Passive potassium transport in the proximal convoluted tubule.

Potassium transport in the isolated proximal convoluted tubule (PCT) of the rabbit was studied to determine the importance of concentration-dependent passive processes in potassium reabsorption. Net potassium flux was measured with an initial perfusate potassium concentration of 4 mM and bath potassium concentration of 2, 4, or 6 mM. When bath concentration was 6 mM, there was net potassium secretion in both superficial (SF) and juxtamedullary (JM) PCT. When bath concentration was 2 mM, there was net reabsorption in both groups of tubules. The apparent permeability coefficients were found to be significantly higher in JMPCT (2.96 +/- 0.37 pmol X mm-1 X min-1 X mM-1) than in SFPCT (1.94 +/- 0.34 pmol X mm-1 X min-1 X mM-1). We also attempted to uncover an active potassium reabsorption process by inhibiting water flux, but not other transport processes, by the imposition of a transtubular osmotic gradient. When the perfusate was made 30 mosmol/kg H2O hypertonic to the bath, there was net fluid entry of 0.38 +/- 0.09 nl/min, accompanied by a significant decline in net potassium flux. The collected fluid-to-perfusate potassium concentration ratio was not significantly less than zero, thereby not providing evidence for active potassium reabsorption. These studies suggest that a primary mode of potassium reabsorption in this segment is due to its movement along a transepithelial gradient established by fluid reabsorption. We have been unable to identify an active component of potassium reabsorption.