Mepyramine inhibits platelet activating factor-induced rabbit platelet aggregation: Role of intracellular histamine

AIM: To study the possible role of intracellular histamine (HA) in platelet activating factor (PAF)-induced platelet activation. METHODS: Washed rabbit platelet suspension was used to test the inhibitory effect of mepyramine (Mep, an H-1 receptor antagonist) on PAF-induced platelet aggregation. The thromboxane B-2 (TXB(2)) generation was measured by radioimmunoassay and the intracellular calcium ([Ca2+](i)) concentration was determined by the specific fluorescence indicator Fura-2. RESULTS: Mep >100 mu mol . L(-1) generated a concentration-dependent inhibition on PAF-induced aggregation, with an IC50 value of 162 (95% confidence limits: 114 - 232 mu mol . L(-1)). Cimetidine, an H-2 receptor antagonist, even up to 400 mu mol . L(-1) had no effect on it. Exogenous HA (10 mu mol . L(-1)) and H-1 receptor agonist. 2-thia-zolylethylamine had no energetic effect. alpha-Fluoro-methylhistidine, an inhibitor of histidine decarboxylase, did not inhibit platelet responses. However, in platelets permeabilized with saponin (8-10 mg . L(-1)), exogenous HA attenuated the inhibitory effect of Mep to about 50% at a concentration of 50 mu mol . L(-1). Preincubation of platelets with Mep (100 or 200 mu mol . L(-1)) resulted in an inhibition on TXB(2) generation [Ca2+](i) elevation induced at PAF. CONCLUSION: Platelets activated by PAF is associated with an intracellular HA synthesis and release via a common pathway of TXB(2) generation and the rise of [Ca2+](i).