AJP - Renal Physiology, Vol 248, Issue 2 228-F232, Copyright © 1985 by American Physiological Society
Passive potassium transport in the proximal convoluted tubule
J. S. Kaufman and R. J. Hamburger
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.
