Cardiotoxic and Inotropic Effects of Ouabain on Atria Isolated From Rabbits of Different Ages

1 In sino-atrial node-right atrial preparations and left atrial preparations obtained from rabbits of different ages (2 to 360 days old), cardiotoxic and inotropic effects of ouabain were compared. Tachyarrhythmias were produced in immature and mature rabbit atria exposed for 6 to 8 min to ouabain (5 x 109(-6) M). The total number of contractions before the arrhythmias developed varied inversely with age (2 to 90 days old). Percentage reductions in the resting membrane potential, overshoot and maximum rate of rise induced by ouabain were greater in mature rabbit atria than in immature rabbit atria. 2 Reduction of Ca2+ in the bathing media suppressed the development of tachyarrhythmias. 3 In electrically-driven left atria isolated from immature and mature rabbits, similar frequency-contractile force curves were obtained, maximum contractions being attained at a frequency of 120/min. Ouabain at 2 x 10(-7) M shifted the curve upward and at 10(-6) M greatly potentiated the contractile force developed at low frequencies (6 to 30/min). Increase in contractions induced by ouabain relative to pre-drug concentrations at a frequency of 60/min were greater in immature rabbit atria (15 days old) but the positive inotropic effect of 2 x 10(-7) M ouabain relative to the effect seen at 10(-6) M was appreciably less. 4 It may be concluded that atria isolated from immature rabbits tolerate higher concentrations of ouabain than those isolated from mature rabbits.