The decreased plasma concentration of insulin-like growth factor-I in protein-restricted rats is not due to decreased numbers of growth hormone receptors on isolated hepatocytes

ABSTRACT The resistance to GH and the low serum concentrations of insulin-like growth factor-I (IGF-I) that occur during fasting are accompanied by decreased GH receptors in liver homogenates. In protein restriction, however, serum IGF-I but not GH receptors are decreased, suggesting that a post-receptor defect exists. Because conclusions about the status of GH receptors during dietary manipulation are based on studies using liver homogenates, the present study was undertaken to determine whether changes in GH binding by homogenates are paralleled by changes in receptors on the cell surface considered to mediate the GH signal. Collagenase-dispersed hepatocytes or liver homogenates from 7-week-old female Wistar rats fed various diets were evaluated for changes in somatogenic receptors. Fasting for 24 h reduced significantly ( P < 0·001) the plasma concentrations of IGF-I (−31%). Likewise, GH-binding sites were decreased on hepatocytes (−55%; P <0·01) and in liver homogenates (−60%; P < 0·001) compared with controls, as was the velocity of initial binding (−77%; P <0·001). Protein restriction for 1 week decreased plasma concentrations of IGF-I (−42%; P < 0·001) but GH-binding sites were not significantly reduced on hepatocytes or in homogenates. The velocity of initial binding was also not decreased. We conclude that observations on changes in homogenate binding of bovine GH during dietary manipulation provide a reliable means of assessing changes in cell-surface GH receptors. The absence of a decline in surface binding during feeding of a low-protein diet supports the hypothesis that the decline in IGF-I during protein restriction is mediated by post-receptor events. Journal of Endocrinology (1990) 124, 159–165