1 We have redetermined the anaesthetic potencies (EC50 s) for a series of primary alkanols, to resolve uncertainties about the molecular dimensions of the anaesthetic site resulting from the use of data from different laboratories. 2 For each alkanol, concentration-response relationships for loss of righting reflex (LRR) were plotted for over one hundred tadpoles, and the median effective concentrations determined. Aqueous concentrations present during potency assays were determined independently, and for alkanols with chain length greater than nonanol, correction was made for depletion from the aqueous phase.3 The EC50 s were found to decrease logarithmically with increasing number of carbon atoms in the hydrocarbon chain of the alkanol (CN), such that, on average, each additional methylene group was associated with an approximately four fold increase in potency. 4 The relationship between log EC50 and CN was best described by the quadratic equation, log EC o = 0.022 (+0.0038) CN2 + 0.76 (±0.051) CN + 3.7 (±0.14) (r2 = 0.9951). 5A previously described correlation between the apparent changes in the free energy of binding of an additional methylene group both to luciferase and to the sites for LRR in tadpoles was not confirmed. 6 A cut-off in potency beyond dodecanol was established in experiments where tadpoles were maintained in supersaturated solutions of tridecanol for 20 h without demonstrable LRR. 7 These findings indicate that the soluble enzyme firefly luciferase does not adequately model the anaesthetic site. Specifically, there are discrepancies in the position of cut-off, and the apparent changes in the free energy of binding, per methylene group, of an alkanol to luciferase do not parallel that for tadpoles.

No abstract

While many theories of general anesthesia postulate a lipid site of action, there has been no adequate explanation for the lack of anesthetic potency of the highly hydrophobic primary alkanols with more than 12 carbons (the cut-off). Some work suggests that these nonanesthetic alcohols do not dissolve in membranes. Other work contradicts this and suggests that an anesthetic site on a protein provides a better explanation. Here we show that both the anesthetic dodecanol and the nonanesthetic tetradecanol are taken up equally well into the tissues of animals and into isolated postsynaptic membranes. When a group of Rana pipiens tadpoles were treated with dodecanol, half were anesthetized by 4.7 /AM (free aqueous concentration), and the corresponding concentration in the tissues was found to be 0.4 mmol per kg wet weight. Prolonged exposure (92 hr) to tetradecanol produced even higher tissue concentrations (0.7 mmol per kg wet weight), yet no anesthetic effects were observed. Furthermore, general anesthetics are thought to act on postsynaptic membranes but both alkanols partitioned into postsynaptic membranes from Torpedo electroplaques. The spin label, 12-doxyl stearate, was incorporated into these membranes. The lipid order parameter it reported was decreased by the anesthetic alcohols (octanol, decanol, and dodecanol), whereas the nonanesthetic alcohols either did not change it significantly (tetradecanol) or actually increased it (hexadecanol and octadecanol). Thus, although lipid solubility is unable to account for the pharmacology of the cut-off in potency of the long-chain alcohols, lipid perturbations provide an accurate description.