Cyclic nucleotides and platelet aggregation. Effect of aggregating agents on the activity of cyclic nucleotide-metabolizing enzymes.

The activities of adenylate and guanylate cyclase and cyclic nucleotide 3':5'-phosphodiesterase were determined during the aggregation of human blood platelets with thrombin, ADP, arachidonic acid and epinephrine. The activity of guanylate cyclase is altered to a much larger degree than adenylate cyclase, while cyclic nucleotide phosphodiesterease activity remains unchanged. During the early phases of thrombin-and ADP-induced platelet aggregation a marked activation of the guanylate cyclase occurs whereas aggregation induced by arachidonic acid or epinephrine results in a rapid diminution of this activity. In all four cases, the adenylate cyclase activity is only slightly decreased when examined under identical conditions. Platelet aggregation induced by a wide variety of aggregating agents including collagen and platelet isoantibodies results in the "release" of only small amounts (1-3%) of guanylate cyclase and cyclic nucleotide phosphodiesterase and no adenylate cyclase. The guanylate cyclase and cyclic nucleotide phosphodiesterase activities are associated almost entirely with the soluble cytoplasmic fraction of the platelet, while the adenylate cyclase if found exclusively in a membrane bound form. ADP and epinephrine moderately inhibit guanylate and adenylate cyclase in subcellular preparations, while arachidonic and other unsaturated fatty acids moderately stimulate (2-4-fold) the former. It is concluded that (1) the activity of platelet guanylate cyclase during aggregation depends on the nature and mode of action of the inducing agent, (2) the activity of the membrnae adenylate cyclase during aggregation is independent of the aggregating agent and is associated with a reduction of activity and (3) cyclic nucleotide phosphodiesterase remains unchanged during the process of platelet aggregation and release. Furthermore, these observations suggest a role for unsaturated fatty acids in the control of intracellular cyclic GMP levels.