Effects of Growth Hormone on Adipose Tissue of Weanling Rats*

Epididymal and parametrial adipose tissues were exised from weanling rats, aged 18–35 days, and tested for responsiveness to GH in vitro. Segments of epididymal fat from rats of the Charles River strain, ranging from 20–32 days of age, responded to GH with an increase in glucose utilization. Tissues from younger rats (18 days old) or older rats (33–35 days old) were unresponsive. Similar results were obtained with rats of the Fisher strain, except that responsiveness of epididymal fat to GH terminated before day 29, while responsiveness of parametrial fat persisted through day 30. Stimulation of glucose metabolism was not always accompanied by increased leucine oxidation, and no antilipolytic effect of GH was ever seen. The insulinlike response, although limited to glucose utilization, was otherwise similar to insulin-like responses seen in adipose tissue of older hypophysectomized rats; it persisted for only 1–2 h and was followed by a period of refractoriness. The minimal concentration of GH needed to produce an insulin-like response was 100 ng/ml, a concentration found within the range of values in serum collected from these rats at the time that they Were killed. The minimum concentration needed for induction of refractoriness was 300 ng/ml, a value obtained in only two of the rats and considerably in excess of that found necessary to induce refractoriness in tissues of older rats. GH increased lipoly$is without the usual lag period in tissues of rats younger than 25 days, and after a lag period of 1 h in rats aged 26– 30 days. The results indicate that GH may produce a different pattern of metabolic responses in weanling rats than in older animals.