Hemodynamic mechanisms influencing sodium excretion during angiotensin infusion.

To examine mechanisms of transition between antinatriuresis and natriuresis, angiotensin II was first infused intrarenally (0.001-0.07 mug/kg-min) in anesthetized dogs; glomerular filtration rate (GFR), sodium excretion, and intrarenal pressure (IRP), indicating tubular pressure, fell as during mechanical aortic constriction. During supplementary intravenous infusion (0.10-0.30 mug/kg-min), renal blood flow (RBF) rose toward control (tachyphylaxis). Tubular reabsorption increased but was still 17.1% below control. Filtration fraction averaging 0.31 remained constant. Sodium excretion and IRP exceeded control but were normalized by restoring renal arterial pressure mechanically. During intrarenal angiotensin infusion, carotid constriction increased blood pressure more, but RBF, IRP, and sodium excretion less than intravenous angiotensin. Intrarenal infusion of angiotensin at 0.10-0.20 mug/kg-min increased RBF and sodium excretion more in infused than in contralateral kidneys. Thus, angiotensin natriuresis depends on increased perfusion pressure and is augmented as tachyphylaxis develops. High correlation between sodium excretion and IRP at unchanged filtration fraction suggests a regulation of sodium excretion by hydrostatic rather than oncotic pressures in glomerular and peritubular capillaries.