Basolateral Na+ pump modulates apical Na+ and K+ conductances in rabbit cortical collecting ducts

Previous studies indicated that an acute elevation of peritubular K+ enhances K+ secretion and Na+ reabsorption in the isolated perfused cortical collecting duct (CCD) from rabbit kidneys [S. Muto, G. Giebisch, and S. Sansom. Am. J. Physiol. 255 (Renal Fluid Electrolyte Physiol. 24): F108-F114, 1988]. To determine the underlying cellular mechanisms, we used microelectrode techniques to assess the membrane properties of collecting duct cells in isolated perfused CCDs of control and desoxycorticosterone acetate (DOCA)-treated rabbits following acute stimulation of the basolateral Na+-K+ pump by rapidly increasing the bath solution from 2.5 to 8.5 mM K+. This induced in both groups of tubules, first, a short-lasting hyperpolarization and, second, a sustained phase of depolarization of transepithelial, basolateral, and apical membrane voltages. Whereas the transepithelial conductance (GT) and fractional apical membrane resistance (fRA) remained unchanged during the initial phase of hyperpolarization, during the depolarization, GT increased and fRA decreased. Perfusion of the lumen with solutions containing either amiloride or Ba2+ attenuated the high K+-induced apical electrical changes, and basolateral strophanthidin abolished both apical and basolateral electrical responses during elevation of K+ in the bath. From these results we conclude the following: 1) acute elevation of basolateral K+ activates the basolateral Na+-K+ pump, which secondarily elevates the apical Na+ and K+ conductances; 2) DOCA pretreatment increases the basolateral K+ conductance and augments the response to the rise of K+ of both basolateral Na+-K+ pump activity and apical cation conductances.