Half-Time of Endogenous Growth Hormone (GH) Disappearance in Normal Man After Stimulation of GH Secretion by GH-Releasing Hormone and Suppression with Somatostatin*

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Abstract
The half-life (t½) of disappearance of endogenous GH from serum was studied using physiological effectors to stimulate and then suppress GH release. GH secretion was stimulated by a single iv injection of GHRH, followed 45 min later by an iv bolus dose and then a 2.5-h infusion of somatostatin (SRIH) to suppress further release. The in vivo t½ of GH in seven men was calculated from serum GH concentrations measured at frequent intervals after beginning the SRIH infusion. The mean t½ of endogenous GH was 18.9 ± 0.8 (±se) min by monoexponential analysis and 3.5 ± 0.7 and 20.7 ± 0.7 min by biexponential fitting. In these normal men, the decline in GH concentrations after GHRH and SRIH administration was similar to that after the administration of GHRH alone, which yielded a t½ of 20.3 ± 1.9 min. We conclude that the physiological kinetics of endogenous GH removal/disappearance can be estimated in vivo in man using GHRH with or without SRIH infusion.

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