Receptor (norepinephrine), P-site (2',5'-dideoxyadenosine), and calcium-mediated inhibition of prostaglandin and forskolin-activated cyclic AMP-generating systems in human platelets.

Prostaglandin D2, 2-chloroadenosine, forskolin and combinations of these agents increase cyclic AMP-levels in intact human platelets. The inhibition of activated cyclic AMP-generating systems by 1) alpha2-adrenergic receptor-mediated hormonal input (norepinephrine), 2) a P-site agent (2',5'-dideoxyadenosine) and 3) a divalent cation (calcium) were examined: 1) Norepinephrine produces non-competitive inhibition of both forskolin and prostaglandin D2(PGD2)-stimulated cyclic AMP-accumulation in intact human platelets. The Ki values for norepinephrine versus forskolin, PGD2 and 2-chloroadenosine are similar in magnitude, while the Ki versus a forskolin-PGD2 combination is approximately 10-fold greater. Onset of inhibition by norepinephrine of the PGD2-response is several fold faster than for the forskolin-response. When platelets stimulated by the forskolin and PGD2 combination are exposed to norepinephrine, there is a transient increase in levels of cyclic AMP due to the potentiation of a minor beta-adrenergic component. This stimulation is followed by inhibition. 2) 2',5'-Dideoxyadenosine produces a non-competitive inhibition of the forskolin-response with a Ki of 110 microM. The inhibition of the PGD2-response by 2',5'-dideoxyadenosine is competitive with a Ki of 6-13 microM, while inhibition of the forskolin-PGD2 response has a Ki of 30 microM. Onset of inhibition by 2',5'-dideoxyadenosine is identical for forskolin or PGD2-stimulated platelets. There is a lag period for inhibition of platelets stimulated with the forskolin-PGD2 combination. The PGD2-forskolin combination appears to stabilize the cyclic AMP-generating system of platelets against inhibition by either norepinephrine or 2',5'-dideoxyadenosine. 3) Calcium ions cause a similar inhibition of cyclic AMP-generation in intact platelets, regardless of the type of stimulation.