Local administration of human growth hormone in vivo to the cartilage growth plate of the proximal tibia of hypophysectomized rats resulted in accelerated longitudinal bone growth. This finding suggests that growth hormone directly stimulates the cells in the growth plate, and does not support the theory that the increase in the plasma concentration of somatomedin that follows growth hormone administration is the cause of this stimulation.

To explore the short term effects of endogenous GH on body growth, glucose metabolism, and insulin responsiveness in adipose tissue, 35- to 40-day-old male rats were treated with a potent goat antiserum against rat GH (ArGHS). The administration of ArGHS, but not normal goat serum, caused a dose-dependent decrease in body weight gain and longitudinal bone growth, as measured by the tetracycline method. Glucose metabolism was measured by determining the production of CO2 from [14C]glucose in epididymal fat pad. Treatment with ArGHS (0.05-0.5 ml, ip, twice daily for 6 days) resulted in increased insulin sensitivity, as determined by the ED50 values for the effect of insulin. An increased response to a submaximal concentration of insulin was observed 3 h after the administration of 0.5 ml ArGHS. Basal levels were not consistently affected by ArGHS treatment. The maximal response to insulin was significantly increased after treatment with low doses of ArGHS (0.1-0.2 ml/day) and was decreased after treatment with high doses of ArGHS (0.8-1 ml/day) for 6 days. The magnitude of the response, as determined by the percent increase in response to 10 mU/ml insulin, was, however, not different compared to that observed in adipose tissue of normal goat serum-treated rats. These results demonstrate that elimination of endogenous GH results in retarded growth in the rat within 24 h. Moreover, the results suggest that GH is important in insulin sensitivity.

The effects of insulin treatment and dietary glucose on the responsiveness of adipose tissue to insulin and GH after hypophysectomy were studied. Male rats, 130-150 g, were hypophysectomized. Glucose metabolism was measured by determining the production of CO2 from [14C]glucose and the incorporation of glucose into lipids in the epididymal fat pad. Basal levels of glucose oxidation as well as the response to insulin were markedly decreased 7 days after hypophysectomy. In hypophysectomized animals given drinking water containing 10% glucose, insulin responsiveness was partially restored, and an enhanced response to the insulin-like effect of GH was observed. Plasma insulin levels decreased after hypophysectomy. Additional glucose caused a significant increase in plasma insulin levels, but these levels were still lower than those in sham-operated animals. To examine the possibility that endogenous insulin levels are important for the capacity of adipose tissue to metabolize glucose and respond to insulin and GH, hypophysectomized rats were injected with different, progressively increasing doses of insulin for 7 days, beginning on the day after the operation. Basal levels of glucose oxidation were decreased in hypophysectomized control animals and gradually increased in a dose-dependent manner in insulin-treated animals. Basal levels were normalized when the total dose of insulin injected was 16.5 U. In these animals, the response to insulin was enhanced, and there was an increase in the magnitude of the response to GH. Similar results were obtained when glucose incorporation into lipids was determined. The decrease in basal and insulin-stimulated glucose oxidation levels after hypophysectomy were most pronounced when measured at a high glucose concentration (50 mM), when glucose transport is not rate limiting. The results indicate that the changes in glucose metabolism and hormonal responsiveness of adipose tissue after hypophysectomy are, at least in part, dependent upon the decrease in endogenous insulin levels.

The ability of GH to regulate its own receptor in adipocytes of hormone-substituted hypophysectomized rats was studied. Male rats (130-150 g) were hypophysectomized and substituted with T4 and cortisone. A fixed dose of GH (bovine GH, human GH, or ovine GH), was administered for 5-6 days in three different ways: 1) two injections/day, 2) four injections/day, or 3) by osmotic minipumps. GH binding was measured in cell aliquots using [125I]human GH. In unsubstituted hypophysectomized animals, GH binding was decreased and was approximately 25% of the binding observed in adipocytes of normal rats. With T4 and cortisone replacement, GH binding was partially restored. When GH was administered in four daily injections or via osmotic minipumps, a further increase in GH binding was observed. This increase was observed if the animals were killed up to 6 h but not 12 h after the last GH injection. GH given in two daily injections had no effect on GH binding even when studied at different time periods after the last GH injection. The GH receptor induced by frequent or continuous administration of GH was mainly somatogenic, since an excess of unlabeled ovine PRL inhibited GH binding only to a minor extent. There was no difference in accumulated body weight gain between the GH-treated groups during the treatment period. The results show that GH regulates its own receptor in adipocytes and that the mode of administration of the hormone is of importance for this effect of GH.

Growth hormone (GH) binding and the effect of GH and insulin on glucose metabolism in rat adipocytes were studied at various time periods following hypophysectomy. Male rats were hypophysectomized at 33-34 days of age. After 6 h, 20 h or 3, 7 and 14 days adipocytes were prepared from epididymal fat pads by mild collagenase digestion (0.5 mg X ml-1, 60 min, 37 degrees C). Glucose metabolism was studied by determining the production of CO2 from [14C]glucose and the incorporation of [14C]glucose into lipids. GH binding was measured in cell aliquots using [125I]hGH. No difference in GH binding to adipocytes was observed between control rats and rats hypophysectomized or sham-operated 6 h earlier. GH binding was significantly decreased 20 h after hypophysectomy and declined further with time after hypophysectomy. Adipose tissue from normal rats is usually refractory to the insulin-like effect of GH. Adipocytes isolated from normal rats were, however, usually responsive to GH immediately after cell isolation, suggesting that refractoriness to the insulin-like effect of GH was lost during the time required for the preparation of adipocytes. The magnitude of the response to GH in adipocytes progressively declined with time after hypophysectomy. The decreased responsiveness to GH with time after hypophysectomy parallelled the decrease in GH binding. The results suggest that the pituitary, directly or indirectly, is necessary for the maintenance of GH binding sites in adipose tissue and that these binding sites are related to the insulin-like effect of GH.