Anesthetic synergy between two n-alkanes.

OBJECTIVE: N-butane and n-pentane can both produce general anesthesia. Both compounds potentiate γ-aminobutyric acid type A (GABA(A)) receptor function, but only butane inhibits N-methyl-d-aspartate (NMDA) receptors. It was hypothesized that butane and pentane would exhibit anesthetic synergy due to their different actions on ligand-gated ion channels. STUDY DESIGN: Prospective experimental study. ANIMALS: A total of four Xenopus laevis frogs and 43 Sprague-Dawley rats. METHODS: Alkane concentrations for all studies were determined via gas chromatography. Using a Xenopus oocyte expression model, standard two-electrode voltage clamp techniques were used to measure NMDA and GABA(A) receptor responses in vitro as a function of butane and pentane concentrations relevant to anesthesia. The minimum alveolar concentrations (MAC) of butane and pentane were measured separately in rats, and then pentane MAC was measured during coadministration of 0.25, 0.50 or 0.75 times MAC of butane. An isobole with 95% confidence intervals was constructed using regression analysis. A sum of butane and pentane that was statistically less than the lower-end confidence bound isobole indicated a synergistic interaction. RESULTS: Both butane and pentane dose-dependently potentiated GABA(A) receptor currents over the study concentration range. Butane dose-dependently inhibited NMDA receptor currents, but pentane did not modulate NMDA receptors. Butane and pentane MAC in rats was 39.4±0.7 and 13.7±0.4 %, respectively. A small but significant (p<0.03) synergistic anesthetic effect with pentane was observed during administration of either 0.50 or 0.75×MAC butane. CONCLUSIONS: Butane and pentane show synergistic anesthetic effects in vivo consistent with their different in vitro receptor effects. CLINICAL RELEVANCE: Findings support the relevance of NMDA receptors in mediating anesthetic actions for some, but not all, inhaled agents.