in cellular systems provided with activatory ( ra-site ) receptors for adenosine , such as rat cerebral microvessels and rat liver plasma membranes , the adenosine-receptor antagonist 8-phenyltheophylline ( 10 microm ) significantly decreased adenylate cyclase activity if atp was the substrate and only if gtp was present .
C0007634
cellular
C0007603
plasma membranes