Mechanism of effect of alpha adrenergic stimulation with norepinephrine on renal water excretion

Abstract

The present study was undertaken to investigate the mechanism whereby alpha adrenergic stimulation with intravenous norepinephrine results in a water diuresis. Renal perfusion pressure was kept constant in all experiments by adjustment of a suprarenal aortic clamp. In hydropenic anesthetized dogs the intravenous infusion of norepinephrine (0.5 mug/kg per min) was associated with a mean decrease in urinary osmolality from 616 to 126 mosmol/kg (P < 0.001) which increased to 532 mosmol/kg (P < 0.001) after the infusion was discontinued. During the same period of time the mean free water clearance increased from -0.437 to 1.59 (P < 0.001) and then returned to -0.314 ml/min (P < 0.001) after cessation of the infusion. This diuretic effect occurred in both innervated and denervated kidneys and was not associated with an increase in glomerular filtration rate or solute excretion. Systemic arterial pressure increased from 121 to 142 mm Hg during the norepinephrine infusion. Studies were also performed in hypophysectomized animals receiving a constant infusion of either 80 mug/kg per min or 20-40 muU/kg per min of vasopressin. In these animals, intravenous norepinephrine was not associated with changes in either urinary osmolality or free water clearance. The intrarenal administration of norepinephrine, in doses comparable with those reaching the kidneys during the intravenous studies, also resulted in no significant change in either urinary osmolality or free water clearance in hypophysectomized animals receiving 20-30 muU/kg per min of vasopressin. These results thus indicate that the water diuresis associated with intravenous norepinephrine is mediated primarily by suppression of vasopressin release rather than by changes in renal hemodynamics, renal innervation, or an effect of norepinephrine on the water permeability of the tubular epithelium.