Gonadotropin binding and stimulation of cyclic adenosine 3':5'-monophosphate and testosterone production in isolated Leydig cells.

TLDR

The study examined gonadotropin binding and its effects on cyclic AMP production and testosterone synthesis in collagenase‑dispersed adult rat testicular interstitial cells. High‑affinity, low‑capacity hCG receptors on Leydig cells showed that testosterone production is maximally stimulated at low hCG concentrations without increasing cyclic AMP, whereas higher hCG levels raise cyclic AMP but do not alter the 20–30 min lag for steroidogenesis, indicating a dissociation between receptor occupancy, cyclic AMP formation, and steroid production.

Abstract

Gonadotropin binding and stimulation of cyclic adenosine 3':5'-monophosphate (cyclic AMP) formation and testosterone synthesis were studied in collagenase-dispersed interstitial cells from the adult rat testis. Binding of 125I-human chorionic gonadotropin (hCG) by isolated Leydig cells was of high affinity (Ka = 10(10) M-1) and low capacity, equivalent to approximately 6000 sites/cell. The binding data were consistent with the presence of a single order of receptors, with no interaction between binding sites. Stimulation of testosterone synthesis by increasing concentrations of hCG was completely dissociated from changes in cyclic AMP formation, and maximum activation of steroidogenesis was induced by hCG concentrations which had no effect upon cyclic AMP production. Kinetic analysis of gonadotropin-induced responses in dispersed Leydig cells also showed a marked dissociation between steroidogenesis and cyclic nucleotide formation. Low concentrations of hCG caused maximum stimulation of testosterone production which was not accompanied by a rise in cyclic AMP formation at any time after addition of gonadotropin. Higher concentrations of hCG caused marked elevations of cyclic AMP at progressively earlier time intervals, but did not alter the 20 to 30 min lag period required for induction of testosterone synthesis. These observations indicated that occupancy of gonadotropin receptors occurs over a much wider range of hCG concentration than that required for maximum steroidogenesis.