Intracellular Ca2+ transients induced by high external K+ and tetracaine in cultured rat myotubes

Abstract

Cultured myotubes from rat neonatal skeletal muscle were used to measure intracellular Ca2+ concentration ([Ca2ri) and membrane potentials (Vm) uslng the Indo-l mlcrofluorimetry method and the nystatin perforated membrane patch technique, respectively. Sudden increases in external [K$ from 5 mM to either 22, 42 or 34 mM elicited transient elevations in [Ca2Q from a resting level of i03.2 ? 10.3 nM (n = 41) to peak values of 297, 409 and 454 nM, respectively. Vm changes Induced by elevated [K$ followed the Nernst equation for [Kyo. The complex Ca2+ release response induced by elevated [K$, can be described by a minimal model involvin 9+ two components with different kinetics. This analysis revealed that the extent of the Ca release by the fast component bears a sigmoidal relationship with Vm (midpoint at -47.5 mV and an effective valence of 4). Furthermore, while the fast transitory component was rather insensitive to [Ca2’10 and nifedipine, the slow cotnponent was profoundly inhibited by the dihydropyridine (10 fl) both in normal and in a Ca deficient medium. Tetracaine (0.05 to 2 mM), a blocker of the charge movement associated with excitation-contraction (E-C) coupling, elicited a fast elevation in [Ca2’]i followed by a rise at a constant rate to levels as high as 1-2 w, and the changes in [Ca2’]i were readily reversible. Simultaneous measurements of Vm and [Ca2+]i suggest that the fast component is coupled to the rapid depolarization of the membrane induced by the anesthetic. We concluded that tetracaine triggers the release of Ca2+ from internal stores by at least two different mechanisms, one of which Is associated with the depolarizing effects of the drug.