Hyperpolarization, but not depolarization, increases intracellular Ca²⁺ level in cultured chick myoblasts

Ca²⁺ influx appears to be important for triggering myoblast fusion. It remains, however, unclear how Ca²⁺ influx rises prior to myoblast fusion. The present study examines a possible involvement of the voltage-dependent Ca²⁺ influx pathways. Treatment with the L-type Ca²⁺ channel blockers, diltiazem, and nifedipine did not alter cytosolic Ca²⁺ levels. Depolarization with high K⁺ solution and activation of Ca²⁺ channel with Bay K 8644, and agonist of voltage dependent Ca²⁺ channels, failed to elicit increases intracellular Ca²⁺ level, indicating the absence of depolarization-operated mechanisms. In contrast, phloretin, an agonist of Ca²⁺-activated potassium (K_Ca) channels, was able to hyperpolarize membrane potential and promoted Ca²⁺ influx. These effects were completely abolished by treatment of charybdotoxin, a specific inhibitor of K_Ca channels. In addition, gadolinium, a potent stretch-activated channel (SAC) blocker, prevented the phloretin-mediated Ca²⁺ increase, indicating the involvement of SACs in Ca²⁺ influx. Furthermore, phloretin stimulated precocious myoblast fusion and this effect was blocked with gadolinium or charybdotoxin. Taken together, these results suggest that induced hyperpolarization, but not depolarization increases Ca²⁺ influx through stretch-activated channels, and in turn triggers myoblast fusion.