EFFECTS OF LIPOPROTEIN, EQUINE LUTEINIZING-HORMONE, EQUINE FOLLICLE-STIMULATING-HORMONE, AND EQUINE PROLACTIN ON EQUINE TESTICULAR STEROIDOGENESIS IN-VITRO

A stallion testicular cell incubation system was developed and used to investigate the regulation of steroidogenesis in stallion testes. Cel ls isolated from testes of 2- to 4-year-old stallions (n = 6) were cul tured for 12 hours in a defined medium with and without varying doses of lipoprotein, equine luteinizing hormone (eLH), human chorionic gona dotropin (hCG), equine follicle-stimulating hormone (eFSH), and/or equ ine prolactin (ePRL). Estrogen conjugate (EC), testosterone (T), and e stradiol-17 beta (E2) production were determined by RIA. Increasing do ses of lipoprotein significantly (P < 0.001) increased basal, hCG- and eLH-stimulated EC production, resulting in a maximal fourfold increas e in each case. A maximal dose of lipoprotein (3 mg/ml) significantly (P < 0.001) increased basal T production threefold, whereas hCG- and e LH-stimulated T production were increased 76- and 30-fold, respectivel y. In the presence of 0.5 mg/ml of lipoprotein, increasing doses of eL H significantly (P < 0.001) stimulated EC, T, and E2 production. The i ncrease in T production (5.6-fold) at a physiological dose of eLH (5 n g/ml) was significantly (P < 0.05) greater than the increase in EC or E2 production (2.1- and 2.3-fold, respectively). However, the total ma ss of EC produced was significantly greater (P < 0.05) than the total amount of T produced at both basal (15 ng vs. 148 pg) or hormone-stimu lated (48 ng vs. 2,427 pg at 5 ng/ml eLH) levels. hCG significantly (P < 0.001) stimulated EC and T production and was 82-fold more active i n stimulating EC production and 41-fold more active in stimulating T p roduction than was eLH. FSH had no significant effect on steroidogenes is either alone or in the presence of eLH, except at the highest dose tested (50 ng/ml), which was above the physiological level of circulat ing FSH (4-7 ng/ml) in the stallion. PRL (1-50 ng/ml) had no significa nt effect on steroidogenesis either alone or in the presence of eLH. T hese data suggest that in the postpubertal stallion, both estrogen and T production are regulated by LH, and this regulation appears to be d ependent on the availability of lipoprotein-derived cholesterol. Furth ermore, the observation that testicular cells produced a larger mass o f EC than T, but responded to eLH with a larger relative increase in T production, suggests that production of these two steroids may be ind ependently regulated.