The present data disagree with earlier suggestions that thrombin's effect on platelets is to cause a decrease in intracellular cyclic 3′,5′-adenosine monophosphate. Washed human platelets or platelet-rich plasma were incubated at 37°C with human thrombin. After centrifugation, the supernates were assayed for nucleotides and calcium released. The platelet pellets, and in some experiments the supernates as well, were assayed by radioimmunoassay for intracellular cyclic AMP. In the washed platelet system, increasing doses of thrombin to 0.5 U/cc induced increasing release of nucleotides and calcium. This was accompanied by an average twofold increase in intracellular cyclic AMP levels. Prostaglandin E1, which inhibited 30-50% of release, induced a four- to fivefold increase in cyclic AMP levels that was additive to the cyclic AMP-stimulatory effect of thrombin. Theophylline, which inhibited only 20-40% of nucleotide release, was synergistic with thrombin in the intracellular accumulation of cyclic AMP. The time-course of cyclic AMP accumulation in response to thrombin was slower than thrombin-induced nucleotide release. Similar findings were made in the platelet-rich plasma system where thrombin stimulation of nucleotide release also resulted in a marked accumulation of intracellular cyclic AMP. Thrombin did not appear to stimulate the release of intracellular cyclic AMP.
Michael J. Droller, Sidney M. Wolfe