Glomerular and tubular antinatriuretic actions of low-dose angiotensin II infusion in man.

OBJECTIVE: The aim was to study the physiological effects of angiotensin II upon the glomerular and tubular handling of sodium. DESIGN: Healthy volunteers were examined before and during infusion with either low-dose angiotensin II (n = 11) or placebo (n = 13). METHODS: Lithium clearance was used to estimate the segmental tubular reabsorption of sodium. RESULTS: During infusion with angiotensin II a sustained and marked fall in renal plasma flow was observed. The glomerular filtration rate (GFR) decreased to a minor extent so that the filtration fraction increased during angiotensin II infusion. Angiotensin II caused an extensive and instantaneous fall in both urinary flow and urinary sodium excretion. Proximal absolute reabsorption of sodium was unchanged despite the fall in GFR, showing that proximal fractional reabsorption was enhanced by angiotensin II. Distal absolute reabsorption was decreased during the entire period of angiotensin II infusion. However, when the distal reabsorption was related to the delivery of sodium from the proximal tubules, distal fractional reabsorption in fact increased after 30 min angiotensin II infusion. None of the measured parameters changed during infusion with placebo. A significant increase in plasma aldosterone was observed 30 min after the start of the angiotensin II infusion. Plasma atrial natriuretic peptide did not change during infusion with either angiotensin II or placebo. CONCLUSIONS: We conclude that physiological increments in angiotensin II affect glomerular haemodynamics and cause a marked antinatriuresis in man. The antinatriuretic effect of angiotensin II is caused initially by a combination of a decrease in the GFR and an increase in proximal fractional sodium reabsorption, and later by the enhanced distal fractional reabsorption of sodium.