Adenosine receptor activation potentiates phosphoinositide hydrolysis and arachidonic acid release in DDT 1-MF2 cells: Putative interrelations

Abstract

Studies were undertaken in an effort to discern possible mechanisms by which the A 1 adenosine receptor agonist cyclopentyladenosine (CPA) enhances the norepinephrine-stimulated (NE-stimulated) hydrolysis of phosphoinositides in DDT 1-MF2 cells. Measurements of arachidonic acid release revealed similar behaviours to those observed in measurements of phosphoinositide hydrolysis. In the presence of NE, both second messenger responses were potentiated by the addition of CPA, whereas in the absence of NE, CPA had little or no effect on either second messenger. The stimulation and potentiation of both second messenger responses were enhanced in the presence of extracellular calcium, and in each case these effects were persistent over time. For either second messenger system the stimulation by NE and the potentiations by CPA appeared to utilize separate mechanisms as evidenced by the fact that the potentiations by CPA were selectively antagonized by a cAMP analogue or by pertussis toxin, whereas the stimulations by NE were essentially unaffected by these agents. Inhibition of phospholipase A 2 (PLA 2) also blocked the potentiation of PLC by CPA, without affecting NE-stimulated phosphoinositide hydrolysis. Furthermore, in the presence of CPA, the exogenous administration of PLA 2 was found to stimulate phosphoinositide hydrolysis in these cells. These data are consistent with a hypothesis whereby the apparent potentiation of NE-stimulated phosphoinositide hydrolysis by CPA is actually due to the stimulation by CPA of a second pathway of phospholipase C activity which is additive to that of NE. The activation of PLC and PLA 2 by NE produces phospholipid products which may play a permissive role in the pathway coupling adenosine A 1 receptors to these phospholipases. The formation of lysophosphatidic acid is suggested as one possible mediator of this permissive effect.