Possible Mechanisms of Epinephrine Actions in Quin-2-Loaded Platelets Refractory to Arachidonic Acid

We evaluated the effect of Quin-2 loading (> 20 μM) on platelet responses such as phosphoinositide turnover, elevation of cytosolic Ca²⁺, phosphorylation of myosin light chain (MLC) and a 47-kDa protein, and aggregation in human platelets stimulated with arachidonic acid (AA) and epinephrine. The formation of inositol phosphates (IP, IP₂, and IP₃) in platelets stimulated with AA was inhibited by 50.4, 59.5, and 61%, respectively, in the presence of Quin-2 (40 μM). A similar degree of inhibition was observed in platelets stimulated with epinephrine (50 μM) and thrombin (0.1 U/ml). Even though Quin-2-induced inhibition of aggregation in response to AA was reversed by epinephrine, its effect on phosphoinositide turnover remained unaffected. Monitoring of cytosolic Ca²⁺ changes further indicates that the ability of epinephrine to restore aggregation in Quin-2-loaded (40 μM) and AA-stimulated platelets is not coupled to an increase in cytosolic Ca²⁺. Quin-2 loading (40 μM) caused a significant inhibition of MLC phosphorylation (20 kDa) in platelets stimulated by AA. However, it had no effect on the phosphorylation of the 47-kDa protein induced by AA. Furthermore, Quin-2 loading (40 μM) exerted no significant effect on shape change, actin filament assembly, and spreading, but caused a significant inhibition of secretion and clot retraction. We conclude that the formation of inositol phosphates, increases in cytosoloic Ca²⁺ and phosphorylation of MLC affected by Quin-2 are not coupled to the mechanisms by which platelets develop stickiness, undergo shape change, spreading, and aggregation in response to epinephrine and AA. It appears that the effect of epinephrine in restoring the aggregation response of refractory platelets is coupled to a calcium-mediated α-adrenergic receptor, and it may serve as a critical salvage pathway in platelets with compromised functions.