Endothelium-dependent phosphorylation of vasodilator-stimulated protein in platelets during coronary passage.

Compounds that elevate intraplatelet guanosine 3',5'-cyclic monophosphate (cGMP) or adenosine 3',5'-cyclic monophosphate (cAMP) stimulate the phosphorylation of a 46- to 56-kDa thrombocyte protein designated "vasodilator-stimulated protein" (VASP), which is most likely involved in the regulation of adhesion/aggregation. We investigated whether endothelium-derived relaxing factor (EDRF)/nitric oxide (NO) and prostaglandin I2 (PGI2) affected VASP phosphorylation in washed human platelets that were injected into the coronaries of saline-perfused rabbit hearts (n = 22) and collected immediately after passage. The endothelial stimulator acetylcholine (ACh; 1 microM) significantly increased the concentration of cGMP (indicating release of EDRF) and PGI2 in the coronary venous effluent, as well as the concentration of cGMP and cAMP in platelets. Phosphorylation state of VASP increased from 32.1 +/- 2.9 to 64.8 +/- 2.7%. Inhibition of EDRF/NO synthesis by NG-nitro-L-arginine (30 microM) completely abolished the ACh-induced cGMP increase, attenuated the cAMP-elevation without affecting PGI2, and caused a 20.5 +/- 5.8% decrease of the phosphorylation state of VASP. Indomethacin (30 microM) virtually abolished ACh-induced increases of PGI2, cAMP (but not cGMP), and phosphorylated VASP. These results indicate that both EDRF/NO and PGI2 contribute to the regulation of VASP phosphorylation in platelets collected after a single coronary passage. Their synergistic inhibitory effects on platelet function may thus be mediated by a common effect on target proteins like VASP as well as by a secondary increase in cAMP in response to cGMP-elevating compounds such as EDRF.