Changes in Oxygen Consumption and Heart Rate of Rats During Growth and Ageing: Role of the Thyroid Gland

Abstract

In rats, the lowest values of 02 consumption/unit metabolic size and heart rate/min. occur at birth, but their values climb to a max. before the animals are a month old, and decline thereafter to old age. The decline in the O2 consumption and heart rate observed during ageing in intact rats is not observed in thyroidectomized rats because the percentage fall in values of these functions due to thyroidectomy is greater in the young than in the old rat. The decline observed during ageing in normal rats is due in part at least to a diminished responsiveness of the O2 consumption and heart rate in the older animal to thyroxine. The 3-wk. old rat consumes significantly less O2 than do 2-wk. old ones, although the former weighs significantly more. Male rats have a significantly higher O2 consumption than females per unit metabolic size, that is, per kg. 0.75 body wt., but there are no significant differences per unit body wt. Also, males have a higher heart rate if correction is made for differences in body size. The increase in the product of the stroke volume and the arteriovenous O2 difference during growth is due chiefly to the increase in stroke volume, the arteriovenous O2 difference remaining relatively unchanged.