PROXIMAL TUBULAR REABSORPTION IN RAT KIDNEY AS STUDIED BY OCCLUSION TIME AND LISSAMINE GREEN TRANSIT TIME TECHNIQUE

Proximal reabsorption rate, determined as the reciprocal of the occlusion time (1/OT), varied in direct proportion to the inulin clearance (Cin) in the range between 1.6 and 0.7 ml/min g KW in nondiuretic rats. Within this range transit time varied from 10 to 24 sec in direct proportion to the occlusion time; the OT/TT ratio averaged 1.10 ± 0.1 (SD). Proximal intra‐tubular pressure averaged 12.4 ± 0.8 mm Hg and did not correlate with Cin Below inulin clearance of 0.7 ml/min g KW transit time (TT) was further increasing from 24 to 44 sec while the OT remained unchanged, thus the OT/TT ratio was decreasing. Proximal intratubular pressure averaged 11.5 ± 0.3 mm Hg. Previous estimates of proximal luminal radius from photomicrographs are revised and a more correct value of about 14 μ was found. The length of the proximal segment, as measured in microdissected nephrons, was 10.4 and 10.5 mm in superficial and juxtamedullary nephrons, respectively. Reflux from more distal segments represented an error of no more than 12 % in estimation of proximal reabsorption rate from the measured OT. The relationship between OT, TT, linear velocity of flow, and tubular fluid/plasma inulin ratio (F/Pin) at 60 % proximal length are derived in quantitative terms. OT/TT = l/ln(F/Pin). From the corrected OT/TT ratio the F/Pin ratio was calculated from values obtained in control rats (“state A”), in rats with partial constriction of the renal artery (“State B”), and in rats with elevated intratubular pressure and urine flow (“State C”). © 1969 Scandinavian Physiological Society