Heterogeneity in electrical activity of the canine ileal circular muscle: interaction of two pacemakers

In cross-sectioned slabs from the muscularis externa or in the isolated circular muscle devoid of longitudinal muscle and myenteric plexus (MyP) of ileum, a slow wave, sigmoidal or triangular in shape, was recorded from microelectrode impalements near the deep muscular plexus (DMP) region in the whole-thickness preparation. From the MyP region of whole-thickness preparations, a slow wave which oscillated at a similar frequency (9-10 cycles min-1) was characterized by a fast upstroke and a square shape. Slow waves of mumixed pattern were recorded in the outer circular muscle (OCM) while triangular slow waves were present near the submucous plexus (SMP). In this preparation but not in isolated circular muscle, the inhibitory junction potentials (IJPs) produced by supramaximal electrical field stimulation triggered slow waves. The amplitudes (15-25 mV) of spontaneous and triggered slow waves (TSWs) were greatest in the MyP region, significantly so compared to those of DMP and SMP regions and to those in all regions of isolated circular muscle. Frequencies of slow waves recorded from the MyP and DMP were slightly but significantly higher than those recorded from either the OCM or the SMP or from all regions of isolated circular muscle. A 10-15 mV gradient in resting membrane potential (more hyperpolarized near MyP) existed across the intact (but not the isolated) circular muscle layer. Both types of slow waves, TSWs and IJPs were unaffected by muscarinic, adrenergic or tachykinergic blockade. We suggest that a MyP pacemaker network generated a plateau-type slow wave while a DMP one induced a triangular slow wave. Each source can function independently but the MyP network may dominate and entrain DMP slow waves.