The Effect of Pulsatile Administration, Continuous Infusion, and Diurnal Variation on the Growth Hormone (GH) Response to GH-Releasing Hormone in Normal Men*

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Abstract
To examine the relative effectiveness of GH-releasing hormone (GHRH) given either as multiple iv pulses or as a continuous iv infusion, we studied the GH response to a nearly equivalent total dose of GHRH-44 administered by both routes in a group of normal men. Further, in view of the pulsatile nature of GH secretion and its augmentation with sleep, we investigated whether a diurnal difference in GH release was present during chronic pulsatile administration of GHRH during day and night. Seven men received six GHRH pulses (1 µg/kg,iv) at 2-h intervals during both day (0900–2100 h) and night (2100–0900 h), and four underwent nighttime placebo pulsing. Eight men received a daytime continuous GHRH infusion (0.15 µg/kg.h for 5 h, followed by 0.75 µg/kg.h for 5 h) and a separate 10-h placebo infusion. The GH response to a bolus dose of GHRH (1 µg/kg, iv) was determined after both continuous GHRH and placebo infusions. No significant difference was found in the GH area response (mean ± SEM) during total day and night GHRH pulsing periods (6095 ± 1192 vs. 6506 ± 1483 ng/min.ml; P = NS). GH secretion was blunted after the initial daytime GHRH pulse (P = 0.02), and only two of seven men had a GH increase after the second pulse; responsiveness was restored after the fourth pulse. In contrast, all subjects responded to the second nighttime GHRH pulse. During continuous GHRH infusions, GH secretion was unsustained and pulsatile. The incrementalGH response to a single GHRH bolus dose was decreased after GHRH infusion compared to that after placebo (4.4 ± 1.8 vs. 10.3 ± 3.4 ng/ml; P < 0.05). No difference was found in the total GH area response to a nearly equivalent dose of GHRH administered as either multiple pulses or continuous infusion followed by a single GHRH bolus dose. The apparent pulsatile nature of GH secretion during continuous GHRH infusion and the lack of asignificant difference in the GH response to a nearly equivalent dose of GHRH administered as either multiple pulses or a continuous infusion suggest that GHRH need not be administered in a pulsatile manner to be an effective therapeutic agent for the stimulation of GH secretion in children with hypothalamic GHRH deficiency.

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