Influence of halothane on phospholipase A(2) and enzymatic methylations inthe rat retinal membranes

Phospholipase A(2) (PLA(2)) and phospholipid methylases (PLM) play signific ant roles in transmitter release and membrane signal transduction, respecti vely. Previous studies have indicated that PLMs occur in the rat brain syna ptosomal and retinal membranes, and they are activated under halothane anes thesia. The influence of halothane on PLA(2) is not known. Therefore, we ha ve investigated the effect of halothane on retinal PLA(2) activity. Rat ret inal sonicates were assayed for PLA(2) activity using 1-palmitoyl-2[1-C-14] arachidonyl-phosphatidylethanolamine (PE, 2.2 nmol) in Tris buffer(10 mM, p H 7.4) at 37 degrees C with and without halothane (0.25-2.0 mM) in the assa y medium. These studies gave the following results: (1) Rat retinal sonicat es contained PLA(2) activity of 4.2 +/- 0.8 pmol PE hydrolyzed/100 ng prote in/hr; (2) Halothane (0.25-2.0 mM) increased PLA(2) activity by 20 to 150% depending upon concentration; (3) The lower concentration of halothane (0.2 5 mM) exhibited high activation of PLA(2) (150%); (4) High concentrations o f halothane (1.0-2.0 mM) caused a low degree of activation of PLA(2) (20%); and (5) During phospholipid methylation of retinal membranes with S-adenos yl-L-methionine in the presence of halothane, increased amounts of fatty ac id methyl eaters (FAME) were formed. This increase in FAME (45%) was possib ly due to the hydrolysis of phospholipids by activated PLA(2), liberating f atty acids which were methylated. This increase in FAME (45%) was inhibited by mepacrine (quinacrine) (10 mu M), an inhibitor of PLA(2). These observa tions suggest that the release of retinal transmitters (dopamine, acetylcho line and others) is affected during halothane anesthesia, due to activation of PLA(2) and enhanced fusogenic activity of vesicular membranes with plas ma membrane and depletion of vesicles.