ACUTE VOLUME EXPANSION: RAT PROXIMAL TUBULAR Na TRANSPORT INHIBITION BY AUTOLOGOUS TUBULAR FLUID WITHOUT CHANGES IN Na PERMEABILITY

Abstract

Sodium permeability (P22Na) and steady state transepithelial concentration differences of Na (delta cNa) were measured in stationary droplets in proximal tubules of rats in both the non-expanded (NE) and subsequent mannitol-saline volume-expanded (VE) state. The test solutions for tubular injection were either an artificial tubular solution (AS) in group I rats or harvested proximal tubular fluid (HTF) in a second group of rats (group II). delta cNa (mmol.kg-1H2O) was significantly reduced (p less than 0.001) with HTF in the VE state (12.1 +/- 0.8 SEM) when compared to HTF in the NE state (17.8 +/- 1.8) and AS in both NE (17.4 +/- 1.0) and VE (18.2 +/- 1.0). P22Na calculated from the half-time of disappearance of 22Na from the droplets, when using paired experiments were 8.1 +/- 1.7 X 10(-5) mm2.sec-1 and 7.3 +/- 1.8 with HTF in NE and subsequent VE and 8.6 +/- 1.1 and 7.7 +/- 0.8 with AS in NE and VE. These permeability results did not differ from each other significantly. In these experiments with zero net Na and water fluxes, the 32% reduction of delta cNa cannot be accounted for by an increased backleak of Na, since P22Na would have had to increase by 32%. These results therefore provide strong evidence for a factor present in proximal tubular fluid consequent upon VE which inhibits Na transport without altering transepithelial PNa, but do not exclude an effect of such a factor on the cellular Na entry step or the Na pump itself.