Mechanism of natriuresis following intravascular and extracellular volume expansion

Abstract

In clearance studies in rats, increases in filtration rate and electrolyte excretion were observed following both intravascular and extracellular fluid volume expansion. The inulin concentration ratio of proximal tubular fluid to plasma was decreased with extracellular expansion. Neither natriuresis nor fractional sodium reabsorption was related to the degree of intravascular expansion. Microperfusion studies demonstrated a decrease in proximal sodium reabsorption only when both intravascular and extravascular volumes were expanded; net sodium transport was not affected by a blood volume increase alone. From the data it is concluded that in the rat an increase in blood volume is followed by a rise of filtration rate and a fall of fractional reabsorption at a site distal to the proximal tubule, resulting in diuresis and natriuresis. If, in addition, the interstitial fluid compartment is expanded, a direct inhibition of the active transport component of proximal Na⁺ reabsorption occurs.