doses of phenylephrine and UK 14304 but not by the low doses of these drugs. These studies show that adrenergic stimulation of renal tubular sodium reabsorption involves the activation of alpha 1- but not alpha 2-adrenoceptors. Further, adrenergic activation of the juxtaglomerular cells to release renin does not appear to involve either alpha 1- or alpha 2-adrenoceptors.