Dihydropyridine Agonist Bay K 8644 Inhibits Platelet Activation by Competitive Antagonism of Thromboxane A2-Prostaglandin H2 Receptor

The dihydropyridine calcium channel antagonists, such as nifedipine, inhibit platelet aggregation in vitro and ex vivo, but the mechanism by which this occurs is uncertain. Bay K 8644 (BAY) is a substituted dihydropyridine that has effects on voltage-dependent calcium channels in cardiac and smooth muscle that are opposite the effects of nifedipine. To evaluate the mechanism responsible for dihydropyridine-induced inhibition of platelet function, we studied the in vitro effects of BAY on human platelet aggregation and secretion plus several related biochemical parameters, including cytoplasmic ionized calcium ([Ca]1). BAY exerted concentration-dependent effects on platelet aggregation and secretion of [C] serotonin. BAY (1–10 μm) inhibited the second wave of platelet aggregation and secretion stimulated by adenosine diphosphate or epinephrine and blocked shape change, aggregation, and secretion induced by the thromboxane A2(TXA2) mimic, U46619. BAY also inhibited U46619-induced phosphorylation of the −40,000-dalton cytoplasmic protein substrate of protein kinase C (40K protein), formation of TXA2, and rise in [Ca]1, all biochemical consequences of platelet activation. The (+)-(R) enantiomer of BAY [BAY(+)] was predominantly responsible for the inhibitory effects of racemic BAY. Nifedipine had the same inhibitory effects on platelet function and biochemistry, except it was −10 times less potent than BAY. Since these results suggested inhibition of the TXA2-prostaglandin H2 (PGH2) receptor, we measured binding of [H]U46619 to intact platelets. BAY, BAY(+), and nifedipine all functioned as competitive antagonists of [H]U46619 binding (BAY K1 = 1.47 μM). They did not inhibit binding of [H]yohimbine to platelet α2-adrenergic receptors. At 1–10 nM BAY, BAY(+) and the (-)-(S) enantiomer of BAY [BAY(-)] all resulted in slight stimulation of platelet function and biochemical events. No significant increase in [H]U46619 binding was demonstrable, however. Therefore, dihydropyridines that function as either calcium channel agonists or antagonists in cardiac or smooth muscle exert concentration-dependent effects on platelet function. In nanomolar concentrations, they augment, and in micromolar concentrations, they inhibit platelet activation induced by TXA2 or U46619. These data indicate that dihydropyridines do not inhibit TXA2-induced platelet activation by an effect on voltage-dependent calcium channels; they define the mechanism of inhibition as competitive antagon