Am~ican ~jouknai, of Piiysiology

WEISFELDT, MYRON L., W. A. LOEVEN, AND N. W. SHOCK. Resting and active mechanical properties of trabeculae carneae from aged male rats. Am. J. Physiol. 220(6): 1921-1927. 1971.-Trabeculae carneae from the left ventricles of 1 lto 13and 26to 3 l-monthold male rats were examined in terms of the active and resting length-tension curves. No significant difference between the two groups was noted for the active length-tension curve. The resting length-tension curves showed that muscles from the older rats had significantly greater resting tension at lengths greater than 115 y. of Lo. Lo was defined as the shortest length with a resting tension above zero. The duration of contraction, time-to-peak tension, and relaxation time at maximum isometric tension were prolonged in muscles from the older group. Stress relaxation was studied using other trabeculae from rats of similar ages. Resting tension was found to decline linearly with the log of time from 0.3 to 240 set after a rapid stretch. Less stress relaxation was exhibited by muscles from 25to 29-month-old male rats than 1 lto 12-month-old rats. This age-associated alteration in stress relaxation accounts in part for the age-associated changes in the resting length-tension curve.