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Left-ventricular heart muscle and pectoralis major muscle of the rat were studied to determine the intracellular localization of lactic dehydrogenase (LDH) isoenzymes. Fixation of tissue for 2 hr in 2% buffered formaldehyde provided the best preservation of the ultrastructure and enzyme activity. Total LDH activity was found diffusely in the ground substance of the sarcoplasm and in the mitochondria of the heart muscle. In skeletal muscle a strong reaction was noted in the sarcoplasmic reticulum, and moderate activity was seen in the ground substance of the sarcoplasm and in the mitochondria. Differentiation of the isoenzymes of LDH was accomplished by addition of 4 M urea or application of heat. Heart-type isoenzymes were mainly localized in the mitochondria and sarcoplasm, whereas muscle-type isoenzymes were localized mainly in the sarcoplasmic reticulum of the skeletal muscle. It is speculated that the sarcoplasmic reticulum of the skeletal muscle is the site of anaerobic glycolysis and that the sarcoplasm and mitochondria are involved primarily in aerobic metabolism of pyruvate.

Employing an isolated perfused rat heart preparation, we investigated the contribution of anaerobic metabolic energy to the performance, recoverability, and ultrastructure of the heart perfused at 32 degrees C in 5% albumin in Krebs-Ringer Bicarbonate solution. During exposure to anoxia for 30 min, inclusion in the perfusate of the anaerobic substrate, glucose, resulted in marked improvement in electrical and mechanical performance of the heart and in enhanced recovery during the subsequent period of reoxygenation. Lactate production was fivefold greater in the glucose-supported anoxic heart than in the anoxic heart without glucose. Electron microscope sections of the hearts exposed to anoxia in the absence of glucose revealed alterations in mitochondrial morphology and dilatation of the longitudinal tubules. These morphologic changes during anoxia were averted by inclusion of glucose in the perfusion fluid. The data are consistent with the hypothesis that anaerobic energy generation plays a significant role in preserving myocardial function and structure and in promoting recoverability of the anoxic mammalian heart.