Effects of age on mechanical and electrical performance of rat myocardium.

Left ventricular papillary muscles were isolated from male Fischer rats at 3, 6, 12, and 24 mo of age. Mechanical performance and transmembrane action potential characteristics were recorded simultaneously. Contractile protein enzyme activity was determined in the same hearts from which papillary muscles were used for acquisition of mechanical and electrical information. There was an age-related increase in the isometric time to peak tension and time to half relaxation with no age-related change in peak isometric developed tension. Resting tension showed a significant increase at both 12 and 24 mo of age. Although no significant change was noted in peak rate of tension rise (+T') or fall (-T'), there was a significant age-related prolongation in the time to +T' and -T'. Isotonically, peak shortening decreased in 24-mo-old animals. Time to peak shortening increased, whereas peak velocity of muscle shortening decreased from 3 to 24 mo of age. A significant and progressive prolongation was noted in the duration of the transmembrane action potential at 50% and 75% repolarization. In addition an age-related progressive decline was seen in contractile protein enzyme activity. These biochemical and electrophysiological alterations may help to explain the contractile modifications during aging.