Lactate dehydrogenase subunit activities in hearts of aging rats

Diets with 40% of calories as fat with poly-unsaturated to saturated ratios of either 0.2 beef tallow (BT) diet or 1.0 mix (M) diet were fed to rats of both sexes. Groups were killed at 9, 12, 15, 18, and 21 mo. of age. To estimate the effects of age, sex and dietary fat on lactate dehydrogenase (LDH), activities of muscle type and heart type subunits were assayed biochemically in heart homogenates using differential substrate concentrations of 1.0 10−2 M and 3 10−4 M pyruvate, respectively. For comparison of methods, LDH activity was also determined histochemically in frozen sections of hearts, using lactate as a substrate. Age significantly (p < 0.05) affected heart type activity, which was highest in males at 9 mo. and females at 12 mo., and lowest in 15 mo. old rats. Activities of heart type LDH were similar in hearts from rats at 9 and 21 mo. of age. Age also significantly (p < 0.05) affected muscle type activity, which generally increased to a maximum in 18 mo. old rats. Histochemical LDH activity was significantly (p <.001) greater in hearts of 9 and 12 mo. old rats than in those of 15 mo. old rats. Fluctuations in the activities of muscle type and heart type LDH subunits seemed to follow a pattern indicative of myocardial damage.