Electromechanical coupling in cat stomach smooth muscle in Ca2+-free EGTA-containing solutions

The electromechanical coupling in the smooth muscles of cat stomach was studied in Ca2+-free solutions containing EGTA. The experiments were carried out on circular muscle strips from the corpus of cat stomach using the single sucrose gap technique. Complete disappearance of the spontaneous electrical and contractile activity of the muscle strips was observed in Ca2+-free solution. Addition of EGTA (0.1 to 1 mM) to the Ca2+-free solution caused a considerable depolarization of the membrane and appearance of fast fluctuations of the membrane potential, grouped in periodically appearing spindles. After 10-15 min the fluctuations of the membrane potential obtained the shape of flat plateau-like waves. This electrical and contractile activity was completely eliminated by Mn2+ (2.5 mM), D600 (10(-6) M) or nifedipine (10(-8) M). Light inactivation of nifedipine-induced restoration of the chelator-evoked potentials and their reduction to the initial values. The chelator-induced electrical activity depended on the concentration of extracellular Na+, and it was not recorded when the Na+ concentration was decreased below 60 mM. These results support the assumption that the chelator-induced flat plateau-like waves are determined by Na ions penetrating through the Ca-channels. These potentials, appearing in Ca2+-free EGTA-containing solution, probably cause the release of Ca2+ from the intracellular stores in amounts sufficient for the realization of the electromechanical coupling.