| Abstract: | The receptor mechanisms underlying vasopressin-induced human platelet activation were investigated with respect to stimulation of phosphoinositide metabolism and changes in the cytosolic free Ca2+ concentration (Ca2+]i). Vasopressin stimulated phosphoinositide metabolism, as indicated by the early formation of 32P]phosphatidic acid (32P]PtdA) and later accumulation of 32P]phosphatidylinositol (32P]PtdIns). In addition, vasopressin elicited a transient depletion of glycerol-3H]PtdIns and accumulation of glycerol-3H]PtdA. The effects of vasopressin on phosphoinositide metabolism were concentration-dependent, with half maximal 32P]PtdA formation occurring at 30 +/- 15 nM-vasopressin. In the presence of 1 mM extracellular free Ca2+, vasopressin induced a rapid, concentration-dependent elevation of Ca2+]i in quin2-loaded platelets: half-maximal stimulation was observed at 53 +/- 20 nM-vasopressin. The V1-receptor antagonist 1-(beta-mercapto-beta, beta-cyclopentamethylenepropionic acid),2-(O-methyl)tyrosine,8-arginine]-vasopressin selectively inhibited vasopressin (100 nM)-induced 32P]PtdA formation I50 (concn. giving 50% inhibition) = 5.7 +/- 2.4 nM] and elevation of Ca2+]i (I50 = 3 +/- 1.5 nM). Prior exposure of platelets to vasopressin rendered them unresponsive, in terms of 32P]PtdA formation and elevation of Ca2+]i, to a subsequent challenge with vasopressin, but responsive to a subsequent challenge with U44069, a thromboxane-A2 mimetic. These results indicate that vasopressin-induced human platelet activation is initiated by combination with specific V1 receptors on the platelet, and that the sequelae of receptor occupancy (stimulation of phosphoinositide metabolism and elevation of Ca2+]i) are equally susceptible to inhibition by receptor antagonists and by receptor desensitization. |
