Influence of halothane on phospholipase A2 and enzymatic methylations in the rat retinal membranes

Phospholipase A2 (PLA2) and phospholipid methylases (PLM) play significant roles in transmitter release and membrane signal transduction, respectively. Previous studies have indicated that PLMs occur in the rat brain synaptosomal and retinal membranes, and they are activated under halothane anesthesia. The influence of halothane on PLA2 is not known. Therefore, we have investigated the effect of halothane on retinal PLA2 activity. Rat retinal sonicates were assayed for PLA2 activity using 1-palmitoyl-2[1-14C]arachidonyl-phosphatidylethanolamine (PE, 2.2 nmol) in Tris buffer (10 mM, pH 7.4) at 37 degrees C with and without halothane (0.25-2.0 mM) in the assay medium. These studies gave the following results: (1) Rat retinal sonicates contained PLA2 activity of 4.2+/-0.8 pmol PE hydrolyzed/100 ng protein/hr; (2) Halothane (0.25-2.0 mM) increased PLA2 activity by 20 to 150% depending upon concentration; (3) The lower concentration of halothane (0.25 mM) exhibited high activation of PLA2 (150%); (4) High concentrations of halothane (1.0-2.0 mM) caused a low degree of activation of PLA2 (20%); and (5) During phospholipid methylation of retinal membranes with S-adenosyl-L-methionine in the presence of halothane, increased amounts of fatty acid methyl esters (FAME) were formed. This increase in FAME (45%) was possibly due to the hydrolysis of phospholipids by activated PLA2, liberating fatty acids which were methylated. This increase in FAME (45%) was inhibited by mepacrine (quinacrine) (10 microM), an inhibitor of PLA2. These observations suggest that the release of retinal transmitters (dopamine, acetylcholine and others) is affected during halothane anesthesia, due to activation of PLA2 and enhanced fusogenic activity of vesicular membranes with plasma membrane and depletion of vesicles.