Testing for circadian differences in lethality for intravenous ouabain in male mice.

Using a randomized, balanced design and double-blind methodology, ouabain octahydrate was administered intravenously to male mice at six clock times. Eight runs were conducted using six constant dosage levels. All the dose-response curves at the clock times of 02:30, 06:30, 10:30, 14:30, 18:30 and 22:30 were parallel and no significant differences were noted between the respective LD50 determinations using nomograph methods. Independent chi-square analysis of all lethality data indicated no significant variation in response between clock times and between runs but a very highly significant difference between doses. Using regression methods, onset time for death was shown to vary inversely with log-dosage, but those periods of possible increased susceptibility could not be correlated with a shortened time to death. These findings are consistent with a random variation in lethality in regard to clock time rather than a true circadian pattern. The pooled (N = 576) intravenous LD50 for ouabain octahydrate was 3.75 mg/kg with 95% confidence limits of 3.60-3.90 mg/kg or, when calculated as anhydrous ouabain, 3.01 (2.89-3.13) mg/kg.