Induction of Endogenous Insulin-Like Growth Factor-I Secretion Alters the Hypothalamic-Pituitary-Testicular Function in Growth Hormone-Deficient Adult Dwarf Mice1

Abstract

To evaluate the influence of growth hormone (GH) on hypothalamic-pituitary-testicular function, GH-deficient adult male Ames dwarf mice were treated (s.c.) twice daily for 8 days with either vehicle or bovine GH (25 micrograms/injection/mouse). Normal male siblings treated with vehicle served as controls. Two in vivo experiments were conducted. In experiment 1, on Day 8, mice were treated (i.p.) with either saline or LHRH (1 ng/g b.w.) in saline. Fifteen minutes later, blood was obtained via heart puncture to assess plasma insulin-like growth factor-I (IGF-I), LH, and testosterone (T) levels by RIAs. In experiment 2, on Day 7, mice were bilaterally castrated and received injections of either oil or T propionate (1 microgram/g b.w.) in oil. Eighteen to twenty hours later, blood was obtained as in experiment 1, and plasma IGF-I and LH levels were determined. In addition to these in vivo experiments, testicular androstenedione and T responses to hCG stimulation in vitro were evaluated. Administration of GH to dwarf mice increased (p < 0.001) plasma IGF-I levels, whereas IGF-I was not detectable in control dwarf mice. Plasma IGF-I concentrations were higher in normal mice than in treated dwarf mice. Basal LH levels were lower (p < 0.025) in dwarf mice than those in normal mice. In dwarf mice, GH treatment increased (p < 0.001) plasma LH levels. The effect of LHRH on LH secretion was increased (p < 0.001) in dwarf mice pretreated with GH, but this LH response was lower than in normal siblings that received vehicle only.(ABSTRACT TRUNCATED AT 250 WORDS)