ML Vance, DL Kaiser, PM Martha Jr, R Furlanetto, J Rivier, W Vale and MO Thorner

Journal of Clinical Endocrinology & Metabolism, Vol 68, 22-28, Copyright © 1989 by Endocrine Society

Department of Internal Medicine, University of Virginia Medical Center, Charlottesville 22908.

In vitro and in vivo studies of somatotroph responsivity to GHRH stimulation indicate that partial loss of GH responsiveness occurs during constant GHRH stimulation. To determine if these observations reflect either a short term effect of GHRH or if the absence of somatostatin effects somatotroph desensitization (as occurred in in vitro studies), we administered GHRH-40 (10 ng/kg.min) by continuous iv infusion for 14 days to five normal men and one GH-deficient boy. Serum insulin-like growth factor I (IGF-I) concentrations were measured at frequent intervals to assess the biological effect of GHRH on GH secretion.

The GH secretory profiles were assessed by measuring serum GH levels every 20 min for 24 h before (day 0), on the 14th GHRH infusion day, and 14 days after discontinuation of the GHRH infusion in the normal men.

The GH-deficient boy was studied before and during the 14th GHRH infusion day. A supramaximal iv GHRH dose was administered at the end of the 24-h sampling period, and the GH responses were compared.

Serum IGF-I concentrations increased on the 14th day of GHRH infusion in the normal men [day 0 mean, 0.84 +/- 0.14 (+/- SE) X 10(3); day 14, 1.74 +/- 0.20 X 10(3) U/L; P less than 0.05] and from 0.20 X 10(3) on day 0 to a maximum of 0.67 X 10(3) U/L on day 3 in the GH- deficient boy; they declined to pretreatment levels after discontinuation of GDRH.

The mean integrated serum GH concentrations in the normal men were 1.44 +/- 0.10 micrograms/L.h on day 0 and 3.11 +/- 0.95 on day 14 of GHRH infusion.