Luteinizing Hormone Receptors and Testosterone Synthesis in Two Distinct Populations of Ley dig Cells*

Dispersed cells from whole testes or from isolated interstitial tissue of mature rats, yielded two distinct populations of Leydig cells when subjected to centrifugation in a 0-40% metrizamide gradient. One population (I) was found in a fractions with a density of 1.085-1.117 g/cm3, and the other population (II) was found in fractions with a density of 1.128-1.145 g/cm3. Binding of 125I-labeled hCG by each population of cells indicated a single class of binding sites with the same high binding affinity and similar concentrations of binding sites per Leydig cell. Testosterone production per fmol gonadotropin receptor site in the absence of gonadotropin stimulation was similar for cells of each population. However, when cells from each population were incubated with increasing concentrations of hCG or dibutyryl cAMP, only Leydig cells from population II exhibited a marked increase in testosterone production. The low responsiveness of Leydig cells in population I did not appear to be a result of either damage to these cells or inhibition by non-Leydig interstitial cells in population I.