Cultured Leydig tumor cells (MA-10) respond to luteinizing hormone (LH
) by synthesizing and secreting progesterone (P). The specificity of t
he response to LH prompted us to develop this system for use as a simp
le and rapid in vitro bioassay for LH. The aims of this study were to
(1) improve sensitivity and reproducibility, and (2) optimize the assa
y for use in diverse animal species. A minimum sensitivity was observe
d at 0.05 mIU/well of LH with 0.5 x 10(4) cells/well for 1.5 hours. At
higher concentrations of LH, shorter incubation periods also signific
antly stimulated P production. Addition of human LH standard or serum
samples resulted in a dose-dependent increase in P production. Paralle
l dose-dependent curves were observed with LH preparations from mammal
ian, avian, and amphibian species. In conclusion, these findings demon
strate that (1) the assay is rapid, sensitive, and reproducible; (2) s
erum LH levels analyzed using this assay and the mouse Leydig cell bio
assay are comparable; (3) shorter incubation times suggest the impleme
ntation of this assay for rapid qualitative determination of LH surges
; and (4) the assay can be used for the analysis of samples from diver
se species, especially those lacking radioimmunoassays. Therefore, thi
s assay system allows for the simple and rapid measurement of circulat
ing bioactive LH levels in humans and diverse animal species, and shou
ld provide insight regarding the role of bioactive LH in physiogical a
nd pathological conditions.