The cardiotoxic effect of
ouabain was explored in rabbits at different ages. The dose of
ouabain necessary to produce
ventricular tachycardia and death in 1-to 4-week-old rabbits was significantly greater than that required in older animals. In the 1 to 3-week-old animals,
ventricular fibrillation occurred only once whereas in animals older than 3 weeks, fibrillation occurred in 23 of 31 animals. A similar pattern of sensitivity to
ouabain was observed in bilaterally vagotomized animals. Neither the initial heart rate nor the
potassium content of the ventricles in the different age groups appeared to have any influence on the change in the cardiotoxic dose of
ouabain. Heart weight increased during the interval when the occurrence of fibrillation increased but it may not fully explain the alteration in the mode of death since fibrillation occurred in some of the younger animals with bilateral
vagotomy before the heart weight increased. The
catecholamine content of ventricles from 4-to 12-week-old rabbits was significantly greater than that in 1- or 3-week-old rabbits. The age-dependent decrease in the dose of
ouabain necessary to produce
ventricular tachycardia,
ventricular fibrillation and death appeared to be associated with an increase oof ventricular
norepinephrine content, although other factors may also play an important part in this response.