HALOTHANE REGULATES G-PROTEIN-DEPENDENT PHOSPHOLIPASE-C ACTIVITY IN TURKEY ERYTHROCYTE-MEMBRANES

The ability of halothane to stimulate phospholipase C (PLC) was examin ed in turkey erythrocyte membranes prepared from [H-3]inositol-labeled turkey erythrocytes by measuring [H-3]inositol phosphate formation ([ H-3]InsP) in the presence and absence of G-protein activation. In the presence of guanosine 5'-3-O-(thio)triphosphate) (GTPgammaS), halothan e (0.5-10 mM) caused a dose-dependent activation of PLC. The EC50 valu e for halothane-induced PLC activation was 2.8 +/- 0.3 mM. Halothane ( 0.1-30 mM) had no effect on PLC activity in the absence of G-protein a ctivation and did not affect Ca2+-dependent PLC activity. The activati on of PLC by GTPgammaS occurred after an initial lag period of 60 s wh ich was followed by a linear increase in [H-3]InsP. Halothane dose-dep endently decreased the lag period for GTPgammaS-induced PLC activation (minimal value 15 s) and increased the rate of [H-3]InsP formation at all time points following this lag. As a result, halothane shifted th e EC50 value for GTPgammaS-induced PLC activation to the left (4-fold) and increased its maximal response. Halothane also caused a dose-depe ndent activation of PLC in the presence of AlF4-. Half-maximal stimula tion of AlF4--activated PLC occurred with an EC50 value of 2.9 +/- 0.4 mM halothane, which is similar to the halothane dose giving half-maxi mal stimulation of PLC in the presence of GTPgammaS. At low doses (0.1 -0.3 mM) halothane inhibited both isoproterenol- and adenosine 5'-O-(2 -thiodiphosphate) (ADPbetaS)-induced [H-3]InsP formation, whereas at h igher concentrations it stimulated PLC independent of the presence of these agonists. At concentrations chosen to reflect their different me mbrane/buffer partition coefficients, both hexanol (5 mM) and benzyl a lcohol (20 mM) fluidized turkey erythrocyte membranes to the same degr ee as halothane (5 mM). However, these agents had no effect on GTPgamm aS- or AlF4-induced PLC activity, indicating that halothane-induced PL C activation was not secondary to changes in bulk lipid fluidity prope rties. Halothane also stimulated [H-3]inositol bisphosphate and [H-3]i nositol trisphosphate formation in intact erythrocytes. These data dem onstrate that the anesthetic halothane can stimulate G-protein-depende nt PLC activity and modify the responsiveness of this signaling system to activation by receptor-linked agonists.