ADENOSINE RECEPTOR ACTIVATION POTENTIATES PHOSPHOINOSITIDE HYDROLYSISAND ARACHIDONIC-ACID RELEASE IN DDT1-MF(2) CELLS - PUTATIVE INTERRELATIONS

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 p hosphoinositides in DDT1-MF2 cells. Measurements of arachidonic acid r elease revealed similar behaviours to those observed in measurements o f phosphoinositide hydrolysis. In the presence of NE, both second mess enger responses were potentiated by the addition of CPA, whereas in th e absence of NE, CPA had little or no effect on either second messenge r. The stimulation and potentiation of both second messenger responses were enhanced in the presence of extracellular calcium, and in each c ase these effects were persistent over time. For either second messeng er system the stimulation by NE and the potentiation by CPA appeared t o utilize separate mechanisms as evidenced by the fact that the potent iations by CPA were selectively antagonized by a cAMP analogue or by p ertussis toxin, whereas the stimulations by NE were essentially unaffe cted by these agents. Inhibition of phospholipase A(2) (PLA(2)) also b locked 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 phosphoinos itide hydrolysis in these cells. These data are consistent with a hypo thesis whereby the apparent potentiation of NE-stimulated phosphoinosi tide hydrolysis by CPA is actually due to the stimulation by CPA of a second pathway of phospholipase C activity which is additive to that o f NE. The activation of PLC and PLA(2) by NE produces phospholipid pro ducts which may play a permissive role in the pathway coupling adenosi ne A(1) receptors to these phospholipases. The formation of lysophosph atidic acid is suggested as one possible mediator of this permissive e ffect.