K+ channels in the renal proximal tubule play an important role in salt reabsorption. Cells of the frog proximal tubule demonstrate an inwardly rectifying, ATP-sensitive K+ conductance that is inhibited by Ba2+, G Ba. In this paper we have investigated the importance of phosphorylation state on the activity of G Ba in whole-cell patches. In the absence of ATP, G Ba decreased over time; this fall in G Ba involved phosphorylation, as rundown was inhibited by alkaline phosphatase and was accelerated by the phosphatase inhibitor F−(10 mM). Activation of PKC using the phorbol ester PMA accelerated rundown via a mechanism that was dependent on phosphorylation. In contrast, the inactive phorbol ester PDC slowed rundown. Inclusion of the PKC inhibitor PKC-ps in the pipette inhibited rundown. These data indicate that PKC-mediated phosphorylation promotes channel rundown. Rundown was prevented by the inclusion of PIP-2 in the pipette. PIP-2 also abrogated the PMA-mediated increase in rundown, suggesting that regulation of G Ba by PIP-2 occurred downstream of PKC-mediated phosphorylation. G-protein activation inhibited G Ba, with initial currents markedly reduced in the presence of GTPγs. These properties are consistent with G Ba being a member of the ATP-sensitive K+ channel family.
The Journal of Membrane Biology – Springer Journals
Published: Jan 1, 2005
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