Effect of the early postnatal administration of cortisol on development of the nervous control of cardiac function in albino rats.

When infant rats were treated with cortisol, in daily s.c. doses of 20 mg X kg-1, between the ages of 2 and 15 days, the noradrenaline content of their heart and spleen, between the ages of 23 and 65 days, was lower than in the controls. The decrease in the noradrenaline content did not diminish with advancing age; on the contrary, it was the most pronounced at 65 days. Cortisol treatment did not affect the noradrenaline content of skeletal muscles. The functional significance of the decrease in the noradrenaline content was studied in nervous control of the sinoatrial node of the heart. In agreement with the drop in noradrenaline concentration, transmural stimulation of the sinoatrial node region of isolated atria led to a mild, but statistically significant reduction of the function of sympathetic nerve endings, whereas parasympathetic innervation of the node showed no signs of impairment. This peripheral functional deficiency of sympathetic innervation of the node is not seen in the intact organism, where it is masked by central nervous mechanisms. Rats given cortisol postnatally had a significantly higher heart rate at 23, 33 and 44 days, because, in the presence of normal sympathetic influence, the tone of the parasympathetic nerves was reduced. The heart rate was highest at 23 days; with advancing age the difference diminished and at 65 days it was statistically nonsignificant.