Postnatal changes in the inotropic effect of ouabain on the rat heart ventricle

The concentration-dependent effects of ouabain on the contractility of postnatally developing rat heart ventricles were studied. Ouabain caused a positive inotropic effect in right ventricular strips of neonatal rats up to the age of about 30 days but a negative inotropic effect in the adult cardiac tissue. When extracellular Ca concentration was lowered from 2.5 to 1.0 mmol/l and the rate of stimulation was simultaneously elevated from 0.2 to 1.0 Hz a clear positive inotropic effect was also generated in the adult rat heart. The positive inotropic effect of ouabain showed a biphasic developmental pattern: the contractile force first grew from birth to about 15 days of age but steeply declined near to the adult level during the 3rd postnatal week. The force response to ouabain occurred within two distinct dose-ranges. In the newborn only the high-dose (above 3 X 10(-6) mol/l) effect was seen but in rats older than 5 days a mixed low-dose (below 3 X 10(-6) mole/l)/high-dose effect was apparent. In both ranges the positive inotropic effect of ouabain seemed to be dependent on caffeine sensitive Ca store, perhaps the sarcoplasmic reticulum. It is suggested that during the 3rd postnatal week a transition from extracellular to intracellular Ca stores occurs in the rat heart, which is reflected as a changing inotropic effect of ouabain on the developing cardiac tissue.