| Abstract: | Exposure of platelets to 1 C led to a transient increase in cyclic AMP levels (determined either by a protein binding method or by radioimmunoassay) within five to ten minutes reaching a maximum 10 to 15 minutes after chilling was begun and returning subsequently to baseline values. Addition of EDTA to the platelet suspension medium prevented this increase. Rewarming at 37 C produced a sudden reduction in platelet cyclic AMP. To determine whether the cold-induced increase in cyclic AMP was due to a transient stimulation of platelet adenylate cyclase or a rapid inhibition of phosphodiesterase, these enzymes were assayed in ruptured platelet suspensions. Platelet adenylate cyclase activity was found to possess certain characteristics similar to those of the enzyme derived from other sources but there was a marked potentiation of fluoride-stimulated adenylate cyclase activity by 0.001 M EDTA. This effect was limited to low EDTA concentrations. Exposure of platelets to 1 C for up to 60 minutes did not increase adenylate cyclase activity but lowered it substantially compared with controls kept at room temperature. Phosphodiesterase activity at 1 C was depressed sooner and to a greater extent than was adenylate cyclase. The transient rise in cyclic AMP levels in chilled platelets appears to be due to a disproportionate reduction of cyclic nucleotide phosphodiesterase activity. |
