EDRF increases cyclic GMP in platelets during passage through the coronary vascular bed.

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Abstract
It was investigated whether endothelium-derived relaxing factor (EDRF) increases cyclic GMP (cGMP) content in platelets passing through the coronary bed. Boluses of washed platelets from healthy human donors were injected into the aortic perfusion line of isolated, saline-perfused rabbit hearts under constant flow conditions (28 +/- 2 ml/min). The coronary effluent was collected over 5 seconds, and the cGMP content of platelets was determined by radioimmunoassay. Platelet cGMP amounted to 0.34 +/- 0.11 pmol/mg protein after passage through the unstimulated coronary bed. During stimulation with acetylcholine (1 microM), it increased to 1.6 +/- 0.5 pmol/mg (p less than 0.01; n = 14). Simultaneously, the platelet recovery (measured over 20 seconds after injection) was enhanced (by 45 +/- 11%; p less than 0.01) during endothelial stimulation with acetylcholine. Treatment with the EDRF inhibitor hemoglobin (6 microM) completely abolished the increase in platelet cGMP (p less than 0.01; n = 11) as well as the enhanced platelet recovery (n = 8). Inhibition of EDRF by hemoglobin reduced also the basal platelet cGMP content to 0.17 +/- 0.11 pmol/mg (p less than 0.01). The data indicate that basally released EDRF is able to increase cGMP in platelets during a single passage through the coronary bed. The enhanced recovery of platelets after EDRF stimulation, which coincides with an increase of platelet cGMP, suggests that EDRF plays an important role as inhibitor of platelet activation in the coronary circulation.