Intermediate-conductance Ca2-activated K channel is expressed in C2C12 myoblasts and is downregulated during myogenesis

port here the expression in C2C12 myoblasts of the intermediate-conductance Ca2-activated K (IKCa) channel. The IKCa current, recorded under perforated-patch configuration, had a transient time course when activated by ionomycin (0.5 M; peak current density 26.2 3.7 pA/pF; n 10), but ionomycin (0.5 M) 5,6-dichloro-1-ethyl-1,3-dihydro-2H-benzimidazol-2-one (100 M) evoked a sta-ble outward current (28.4 8.2 pA/pF; n 11). The current was fully inhibited by charybdotoxin (200 nM), clotrimazole (2 M), and 5-nitro-2-(3-phenylpropylamino)benzoic acid (300 M), but not by tetraethylammonium (1 mM) or D-tubocurarine (300 M). Congruent with the IKCa channel, elevation of intracellular Ca2 in inside-out patches resulted in the activation of a voltage-insensitive K channel with weak inward rectification, a unitary conductance of 38 6 pS (at negative voltages), and an IC50 for Ca2 of 530 nM. The IKCa channel was activated metabotropically by external application of ATP (10