Effects of acetylcholine and substance P on electrical activity of intact toad gastric muscles

By using isolated gastric muscle cells of Bufo marinus, others have studied the mechanism of action of muscarinic agonists and substance P (SP). To compare responses of isolated cells with those of intact muscles, we have studied the effects of acetylcholine (ACh) and SP on membrane potentials of circular muscle cells in strips of intact muscle from the toad gastric corpus region. These cells had average resting potentials of -69 +/- 0.7 mV. Membrane potential rhythmically depolarized, producing slow waves at an average frequency of 1/min and average amplitude of 25 +/- 2.2 mV. The major effect of ACh (10(-7) to 10(-4) M) was chronotropic; the frequency of slow waves was increased by 88 +/- 11% by 10(-6) M ACh. The amplitudes and rates of rise of slow waves were decreased by ACh. SP had effects similar to ACh; its major effect was chronotropic. The data suggest that ACh and SP primarily affect the pacemaker mechanism in gastric muscles. Since rhythmicity is apparently not expressed in isolated gastric myocytes, it is possible that this effect of these agonists may have been missed in studies of dispersed cells. Our data suggest that the excitatory effects of ACh and SP on contractions may be due to summation of Ca2+ signals, a partial tetanus-like effect.