Regulation of phospholipase C responses by agents which alter the cellular cyclic AMP profile

In DDT$\sb1$-MF$\sb2$ smooth muscle cells the addition of norepinephrine resulted in a robust phospholipase C response, as detected by inositol phosphate accumulation, and in the activation of adenylyl cyclase, as detected by cAMP accumulation. The addition of an adenosine agonist resulted in both the inhibition of forskolin- or norepinephrine-stimulated cAMP accumulation and the potentiation of norepinephrine-stimulated phosphoinositide hydrolysis. Pharmacologic analysis indicated the involvement of an A$\sb1$ adenosine receptor subtype in both of these responses. In the absence of norepinephrine and extracellular calcium, the activation of the adenosine receptor did not directly stimulate phosphoinositide hydrolysis. The adenosine receptor-mediated augmentation of norepinephrine-stimulated phosphoinositide hydrolysis was pertussis toxin-sensitive and was selectively antagonized by agents that mimicked cyclic AMP (8-Bromo cyclic AMP) or raised cellular cyclic AMP levels (forskolin). This initially suggested that cyclic AMP might partially regulate the magnitude of the phospholipase C response to norepinephrine and that adenosine agonists might enhance the phospholipase C response by reducing cyclic AMP levels. However, neither the reduction of cellular cyclic AMP levels by other agents nor the inhibition of cyclic AMP-dependent protein kinase were sufficient to replicate the action of adenosine receptor activation on phosphoinositide hydrolysis. Thus, in the presence of norepinephrine, adenosine receptor agonists appear to stimulate phosphoinositide hydrolysis via a pathway that is separate from, but dependent upon, that of norepinephrine. Measurements of arachidonic acid release revealed analogous behaviors to that of phosphoinositide hydrolysis. Inhibitors of arachidonic acid release were found to block the potentiating effect of CPA on NE-stimulated phosphoinositide hydrolysis. Furthermore, the exogenous administration of arachidonic acid was found to stimulate phosphoinositide hydrolysis in these cells. These data are consistent with a hypothesis whereby adenosine receptor agonists may potentiate NE-stimulated phosphoinositide hydrolysis via the activation of PLA$\sb2$ and the subsequent generation of a bioactive metabolite(s) of arachidonic acid