SELECTIVE MODULATION OF THE HUMAN PLATELET THROMBOXANE-A(2) PROSTAGLANDIN-H2 RECEPTOR BY EICOSAPENTAENOIC AND DOCOSAHEXAENOIC ACIDS IN INTACT PLATELETS AND SOLUBILIZED PLATELET MEMBRANES

We previously demonstrated that nonesterified as well as esterified ei cosapentaenoic acid (20:5n-3) and docosahexaenoic acid (22:6n-3) inhib it U46619-induced platelet aggregation and [H-3]U46619 specific bindin g to washed human platelets. It was also demonstrated that esterificat ion of these fatty acids resulted in a decrease in the affinity of [H- 3]U46619 for the thromboxane A2/prostaglandin H-2 (TXA2/PGH2) receptor . In order to investigate the specificity of this inhibition, the effe cts of 20:5n-3 and 22:6n-3 on the function and binding of the platelet alpha(2)-adrenergic receptor were studied. It was found that neither 20:5n-3 nor 22:6n-3 (nonesterified or esterified) altered epinephrine- induced aggregation or [H-3]yohimbine specific binding. Moreover, Scat chard analysis revealed that esterification with either 20:5n-3 or 22: 6n-3 did not alter the dissociation constant for [H-3]yohimbine bindin g. Modulation of the TXA2/PGH2 receptor by 20:5n-3 and 22:6n-3 was nex t evaluated using CHAPS- and digitonin-solubilized platelet membranes. [H-3]SQ29,548 dissociation constants of 26.5 nM and 20.8 nM were meas ured for CHAPS and digitonin-solubilized membranes, respectively. Comp etitive binding experiments in these solubilized preparations revealed that 20:5n-3 or 22:6n-3 blocked [H-3]SQ29,548 binding with IC50 value s in the range of 6-15-mu-M, while concentrations of these fatty acids of up to 100-mu-M showed no effect on [H-3]yohimbine binding. On the other hand, the IC50 values for inhibition of [H-3]SQ29,548 binding by linoleic acid (18:2n-6) and gamma-linolenic acid (18:3n-6) were in th e range of 150-mu-M. Furthermore, 18:2n-6 and 18:3n-6 showed similar i nhibitory effects on [H-3]yohimbine binding. Finally, competition bind ing studies performed in a partially purified TXA2/PGH2 receptor prepa ration also demonstrated inhibition of [H-3]SQ29,548 binding by 20:5n- 3 and 22:6n-3. Collectively, these findings support the notion that 20 :5n-3 and 22:6n-3 can selectively and directly modulate TXA2/PGH2 rece ptor function, and that this mechanism of action may contribute to the antiplatelet activity associated with diets rich in these fatty acids .