Structure and organization of electrical activity of canine distal colon

The morphology and electrophysiology of the canine distal colon were studied to compare this region with the proximal colon. Many morphological characteristics were similar including the presence of interstitial cells at the submucosal surface of the circular layer. Muscle cells near the submucosal surface had resting membrane potentials (RMPs) of -79 +/- 1 mV, and slow waves were generated in this region. Slow waves had similar waveform characteristics to those of the proximal colon, but rapid oscillations were superimposed on slow waves of some preparations. RMPs and slow waves decreased with distance from the submucosal surface. The latter were not resolvable in the myenteric half of the circular layer. Cells at the myenteric border had RMPs of -49.5 +/- 2 mV and a higher frequency oscillation of 16 min-1. Acetylcholine increased slow-wave amplitude and duration and caused fast oscillations on the plateau phase of slow waves. Isolated circular myocytes were studied with the patch-clamp technique. Cells from the submucosal border displayed voltage-dependent inward and outward currents. With outward currents blocked, the inward current was composed of two components. Nifedipine (10(-6) M) blocked a portion of the inward current but left a substantial transient component. The effect of nifedipine correlated with its effects on tissues, suggesting that two components of Ca2+ current participate in slow waves. These studies describe numerous similarities in the structure and activity of the proximal and distal portions of the colon but also show some potentially important differences between these regions.