Modulation of the growth hormone-releasing activity of thyrotropin-releasing hormone in the chicken by its gene-related peptide preproTRH((160-169)) (Ps4): enhanced somatostatinergic tone?

Recent research demonstrated that endocrine actions of thyrotropin (TSH)-re leasing hormone (TRH) are modulated by gene-related products within proTRH. In the present report we show that the growth hormone (GH) response to TRH is clearly inhibited after the preincubation of chicken pituitary glands w ith preproTRH((160-169)) Ps4, whereas the TSH response is not impaired. Bin ding sites for I-125-[Tyr(0)]-Ps4 were, however, not detected on chicken pi tuitary membranes, although (as a control) they were readily detectable on membranes from rat pituitary glands. An indirect action may therefore take place within the pituitary by modulating the action of somatostatin (SRIH), the inhibitor of GH release in the chicken. This hypothesis is strengthene d by the observation that Ps4 increases the binding of I-125-[Tyr(1)]-SRIH to chicken pituitary membranes in a dose-related way. Since Ps4 is also pro duced by pituitary tissue, this may reflect a local or paracrine action on the regulation of GH release. (C) 2000 Harcourt Publishers Ltd.