Studies of sarcoplasmic reticulum function and contraction duration in young adult and aged rat myocardium

Kinetic measurements of Ca⁺² uptake in microsomal fractions from rat myocardium demonstrated significantly lower rates of oxalate-facilitated accumulation in preparations from aged (24 to 25 month) hearts as compared to those from young adult (6 to 8 month) hearts. The per cent decline in transport activity in microsomes from aged hearts varied with Ca²⁺ concentration decreasing from 57% at 0.33 μm Ca²⁺ to 24% at 1.21 μm Ca²⁺. Double-reciprocal plots of the dependence of the velocity of accumulation on Ca²⁺ concentration showed upward curvature in both age groups indicating the presence of multiple Ca²⁺ binding sites. Mechanical studies using muscles isolated from the same hearts used to prepare sarcoplasmic reticulum demonstrated prolonged contraction duration in aged myocardium in agreement with previous findings. The lower in vitro rates of Ca²⁺ accumulation in aged microsomes suggest a possible biochemical mechanism to account for the observed increase in the time-course of cardiac relaxation.