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

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 less than 0.001) the plasma concentrations of IGF-I (-31%). Likewise, GH-binding sites were decreased on hepatocytes (-55%; P less than 0.01) and in liver homogenates (-60%; P less than 0.001) compared with controls, as was the velocity of initial binding (-77%; P less than 0.001). Protein restriction for 1 week decreased plasma concentrations of IGF-I (-42%; P less than 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.(ABSTRACT TRUNCATED AT 250 WORDS)