Indian Journal of Experimental Biology Vol. 39, Jan uary 2001 , pp. 29-34 Myosin ATPase in young and old mammalian myocardium: Responses to exercise in cold and thermo neutral environment Sharon Christopher & S Asha Devi* Department of Zoology, Bangalore Universit y, Bangalore 560 056, India Recei ved 15 February 2000; 8 November 2000 Resting le vels of blood lactate (La) were significantly lower at 35°C th an at 25°C in the trained (Tr) rats while untr ai ned (UTr ) rats maintained higher levels. Extent of decrease in blood gl ucose (Giu) in Tr rats was lower in cold (C) than in th ermo neutral (N) water in young but not in old. Training inC had no impact on young right and le ft ventricles ( RY a nd LV ). UTr old rats showed lo ss in LV Glu at both the temperatures. Old trainees had reduced RV Glu inC while no change was observed at N. Substrate changes were concomitant with altered myosin ATPase activity. Young ventricles showed hi gher ATPase activity in LV than in RV, and training inC evoked elevations in RV enzyme. The results suggest that training may enhance the functional and biochemical ac ti vi ty in terms of higher myosin ATPase activity in , as against a C environment, and the effect is better pronounced in LV. Anaerobic condition is felt more in C as seen through elevated blood La .. Physical exercise when performed in different environmental temperatures results in age-related regulation 1 of carbohydrate (CHO) utilization 2 and differential time frame for attainment of fatigue. Studies on rats have demonstrated age-dependent responses of blood CHO substrates to different types of exercise such as swimming and running 3 .4 . Further, low blood lactate (La ) levels were observed in trained female rats which swam at near body temperature (thermo neutral, 35°C) and these changes were in co- existence with significant changes in lactate dehydrogenase (LDH) isoenzymes in right ventricular myocardium 5 . The myocardial functional capacity varies in left and right ventricles (LV and RV) due to their differential anatomy in young and old Wistar rats and the responses of these regions do vary under conditions such as physical training when initiated in young, middle-age and old 6 . One of the factors that may influence swimmjng endurance and thereby trainability of animal models and humans in obtaining maximum benefits in terms of improved cardiac function even during later stages of life is medium temperature 7 . The primary objective of the present study is to investigate the effects of swim training the rats in cold (25°C) and at near body temperature (3 7°C) on resting levels of blood glucose and lactate. For this rats belonging to adult and old age groups were considered. *Correspondent author Phone: 91-080-321 4001/214 e-mail: asuba@blr. vs nl.net.in The secondary purpose of this study is to examine the effect of training on myosin ATPase activity of LV and RV as swimming is known to cause an improvement in cardiac muscle contractilit/ · 9 . In earlier studies the effects of training were eva lu ated by comparing the data procured from rats exercising e ith er by swimming or running 4 ' 5 at 35°C. Howe ver, untrained swimmers may face an inadequate oxygenation of working muscles due to reduced circulation. Hence, in the present study, the postulate that benefits derived from swim training are a function of age and swim temperature has been examined. Materials and Methods Chemicals- Lithium lactate, sodium pyruvate, NADH, ATP and tris were purchased from Sigma (MO, USA) . Standard albumin was obtained from PIERCE (Rockford, Illinois). All other biochemicals were of reagent grade. Animal care and training program- Pathogen- free Wistar albino rats (2 months- and 18 months-o ld ) were housed two to three in polypropylene cages of 30 x 22 x 14 em in a room maintained at 28° ± 1 °C, 28% R.H and with 12 hr L:D cycle. They had fr ee access to tap water and standard commercial diet (Lipton India, Calcutta). Animals were handled o nl y when cages were changed or during weekly weighing and during training. The rats were divided in to following 5 groups. (A) sedentary controls (SE-C), (B) trained swimmers at 25°C (T-25), (C) untrain ed