Effect of repeated exercise on postnatal development of the noradrenaline content of the albino rat heart, spleen and skeletal muscle.

The authors studied the effect of repeated elevation of sympathetic activity on the postnatal development of the noradrenaline content of tissues of the albino rat. Between the ages of 15 and 29 days, young rats were forced to swim in water heated to 25 degrees C, 3 X 30 min on weekdays and 1 X 30 min on Saturdays and Sundays. At 30, 45 and 65 days, the noradrenaline content of the tissues was determined spectrofluorometricaLly by the trihydroxyindole method. The noradrenaline content of the heart of trained rats was higher than in the controls in all the given age groups and the size of the absolute difference rose with advancing age. The noradrenaline content of the spleen developed similarly. Repeated exercise did not lead to an increase in the noradrenaline content of skeletal muscle. The results show that the repeated elevation of the activity of sympathetic adrenergic neurones which occurs in young rats during exercise is a long-term factor stimulating the development of sympathetic innervation of the heart and spleen. The development of the neurones innervating skeletal muscle was not stimulated, probably because the activity of these neurones is not increased by stress.