Evidence to suggest that rat growth hormone is modified when secreted by the pituitary gland

The relationships were examined between the concentrations of GH determined by bioassay (rat tibia test) and radioimmunoassay in homogenates of rat adenohypophysial tissue and in the incubation medium in which the tissue had been maintained. Adenohypophyses from young male rats were incubated in Medium 199 with or without a putative synthetic GH-releasing factor (GRF) to achieve a dynamic state of GH secretion, but the GRF did not consistently stimulate the release of GH as measured by either bioassay or radioimmunoassay. Both assays gave closely similar values for the concentration of GH in homogenates of the adenohypophyses of undisturbed rats. When the two assays were used to measure the concentrations of GH in incubated pituitary tissue, the values showed no correlation, although the concentrations of GH in the incubation medium were again highly correlated. However, the bioassay : radioimmunoassay ratio for the secreted hormone [2.14 +/- 0.09 (S.E.M.)] was significantly higher than that for the intrapituitary form from the unincubated gland (0.93 +/- 0.06). By radioimmunoassay, secreted rat GH was found to be less stable than purified rat GH when the two forms were incubated in vitro with slices of various rat tissues. Similarly, the rate of clearance of secreted rat GH from the plasma of hypophysectomized rats, as determined by radioimmunoassay, was much faster than that of the purified GH. The biological activity of purified bovine GH was blocked by aprotinin, an inhibitor of proteolysis, but the drug had no inhibitory effect on the bioactivity of rat GH secreted into the incubation medium. Overall, these results indicate that secreted rat GH has properties intermediate between those of the intrapituitary form and the form found in the circulation.