Divergent Responses of Serum Insulin-Like Growth Factor-I and Liver Growth Hormone (GH) Receptors to Exogenous GH in Protein-Restricted Rats*

Abstract

Protein deprivation in young rats retards growth and decreases serum insulin-like growth factor-I (IGF-I) concentrations, neither of which is prevented by injections of GH once daily. Since four time daily injections of GH in hypophysectomized rats increase serum IGF-I concentrations more efficiently than single daily injections, we assessed whether this mode of GH delivery could overcome the GH resistance of protein malnutrition. Also, we evaluated whether continuous GH infusion could override this GH resistance. We fed 4-week-old female Wistar rats a low (5%) protein diet (P5) or a normal (15%) protein diet (P15) for 7 days. In a first experiment, rats fed a P5 diet received 40 or 400 micrograms/100 g BW.day rat GH (rGH) in four daily sc injections, while control P5 rats were injected at the same frequency with vehicle. In a second experiment, rats fed a P5 diet received 200 micrograms rGH/100 g BW.day by continuous infusion, while P5 sham-operated rats served as controls. IGF-I was measured by RIA on extracted serum, and free and total liver GH binding were determined by incubation of [125I]bovine GH with water- or MgCl2-treated homogenates, respectively. Neither continuous infusion nor repeated injections of rGH normalized the indices of growth or restored the serum IGF-I level to P15 control values. Injections of 400 micrograms rGH increased serum IGF-I 2-fold (P less than 0.01), but did not promote growth. Continuous GH infusion increased total and free liver GH binding to P15 control values, but had no effect on serum IGF-I. The discordance between liver GH binding and IGF-I confirms that a postreceptor defect is responsible for the GH resistance in protein restriction. These observations demonstrate that the consequences of protein restriction on growth are not overridden by intermittent or continuous administration of GH. The increase in IGF-I in response to 400 micrograms GH given intermittently in the absence of growth-promoting effects suggests that nutritional sufficiency is essential for IGF-I to promote growth.