Objective: To clarify the mechanism by which galanin, a 29-amino-acid peptide, increases GH secretion in man.
Design: We studied the GH-releasing effect of this neurohormone (galanin, 15 micrograms/kg) infused over 60 minutes after 120 minutes of saline, following a previous GHRH bolus (GHRH 1 microgram/kg i.v. at 0 minutes, galanin infused from 120 to 180 minutes) and coadministered with the second of two consecutive GHRH boluses (GHRH every 120 minutes, galanin infused from 120 to 180 minutes).
Patients: Fourteen healthy male subjects, aged 20-34 years, in two groups (group A, 20-31 years (n = 8); group B, 25-34 years (n = 6)) were studied.
Measurement: Blood samples were drawn every 15 minutes of 255 minutes. Serum GH was measured in duplicate by IRMA. Statistical analysis of the data was carried out by non-parametric ANOVA test.
Results: The GH response to galanin infused 120 minutes after saline overlapped with that induced by the neuropeptide infused following previous GHRH bolus (AUC, mean +/- SEM: 317.3 +/- 73.2 vs 326.8 +/- 54.2 micrograms/l/h). The GH-releasing effect of the second GHRH bolus (126.9 +/- 32.3 micrograms/l/h) was lower than that of the first one (503.4 +/- 41.3 micrograms/l/h; P = 0.0002). Galanin markedly enhanced the GH responses to the second GHRH bolus (1118.0 +/- 212.7 micrograms/l/h; P = 0.0002 vs second GHRH bolus alone) so that it did not significantly differ from the first one (710.9 +/- 107.8 micrograms/l/h).
Conclusions: Our results show that the GH-releasing effect of galanin is not modified by GHRH pretreatment and that the neuropeptide reinstates the GH response to the repeated GHRH stimulation in man. They suggest that these effects are due to the inhibition of hypothalamic somatostatin release.