Functional pool of cyclic adenosine 3',5'-monophosphate in rabbit platelets

Accumulation of the newly formed 14C-cyclic adenosine 3',5'-monophosphate (cyclic AMP) was found in the P1 (1.0) fraction, i.e. a platelet plasma membrane fraction which was obtained from 14C-adenine-labeled platelets. On the other hand, total cyclic AMP as determined simultaneously was located mainly in the platelet soluble fraction. Furthermore, the highest value of the cyclic AMP-binding capacity was found in the P1 (1.0) fraction. The cyclic AMP-binding activity of platelet membranes was attributed to two proteins with molecular weights of approximately 48,000 and 68,000. The treatment of 14C-adenine-prelabeled platelets with thrombin (1 unit per ml) led to about 40% decrease in the newly formed 14C-cyclic AMP level and 18% reduction of 14C-adenosine triphosphate level in whole platelets within 10 sec. On the other hand, the 14C-cyclic AMP level in the P1 fraction decreased by about 80% of the control value while the total cyclic AMP in this fraction was almost unchanged. This rapid and striking fall in the membrane 14C-cyclic AMP level could be correlated with the more than 2fold stimulation of the membrane-bound cyclic AMP phosphodiesterase, together with the more than 20% inhibition of both the cyclic AMP-binding capacity and the adenyl cyclase in platelet membranes by thrombin treatment. These observations suggest the possibility that functional pool of cyclic AMP related to thrombin-induced aggregation is located in rabbit platelet plasma membrane.