Post-natal developmental changes in myocardial contractility of the female Wistar rat.

The intrinsic contractile properties of papillary muscles isolated from female Wistar rats during post-natal development were examined. In addition, the myocardial responsiveness to certain inotropic interventions was studied. In muscles from 60-day-old animals, the velocity of shortening was markedly greater than muscles from animals greater than or equal to 180 days old. These findings occurred in the presence and absence of added norepinephrine or calcium and over a wide range of contraction frequency. No major age-related differences in responsiveness to the various inotropic agents was observed. Isometric tension was significantly higher in muscles from animals greater than or equal to 180 days of age when compared to 60-day-old rats. Thus, post-natal development profoundly affects the intrinsic contractile state of cardiac muscle, with the force of contraction increasing and the speed of shortening decreasing as animals age.