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

Abstract

We report 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 μM; peak current density 26.2 ± 3.7 pA/pF; n = 10), but ionomycin (0.5 μM) + 5,6-dichloro-1-ethyl-1,3-dihydro-2 H-benzimidazol-2-one (100 μM) evoked a stable outward current (28.4 ± 8.2 pA/pF; n = 11). The current was fully inhibited by charybdotoxin (200 nM), clotrimazole (2 μM), and 5-nitro-2-(3-phenylpropylamino)benzoic acid (300 μM), but not by tetraethylammonium (1 mM) or d-tubocurarine (300 μM). 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 (100 μM), an intracellular Ca2+ mobilizer. Under current-clamp conditions, ATP application resulted in a membrane hyperpolarization of ∼35 mV. The IKCa 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 IKCa channel by charybdotoxin did not inhibit bFGF-sustained myoblast proliferation. These observations show that in C2C12 myoblasts the IKCa channel expression correlates inversely with differentiation, yet it does not appear to have a role in myoblast proliferation.