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.
The Journal of Membrane Biology – Springer Journals
Published: Jan 1, 2005
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