Abstract
An intravenous injection of human GH-releasing factor (GRF) was evaluated as a test of GH reserve in 26 healthy male subjects ranging in age between 26 and 53 years old. Results obtained were compared with those after insulin-induced hypoglycemia (ITT) and oral administration of L-dopa. After a dose of 0.5 or 2μg GRF/kg BW, the mean peak GH levels were 15.4±4.5 and 19.4±5.0 (± SE) ng/ml in healthy male subjects, respectively. Eight out of 22 healthy male subjects failed to attain GH levels exceeding 10ng/ml following 2μg GRF/kg BW. The overall plasma GH response to GRF was similar to that after L-dopa (500mg). In contrast, ITT resulted in a consistent stimulation of GH that exceeded long/ml in 22 out of 23 healthy male subjects. The mean peak GH level after ITT was 35.3±5.3 ng/ml, which was significantly higher than that after 2μg GRF/kg BW.
Eight patients with GH deficiency due to various hypothalamic-pituitary diseases were subjected to ITT and GRF (lug/kg BW) tests. Plasma GH levels after ITT did not rise in any of the patients, while clear-cut plasma GH elevations were observed in 6 out of the 8 patients after GRF.
GRF test has the potential disadvantages of failing to elicit GH release in healthy male subjects. However, a combination of GRF test with other traditional GH provocation tests such as ITT could add useful information on the pathophysiology of GH deficiency. An additional important finding of the present studies was that a significant divergence existed in plasma GH responses between GRF and ITT in healthy male subjects. This suggests that the stimulation of GH release following ITT is not solely mediated by endogenous GRF.
