The intact stomach of the toad initiates rhythmic slow-spikes of 5–15 s duration and frequency of 3-5 min−1. The spontaneous electrical waves originate in the longitudinal muscle layer; isolated circular muscle is quiescent. Aboral conduction velocity is 0.12–0.9 mm s−1. Reduction of external sodium concentration from 89.5 to 15 mM produced no effect on slow spikes, although further reduction to 1.5 mM increased frequency and decreased amplitude. Slow-spikes were unaffected by ouabain or by incubation in potassium-free solution. When calcium in the medium was reduced, slow-spike amplitude and frequency decreased. Slow-spikes exhibited a change in amplitude of 16 mV per decade change in CaO2+; slow-spikes were eliminated at 10−8 M CaO2+ and by blockers of calcium conductance channels.
Intact intestine of toad demonstrated slow-waves which resembled those of mammalian intestine. These were sensitive to changes in external sodium and were eliminated by 1 × 10−4M ouabain.
It is suggested that rhythmic slow-spikes of longitudinal smooth muscle of amphibian stomach may result from periodic changes in Ca conductance whereas endogenous electrical waves of intestine may result from rhythmic extrusion of sodium.