Proximal tubular function in rats with low inulin clearance.

Abstract

Proximal tubular salt and water reabsorption were studied in relation to the inulin clearance (Cin) in anesthetized rats maintained on a standard diet with and without partial clamping of the renal artery and in rats on a sodium deficient diet. The rate of proximal reabsorption, determined as the reciprocal of the occlusion time (OT), was linearly related to Cin above clearance values of 0.7 ml/min g kidney weight (k.w.). Below this value, proximal reabsorption rate reached a fixed minimum corresponding to an OT of 25–26 sec. Late proximal fluid to plasma inulin ratios averaged 3.0 at Cin greater than 0.65 ml/min g k.w. and 5.1 when Cin was 0.55 ml/min or less. This significant difference in fractional reabsorption was predicted from the OT measurements. Factors leading to a decrease in caliber of the proximal tubular lumen following retrograde microinjection of the distal convolution of the same nephron were also studied. When seen, proximal collapse appeared to be due to leakage of tubular fluid at a previous proximal puncture site and did not occur when procedures were taken to minimize leakage.