Acute effects of hypophysectomy and administration of pancreatic and thyroid hormones on circulating concentrations of somatomedin-C in young chickens: relationship between growth hormone and somatomedin-C.

The short-term control of plasma concentrations of somatomedin C (SmC) in young chicks was examined by either surgical removal of the pituitary gland or by the administration of hormones which affect plasma concentrations of growth hormone (GH). As expected, removal of the source of GH by hypophysectomy reduced plasma concentration of GH, these being suppressed by 95.7% within 1 hour. Hypophysectomy was rapidly followed by reductions in the plasma concentration of SmC. For instance, plasma concentrations of SmC were decreased to 53% of pretreatment one hour following hypophysectomy. This suggests both that SmC has a short half life and that the release of SmC into the circulation is tightly coupled to the presence of pituitary hormone(s), presumably including GH. Sham surgery also decreased plasma concentrations of GH but were without effect on plasma concentrations of SmC. The short term control of plasma concentrations of SmC was also examined by the acute administration of hormones, which affect GH secretion in vivo. Injections of thyroxine or triiodothyronine decreased the plasma concentration of GH but were without effect on the plasma concentration of SmC. On the other hand, the administration of either glucagon or insulin decreased the plasma concentration of both GH and SmC. The present data suggest that plasma concentrations of SmC do not simply reflect the GH status in young chickens.