Eugenio E. Muller

Growth hormone-releasing activity in the hypothalamus and plasma of rats subjected to stress.

Summary Experiments were performed in the rat to determine whether the increased secretion of growth hormone (GH) during stress involves augmentation of synthesis and/or release of hypothalamlc GH-releasing factor (GRF). Two different stimuli were used as stress: injection of formalin, which was previously shown to be ineffective in inducing GH release; and cold exposure, which evoked depletion of pituitary GH. Exposure of rats to cold (4°C) for 1 hr resulted in a significant depletion of pituitary GH, which was accompanied by disappearance of hypothalamic GRF and appearance of GRF activity in plasma. Cold exposure for 5 minutes was ineffective. Injection of 10% formalin did not induce pituitary GH depletion, or affect hypothalamic and plasma GRF activity. These results indicate that some stressful stimuli, such as cold exposure, capable of releasing GH, induce this effect via the hypothalamus; other stresses, like formalin injection, which do not release GH, exert little effect on the GH-release mechanism of the hypothalamus.

Increase in Plasma Growth Hormone (GH)-Like Activity After Administration of Porcine GH-Releasing Hormone

Summary The ability of pure growth hormone (GH)-releasing hormone (GH-RH) to deplete pituitary GH content and simultaneously to increase plasma GH-like activity was investigated in the rat using the “tibia test” method. GH-RH (350 ng) induced a significant fall in pituitary GH content which was accompanied by increased GH-like activity in plasma. GH-RH administered at two dose levels, 80 and 400 ng, induced a dose-related fall in pituitary GH. There was a concomitant rise in plasma GH-like activity of rats given the larger dose. Plasma GH-like activity in rats injected with saline was not detectable. These results indicate that the pituitary depletion of GH content in rats caused by GH-RH reflects true release of the hormone.