Role of cyclic AMP in adrenergically-induced tracheal muscle relaxation

Studies were performed on the isolated bovine tracheal muscle preparation to assess the possible role of adenosine 3',5'-cyclic monophosphate (cyclic AMP) in adrenergically-induced smooth muscle relaxation. Isoproterenol, epinephrine, terbutaline and soterenol were found to relax the tracheal muscle and to increase its cyclic AMP content. The two events generally paralleled one another when analyzed in terms of dose-response and time-response relationships. Propranolol antagonized the relaxation and the increase in cyclic AMP produced by isoproterenol. However, other findings suggested that there may not be a cause and effect relationship between the biochemical and mechanical responses. Thus, salbutamol and carbuterol failed to increase muscle cyclic AMP even in concentrations which produced maximum relaxation. Additionally, butoxamine and H35/25 antagonized the biochemical response but had additive effects (instead of antagonistic effects) on the mechanical response to isoproterenol. Furthermore, it was observed that low concentrations of H35/25 produced tracheal muscle relaxation associated with an increase in tissue cyclic AMP; however, use of a high concentration of H35/25 paradoxically resulted in autoblockade of the increase in cyclic AMP but not the smooth muscle relaxation.