Myenteric Plexus Region Research Articles

Direct actions of catecholamines on subsections of the canine colon wall

DIRECT ACTIONS OF CATECHOLAMINES ON SUBSECTIONS OF THE CANINE COLON WALL. K.D. Keef, B.W. Frey and T.K. Smith. Department of Physiology, University of Nevada, Reno, NV, 89557. The purpose of the present study was to characterize the effects of epinephrine (Epi) and norepinephrine (NE) on the contractile activity of colonic smooth muscle. The wall of the colon is functionally complex (e.g., Smith et al, 1987) thus the actions of catecholamines were tested on subsections which contained only one muscle type and one pacemaking/plexus region. These segments included: 1) Longitudinal muscle (LM), 2) Myentenk circular muscle (MCM) and 3) Submucosal circular muscle (SCM). Segments of LM and MCM contained the myenteric plexus region whereas SCM contained the submucosal plexus region. Responses in these segments were compared to: 4) Whole circular muscle (WCM) which included the entire circular muscle layer and both plexus regions. All experiments were carried out in the presence of 5x16’M tetrodotoxin. Epi addition (10~7-10~sM) led to inhibition of spontaneous contractile activity in LM and MCM. In SCM and WCM Epi produced an initial excitation which declined with time and was followed by inhibition. Excitation never exceeded 10% of the maximum response elicited with ACh (1tiM). Addition of Epi to tissues rhythmically contracting with ACh (5xlw’M) led to inhibition in all segments. Similar results were obtained for NE. Inhibition was found to be due to both alpha and beta receptor stimulation whereas excitation was due specifically to alpha receptor stimulation. The results indicate that the actions of catecholamines on WCM are largely a consequence of the events which occur in the s;lbmucosal pacemaking region. These actions include weak alpha receptor mediated excitation and strong inhibition induced by both alpha and beta adrenergic receptors. Grant support DK 41315.