Postnatal Changes in Rat Ventricular Function

Experiments were designed to assess the intrinsic changes in myocardial function with age and to determine the role of increasing left ventricular mass in this process. Isometric ventricular pressure-volume curves and length-tension curves were measured in isolated perfused rat hearts of different ages and masses. Incremental tension, peak active tension, and peak passive tension were less for 10-day-old hearts than they were for older hearts subjected to proportionately equal increases in length. At resting length, peak active tension developed by hearts at least 16 days old was 78% higher than that developed by 10-day-old hearts, and peak passive tension was 133% higher. There was no significant difference between 16-day-old rats and older rats. Thus, by the sixteenth postnatal day, the heart had developed functional maturity. These functional changes appeared to be related to age rather than to left ventricular mass. Histological studies of the left ventricle showed that, at birth, the myofibrils were disoriented with a high ratio of nonmyofibrillar elements to myofibrillar elements. Within 16-20 days after birth, the structural organization and the ratio of nonmyofibrillar elements to myofibrillar elements were similar to the organization and the ratio found in an adult. Thus a direct relationship existed between the functional properties and the anatomical development of the myocardium. Mass per se was not the sole factor that contributed to improved myocardial function with age.