Is swimming exercise or cold exposure for rats?

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Abstract
Rats were trained by daily swimming sessions (up to 3 h per day) for at least 6 weeks in water at30, 36 and 38 oC. After this training, the adaptive changes obtained were compared with those typical of cold‐acclimated (cold‐specific changes) and running‐trained (training‐specific changes) rats. The most typical training‐specific change, an increased activity of oxidative muscle enzymes was negligible for swimming‐trained rats, while the lowered activity of muscle lactate dehydrogenase was evident for all trained groups. Cold‐specific changes, such as increased food intake, increased calorigenic response to injected noradrenaline, an increase both in mass and metabolic capacity of brown adipose tissue, and maintenance of the stores of ascorbic acid and muscle glycogen during cold exposure, were observed for rats trained at 36 and 30 oC. The cold tolerance test in cold air did not make any distinct difference between the rats trained at different water temperatures, while in cool water the 30 oC ‐swimmers were clearly superior to other groups, that is, their cooling rate was slowest. Other adaptive changes were found, to a variable extent, for all trained groups. These included loss of body fat, cardiac hypertrophy, reduced urinary catecholamine excretion after test swimmings either in cold or warm water, increased tail‐skin temperature response to isoprenaline, and a higher tail‐skin temperature in response to cold. Generally, however, the adaptive changes observed for 30‐ and 36 oC‐swimmers were similar, while the changes observed for 38 oC‐swimmers were different. The latter group neither displayed any cardiac enlargement nor any cold‐specific changes. The present results support the conclusion that the adaptive changes produced by swimming‐training in rats, more closely resemble those produced by cold‐acclimation rather than those resulting from running exercise.