Effects of age on metabolic responses to endurance training in rats ROBERT S. MAZZEO, GEORGE A. BROOKS, AND STEVEN M. HORVATH Institute of Environmental Stress, University of California, Santa Barbara 93106; and Exercise Physiology Laboratory, Department of Physical Education, University of California, Berkeley, California 94720 MAZZEO,ROBERTS.,GEORGE A. BROOKS,AND STEVEN M. HORVATH. Effects of age on metabolic responses to endurance training in rats. J. Appl. Physiol.: Respirat. Environ. Exercise Physiol. 57(5): 1369-1374, 1984.-The purpose of this investi- gation was to determine the age-relatedmetabolic responses to chronic exercise. Female Fischer 344 rats 3, 12, and 24 mo of age were trained for 8 wks by treadmill running at 75%of their maximal oxygen uptake (OO~,,,). Values of VoZmax were 74.5 t 3.1,63.3 t 3.9, and 54.6 t 2.3 ml l kg-l. min-’ in the untrained 3-, l2-, and 24-mo groups,respectively. In response to training, VO 2 max increased significantly (P < 0.05) by 11.9, 18.6, and 20.4% in the three groups. The 24-mo animals also demon- strated the greatest improvement in the treadmill speed elicit- ing VO2max (71%) when compared with the 12-mo(64%) and 3- mo (57%) agegroups.While large increases in endurancetimes were recorded for all trained groups (456, 806, and 324% in 3, 12, and 24 mo, respectively) relative to the controls, the older animals exhibited a diminished response, comparatively. Body weights did not differ between the trained (Tr) and sedentary controls for 3- and 12mo animals, but the 24-mo Tr rats demonstrated a more rapid decline in body weight with age than did the age-matched controls. While no differences in absoluteor relative heart weights existed for the 3- and 12-mo groupswith training, both of theseparametersin the 24-moTr animals were significantly greater than sedentary controls. It was concludedthat comparedwith younger animals the 24-mo group demonstrateda similar relative adaptation to training in terms of aerobic power but a diminished response compara- tively in endurance capacity. Further, the 24-mo animalsdem- onstrated the ability to better maintain functional myocardial mass into senescence than their age-matched controls. these elderly subjects for health and safety reasons do not train at as high relative intensity as do young indi- viduals. Alternatively, the diminished response to train- ing may be related to the aging process itself. The rat is the animal model most frequently used to study the process of aging, and a number of investigators have determined the traditional cardiovascular adapta- tions of young rats to endurance training (2, 5, 13, 14). Recent studies have demonstrated that animals of ad- vanced age are capable of adapting to physical training through myocardial alterations. Improvements in my- ocardial function (34), energy metabolism (34), and cor- onary blood flow (34), as well as a reduction in myocar- dial stiffness and contraction duration (32) have been reported. However, the adaptive response to endurance training in the older animal is not always of the same magnitude as seen in younger animals (8). The purpose of the present investigation was to determine and com- pare the metabolic responses to endurance training in rats 3, 12, and 24 months of age. exercise;aging; Fischer 344 rats; maximal oxygen consumption; myocardial mass Animals. Thirty-five virgin female Fischer 344 rats, obtained through the National Institute of Aging (Charles River Breeding Laboratories, Wilmington, MA), were received in one shipment at the ages of 3, 12, and 24 mo. All animals were individually housed in climate-controlled quarters (25°C 12-h light-dark pho- toperiod) and received Purina rodent chow and water ad libitum. A NUMBER OF CARDIOVASCULAR variables have been shown to decline with age, especially when the organism is challenged by physical stress. Included among these responses are decreases in maximal oxygen consumption (1, 11, 15, 28), coronary blood flow (34, 38), capillary/ fiber ratio (24, 35), cardiac output (18, 30), and mito- chondrial function (7, 21, 33, 37). The beneficial effects of endurance training on such cardiovascular functions have been thoroughly examined in relatively young pop- ulations (16 25, 27). However, the ability of individuals of advanced age to elicit similar training adaptations remains in question (12,26,29). The intensity of training may be one factor responsible for such differences as 0161-7567/84 $1.50 Copyright 0 1984 the American Physiological Society A4aximalO~ consumption and training. To train all age groups at the same relative intensity a Vozmax test was performed on all animals prior to training. Maximal O2 consumption was measured using the rapid flow open- circuit system described previously by Brooks and White (5). Animals were placed in a Plexiglas metabolism chamber that allowed a unidirectional flow of gas. Am- bient air was drawn through the chamber at 5,800 ml. min-’ (STPD). One hundred and twenty-five milliliters per minute of gas leaving the chamber was dried by CaS04 and then continuously analyzed by Beckman LB- 2 and Applied Electrochemistry S-3A O2 analyzers. Both analyzers were on-line with a Mint 11 computer (Digital) allowing for continuous calculation of 02 consumption (VOW), COa production (VCO~), and the respiratory ex- METHODS 1369