Regulation of A-type potassium channels in murine colonic myocytes by phosphatase activity

A rapidly inactivating K+ current (A-type current) participates in the regulation of colonic muscle excitability. We found 19-pS K+ channels in cell-attached patches of murine colonic myocytes that activated and inactivated with kinetics similar to the A-type current. The A-type current in colonic myocytes is regulated by Ca2+/calmodulin-dependent protein kinase II. Therefore, we studied regulation of the 19-pS K+ channels by Ca2+-dependent phosphorylation/dephosphorylation. The rates of inactivation of ensemble-averaged currents resulting from 19-pS K+ channels were increased by the calmodulin antagonist W-7. Inhibitors of calcineurin, cyclosporin A and FK-506, slowed the inactivation of the 19-pS K+ channels. Okadaic acid, an inhibitor of the calcineurin/inhibitor-1/protein phosphatase 1 cascade, also slowed inactivation of the 19-pS K+ channels. Polymerase chain reaction detected transcripts encoding calcineurin A in isolated colonic smooth muscle cells, and immunohistochemical studies demonstrated specific expression of calcineurin A-like immunoreactivity in colonic muscle tissues and in colonic myocytes. These data, when considered with previous findings, suggest that Ca2+-dependent phosphorylation/dephosphorylation regulates the A-type current in murine colonic smooth muscle cells.