The mechanical and biochemical responses of the canine trachealis to SK&F 94836 [2-cyano-1-methyl-3-[4-(4-methyl-6-oxo- 1,4,5,6-tetrahydropyridazine-3-yl)phenyl]guanidine], a selective inhibitor (ki = 1-3 microM) of the low km cyclic AMP (cAMP) phosphodiesterase, were assessed. Time course studies indicated that SK&F 94836-induced relaxation of trachealis strips contracted with 0.1 microM methacholine was accompanied by an activation of cAMP-dependent protein kinase (cAMP-PK). In subsequent experiments, trachealis strips were contracted with three concentrations of methacholine (0.1, 1.0 or 3.0 microM) or two concentrations of histamine (10 or 300 microM) before being relaxed by the cumulative addition of SK&F 94836. The relaxant response to SK&F 94836 (EC50 = 1-10 microM) decreased progressively as tissues were contracted with higher concentrations of methacholine. In parallel with its inhibitory effect on SK&F 94836-induced relaxation, methacholine suppressed the ability of SK&F 94836 to activate cAMP-PK. Interestingly, the inhibition of cAMP-PK activity was not accompanied by a significant inhibition of SK&F 94836-stimulated cAMP accumulation. Unlike the results with methacholine, the concentration of histamine used to contract tissues had no effect on SK&F 94836-induced relaxation or cAMP-PK activation. To determine the effect of SK&F 94836 on the mechanical and biochemical responses to the beta adrenoceptor agonist isoproterenol, tissues were first contracted with 3.0 microM methacholine and then incubated with 0, 0.3, 3.0 or 30 microM SK&F 94836 before being relaxed by the cumulative addition of isoproterenol. In these experiments, SK&F 94836 potentiated isoproterenol-induced relaxation, cAMP accumulation and cAMP-PK activation in a concentration-dependent manner.(ABSTRACT TRUNCATED AT 250 WORDS)