ROLE OF 1,2-SN DIACYLGLYCEROL IN AIRWAY SMOOTH-MUSCLE STIMULATED BY CARBACHOL

Activation of muscarinic receptors in airway smooth muscle leads to br eakdown of membrane polyphosphoinositides. In agreement with others, w e show here that muscarinic stimulation elicits inositol-1,4,5-triphos phate formation. In addition, we show that carbachol also elicits tota l diacylglycerol and 1,2-sn diacylglycerol accumulation in a specific and dose-dependent manner (EC(50) values: 2.1 x 10(-8) M for 1,2-sn di acylglycerol). The time-course of inositol-1,4,5-triphosphate accumula tion is compatible with that of the clonic phase of muscle contraction . Since this derivative can mobilize intracellular Ca2+ stores, it may play a second-messenger role in the initial phase of contraction. The time-course of diacylglycerol accumulation is compatible with the mus carinic-induced tonic phase of smooth-muscle contraction. Carbachol in duces similar dose-dependent reductions in isoproterenol-induced muscl e relaxation (EC(50) values for relaxation concentration-response curv es to isoproterenol: 3x10(-6) M and 2.1x10(-5) M, with carbachol at 10 (-7) M and 10(-4) M, respectively), and increases in adenylate cyclase activity (EC(50) values for the concentration response to isoproteren ol: 1.2 x 10(-6) M and 1.5 x 10(-5) M, with carbachol at 10(-7) M and 10(-4) M, respectively). Since it is known that carbachol-induced unco upling of beta(2)-adrenergic receptors is proportional to the breakdow n of polyphosphoinositides, and that 1,2-sn diacylglycerol is a potent activator of protein kinase C, 1,2-sn diacylglycerol could be mediati ng the uncoupling of beta(2)-adrenergic receptors, via activation of a lpha-protein kinase C and subsequent phosphorylation of receptor, and/ or cyclase, and/or G proteins. Measurement of diacylglycerol might be a useful tool for studying the effect of spasmogens and other activato rs of polyphosphoinositide breakdown, such as inflammatory mediators, in lung cells in asthma.