Modification of training-induced responses by repeated norepinephrine injections in rats

Rats were subjected to daily (5 times/wk) running exercises, norepinephrine injections (0.5 mg/kg sc), or both for 9 wk; one rat group served as the control. Norepinephrine injections were used to potentiate the effect of the endogenous amine, the release of which is known to be stimulated by exercise. Changes observable in all treated rat groups included reduced weight gain, reduced food intake, decreased amount of epididymal fat, hypertrophy of the heart and adrenals, and increased tail skin temperature response to isoproterenol (0.2 mg/kg sc). The training-specific changes (increased activity of oxidative muscle enzymes) were not potentiated by training under the influence of exogenous norepinephrine administration. The changes produced by repeated norepinephrine injections alone (hypertrophy of the brown adipose tissue and increased colonic temperature response to isoproterenol) failed to develop if the rats underwent physical training in addition to drug treatment. Lower tail skin temperature in a cool environment (15 degrees C) was associated with norepinephrine treatment, whether or not there was simultaneous training. These results suggest that the chronic norepinephrine treatment-physical training combination led to complex adaptive changes that could not be systematically associated with either norepinephrine treatment or physical training alone.