Does gonadotropin receptor complex have an amplifying role in cAMP/testosterone production in Leydig cells?
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 x 10(-11)mol/L and 5.6 x 10(-13)mol/L, respectively. This suggests that extremely low levels of hormone in the range of 10(-13)mol/L hCG are sufficient to generate enough cAMP (5.66 pmol; 2.83 x 10(-9)mol/L) for steroidogenesis, thereby preserving the catalytic potential of the receptor-cyclase system. Most of the cAMP formed at 10(-13)mol/L hCG was released into the medium, and the intracellular cAMP was much less and barely detectable (0.98 x 10(-9)mol/L; 1.96 pmol/2 x 10(6) cells). The specific binding of 125I-labeled hCG to purified Leydig cells at a correspondingly higher hCG concentration (3 x 10(-10)mol/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 x 10(6) 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.
Browne
E S
ES
Department of Physiology, Medical College of Georgia, Augusta, Georgia 30912.
Bhalla
V K
VK
D000818:Animals; D006063:Chorionic Gonadotropin / Q000378:metabolism / Q000494:pharmacology; D000242:Cyclic AMP / Q000096:biosynthesis*; D066298:In Vitro Techniques; D007700:Kinetics; D007985:Leydig Cells / Q000187:drug effects / Q000378:metabolism*; D008297:Male; D051381:Rats; D011919:Rats, Inbred Strains; D011974:Receptors, LH / Q000187:drug effects / Q000378:metabolism*; D013739:Testosterone / Q000096:biosynthesis*
D006063:Chorionic Gonadotropin; D011974:Receptors, LH; D013739:Testosterone; D000242:Cyclic AMP