Mechanisms Underlying Regulation of a Barium -sensitive K+ Conductance by ATP in Single Proximal Tubule Cells Isolated from Frog Kidney

K⁺ channels play an important role in pump-leak coupling and volume regulation in the renal proximal tubule. Previous experiments have identified a barium-sensitive K⁺ conductance (G_Ba) in proximal tubule cells isolated from frog kidneys. In this paper we examine the regulation of G_Ba by ATP. G_Ba was measured in single cells isolated from frog kidney using the whole-cell patch-clamp technique. G_Ba was activated by 2 mM intracellular ATP. This activation was enhanced by inhibition of protein kinase C and attenuated by inhibition of protein kinase A, indicating reciprocal regulation by these kinases. Activation by ATP was reduced in the presence of a hypertonic bath solution, suggesting that cell swelling was required. However, after activation to steady-state, G_Ba was not sensitive to cell-volume changes. Hypotonic shock-induced volume regulation was inhibited by barium and quinidine, inhibitors of G_Ba. The effect of maximal inhibitory concentrations of barium and quinidine on volume regulation was similar and addition of both blockers together did not augment the inhibitory response. G_Ba was also activated by ADP, via a mechanism dependent on the presence of Mg²⁺. However, the responses to ADP and ATP were not additive, suggesting that these nucleotides may share a common mechanism of activation. The regulation of G_Ba by ATP was biphasic, with a half-maximal activating concentration of 0.89 mM and a half maximal inhibitory concentration of 6.71 mM. The sensitivity to nucleotides suggests that G_Ba may be regulated by the metabolic state of the cell. Furthermore, the sensitivity to solution osmolality, coupled with the blocker profile of inhibition of volume regulation, suggests that G_Ba could play a role in volume regulation.