Repeated GH-releasing hormone administration unravels different GH secretory patterns in normal adults and children

Abstract

In normal adults, repeated GHRH administration leads to progressively decreasing somatotrope responses. To verify whether this GH secretory pattern also connotes normal growing children, we have studied the effects of two consecutive (every 120 min) 1 microgram/kg iv GHRH boluses on GH release in normal adults (N = 7, age 23.2-30.6 years) children (N = 6, age 10.4-13.2 years). In the adults, the GH response to the second GHRH bolus (peak, mean +/- SEM; 2.9 +/- 0.8 micrograms/l) was lower (P less than 0.02) than that to the first bolus (15.9 +/- 2.4 micrograms/l). Conversely, in children the GH response to the second GHRH bolus (25.6 +/- 6.3 micrograms/l) overrode the first one (13.6 +/- 6.5 micrograms/l), but this difference did not attain statistical significance. In adults cholinergic enhancement by pyridostigmine, a cholinesterase inhibitor, was previously shown to re-instate, even to potentiate somatotrope responsiveness to consecutive GHRH boluses. Thus, in 5 children GH response to repeated GHRH boluses was retested administering pyridostigmine (60 mg orally) 30 min before the second GHRH bolus. In these subjects, pyridostigmine failed significantly to potentiate the GH responsiveness to the second GHRH bolus (30.3 +/- 4.6 vs 25.0 +/- 7.6 micrograms/l). These data indicate that differently from in adults, in children repeated GHRH administration does not reduce somatotrope responsiveness and that cholinergic enhancement fails to potentiate GH responsiveness to the second GHRH bolus.