Differences in anesthetic potency between Sprague-Dawley and Long-Evans rats for isoflurane but not nitrous oxide

The mechanisms of action of halogenated (isoflurane) and inorganic (nitrous oxide (N2O)) anesthetic gases are postulated to differ. We tested the null hypothesis that there is no difference in anesthetic potency, as determined by MAC (the minimum alveolar anesthetic concentration preventing purposeful movement to supramaximal noxious stimulation in 50% of animals) between the Long-Evans (LE) and Sprague-Dawley (SD) rat strains for isoflurane and N2O. MAC values were determined using a 45-second standard supramaximal electrical stimulation. MAC of isoflurane for SD rats was 1.28 +/- 0.20 vol% (mean +/- standard deviation), but 0.98 +/- 0.14 vol% for LE rats (p = 0.001). MAC values for N2O were the same: 1.53 +/- 0.1 and 1.58 +/- 0.14 ata for SD and LE rats, respectively. The interstrain MAC difference for isoflurane, but not N2O, suggests different mechanisms of action for the two agents.