Intracellular Na+ controls cell surface expression of Na,K-ATPase via a cAMP-independent PKA pathway in mammalian kidney collecting duct cells

In the mammalian kidney the fine control of Na+ reabsorption takes place in collecting duct principal cells where basolateral Na,K-ATPase provides the driving force for vectorial Na+ transport. In the cortical collecting duct (CCD), a rise in intracellular Na+ concentration ([Na+](i)) was shown to increase Na,K-ATPase activity and the number of ouabain binding sites, but the mechanism responsible for this event has not yet been elucidated. A rise in [Na+](i) caused by incubation with the Na+ ionophore nystatin, increased Na,K-ATPase activity and cell surface expression to the same extent in isolated rat CCD. In cultured mouse mpkCCD(c14) collecting duct cells, increasing [Na+](i) either by cell membrane permeabilization with amphotericin B or nystatin, or by incubating cells in a K+-free medium, also increased Na,K-ATPase cell surface expression. The [Na+](i)-dependent increase in Na,K-ATPase cell-surface expression was prevented by PKA inhibitors H89 and PKI. Moreover, the effects of [Na+](i) and cAMP were not additive. However, [Na+](i)-dependent activation of PKA was not associated with an increase in cellular cAMP but was prevented by inhibiting the proteasome. These findings suggest that Na,K-ATPase may be recruited to the cell membrane following an increase in [Na+](i) through cAMP-independent PKA activation that is itself dependent on proteasomal activity.