Changes in Serum Growth Hormone Levels from Fetal to Adult Age in the Rat 1

Abstract

Summary

The concentration of immunoreactive growth hormone (GH) in the serum of rats increased significantly from the 19th to the 21st day of fetal life, began to decrease on the last day of gestation, reached minimum values on the 20th day of extrauterine life and increased again after the pups were weaned. The relative rate of weight gain was greatest between the 19th and the 20th day of fetal life. After this time, it declined progressively, except for a brief period between the 5th and 10th day after birth. Thus, this parameter of growth followed a course that was more closely parallel to previously published changes in the level of serum insulin than in the changes in serum GH observed in these experiments. It is possible that, in fetal and young rats, growth may depend upon a combined action of insulin and GH in which the latter, rather than the former, plays the role of permissive factor.

Addendum. Since this manuscript was submitted for publication, we found that the oral administration of glucose (2 g/kg) or of a 10-amino acids mixture (41) (1 g/kg) had no effect on the serum growth hormone levels of 19-day-old rat fetuses and of 1-hr, 5-, 10-, 20-, 30-, and 45-day-old pups. These results support the notion that, in the rat, the release of growth hormone does not respond to stimuli that are effective in other species (42, 43). We wish to call the reader's attention to a similar study published very recently (44).