Renal haemodynamics and sodium handling after hyperoncotic albumin infusion in sodium‐restricted normal man

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Abstract
The effect of plasma volume expansion with hyperoncotic albumin (HA) on renal haemodynamics and segmental tubular sodium (Na) handling was studied in eight water‐loaded normal men. Clearance studies were performed before, during and after a 60‐min infusion of 300 ml 20% HA. Sodium intake was restricted (20 mmol day‐1) to mimic clinical conditions of relative hypovolaemia in which HA is often applied. Plasma volume rose by 740±160 ml after infusion and colloid oncotic pressure (COP) by 3·6±1·3 mm Hg. No natriuresis was induced. In fact, Na output fell from 43±29 to 28±25 μmol min‐1 during infusion, returning to 41±36 μmol min‐1 afterwards. Corresponding clearances of inulin were 121±19, 110±19 and 117±21 ml min‐1 and of p‐aminohippurate (CPAH) 602±87, 637±100 and 751±148 ml min‐1. Fractional reabsorption of sodium in the proximal tubules (FPRNa), assessed from maximal free‐water and lithium clearance, rose consistently during infusion but diverging patterns (further rise, no change or fall) were seen afterwards. Fractional distal reabsorption remained unchanged both during and after HA infusion. Changes in plasma volume and CPAH in the post‐infusion period were correlated. These results indicate that, despite considerable plasma volume expansion, HA infusion does not enhance Na excretion in Na‐restricted subjects. This is mainly due to elevation of plasma COP, which reduces glomerular filtration and enhances proximal reabsorption. The biphasic response in FPRNa, an initial increase during infusion that abates afterwards, can be explained from a secondary rise in peritubular capillary hydrostatic pressure related to an increased renal perfusion.

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