A Sensitive Gonadotropin Responsive System: Radioimmunoassay of Testosterone Production by the Rat TestisIn Vitro

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Abstract
The steroidogenic response of the isolated rat testis to gonadotropic hormones was measured by radioimmunoassay of testosterone released during incubation in vitro with human chorionic gonadotropin (HCG) and luteinizing hormone (LH). Measurement of testosterone production by radioimmunoassay of incubation media was validated by reference to specific testosterone assays employing extraction and chromatography prior to measurement by competitive protein binding and radioimmunoassay. During incubation of individual testes in vitro, testosterone production rose slowly to the basal level of 100-200 ng/testis/4 hr. Addition of 20 ng (7.2 X 10-11M) HCG or 10-3M dibutyryl cyclic AMP caused a marked rise in testosterone production, detectable after IS min and increasing throughout the period of incubation. The decapsulated adult rat testis was extremely sensitive to gonadotropin stimulation in vitro, responding with increased testosterone release to as little as 1 mlU (0.2 ng) HCG, 2.5 mlU (0.7 ng) human LH, and 0.6 mlU (1 ng) ovine LH. Doseresponse curves with increasing gonadotropin concentration were observed over the range 1-10 mlU (0.7-7.2 X 1 0-112M) HCG, 2.5-20 mlU (1.2-9.5 -11M) human LH, and 0.6-3 mlU (0.8-4.2 x -11M) ovine LH. No rise above the maximum testosterone response of 800-1200 ng/testis/hr was observed on addition of 100 IU (7 X -8MM) HCG or 10-3MM dibutyryl cyclic AMP. Purified human LH (LER-1486-2) and human pituitary gonadotropin (LER 907) induced comparable testosterone responses on the basis of the LH activity of each preparation as determined by bioassay. Ovine FSH and prolactin were devoid of intrinsic activity on testosterone production in vitro, and showed no synergistic actions when added simultaneously with LH or HCG. Measurement of testosterone production by the isolated rat testis provides a sensitive and specific system for determination of the biological activity of intact and structurally modified gonadotropins at the target cell level. (Endocrinology90: 1032, 1972)