Growth Hormone and Insulin-Like Growth Factor I Treatment Increase Testicular Luteinizing Hormone Receptors and Steroidogenic Responsiveness of Growth Hormone Deficient Dwarf Mice*

Abstract

To test the hypothesis that insulin-like growth factor (IGF-I) is required for the in vivo development of testicular Leydig cell function, either recombinant human GH [(hGH) (1.5 micrograms/g BW) or recombinant IGF-I (1 microgram/g BW) was injected three times daily into immature Snell dwarf mice (dw/dw) and into phenotypically normal control (Dw/-) for 7 days. In dw/dw mice hGH enhanced significantly body, liver, kidney, and testicular weight. In addition, hGH increased testicular LH receptors and the acute steroidogenic response to human CG, but there was no significant effect on basal plasma testosterone or plasma LH levels. The effects of IGF-I in body and kidney weight were less pronounced than those produced by hGH, but its effects on testicular weight and LH receptors, as well as on the acute steroidogenic response to human CG, were similar to that observed after hGH treatment. In Dw/- mice hGH had no effect on either body or organ weight or on testicular function, despite the fact that it induced a significant increase in plasma IGF-I levels. These results indicated that IGF-I is able to induce the maturation of Leydig cell function and that the effects of hGH on the testis are probably mediated by IGF-I. They also suggest that the delayed puberty associated with GH deficiency or resistance is most likely related to an IGF-I deficiency.