Alterations in mechanical properties of rat papillary muscle during maturation.

Myocardial performance was measured during maturation using isolated left ventricular papillary muscles from female Wistar rats. Muscles were studied isometrically and isotonically, stimulated at 0.1 Hz, perfused with Tyrode solution having an external calcium concentration of 2.4 mM, and maintained at 30 degrees C. Heart and body weight increased rapidly during the first 6 mo and then stabilized. Peak isometric developed tension, time to peak isometric developed tension, and time to half relaxation increased substantially during the 1- to 10-mo period with little change in the peak rate of isometric tension rise and peak rate of isometric tension fall. No difference in resting tension was noted among any groups. Force-velocity curves from muscles from 2-, 6-, and 10-mo-old animals show a significant age-related decrease in the isotonic velocity of shortening by as much as 21% between 2 and 10 mo of age. Thus maturation profoundly affects the contractile state of cardiac muscle, with force of contraction increasing and velocity of shortening decreasing as a function of age in the rat.