ENDOTHELIN-3, CA2-GMP CONTENT IN HUMAN PLATELETS( MOBILIZATION AND CYCLIC)

As previously described for endothelin-3, platelet exposure to cyclic GMP-elevating agents such as sodium nitroprusside and M&B-22948 (2-o-p ropoxyphcnyl-8-azapurin-6-one), a cGMP phosphodiesterase inhibitor, lo wered Ca2+ mobilization in response to thrombin. Interestingly, when c GMP phosphodiesterases were blocked, endothelin-3 produced a dose-depe ndent cGMP accumulation (P < 0.001). Since endothelin-3 has been propo sed to decrease the activity of Ca2+ accumulating pumps, we examined w hether this latter a rise in cGMP content. Cyclic GMP decreased in a d ose-dependent manner the initial rate and plateau value of the ATP-dep endent Ca-45(2+) uptake in platelet membrane vesicles (P = 0.006 for e ach). Furthermore, combined treatment with endothelin-3 and M&B-22948 or a moderate concentration of Nai-nitroprusside further reduced the t hrombin-evoked Ca2+ discharge (P = 0.004 and 0.01, respectively), sugg esting that endothelin-3 pre-exposure had reduced the amount of mobili zable Ca2+. We propose that the depletion of platelet Ca2+ stores and the reduction of Ca2+ release evoked by endothelin-3 could be due, at least in part, to the elevation of cGMP content and to a decrease in C a2+ accumulating pump activity.