Abstract
Studies in vitro, in animals, and in adults have suggested that SM-C, operating at both CNS and pituitary levels, mediates a feedback inhibition of GH secretion. To test this hypothesis in children we studied 11 short (height < 3rd%) boys, ages 7-14 with delayed bone ages and normal GH responses to standard tests. On successive days 2-hour clonidine (5ug/kg,p.o.) and GRF (0.3ug/kg, i.v.) tests were performed in the fasting state. Daily GH (0.1 U/kg, i.m.) was then given with a repeat clonidine test 20 h after the second dose and a GRF test 20 h after the third.Results are shown below:*Differs from baseline, p<0.05Pre-treatment with GH thus strongly inhibited the GH response to clonidine and GRF, acting at the hypothalamus and pituitary respectively. Although SM-C rose sharply, individual rises did not correlate with suppression of the GH response to clonidine (r=0.35;p=0.3) or GRF (r=-.05).Conclusion: inhibition by GH of its own secretion operates, at least in part, at the pituitary level. While the inhibition may be modulated by SM-C, its extent does not depend directly on plasma SM-C concentrations.
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