DIFFERENT INHIBITORY EFFECT OF ADRENALINE ON PLATELET ADENYLATE-CYCLASE IN THE PRESENCE OF GTP PLUS CHOLERA-TOXIN AND OF STABLE GTP ANALOGS

GTP is generally required for hormonal stimulation of adenylate cyclase. On the other hand, the presence of GTP is essential for hormone-induced inhibition of adenylate cyclase in cell-free preparations of human platelets and other cells. In order to differentiate the dual roles of GTP in hormonal stimulation and inhibition of adenylate cyclase, we have studied the effect of adrenaline on the platelet enzyme under conditions where guanine mucleotides caused marked stimulation. In the presence of GTP (≧1 μM), which by itself had no or only a small stimulatory effect on adenylate cyclase, adrenaline inhibited the basal and prostaglandin E1-stimulated forms of the enzyme. In contrast, the stable GTP analogues, GMP-P(NH)P and GTP-γ-S, which caused a time-dependent, persistent activation of the enzyme, reversed or prevented the inhibitory effect of adrenaline. Cholera toxin, which activates adenylate cyclase presumably by inhibition of a specific GTPase, increased cyclase activity in platelet membranes up to 4-fold, and GTP addition (0.1-30 μM) augmented this activation about 2-fold. The α-adrenergic component of adrenaline (0.1-100 μM), in a concentration-dependent manner, prevented the GTP-induced increase in cholera toxin-stimulated activity. The inhibition was also observed in enzyme preparations that had been fully activated by pretreatment with cholera toxin. These data suggest that α-adrenergic agonists may inhibit platelet adenylate cyclase through increased inactivation of the enzyme, possibly involving a stimulation of the GTPase connected to the adenylate cyclase system. © 1979 Springer-Verlag.