The Anesthetic Potencies of Alkanethiols for Rats: Relevance to Theories of Narcosis

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Abstract
Meyer and Overton suggested that anesthetic potency correlates inversely with lipophilicity. Thus, MAC times the olive oil/gas partition coefficient equals an approximately constant value of 1.82 ± 0.56 atm (mean ± sd). MAC is the minimum alveolar concentration of anesthetic required to eliminate movement in response to a noxious stimulus in 50% of subjects. Although MAC times the olive oil/gas partition coefficient also equals an approximately constant value for normal alkanols from methanol through octanol, the value (0.156 ± 0.072 atm) is 1/10th that found for conventional anesthetics. We hypothesized that substitution of sulfur for the oxygen in n-alkanols would decrease their saline/gas partition coefficients (i.e., decrease polarity) while sustaining lipid/gas partition coefficients. Further, we hypothesized that these changes would produce products of MAC times olive oil partition coefficients that approximate those of conventional anesthetics. To test these predictions, we measured MAC in rats, and saline and olive oil solubilities for the series H(CH2)nSH, comparing the results with the series H(CH2)nOH for compounds having three to six carbon atoms. As hypothesized, the alkanethiols had similar oil/gas partition coefficients, 1000-fold smaller saline gas partition coefficients, and MAC values 30 times greater than for comparable alkanols. Such findings are consistent with the notion that the greater potency of many alkanols (greater than would be predicted from conventional inhaled anesthetics and the Meyer-Overton hypothesis) results from their greater polarity. Implications The in vivo anesthetic potency of alkanols and alkanethiols correlates with their lipophilicity and hydrophilicity.