Detection of steroidogenic acute regulatory protein in equine ovaries

A steroidogenic acute regulatory (StAR) protein has been identified in seve ral species as a probable important rate-limiting step in steroidogenesis. This protein is believed to be responsible for transporting cholesterol fro m the outer to the inner mitochondrial membrane. It is known that equine ch orionic gonadotrophin (eCG) stimulates steroidogenesis in the corpora lutea of early pregnant mares and that eCG also upregulates StAR mRNA in bovine ovaries. In the present study, ovarian tissue from cyclic and early pregnan t mares was immunostained to detect the distribution of the StAR protein. W estern blot analysis was performed, followed by phosphor imaging to establi sh whether the onset of eCG secretion in pregnancy was associated with incr eased expression of the StAR protein. Immunostaining for StAR was confined to the theca interna of growing and preovulatory follicles, but 24 h after treatment with hCG, some granulosa cells were positively stained. Positive staining was confined to the large luteal cells of the equine corpus luteum . There was no difference in the distribution of immunostaining before or a fter onset of eCG secretion in pregnant mares, but increased amounts of StA R were detected in corpora lutea from mares at day 40 or day 41 of pregnanc y compared with non-pregnant mares and mares at days 20-30 of pregnancy.