Mechanism of Inhibition of Proximal Tubule

occur in rats in response to saline loading are blunted markedly when renal perfusion pressure is reduced immediately before, but not after, the volume load. To ascertain the mechanism responsible for these differences between early clamp (EC) vs. late clamp (LC), intracapillary and interstitial determinants of peritubular capillary uptake of APR were measured in seven LC and seven EC Munich-Wistar rats before and after isotonic saline loading (8% body wt). With volume expansion in LC animals, we observed a marked decline in APR (averaging 11 ± 1 nl/min), associated with large increases in urinary sodium excretion rate, which averaged 8±2 ,ueq/min. In EC, the changes in urinary sodium excretion rate (+1±0 ,eq/min) and APR (-3± 1 nl/min) with volume expansion were smaller in magnitude. Since peritubular capillary reabsorption coefficient and mean peritubular transcapillary hydraulic pressure difference did not change with saline loading in LC, the marked fall in APR was attributed primarily to a measured large decline in mean peritubular transcapillary oncotic pressure difference (AII). Despite an equivalent mean fall in AH with volume expansion in EC, near-constancy of APR was found to be associated with a simultaneous and equivalent decline in mean peritubular transcapillary hydraulic pressure difference (a consequence of decreased mean peritubular capillary hydraulic pressure), which effectively offset the fall in All. These Portions of these studies were presented at the Annual meeting of the Association of American Physicians, Washington, D. C., 7 May 1979 and published in abstract form (1979. Clin. Res. 27: 520A.). Address reprint requests to Dr. Ichikawa. Received for publication 27 March 1979 and in revised form 8 June 1979. 1466 J. Clin. Invest. C) The American Society for results demonstrate the importance of hydraulic pressure patterns of the peritubular capillaries in modulating APR and are consistent with the view that Starling forces across the postglomerular microcirculation play a fundamental role in determining APR.