DIFFERENTIAL EFFECT OF HALOTHANE AND FORSKOLIN ON PLATELET CYTOSOLIC CA2+ MOBILIZATION AND AGGREGATION

Background: Previous works have suggested that the impairment of plate let aggregation by halothane was partly related to a stimulation of cy clic adenosine monophosphate (cAMP) production, to an inhibitory effec t on Ca2+ signaling, or both. Intracellular Ca2+ measurements therefor e were undertaken, first to determine the critical steps in the platel et Ca2+ signaling cascade most likely to be affected by halothane or b y an increase in cAMP production, and second to establish if the effec t of halothane involves aggregation-related biochemical pathways trigg ered by an increase in internal Ca2+. Methods: Human washed platelets were treated with halothane or forskolin for 5 min before application of either platelet-activating factor, thrombin, U46619, or thapsigargi n. The cytosolic Ca2+ concentration ([Ca2+](i)) was measured with the fluorescent Ca2+ indicator fura-2. Nephelometric measurements were als o performed to assay the aggregation process. Results: Our results ind icate that pretreating platelets with halothane leads to a partial imp airment of the [Ca2+](i) increase induced either by U46619, thrombin, or platelet-activating factor, but this had no significant effect on t he [Ca2(+)](i) response triggered by thapsigargin, In addition, our re sults show that halothane inhibits platelet aggregation triggered by U 46619, but not by thapsigargin, Conversely, forskolin completely inhib ited the [Ca2+](i) response to U46619 and thapsigargin and prevented p latelet aggregation induced by both agonists. Conclusions: These resul ts suggest that halothane and cAMP exert their effects on platelet agg regation and Ca2+ signaling through different mechanisms, and that hal othane cannot impair platelet aggregation independently of phospholipa se C stimulation.