The rat heart demonstrates marked alterations in its responses to ouabain and increased frequencies of stimulation and in the duration of its action potential during the initial 21 days of life. At an age of 6.2 days 5 times 10- minus 5M ouabain produced a 158.2% increase in dP/dt compared with a 17.2% increase at 21.1 days (P less than 0.001). At 6.2 days dP/dt increased by 53.4% when the heart rate was accelerated from 30 to 90 beats/min compared with an increase of 12.2% at 21.1 days (P less than 0.005). The positive glycoside and staircase responses at the younger age were virtually eliminated when the hearts were perfused with a solution containing 50% [Na+]o and 25% [Ca-2+]o ([Ca-2+]o/[Na+]o-2 maintained constant). The duration of the ventricular action potential progressively decreased from 350-400 msec at birth to 100-150 msec at 21 days of life. This decrease was due to a shortening and a decrease in the potential level of the plateau phase. The prominent plateau typical of the early neonatal period was significantly diminished by perfusion with 50% [Na+]o. The results suggest that Na+ flux through a slow membrane channel plays a significant role in the positive staircase and glycoside responses of the early neonatal rat heart. As the heart matures and becomes functionally anomalous relative to other mammalian species, the slow channel progressively closes.

Langer, G. A., Brady, A. J., Tan, S. T., & Serena, D. (1975). Correlation of the glycoside response, the force staircase, and the action potential configuration in the neonatal rat heart. Circulation Research, 36(6), 744–752.