VOLATILE ANESTHETICS INHIBIT NMDA-STIMULATED CA-45 UPTAKE BY RAT-BRAIN MICROVESICLES

We have previously shown that volatile anesthetics inhibit glutamate-stimulated [H-3]MK-801 binding to the ionophore of NMDA receptor complexes in rat brain. In the present study, we examined the influence of enflurane and halothane on NMDA-stimulated Ca-45 uptake by a microvesicle fraction isolated from rat brain. NMDA stimulated Ca-45 uptake (30 sec) by rat brain microvesicles by up to 70% with an EC(50) of 1.4 +/- 0.5 mu M. The NMDA-stimulated Ca-45 uptake was inhibited by MK-801 and D-AP-5 with IC50's of approximate to 10 mu M Enflurane and halothane inhibited Ca-45 uptake stimulated by 100 mu M NMDA by as much as 60-80% with IC50's of 0.2-0.3 mM, concentrations achieved during routine clinical use. Basal Ca-45 uptake measured in the absence of agonist was not affected by the anesthetics. Glycine did not affect the level of NMDA-stimulated Ca-45 uptake, but markedly reduced the inhibition of uptake caused by enflurane and halothane. Preincubation of microvesicles with NMDA resulted in a desensitization of NMDA-stimulated Ca-45 uptake, with a t(1/2) of approximate to 20 sec. Enflurane and halothane diminished both the extent and rate of development of this desensitization, as did glycine. These findings support the idea that volatile anesthetic interference with neurotransmission at NMDA receptor complexes contributes to the development of the anesthetic state.