Canine colonic circular muscle generates action potentials without the pacemaker component.

Abstract

Two dominant types of action potentials in canine colon are slow wave type action potentials (slow waves) and spike-like action potentials (SLAPs). The slow waves, originating at the submuscular surface where a network of interstitial cells of Cajal (ICCs) is found, possess a pacemaker component. Activation of the pacemaker component is insensitive to voltage changes and L-type calcium channel blockers, and is postulated to involve a metabolic clock sensitive to cyclic AMP. SLAPs are more prominent in the longitudinal muscle. To understand the contribution circular muscle cells make to the generation of these action potentials, a circular muscle preparation (devoid of the submuscular ICC-smooth muscle network, longitudinal muscle, and myenteric plexus) was developed. Circular muscle preparations were spontaneously quiescent, with a resting membrane potential of -62.9 +/- 0.6 mV. Ba2+ (0.5 mM) depolarized the cells to -51.8 +/- 0.6 mV and induced electrical oscillations with a frequency, duration, amplitude, and rate of rise equal to 6.6 +/- 0.4 cpm, 2.2 +/- 0.2 s, 19.4 +/- 0.9 mV, and 21.8 +/- 1.7 mV/s, respectively. In most cases, Ba(2+)-induced oscillations were preceded by a prepotential of 4.4 +/- 0.3 mV, with a rate of rise of 1.1 +/- 0.1 mV/s. Ba(2+)-induced oscillations were abolished by 1 microM D600 as well as by repolarization of 6-12 mV. Addition of 0.1 microM Bay K8644 in the presence of Ba2+ further depolarized circular muscle cells to -42.4 +/- 0.8 mV and increased the oscillation frequency to 16.8 +/- 1.8 cpm. The electrical oscillations induced in circular muscle preparations by Ba2+ and Bay K8644 were similar to the SLAPs exhibited by the isolated longitudinal muscle layer, indicating that generation of SLAPs is an intrinsic property of smooth muscle cells. Forskolin (1 microM), previously shown to dramatically decrease the frequency but not the amplitude of slow waves in preparations including the submuscular ICC network, decreased the amplitude of the Ba(2+)-induced oscillations in circular muscle preparations without changing the frequency. These results provide strong evidence for the hypothesis that the submuscular ICC-smooth muscle network is essential for the initiation of the pacemaker component of the colonic slow waves. The mechanism for regulating the frequency of slow waves is different from that responsible for the Ba(2+)-induced oscillations in circular muscle preparations. Circular muscle cells are shown to be excitable and capable of generating oscillatory activity dominated by L-type calcium channel activity, which is regulated by K+ conductance.