Rhythmic electrical and mechanical activity in stomach of toad and frog

Rhythmic slow spikes and contractions at 2-5/min occur in frog/toad stomach when longitudinal muscle is present, not in the circular layer after removal of the longitudinal layer. Interstitial cells of Cajal occur in frog stomach in the longitudinal layer and in toad stomach mainly in the longitudinal layer, with some between layers. They were identified by staining with methylene blue, the fluorescent dye rhodamine 123, toluidine blue, and by electron microscopy. Interstitial cells resemble in ultrastructure those observed in mammalian intestine by small cell diameter, long processes forming a network, large nuclei, mitochondria, caveolae, endoplasmic reticulum, and absence of fibrillae. Rhythmic activity in stomachs stained with methylene blue is abolished by bright illumination. Rhythmic slow spikes and contractions are reduced and frequency is lowered when Ca2+ concentration in the medium is significantly decreased; in high Ca2+ these functions are increased. Threshold concentration of Ca2+ for maintaining slow spikes was between 10(-7) and 10(-6) M Ca2+. Ba2+ and 2.5 mM increases slow spike frequency and amplitude; Ba2+ increases contractions proportionately more than spikes. Slow spikes are reduced or blocked by verapamil, nifedipine, and Cd2+ and are enhanced by BAY K 8644. Quaternary ammonium compounds tetraethylammonium and tetrapentyl-ammonium bromide prolong slow spikes and enhance contractions. Replacement of NaCl by LiCl, methyl glutamine, or treatment with amiloride reduces slow spike amplitude and to a lesser extent frequency. This indicates efflux of Ca2+ by Na+/Ca2+ exchange. Contractions, but not spikes, are reduced by the calmodulin blocker trifluoroperazine. Contractions are enhanced, but slow spikes are not altered by ouabain. This suggests retention of Ca2+ when a tonic Na+/K+ pump is blocked. A model for rhythmicity includes influx of Ca2+ via L-channels, repolarization by IKCa2+, efflux of Ca2+ by Na+/Ca2+ exchange, and efflux of Na+ by a Na+/K+ pump. Frequency is determined by rate constants for both influx and efflux of Ca2+.