Involvement of K_Ca Channels and stretch-activated channels in calcium influx, triggering membrane fusion of chick embryonic myoblasts

Calcium influx is known to be prerequisite for membrane fusion of myoblasts. However, little is known about the channels that are responsible for the entry of calcium into the cells. Here we show that K_Ca channels and stretch-activated channels are involved in the calcium influx. Upon analysis of single-channel recordings, calcium sensitivity of K_Ca channels in myoblasts was found to be about sixfold higher than that in myotubes. Their density in myoblasts (1.68 μm^-2) was also about sixfold higher than that in myotubes (0.27 μm^-2). In addition, the opening of the calcium-permeable cationic channels in myoblasts was found to increase with membrane stretching and could be blocked by gadolinium. The density of stretch-activated channels was 0.22 μ^-2 for myoblasts, and the relative permeability of calcium to potassium was P_Ca/P_K ≅ 3.6. The channels could generate inward calcium currents to open K_Ca channels in physiological solution. Furthermore, the activation of K_Ca channels by phloretin dramatically hyperpolarized the resting membrane potential of myoblasts and this effect could be reversed upon treatment of tetraethylammonium. While phloretin induced precocious fusion, tetraethylammonium or gadolinium blocked not only the phloretin-induced precocious fusion but also the spontaneous fusion of myoblasts. These results suggest that hyperpolarization generated by reciprocal activation of stretch-activated channels and K_Ca channels is involved in the calcium influx that triggers myoblast fusion.