Does Gonadotropin Receptor Complex Have an Amplifying Role in cAMP/Testosterone Production in Leydig Cells?

Abstract

ABSTRACT: The dose—response relationship between luteinizing hormone/human chorionic gonadotropin (LH/hCG)‐stimulated biological response and 125I‐labeled hCG binding was studied in purified Leydig cells from adult rat testes. The concentration of hCG needed for one‐half maximal stimulation of cyclic adenosine monophosphate (cAMP) and testosterone production (ED50) was 2.16 × 10−11mol/L and 5.6 × 10−13mol/L, respectively. This suggests that extremely low levels of hormone in the range of 10−13mol/L hCG are sufficient to generate enough cAMP (5.66 pmol; 2.83 × 10−9mol/L) for steroidogenesis, thereby preserving the catalytic potential of the receptor‐cyclase system. Most of the cAMP formed at 10−13mol/L hCG was released into the medium, and the intracellular cAMP was much less and barely detectable (0.98 × 10−9mol/L; 1.96 pmol/2 × 106 cells). The specific binding of 125I‐labeled hCG to purified Leydig cells at a correspondingly higher hCG concentration (3 × 10−10mol/L) was extremely low and did not display a dose‐dependent increase in binding. Assuming the specific binding to represent 100% occupancy of high affinity receptors (14.2 fmol/ 2 × 106 cells per 2 ml), each mole of bound hCG generated 15,423 mol cAMP and 12,817 mol testosterone. The results show that the hormone interacts with cellular receptors as a catalyst to generate the biological response. Moreover, the true affinity of hormone‐receptor interaction responsible for the physiologic action is possibly much greater than previously reported for this system. This information should prove useful for reconstitution studies using the hormone receptor/G‐protein/adenylate cyclase system in vitro in soluble form.