Ouabain-induced changes in electrophysiologic properties of neonatal, young and adult canine cardiac Purkinje Fibers.

Young patients frequently require higher concentrations of digitalis per kilogram of body weight or per square meter of surface area than adults to induce digitalis effect or toxicity. In order to investigate the electrophysiologic basis for this observation, we used standard microelectrode techniques to study the effects of ouabain on action potential characteristics of cardiac Purkinje fibers of neonatal (1-7 day), 4 to 7 week and adult dogs. Purkinje fibers were stimulated at a 500-msec cycle length. Action potentials were recorded during control Tyrode's perfusion and 23.5 minutes after onset of superfusion with ouabain, 2 X 10(-7) M (10% ouabain-3H). The Purkinje fibers were then assayed for ouabain uptake. Ouabain accelerated repolarization and decreased action potential amplitude, resting membrane potential and maximum upstroke velocity of phase O. Changes were greatest in the adults, least in the neonates. Ouabain increased the slope of phase 4 depolarization in 10 to 12 adults, 7 of 16- 4 to 7-week puppies and 0 of 12 neonates. Hence, ouabain induced lesser changes in the action potentials of Purkinje fibers taken from young animals. These results suggest that Purkinje fiber sensitivity to ouabain effects increases with age.