Tissue lactic dehydrogenase isozymes: variation in rats during prolonged cold exposure

¹ Pharmacology and Biochemistry Laboratories, U.S. Army Research Institute of Environmental Medicine, Natick, Massachusetts

Total lactic dehydrogenase (LDH) activity as well as the relative distribution of heart- and muscle-type subunits was assayed in tissues of rats chronically exposed to cold (6 weeks, 5 C). A depression of LDH activity was noted in heart and liver following 1 week at 5 C; restoration to normal levels occurred in weeks 2, 3, and 4 which persisted for heart tissue; liver values fell below normal limits for the final 2 weeks of study. No significant changes in total activity or in subunit distribution were seen in kidney, muscle, plasma, or red blood cells. A progressive increase in the ratio of heart to muscle type was noted in heart muscle with a final level almost twice that of the control value. The changes in total activity as well as the relative distribution of the subunits may be related to substrate availability, tissue oxygen supply, or hormone action; the study did not permit assessment of any of these parameters as the causative factors. The adaptive change observed in heart muscle affords a possible example of the modification of genic expression by environmental stress.

Key Words: variation in tissue enzymes • adaptation to cold • modification of genic expression

Submitted on March 24, 1965