Growth hormone binding protein in the rat: effects of gonadal steroids

Abstract

In normal rats, females have higher circulating GH-binding protein (GHBP) levels than males, whereas in the GH-deficient dwarf (Dw) rat, there is no sexual dimorphism in plasma GHBP, suggesting that GH secretion may be involved in this difference. In order to study the relationship between gonadal steroids and GH on GHBP and GH receptor regulation, the levels of plasma GHBP, hepatic bovine GH, and human GH (hGH) binding as well as GHBP and GH receptor messenger RNA (mRNA) have now been studied in normal, Dw, hypophysectomized (Hx), or ovariectomized (Ovx) rats, subjected to different GH and gonadal steroid exposure. In normal male rats, estradiol (E2, 12.5-25 micrograms/day for 1 or 2 weeks) markedly increased plasma GHBP and hepatic hGH, and bGH binding. These effects of E2 were diminished in Dw rats, absent in Hx rats, but restored in Hx rats given exogenous hGH. Plasma GHBP rose in female rats given E2, and fell in females given the anti-estrogen tamoxifen. Ovx animals had lower plasma GHBP and hepatic GH binding which was reversed by E2, but not testosterone treatment. Continuous hGH infusions in Ovx rats restored hepatic GH binding, and increased plasma GHBP. In Dw males, hGH increased plasma GHBP and hepatic GH binding, whereas testosterone had no effect on GHBP or GH receptors and did not affect their up-regulation by hGH. Hepatic levels of GHBP-, and GH receptor mRNA transcripts showed the same trends in response to steroid or GH treatment, but the differences were rarely significant, except in Ovx animals which had higher GHBP mRNA transcripts after GH or E2 treatment. Thus E2 and GH increase both plasma GHBP and hepatic GH receptor binding. GH up-regulates GHBP in the absence of E2, whereas E2 treatment does not raise GHBP in the absence of GH. Whereas some of the effects of estrogen could be mediated via alterations in GH secretion, estrogen may also directly influence GHBP production at the liver, but only in the presence of GH.