Pharmacologic characterization of the changes in cholinergic sensitivity of rat jejunal circular muscle after...

Neurons located in the myenteric plexus are generally believed responsible for motor control of intestinal circular muscle. The in vitro isometric responses of naive and myenterically denervated (MD) rat jejunal circular muscle to bethanechol and carbachol, alone and in the presence and absence of neuronal antagonists (hexamethonium bromide, tetrodotoxin and Botulinum toxin A) 15 and 30 days after myenteric plexus ablation, were determined. The responses to bethanechol indicated no differences in muscarinic sensitivity between naive and MD tissue. The relative potency of carbachol, which acts at both muscarinic and nicotinic receptors, in MD tissue 15 days after denervation was significantly higher than that in naive tissue. However, 30 days after denervation, the relative potencies of carbachol in naive and MD circular muscle were comparable. The presence of neuronal antagonists had no effect on the relative potency of carbachol 15 days after myenteric denervation, but altered significantly the responses 30 days after denervation. The effects produced by the neuronal antagonists 30 days after myenteric denervation were qualitatively and quantitatively different than those produced in naive tissue, suggesting that the nature of the innervation in these tissues was different. These results demonstrate that circular muscle was denervated initially after myenteric plexus ablation but reinnervation occurred within 30 days. The reinnervation observed is likely due to neurons located in the submucosal plexus.