Modulation of the Pacemaker Activity in Circular Muscle of the Canine Colon

The smooth muscle cells of the circular muscle layer of the canine colon exhibit periodic waves of depolarization (slow waves) which serve a pacemaker function similar to that of their counterparts in the small intestine (1). Slow waves bring the membrane potential periodically close to or above threshold for spiking activity. Slow waves with spiking activity on their peak depolarizations are associated with phasic contractions (1,2). In addition to these rhythmic contractions, the circular muscle is capable of generating prolonged forceful contractions for periods which would normally span several slow wave cycles (3,4). These contractions are most noticeable under stimulated conditions, i.e. with neostigmine, and appear to perform a propulsive function. Such contractions cannot easily be explained in the framework of the control function of the slow waves (3). Therefore the possibility was examined that endogenous excitatory substances of either neural or hormonal origin could alter the regular pacemaker activity to allow such prolonged contractions to occur. The present paper deals with the effects of two excitatory substances, the acetylcholine analog carbachol and substance P, on the electrical and mechanical activities of circular muscle of the dog colon.