AJP - Cell Physiology, Vol 263, Issue 3 C691-C699, Copyright © 1992 by American Physiological Society
Ionic basis for spontaneous depolarizations in isolated smooth muscle cells of canine colon
J. M. Post and J. R. Hume
Department of Physiology, University of Nevada School of Medicine, Reno 89557.
The type of cell that serves as the pacemaker in the colon is presently
unknown. This study evaluated the ionic basis of spontaneous
depolarizations in circular smooth muscle cells isolated from canine colon
using whole cell voltage and current clamp techniques. Increasing
temperature increased the probability of observing spontaneous
depolarizations, depolarized the resting membrane potential (RMP), and
increased Ca2+ and K+ currents. Spontaneous depolarizations occurred as
rhythmic events, in bursts, or as isolated events. Varying the holding
potential from -100 to -40 mV inhibited a component of inward current
thought to be necessary for spontaneous depolarizations. The Ca2+ channel
blockers, nickel and nisoldipine, inhibited spontaneous depolarizations.
Nickel caused a hyperpolarization, whereas nisoldipine did not affect RMP.
Ouabain depolarized the RMP and inhibited spontaneous depolarizations. The
K+ channel blocker, tetraethylammonium, depolarized the RMP and lengthened
the duration of spontaneous depolarizations. The key finding is that single
colon circular smooth muscle cells are capable of generating spontaneous
depolarizations similar to those described for slow waves in intact tissues
and that a temperature- and nickel-sensitive inward current is essential
for spontaneous activity.
