Influence of Endogenous Somatostatin on Growth Hormone and Thyrotropin Secretion in Neonatal Rats*

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Abstract
When gestating rats were injected iv with an antiserum to somatostatin (SRIF-AS) during the last week of gestation, serum GH levels in fetuses and 6-h-old newborn rats were not significantly different from controls. Similary, 2 h after the ip administration of SRIF-AS, no change in serum GH concentration was observed in 2-h-old rats. However, under the same conditions, a significant increase in serum GH was observed in 24-h-old rats and in 2-to 60-day-old rats. The injection of SRIF-AS neither changed basal serum TSH levels during the neonatal development nor in the adult stage. A significant increase in TRH-induced TSH release was observed after the third postnatal day. It is concluded that endogenous SRIF plays a physiological role in GH release by 24 h of age in the rat and that the fall in GH secretion that normally occurs during the first week of life is due to the development of inhibitory mechanisms mediated by hypothalamic SRIF. Additionally the results suggest that the influence of SRIF upon TSH secretion is present before that of TRH.