In the present model, it is likely that an increase of local P4 concentration by LH stimulation accelerates the cascade of steroid biosynthesis and that this influence could overcome the well-known direct inhibitory effect of LH on the E2 secretion, thus resulting in the increase in both A and E2 secretion. Additionally, the present results provide evidence that LH stimulates the release of ET-1 from the theca layer of bovine mature follicles in vitro. Similar findings were shown in a study by Girsh et al. in which LH stimulated the ET-1 content in a bovine corpus luteum slice incubation. The effect may be a direct stimulation of LH on the endothelial cells in the theca layer, since we could not find measurable concentrations of ET-1 in the medium used to culture bovine granulosa and theca cells (unpublished results). To support this hypothesis, mRNA for LH receptor must be detected in endothelial cells in the theca layer, which requires further investigation. buy diabetes drugs
As expected, a significant concentration of ET-1 was found in the perfusate from the theca layer. Based on the ET-1 concentration in the perfusate (2-7 pg/ml) and the transfer capacity of the microdialysis membrane for ET-1 (0.1%), the possible concentration of ET-1 in the extracellular fluid in the theca layer can be calculated to be in the range 2-7 ng/ml. This range is almost the same as that of the intraluteal ET-1 concentration as previously reported but about 100-fold higher than plasma ET-1 concentrations in the cyclic cow. These findings strongly support the concept that the bovine mature follicle is also a site of ET-1 production. A recent study by our group revealed that plasma ET-1 concentration increases during PGF2a-induced luteolysis as well as during the periovula-tory period. These findings suggest that ET-1 may play an important role in ovarian functions in relation to acute changes in blood capillary functions during those periods.