Effect of n-alkane kinetics in rats on potency estimations and the Meyer-Overton hypothesis.
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.
Liu
J
J
Department of Anesthesia, University of California, San Francisco 94143-0464.
Laster
M J
MJ
Taheri
S
S
Eger
E I
EI
Chortkoff
B
B
Halsey
M J
MJ
D000473:Alkanes / Q000097:blood / Q000493:pharmacokinetics* / Q000494:pharmacology*; D000818:Animals; D055598:Chemical Phenomena; D002627:Chemistry, Physical; D008297:Male; D000069463:Olive Oil; D010313:Partial Pressure; D010938:Plant Oils; D011650:Pulmonary Alveoli / Q000378:metabolism*; D051381:Rats; D017207:Rats, Sprague-Dawley; D012995:Solubility
D000473:Alkanes; D000069463:Olive Oil; D010938:Plant Oils