Prorenin and active renin concentrations in ovarian follicular fluid increase after the LH peak in superovulated heifers

Bovine ovarian cells have (pro)renin receptors and prorenin induces resumption of meiosis in vitro

The discovery of a receptor that binds prorenin and renin in human endothelial and mesangial cells highlights the possible effect of renin-independent prorenin in the resumption of meiosis in oocytes that was postulated in the 1980s.This study aimed to identify the (pro)renin receptor in the ovary and to assess the effect of prorenin on meiotic resumption. The (pro)renin receptor protein was detected in bovine cumulus-oocyte complexes, theca cells, granulosa cells, and in the corpus luteum. Abundant (pro)renin receptor messenger ribonucleic acid (mRNA) was detected in the oocytes and cumulus cells, while prorenin mRNA was identified in the cumulus cells only. Prorenin at concentrations of 10(-10), 10(-9), and 10(-8)M incubated with oocytes co-cultured with follicular hemisections for 15h caused the resumption of oocyte meiosis. Aliskiren, which inhibits free renin and receptor-bound renin/prorenin, at concentrations of 10(-7), 10(-5), and 10(-3)M blocked this effect (P<0.05). To determine the involvement of angiotensin II in prorenin-induced meiosis resumption, cumulus-oocyte complexes and follicular hemisections were treated with prorenin and with angiotensin II or saralasin (angiotensin II antagonist). Prorenin induced the resumption of meiosis independently of angiotensin II. Furthermore, cumulus-oocyte complexes cultured with forskolin (200μM) and treated with prorenin and aliskiren did not exhibit a prorenin-induced resumption of meiosis (P<0.05). Only the oocytes' cyclic adenosine monophosphate levels seemed to be regulated by prorenin and/or forskolin treatment after incubation for 6h. To the best of our knowledge, this is the first study to identify the (pro)renin receptor in ovarian cells and to demonstrate the independent role of prorenin in the resumption of oocyte meiosis in cattle.

Renin and ovarian vascularization in cows with follicular cysts after epidural administration of a GnRH analogue

The ovarian renin-angiotensin system may play an important role in follicular growth and maturation, as well as in the process of ovulation. The aim of this study was to investigate the effects of administration of a GnRH analogue to cows with ovarian follicular cysts on plasma renin concentrations and ovarian vascularization. This study was performed with 60 Friesian cows, which were diagnosed with follicular cysts, and randomly allocated into two groups: group A (treatment; n=30) received 2ml of lecirelin (Dalmarelin((R)) - Fatro), per head via sacro-coccygeal epidural, and group B (control; n=30) received 2ml saline solution (0.9% NaCl) per head by the same route. Blood samples were immediately collected prior to administration (T0) and then 24h (T1), 48h (T2) and 8 days (T3) after administration of the treatment, for both groups. Ovarian vascularization was evaluated utilizing Power Doppler on these same days in 10 animals from each group. The number of pixels detected by Power Doppler was used as an indicator of the degree of vascularization. Plasma renin concentrations remained relatively constant for the control (group B) animals, but increased as the sampling period progressed (NS) for the treated cows (group A). Similarly, there were no changes in ovarian vascularization (number of pixels) for the control cows, but vascularization increased throughout the sampling period in the treated animals. The number of pixels associated with cysts was significantly higher for treated compared to control cows at 24h after treatment (P<0.001). The epidural administration of a GnRH analogue was determined to be a highly effective therapy for follicular cysts (regression occurred in 82% of treated cows within 8+/-2 days after treatment, but in none of the control cows), which also enhanced ovarian vascularization.

Physiology of Local Renin-Angiotensin Systems

Since the first identification of renin by Tigerstedt and Bergmann in 1898, the renin-angiotensin system (RAS) has been extensively studied. The current view of the system is characterized by an increased complexity, as evidenced by the discovery of new functional components and pathways of the RAS. In recent years, the pathophysiological implications of the system have been the main focus of attention, and inhibitors of the RAS such as angiotensin-converting enzyme (ACE) inhibitors and angiotensin (ANG) II receptor blockers have become important clinical tools in the treatment of cardiovascular and renal diseases such as hypertension, heart failure, and diabetic nephropathy. Nevertheless, the tissue RAS also plays an important role in mediating diverse physiological functions. These focus not only on the classical actions of ANG on the cardiovascular system, namely, the maintenance of cardiovascular homeostasis, but also on other functions. Recently, the research efforts studying these noncardiovascular effects of the RAS have intensified, and a large body of data are now available to support the existence of numerous organ-based RAS exerting diverse physiological effects. ANG II has direct effects at the cellular level and can influence, for example, cell growth and differentiation, but also may play a role as a mediator of apoptosis. These universal paracrine and autocrine actions may be important in many organ systems and can mediate important physiological stimuli. Transgenic overexpression and knock-out strategies of RAS genes in animals have also shown a central functional role of the RAS in prenatal development. Taken together, these findings may become increasingly important in the study of organ physiology but also for a fresh look at the implications of these findings for organ pathophysiology.

Vascular control of ovarian function: ovulation, corpus luteum formation and regression

Hemodynamic changes are involved in the cyclic remodeling of ovarian structures. A transrectal color Doppler ultrasonography was used to assess the blood flow and changes in the vasculature that take place in the follicle wall and within the corpus luteum (CL) during specific physiological events such as ovulation, CL development, and CL regression in cows. To investigate the local release of vasoactive peptides, steroid hormones, and prostaglandins (PGs) in the ovarian microenvironment, the capillary membranes (0.2mm diameter and 5-10mm length) of a microdialysis system (MDS) were implanted into the follicle wall and the CL in vitro. Furthermore, in vivo experiments were conducted with the same MDS membranes surgically implanted in follicle wall or on CL along with ovarian venous and jugular catheters to collect simultaneous, real-time information on the ovarian and systemic changes in the secretion of factors regulating vascular function. Based on the results obtained from the series of in vitro and in vivo experiments, we propose that a functional "cross-talk" occurs between the vascular components (endothelial cells) and steroidogenic cells to control follicular and luteal functions in the bovine ovary.

Angiotensin converting enzyme in bovine ovarian follicular fluid and its relationship with oestradiol and progesterone

CONTENT

The purpose of the present study was to identify angiotensin converting enzyme (ACE) in bovine ovarian follicular fluid and to relate the ACE activity to the phase of the oestrous cycle, pregnancy, and the follicular fluid concentrations of oestradiol and progesterone. The ACE activity was similar to that found in bovine serum and was completely inhibited by the specific ACE inhibitor captopril. The 50% inhibitory concentration (IC50) was 1.4 x 10(-8) mol/l (range 0.8 x 10(-8) to 5.0 x 10(-8) mol/l; n=6), which is similar to that found in bovine and human serum. The ACE activity did not differ in the pre-ovulatory and luteal phase, pregnancy or cystic follicles. It correlated with the follicular fluid concentration of progesterone in cycling cows (rho=0.476; p < 0.005; n=36), but did not correlate with the diameter of the follicles, the follicular fluid concentration of oestradiol or the ratio between the oestradiol and progesterone concentrations. The demonstration of ACE in bovine ovarian follicular fluid provides further evidence for the presence of a local renin-angiotensin system in the bovine ovary.

Periovulatory changes in the local release of vasoactive peptides, prostaglandin f(2alpha), and steroid hormones from bovine mature follicles in vivo

We previously proposed that an endothelin-angiotensin-atrial natriuretic peptide system may contribute to inducing ovulation of mature bovine follicles by modulating follicular secretion of steroids and prostaglandins (PGs). Thus, this study aimed to determine the real-time changes in the local release of angiotensin II (Ang II), endothelin (ET), atrial natriuretic peptide (ANP), PGF(2alpha), and steroid hormones from bovine mature follicles during the periovulatory period in vivo. Seven cows were treated for superovulation using FSH and PGF(2alpha) injections. Two dialysis capillary membranes per follicle were surgically implanted into the theca layer of mature follicles and connected to a microdialysis system (MDS). Fractions of the perfusate were collected from Day -1 (Day 0 = LH surge) to Day 3. Five out of seven treated cows were normally ovulated, and the newly formed corpora lutea were observed at the end of the experiment. In these five ovulated cows, the release of estradiol, androstenedione, and progesterone in the theca layer increased (P < 0.05) synchronously with the LH surge. Acute increases in PGF(2alpha) and Ang II concentrations in the ovarian venous plasma (OVP) were observed at 24-48 h after the peak of the LH surge, when multiple ovulations were expected to occur. The follicular Ang II release was low during the pre-LH surge period and rose (P < 0.05) at the beginning of the increase in the LH surge. On the other hand, ET-1 release dropped (P < 0.05) when plasma LH started to increase. However, no clear changes in ANP concentration in the MDS perfusate and plasma were observed. The above local changes in Ang II, PGF(2alpha), as well as steroid hormones were not observed in cows (n = 2) that did not show an LH surge and ovulation. The present results demonstrate for the first time the local release of Ang II, ET-1, and ANP from the bovine mature follicle in real-time in vivo and show that Ang II and PGF(2alpha) concentrations in the OVP acutely increase around the time of ovulation. The overall results support the concept of a local functional ET-Ang-ANP system in the bovine mature follicle that may be involved in the ovulatory process.

Gonadotropins induce higher active renin levels in the follicular fluid of normal and hyperstimulated cycles

The objective of this study was to analyze follicular fluid active renin and its relationship to steroid hormones throughout the normal and gonadotropin-stimulated menstrual cycle. Active renin was measured in the follicular fluid of patients undergoing tubal sterilization (n = 16) and in vitro fertilization (IVF) (n = 25); IVF patients were either in a natural cycle (n = 7) or undergoing controlled ovarian hyperstimulation (n = 18). The largest visible follicle was aspirated at the time of laparoscopic tubal sterilization; ultrasound guided transvaginal follicular aspiration was used in the IVF group. Follicular fluid active renin, estradiol and progesterone levels were measured with immunoradiometric and fluoroimmunoassays. The cycle day was correlated with the spontaneous luteinizing hormone (LH) surge or human chorionic gonadotropin (hCG) administration, as well as active renin, estradiol, progesterone levels and the estradiol/progesterone ratio using simple and multiple regression and analysis of variance (ANOVA). Cycle day independently influenced active renin, progesterone and the estradiol/progesterone ratio in a statistically significant manner (p < 0.0001). The active renin and progesterone levels were highest during the periovulatory period (p < 0.0001 and p < 0.002, respectively) and the estradiol/progesterone ratio correlated inversely with cycle day (p < 0.003). Although the follicular fluid active renin, estradiol and progesterone levels were higher after controlled ovarian hyperstimulation when compared to natural cycles, this difference did not reach statistical significance. Our findings suggest that active renin levels in follicular fluid increase in the follicular phase of the menstrual cycle, reaching peak levels in the periovulatory period following the LH surge or hCG administration, providing indirect support for the hypothesis that the ovarian renin-angiotensin system (RAS) is under gonadotropin control.

Angiotensin II: a reproductive hormone too?

It has long been known that angiotensin II (Ang II) can affect reproductive tissues such as the uterus. However, the existence of a local renin-angiotensin system (RAS) in female as well as male reproductive tissues is a relatively recent observation. Of great interest is the discovery that all components of the RAS are present in the ovary, that the ovary secretes components of the RAS into the bloodstream, and that the ovary itself is responsive to Ang II. Recent studies suggest that the primary role of Ang II in the ovary is to cause atresia in non-ovulatory follicles; however, there is also compelling data to suggest that Ang II facilitates ovulation. Male reproductive structures also contain all of the components of the RAS, gonadotropins regulate the activity of these components, and these tissues have Ang II receptors. Of great interest is the expression of testis-specific angiotensin-converting enzyme (ACE), which is located on germ cells. Recent studies using gene knock-out techniques indicate that testis ACE plays an important role in male fertility. However, the overall significance of the RAS for normal reproductive function remains questionable. There is now a body of evidence implicating the RAS in pathophysiologies associated with reproductive function, which gives rise to the possibility that drugs acting on the RAS might ameliorate some of these disorders. Considerable work remains to determine the role of Ang II in reproductive functions.

The ovarian renin-angiotensin system in reproductive physiology

The identification of the presence of prorenin, renin, angiotensinogen, angiotensin-converting enzyme, angiotensin II (Ang II), and Ang II receptors in the ovary suggests that there is a functional ovarian renin-angiotensin system (RAS). It could play a significant role in such areas of ovarian physiology as follicular development, steroidogenesis, oocyte maturation, ovulation, and follicle atresia. Expression of the ovarian RAS is regulated by gonadotropins. Ang II, a bioactive octapeptide of RAS, has important effects as a paracrine/autocrine regulator at different stages of the reproductive cycle. Ang II modulates ovarian steroidogenesis and formation of the corpus luteum and also stimulates oocyte maturation and ovulation via Ang II receptors on granulosa cells. In addition, increasing evidence demonstrates that Ang II is a major factor in regulating the function of atretic follicles. In any physiologic system, aberrations result in the development of pathologic states. Disturbances in the ovarian RAS can be the cause or the result of such reproductive disorders as polycystic ovary syndrome, ovarian hyperstimulation syndrome, ovarian tumors, and ectopic pregnancy. Data support the concept of an active and regulated RAS in ovarian follicles. Species differences observed in the expression of ovarian RAS suggest varying functional roles among species with respect to ovarian physiology.