We report here the expression in C2C12 myoblasts of the intermediate-conductance Ca2+-activated K+ (IK(Ca)) channel. The IK(Ca) current, recorded under perforated-patch configuration, had a transient time course when activated by ionomycin (0.5 microM; peak current density 26.2 +/- 3.7 pA/pF; n = 10), but ionomycin (0.5 microM) + 5,6-dichloro-1-ethyl-1,3-dihydro-2H-benzimidazol-2-one (100 microM) evoked a stable outward current (28.4 +/- 8.2 pA/pF; n = 11). The current was fully inhibited by charybdotoxin (200 nM), clotrimazole (2 microM), and 5-nitro-2-(3-phenylpropylamino)benzoic acid (300 microM), but not by tetraethylammonium (1 mM) or D-tubocurarine (300 microM). Congruent with the IK(Ca) 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 IK(Ca) channel was activated metabotropically by external application of ATP (100 microM), an intracellular Ca2+ mobilizer. Under current-clamp conditions, ATP application resulted in a membrane hyperpolarization of approximately 35 mV. The IK(Ca) current downregulated during myogenesis, ceasing to be detectable 4 days after the myoblasts were placed in differentiating medium. Downregulation was prevented by the myogenic suppressor agent basic FGF (bFGF). We also found that block of the IK(Ca) channel by charybdotoxin did not inhibit bFGF-sustained myoblast proliferation. These observations show that in C2C12 myoblasts the IK(Ca) channel expression correlates inversely with differentiation, yet it does not appear to have a role in myoblast proliferation.