Regulation of M-type potassium channels in mammalian sympathetic neurons: action of intracellular calcium on single channel currents.

Currents through single M-type potassium channels were recorded in membrane patches excised from rat superior cervical sympathetic neurons. Application of Ca+ to the internal face of inside-out patches produce two forms of M-channel inhibition: a slow, all-or-nothing suppression of activity; and a fast block associated with a concentration-dependent shortening of open times compatible with open-channel block. Both forms of block were enhanced by patch depolarization. Neither was replicated or affected by Mg2+, and both could be recorded in the absence of intracellular ATP, implying that they did not involve phosphorylation. Since the block was reversible in the absence of ATP and since alkaline phosphatase did not reduce channel activity, block was unlikely to have resulted from dephosphorylation. In cell-attached patch recording, M-channel activity increased during exposure of the cell to Ca2(+)-free solution and was rapidly reduced on applying 2mM Ca2+ to the extra-patch solution. This suggests that M-channel activity in these neurons may be tonically regulated by variations in resting intracellular [Ca2+].