Perfusion rate-dependence of transepithelial osmosis in isolated proximal convoluted tubules: Estimation of the hydraulic conductance

Abstract

Perfusion rate-dependence of transepithelial osmosis in isolated proximal convoluted tubules: Estimation of the hydraulic conductance. We estimated the hydraulic conductance (P f , μm · sec -1 ) of isolated superficial proximal convoluted tubules by measuring the absolute rate of osmotic water flow (′J v , nl · min -1 ) in the presence of a transepithelial osmolality difference. Tubule segments were bathed in a phosphate-buffered sodium chloride solution with an osmolality of 290 mOsm/kg H 2 O and perfused with a similar sodium chloride solution having an osmolality of 270 mOsm/kg H 2 O. The experiments were conducted at 25° C, and in the absence of an imposed transepithelial osmolality difference, no net volume absorption was observed. With an osmolality difference, however, axial osmotic equilibration tends to occur because of transepithelial osmosis and solute movement, and consequently, ′J v is expected to be a function of the perfusion rate (V 0 , nl min -1 ). Observed values of ′J v were 0.75 ± 0.05 (SEM), 1.12 ± 0.09, 1.35 ± 0.08, and 1.90 ± 0.15 at perfusion rates of 11.85, 21.20, 30.75, and 41.80 nl · min -1 , respectively. A minimum estimate of the hydraulic conductivity, 1,900 µm ·sec -1 , was calculated from the volume absorption measured at the maximal perfusion rate. A direct estimate of P f was obtained by plotting 1/′J v as a function of 1/V. The zero intercept of this plot gave an estimate of ′J v at an infinite perfusion rate; from this ′J v , P f was calculated to be 3,560 ± 787 (SEM) µm · sec -1 (95% confidence limits: 2,030 to 5,100 µm · sec -1 ). Given this high hydraulic conductivity, effectve transepithelial osmotic gradients in the range of 2.2 to 5.5 mOsm/kg of water provide a sufficient driving force for the rates of proximal tubular fluid absorption observed either in vitro or in vivo .