Binding sites for growth hormone releasing factor on rat anterior pituitary cells

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Abstract
Growth hormone releasing factors (GRFs) have been isolated from human pancreatic tumours (hGRF)1–3 and rat hypothalamus (rhGRF)4. The response to GRF at the pituitary level can be modulated by other factors, including glucocorticoids5, thyroid hormones5, somatostatin and other neuropeptides5,6 and somatomedins7,8. Glucocorticoids enhance GRF-induced growth hormone (GH) secretion in primary cultures of rat anterior pituitary cells5,9, and the synthetic glucocorticoid dexamethasone has recently been shown to increase the amounts of GH released in freely moving rats in response to submaximal doses of intravenous GRF10. To investigate whether somatotroph sensitivity to GRF is modulated at its receptor level, we have developed a radioreceptor assay using an iodinated analogue of hGRF as radioligand. We report here that the relative binding affinities of rGRF, hGRF and the two analogues are correlated with their in vitro biological potencies. Further, the number of GRF binding sites is drastically decreased in cells deprived of glucocorticoids either in vivo or in vitro.