Measurement of growth hormone-releasing hormone and somatostatin in hypothalamic-portal plasma of unanesthetized sheep. Spontaneous secretion and response to insulin-induced hypoglycemia.
To elucidate the role of growth hormone (GH)-releasing hormone (GRH) and somatostatin (SRIH) in the regulation of the growth hormone ( GH) secretory pattern, we collected portal blood from five unanesthetized ovariectomized ewes for repeated measurements of GRH and SRIH simultaneous with those of peripheral GH. Hormones were measured at 10-min intervals for 5.5 h and their interrelationships analyzed. Mean portal GRH was 20.4 +/- 6.7 (SD) pg/ml and the estimated overall secretion rate was 13 pg/min. GRH secretion was pulsatile with peaks of 25-40 pg/ml and a mean pulse interval of 71 min. Mean portal SRIH was 72 +/- 33 pg/ml and the estimated overall secretion rate was 32 pg/min. SRIH secretion was also pulsatile with peaks of 65-160 pg/ml and a mean pulse interval of 54 min. The GH pulse interval was 62 min. A significant association was present between GRH and GH secretory peaks though not between GRH and SRIH or SRIH and GH. Insulin hypoglycemia resulted in a rapid and brief stimulation of SRIH secretion followed by a decline in GH levels. No effect was observed on GRH secretion until 90 min, when a slight increase occurred. The results suggest (a) the presence of an independent neural rhythmicity of GRH and SRIH secretion with a primary role of GRH in determining pulsatile GRH secretion, and (b) that the inhibitory effects of insulin hypoglycemia on GH in this species are attributable to a combination of enhanced SRIH secretion and possibly other factors, though without significant inhibition of GRH.[1]References
- Measurement of growth hormone-releasing hormone and somatostatin in hypothalamic-portal plasma of unanesthetized sheep. Spontaneous secretion and response to insulin-induced hypoglycemia. Frohman, L.A., Downs, T.R., Clarke, I.J., Thomas, G.B. J. Clin. Invest. (1990) [Pubmed]
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