Journal of Clinical Endocrinology & Metabolism, Vol 67, 449-454, Copyright © 1988 by Endocrine Society

ARTICLES

A persistent pattern of varying pituitary responsivity to exogenous growth hormone (GH)-releasing hormone in GH-deficient children: evidence supporting periodic somatostatin secretion

PM Martha Jr, RM Blizzard, JA McDonald, MO Thorner and AD Rogol
Department of Pediatrics, University of Virginia Medical Center, Charlottesville 22908.

The pattern and degree of variation in pituitary responsivity to GHRH was examined in four GH-deficient children (two boys and two girls, aged 4 3/12 to 10 4/12 yr). All children were studied before and on multiple (three to six per child) occasions during long term GHRH therapy (1 or 2 micrograms/kg, sc, every 3 h) in an identical fashion. Each study comprised withdrawal of blood for serum GH measurements every 20 min between 2000 and 0800 h. All subjects received GHRH at 2000, 2300, 0200, and 0500 h as well as at 0800, 1100, 1400, and 1700 h throughout the long term treatment period (6-18 months). Although all children had low level (less than 7.0 micrograms/L) pulsatile GH secretion during baseline studies, the maximal peak values occurred at times other than 0500 h. Before GHRH treatment, serum GH levels rose significantly in response to 91% (62 of 68) of the GHRH doses administered. GH pulse amplitudes varied throughout the studies in all children, and this variability persisted despite 1300-3600 consecutive doses in each child. In all 17 study periods the highest serum GH concentration occurred shortly after the 0500 h GHRH dose. The mean peak GH concentration after the 0500 h GHRH dose [18.4 +/- 3.5 (+/- SE) micrograms/L] was significantly higher than those after the 2000 h (5.3 +/- 1.0 micrograms/L; P = 0.0001), 2300 h (7.4 +/- 2.1 micrograms/L; P = 0.0003), and 0200 h (10.9 +/- 2.5 micrograms/L; P = 0.011) doses. These results demonstrate that the responsivity of the pituitary to GHRH varies throughout the night in some GH-deficient children. There appears to be a direct relationship between the time of night and the degree of pituitary responsivity to GHRH. We suggest that this variable responsivity may be due to intermittent hypothalamic somatostatin secretion.

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