Distribution of beta-adrenoceptors associated with cAMP-generating system in cat colon

The localization of beta-adrenoceptor associated with the adenosine 3',5'-cyclic monophosphate (cAMP)-generating system was determined by an assessment of the smooth muscle cell contractility and by assay of cAMP in individual layers of the cat colon. Isoproterenol inhibited the acetylcholine (ACh)-induced contractions of isolated longitudinal and circular muscle cells in a dose-dependent manner. The response of the longitudinal muscle cells to isoproterenol was double that seen in the circular muscle cells, and this inhibition was antagonized by sotalol. The basal content of cAMP was 3.42 pmol/mg protein in longitudinal muscle layers, 3.09 pmol/mg protein in circular muscle layers, and 2.75 pmol/mg protein in Auerbach's plexus. Isoproterenol produced a dose-dependent elevation of cAMP levels in longitudinal and circular muscle layers, which were antagonized by sotalol and potentiated by theophylline. By contrast, isoproterenol had no effect on cAMP in Auerbach's plexus. Phenylephrine did not alter the cAMP levels in any layer. Therefore, beta-adrenoceptors associated with the cAMP-generating system probably exist in the longitudinal and circular muscles of the cat colon.