Preovulatory changes in the angiotensin II system in bovine follicles

The present study evaluated whether the gonadotrophin surge modulates components of the renin-angiotensin system and whether angiotensin II (Ang II) plays a role in the production of hormones by follicular cells during the ovulatory process. In Experiment 1, cows were ovariectomised at various times (0, 3, 6, 12 and 24 h) after GnRH injection to obtain preovulatory follicles. The concentration of Ang II in follicular fluid increased after GnRH and reached a peak at 24 h, concomitant with the peak of angiotensinogen (AGT) mRNA expression in granulosa cells. AGT mRNA was not expressed in theca cells. Ang II receptor type 2 and angiotensin-converting enzyme mRNA levels were transiently upregulated in theca cells. In Experiment 2, an in vitro culture was used to determine whether Ang II could modulate hormone production by healthy dominant follicles. In the absence of LH, Ang II did not alter hormonal production by either theca or granulosa cells. Ang II plus LH increased progesterone and prostaglandin secretion by granulosa cells. In summary, the renin-angiotensin system is actively controlled during the preovulatory period and Ang II amplifies the stimulatory effects of LH on the secretion of progesterone and prostaglandins by granulosa cells.