The effect of changing somatostatin tone on the pituitary growth hormone and thyroid-stimulating hormone responses to their respective releasing factor stimuli.
Somatostatin (SS) inhibits GH and TSH secretion, but its role in modulating their pulsatility is unclear. We studied GH and TSH responses to GH-releasing hormone (GHRH) and TRH stimulation upon a variable background infusion of saline, SS-(1-14) at 20 and 100 micrograms/m2.h, and oral pyridostigmine (30 and 60 mg) in six adult males. Basal GH levels were unaffected by SS-(1-14). Deconvolution analysis of serum GH values demonstrated that the pituitary responded to two GHRH stimuli 90 min apart without attenuation of the second response. The higher dose of SS-(1-14) significantly blunted the first GH response; second GH responses were further attenuated by both SS-(1-14) doses. Maximum GH release and "switch-off" rates for both stimuli were reduced without changes in the 50% secretion time. Pyridostigmine enhanced the first GH response to GHRH with an increase in the GH release rate; second GH responses were not augmented. GH secretion was prolonged by pyridostigmine, although the 50% secretion time remained unchanged. Peak stimulated serum TSH was attenuated by both SS-(1-14) doses, but pyridostigmine had no effect. All other TSH parameters examined were unaffected. We conclude that the GH response to GHRH is dependent on SS tone, but that the thyrotroph is not tonically inhibited by SS. SS attenuates the rate of GH release without changing the duration of secretion and appears important in terminating GH secretion.
Kabi International Growth Research Centre, Middlesex Hospital, Oxford, United Kingdom.
H A
HA
Spoudeas
D R
DR
Matthews
C G
CG
Brook
P C
PC
Hindmarsh
D000328:Adult; D001786:Blood Glucose / Q000032:analysis; D004305:Dose-Response Relationship, Drug; D013006:Growth Hormone / Q000097:blood / Q000378:metabolism*; D013007:Growth Hormone-Releasing Hormone / Q000494:pharmacology*; D006801:Humans; D007328:Insulin / Q000097:blood; D007334:Insulin-Like Growth Factor I / Q000032:analysis; D008297:Male; D011729:Pyridostigmine Bromide / Q000494:pharmacology; D013004:Somatostatin / Q000502:physiology*; D013972:Thyrotropin / Q000097:blood*; D013973:Thyrotropin-Releasing Hormone / Q000494:pharmacology*
D001786:Blood Glucose; D007328:Insulin; D013004:Somatostatin; D013973:Thyrotropin-Releasing Hormone; D007334:Insulin-Like Growth Factor I; D013972:Thyrotropin; D013006:Growth Hormone; D013007:Growth Hormone-Releasing Hormone; D011729:Pyridostigmine Bromide