Alteration of the intrinsic heart rate and autonomic nervous tone during the growing process of rats and pigs.

Abstract

Alterations of the heart rate with and without autonomic blockade were traced through the growing period in rats and pigs. The heart rates following the atropine and/or propranolol administration were measured to estimate the autonomic nervous tone. The heart rate per body weight was calculated to make a comparison between rats and piglets. The heart rate in rats under autonomic blockade (a combined injection of atropine and propranolol) increased progressively during 2 to 4 weeks of age and thereafter decreased gradually with advancing age. On the other hand, the heart rate in piglets under the same treatment decreased gradually with age tracing a similar changing pattern as of the control heart rate. The parasympathetic tone of rats was low at 2 weeks of age and became higher during 2 to 4 weeks of age, whereas that of piglets at 2 weeks of age was at almost the same high level as other weeks of age. The sympathetic tone of piglets as well as rats was highest at 2 weeks of age. In piglets, the net autonomic tone, expressed as a sum of the sympathetic and the parasympathetic tones, was maintained at the sympathetic side through the experimental period. Although the net autonomic tone of rats was also in the sympathetic side at 2 weeks of age, it inverted to the parasympathetic side later on.