Differential inhibitory effects of forskolin, isoproterenol, and dibutyryl cyclic adenosine monophosphate on phosphoinositide hydrolysis in canine tracheal smooth muscle

Abstract

A characteristic feature of airway smooth muscle is its relative sensitivity to relaxant effects of beta adrenergic agonists when contracted by inflammatory mediators, such as histamine, vs. resistance to these relaxant effects when contracted by muscarinic agonists. Because contractions presumably depend upon the hydrolysis of membrane phosphoinositides (PI) and the generation of inositol phosphates (IP), our goal was to test for the effects of forskolin, isoproterenol, and dibutyryl cAMP on histamine- vs. methacholine-induced IP accumulation in canine tracheal smooth muscle. Methacholine (10-3 M) was a more effective stimulant of IP accumulation (9.6 ± 2.1-fold increase) than equimolar histamine (3.6 ± 0.5-fold increase) in this tissue. When responses to equieffective methacholine (4 x 10-6 M) and histamine (10-3 M) were compared, neither forskolin, isoproterenol, nor dibutyryl cAMP significantly decreased IP accumulation in response to methacholine. In contrast, each of these three agents significantly decreased responses to histamine (by 56 ± 9, 52 ± 2, and 61 ± 2%, respectively). We conclude that, in canine tracheal smooth muscle, increased cAMP is associated with inhibition of PI hydrolysis in response to histamine but not methacholine. The findings suggest a novel mechanism for selective modulation by cAMP of receptor-mediated cellular activation.