DIRECT INHIBITORY MECHANISMS OF HALOTHANE ON HUMAN PLATELET-AGGREGATION

Background: Although halothane directly inhibits platelet aggregation, the mechanisms of this effect are still unknown. The current study ai med to clarify the inhibitory mechanisms of halothane on thrombin-indu ced human platelet aggregation by measuring (1) platelet-surface glyco protein Ib expression, (2) the concentration of intracellular free Ca2 + ([Ca2+](i)) measured simultaneously with aggregation, (3) the concen tration of intracellular inositol 1,4,5-triphosphate, and (4) the conc entration of intracellular cyclic 3',5'-adenosine monophosphate ([cAMP ](i)). Methods: Washed platelet suspensions, obtained from healthy vol unteers, were preincubated with halothane (0-2 mM) for 2 min and then exposed to 0.02 units/ml thrombin for 3 min. The glycoprotein Ib bound to fluorescein-labeled antibody was measured by fluorescence flow cyt ometry. [Ca2+](i) was measured, simultaneously with aggregation, in Fu ra-2 (Ca2+ indicator)-loaded platelets by use of a fluorometer. Inosit ol 1,4,5-triphosphate and [cAMP](i) were measured by radioimmunoassay. Results: Halothane had no effect on glycoprotein Ib expression with o r without thrombin. Halothane decreased the thrombin stimulated [Ca2+] (i) transient and inhibited platelet aggregation in a dose-dependent m anner, both in the presence and in the absence of external Ca2+. Isofl urane had no apparent effect on either platelet aggregation or [Ca2+]( i) in the absence of external Ca2+. Halothane inhibited the increase i n inositol 1,4,5-triphosphate induced by thrombin. Halothane moderatel y but significantly increased [cAMP](i), but the adenylate cyclase act ivator forskolin (which has the same inhibitory ability on aggregation as halothane) increased [cAMP](i), to a much greater extent than did halothane, Conclusions: Halothane inhibits thrombin-induced human plat elet aggregation by decreasing [Ca2+](i) without inhibiting agonist-re ceptor binding; the inhibitory effect of halothane on [Ca2+](i) might be mediated by a decrease in inositol 1,4,5-triphosphate and in part b y an increase in [cAMP](i).