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 Cart 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. (C) 2002 Elsevier Science (USA).