Effect of n-Alkane Kinetics in Rats on Potency Estimations and the Meyer-Overton Hypothesis

Abstract

Neither lipophilicity nor vapor pressure of larger n-alkanes appear to correlate with their anesthetizing partial pressures in inspired gas. Such results suggest that the Meyer-Overton hypothesis and Ferguson's rule may not apply to these compounds. An alternative explanation might be that a large difference in inspired-to-arterial partial pressure exists, i.e., that the inspired partial pressure misrepresents the effective partial pressure. To test this explanation, we investigated the kinetics of five consecutive even-numbered n-alkanes (C2H6 to C10H22) in rats. The ratio of end-tidal-to-inspired (PA/PI), arterial-to-end-tidal (Pa/PA), and arterial-to-inspired (Pa/PI) partial pressures decreased with increasing carbon chain length, consistent with our separate finding that blood solubility increased. Using Pa/PI and the minimum inspired concentration (MIC) obtained previously, we calculated the true effective potency, minimum alveolar anesthetic concentration (MAC); of these n-alkanes as (Pa/PI)(MIC). This markedly improved, but did not perfectly correct, the correlation of MAC with lipid solubility (the Meyer-Overton hypothesis) and vapor pressure (Ferguson's rule). A coefficient of variation of 76.7% was found for the product of MAC and the olive oil/gas partition coefficient. More importantly, the correlation of the logarithm of MAC and oil solubility had a slope of -0.724 (i.e., deviated from -1.0), whereas the slope for eight conventional anesthetics was -1.046 (approached-1.0). These data imply that olive oil does not adequately mimic the nature of the anesthetic site of action of n-alkanes.