Direct stimulatory effects of insulin-like growth factor-I on Leydig cell steroidogenesis in primary culture

Insulin-like growth factor-I (IGF-I) in concentration as low as 10 ng/ml significantly increased basal testosterone formation and 100 ng/ml of IGF-I increased testosterone production more than two fold in primary cultures of purified mature Leydig cells. IGF-I also markedly potentiated hCG-induced testosterone formation in a dose-dependent manner. Furthermore, IGF-I enhanced 8-bromo cyclic AMP induced steroidogenesis and hCG-stimulated cyclic AMP formation. The binding of ¹²⁵I-IGF-I to purified Leydig cells was linear with a binding affinity of 0.56 ± 0.07×10⁹ M^-1 and a capacity of 167 ± 10.2 fmol/mg protein. Insulin and multiplication-stimulating activity were less potent than IGF-I in competing the binding of ¹²⁵I-IGF-I to purified Leydig cells. This suggests that Leydig cells contain specific type I IGF receptor and IGF-I could modulate Leydig cell steroidogenesis.