Colitis-induced alterations in adrenergic control of circular smooth muscle in vitro in rats

The present study investigated inflammation-induced changes in adrenergic regulation of smooth muscle. Colitis was induced in rats by intrarectal administration of trinitrobenzenesulfonic acid in ethanol. After 4 h (acute) or 7 days (chronic), in vitro isometric tension was measured in strips of circular smooth muscle taken from the distal colon. In controls, the major inhibitory control of smooth muscle responses to nerve stimulation was mediated by nitric oxide and beta adrenergic receptors. There was less evidence of alpha adrenergic control. Studies with the beta3 receptor antagonist cyanopindolol and the beta3 receptor agonist BRL37344 revealed that beta adrenergic regulation of spontaneous contractions and responses to nerve stimulation were mediated primarily by the beta3 adrenoreceptor. Both acute and chronic colitis significantly increased responses to electrical field stimulation. This effect was attributed to a loss of inhibitory nitrergic regulation as well as to selective changes in the beta adrenergic control of colonic circular smooth muscle. Inflammation did not alter alpha adrenergic control. Chronic colitis also decreased the sensitivity to nerve stimulation and pharmacological contractile agents. Acute and chronic inflammation reduced the ability of BRL37344 to inhibit contractions in response to nerve stimulation. In addition, in inflamed colon, BRL37344 was less effective in relaxing carbachol-induced precontractions. Finally, inflammation resulted in a loss of the ability of the cyanopindolol to increase the amplitude of both spontaneous contractions and contractions in response to nerve stimulation. These effects indicated that colitis induced a down-regulation of inhibitory beta3 adrenergic control of colonic smooth muscle function. This loss of adrenergic regulation may contribute to the diarrhea in inflammatory bowel disease.