Neurogenic control of renal function in response to graded nonhypotensive hemorrhage in conscious dogs

The reflex control of plasma renin activity (PRA) and urinary sodium excretion (U(Na)V) was evaluated in 13 dogs instrumented for chronic study and maintained on a normal sodium intake (40 meq/day). Graded blood volume depletion of 14 (BVD₁) and 21% (BVD₂) of the estimated total blood volume was used to activate renal sympathetic nerve activity (RSNA), and experiments were conducted before and after bilateral renal denervation (DNX). In dogs with innervated kidneys, nonhypotensive BVD₁ increased RSNA 40.9 ± 10.9% (P < 0.05) above control. Blood volume depletion increased PRA from 1.95 ± 0.52 to 3.5 ± 0.57 ng · ml^-1 · h^-1 and decreased U(Na)V from 58.2 ± 10.1 to 35.5 ± 4.3 μeq/min without changing renal blood flow or glomerular filtration rate. BVD₂ failed to further activate RSNA (52.0 ± 16.7%) but did increase PRA to 4.85 ± 0.83 ng · ml^-1 · h^-1 and decreased U(Na)V to 17.9 ± 2.7 μeq/min. Renal DNX (n = 13) abolished both the PRA and antinatriuretic responses to BVD₁ and BVD₂. Thus volume-invoked reflex activation of RSNA, but not altered renal hemodynamics, mediates activation of PRA and antinatriuresis. This neurogenic control of renal function may be critical to the rapid regulation of extracellular fluid volume, via alterations in urinary excretion.