Participation of Na+ in the excitation-contraction coupling of the phasic and tonic contractions in stomach smooth muscles.

The sucrose gap method was used to study the effect of Na+ on the electrical and contractile activity of smooth-muscle strips from the antrum and fundus of cat stomach. Na+ reduction or elimination from the nutrient medium leads to reduction of the depolarization phase of the plateau-action potentials and to a transient increase in the amplitude of the phasic contractions of the antrum, followed by their complete disappearance. In Na-free medium direct electrical stimulation leads not to the appearance of plateau-action potentials, but of a group of Ca2+-dependent spike-potentials and high-amplitude phasic contractions. In fundic preparations the decrease in the Na+-concentration of the nutrient solution results in a gradual rise of the tonic tension, without appearance of action potentials. This effect of the reduced Na+ is inhibited by Ca-antagonists La3+ (10(-2) mol/l) D 600 (10(-6) - 10(-5) mol/l) and by sodium nitroprusside (10(-6) mol/l). On the other hand, an increase in the Ca-conductivity of the membrane of the fundic smooth muscle cells was observed in Na+-free medium. It is assumed that at the level of the fundic smooth muscle cell membrane there is competition between Na+ and Ca2+ with respect to slow Ca-channels.