Am J Physiol Cell Physiol AJP: Regulatory, Integrative and Comparative Physiology
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Am J Physiol Cell Physiol 263: C691-C699, 1992;
0363-6143/92 $5.00
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AJP - Cell Physiology, Vol 263, Issue 3 C691-C699, Copyright © 1992 by American Physiological Society

ARTICLES

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.


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