Heterogeneity in Spontaneous and Tetraethylammonium Induced Intracellular Electrical Activity in Colonic Circular Muscle

Marked differences were observed in the intracellular electrical activities (spontaneous and TEA-induced) comparing the submucosal and myenteric plexus surfaces of the circular muscle of the dog colon. Distinct characteristics of the cells at the myenteric plexus surface were: a less (10 mV) polarized membrane, a lower amplitude slow wave, and the occurrence of burst type spiking activity. However, slow waves with a high upstroke amplitude (approximately 2.5 times higher than the plateau) were observed in 40% of the preparations. This high upstroke amplitude was dependent on the occurrence of a regenerative membrane potential change (a spike) during the slow wave propagation into the myenteric plexus surface. Such a spike was mediated by Ca2+-influx and could be evoked or enhanced by electrical pulses or by blocking a TEA-sensitive potassium conductance. In the presence of TEA, spikes occurred in bursts. Both slow waves and spiking activities generated contraction. In conclusion, at least two types of cells exist in the circular muscle layer with marked differences in electrophysiological properties. Slow waves are generated at the submucosal surface, passively propagated to the outermost circular muscle where they induce regenerative membrane potential changes.