Lack of inhibition of thrombin-induced rise in intracellular Ca²⁺levels and 5-hydroxytryptamine secretion by 1-oleoyl-2-acetylglycerol in human platelets

The effect of 1-oleoyl-2-acetylglycerol (OAG) on the thrombin-induced rise in intracellular Ca²⁺ levels ([Ca²⁺]_i) and 5-hydroxy[¹⁴C]tryptamine ([¹⁴C]5HT) secretion was studied. In washed human platelets prelabelled with [¹⁴C]5HT and quin 2, OAG (10–50 ) induced no significant aggregation, [¹⁴C]5HT secretion or rise in [Ca²⁺]_i in the presence or absence of fibrinogen. However, addition of OAG (10–50 ) 10 s to 5 min before or 10–60 s after addition of threshold concentrations of thrombin ( < 0.03 ) resulted in a significant potentiation of aggregation and [¹⁴C]5HT secretion without any effect on the thrombin-induced rise in [Ca²⁺]_i. Both EGTA, which abolished the latter and creatine phosphate/creatine phosphokinase, the ADP scavenger, totally inhibited the aggregation but only partially reduced [¹⁴C]5HT secretion in response to thrombin plus OAG. At higher concentrations of thrombin, neither the rise in [Ca²⁺]_i nor the extent of [¹⁴C]5HT secretion was significantly altered by OAG addition. The results demonstrate that, unlike phorbol esters, OAG has no inhibitory effect on thrombin-induced [Ca²⁺]_i mobilisation but can synergize with low concentrations of thrombin in potentiating [¹⁴C]5HT secretion even at basal [Ca²⁺]_i.