Beta-adrenergic relaxation of dog trachealis: contractile agonist-specific interaction.

The phenomenon of contractile agonist-dependent relaxation by isoproterenol (ISO) of active tension elicited by acetylcholine (ACh), histamine (HIS), serotonin (5-HT), and potassium chloride-substituted Krebs-Henseleit solution (KCl) was studied in 210 tracheal smooth muscle (TSM) strips from 28 mongrel dogs in vitro. All TSM strips were contracted to similar active tensions [target tension (TT) = 50% of the maximal active tension elicited by 127 mM KCl] with ACh, HIS, 5-HT, or KCl and relaxed with either ISO, forskolin (FSK), N6,2'-O-dibutyryladenosine 3',5'-cyclic monophosphate (db-cAMP), or 3-isobutyl-1-methylxanthine (IMX). The concentrations of ISO causing 50% relaxation from TT (RC50) were ACh (2.9 +/- 1.1 x 10(-6) M) greater than 5-HT (8.4 +/- 1.5 x 10(-8) M) approximately KCl (8.1 +/- 2.1 x 10(-8) M) greater than HIS (1.6 +/- 0.2 x 10(-8) M). FSK and IMX relaxed TSM in the same rank order of potency as ISO. In contrast to the contractile agonist-dependent relaxation elicited by ISO, FSK, and IMX, db-cAMP was nearly equipotent in relaxing similarly contracted strips. These results are consistent with contractile agonist-specific interaction with cAMP production by ISO and FSK. These data demonstrate that the phenomenon of contractile agonist-dependent relaxation by ISO is not related specifically to the beta-adrenoceptor.