Ouabain-induced inotropism of isolated newborn and adult rabbit myocardium.

Relative sensitivities tp ouabain of atrial and ventricular muscles from 1-week and 4-month rabbits were compared in vitro. Ventricular muscles were also studied with prior treatment that minimized unphysiological, stimulation-induced release of norepinephrine (propanolol and/or reserpinization). Atrial and pretreated ventricular muscles from the newborn showed (a) higher sensitivity to ouabain and (b) the same "relative' positive inotropic effect (RPIE) when compared to those of the adult (RPIE is ouabain-induced inotropism relative to the maximal calcium-induced inotropism and represents ouabain-specific-inotropism). With no prior treatment, ventricular muscles from the newborn showed a nonsignificant tendency toward a lower sensitivity to ouabain and a significantly lower RPIE than those of the adult. These results suggest that under normal physiological conditions, the newborn myocardium may be more sensitive to ouabain than that of the adult.