Growth hormone receptor regulation in growth hormone-deficient dwarf rats

In the rat, many actions of GH depend upon the sexually dimorphic pattern of exposure to GH. Hepatic human GH (hGH) receptor binding differs between the sexes and is sensitive to GH deficiency, but this has mostly been studied in acutely hypophysectomized rats, which lack all pituitary hormones. We have used a strain of GH-deficient dwarf (Dw) rats to determine whether chronic GH deficiency alters the normal developmental pattern and sexually dimorphic expression of hepatic GH receptors. Adult female Dw rats had lower levels of 125I-labelled hGH binding (reflecting predominantly lactogenic receptors) than their normal counterparts whereas there was no difference between adult Dw and normal males; binding capacity increased from 25 days of age, becoming sexually dimorphic from 40 days to adulthood in both strains (% specific binding/mg protein: normal males 1.6 +/- 0.3, normal females 13.2 +/- 1.1, Dw males 2.1 +/- 0.4, Dw females 10.0 +/- 0.6). In contrast, hepatic 125I-labelled bovine GH (bGH) binding (somatogenic receptors) was much lower, and similar in both Dw and normal animals. A sex difference in 125I-labelled bGH binding was only seen in adult animals, and was considerably less marked in Dw rats compared with normal animals (normal males 1.3 +/- 0.1, normal females 2.5 +/- 0.2, Dw males 1.9 +/- 0.2, Dw females 2.4 +/- 0.2%/mg protein). Continuous hGH infusion stimulated growth in female Dw rats, and raised somatogenic and lactogenic GH binding (3.2 +/- 0.4 and 19.6 +/- 2.5%/mg protein) compared with sham-infused controls (2.4 +/- 0.2 and 7.9 +/- 0.6%/mg protein).(ABSTRACT TRUNCATED AT 250 WORDS)