Effect of gonadotropins and pregnancy on prorenin and renin in the rat

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.

First report of active renin in rat amniotic fluid

1. Epidemiological studies indicate that a reduced birthweight increases the likelihood of human cardiovascular disease later in life. The role of hormonal factors in this finding is not known. Given that angiotensin II is believed to be a fetal regulator of growth, we have examined in the hypertensive Ren-2 transgenic rat whether it has active renin in its amniotic fluid and whether this is associated with fetal underdevelopment. 2. We found that while the Sprague-Dawley rat contained no active renin in its amniotic fluid near term (20 days), Ren-2 amniotic fluid contains high levels of active renin and is associated with a reduced fetal weight. 3. This is the first report of active renin in the rat and allows the possibility that renin overproduction plays a role in reduced fetal growth and the prenatal 'programming' of essential hypertension that has been proposed to occur in humans.

Effect of nitric oxide synthase inhibition on cardiovascular and hormonal regulation during pregnancy in the rat

Recent studies have shown that nitric oxide (NO) biosynthesis increases in pregnancy and that inhibition of nitric oxide synthase (NOS) induces some pathological processes characteristic of preeclampsia. The current project sought to study the effect of the NOS inhibitor Nω-nitro-L-arginine methyl ester (L-NAME, 10 µg·min-1, sc for 7 days) on plasma volume, plasma atrial natriuretic factor (ANF), plasma endothelin-1 (ET), and plasma renin activity (PRA) during gestation in conscious rats. NOS inhibition caused mean arterial pressure to increase in both virgin and 21-day pregnant rats. Plasma volume fell in the pregnant rats [L-NAME, 4.5 ± 0.3 mL·100 g-1 body wt. (n = 7) vs. D-NAME, 6.8 ± 0.2 mL·100 g-1 body wt. (n = 10); P < 0.05] but not in the virgin rats [L-NAME, 4.3 ± 0.1 mL·100 g-1 body wt. (n = 6) vs. D-NAME, 4.8 ± 0.2 mL·100 g-1 body wt. (n = 8)]. There was no effect of NOS inhibition on plasma ANF levels or PRA in either the virgin or pregnant rats. However, L-NAME did decrease plasma ET levels in the pregnant rats [L-NAME, 19.6 ± 1.6 pg·mL-1 (n = 8) vs. D-NAME, 11.6 ± 2.5 pg·mL-1 (n = 9); P < 0.05]. Our results confirm that NO is involved in cardiovascular homeostasis in pregnancy; NOS inhibition selectively reduces plasma volume in pregnant rats, thus mimicking a major pathophysiological perturbation of preeclampsia. However, it does not induce the hormonal changes characteristic of preeclampsia, namely the decrease in PRA and increase in plasma ET and ANF levels.

Key words: plasma volume, preeclampsia, endothelin, atrial natriuretic factor, plasma renin activity.

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.

Reproduction and the renin-angiotensin system

A unique aspect of the circulating renin-angiotensin system and the many independent tissue renin-angiotensin systems is their interactions at multiple levels with reproduction. These interactions, which have received relatively little attention, include effects of estrogens and possibly androgens on hepatic and renal angiotensinogen mRNA; effects of androgens on the Ren-2 gene and salivary renin in mice; the prorenin surge that occurs with but outlasts the LH surge during the menstrual cycle; the inhibitory effects of estrogens on thirst and water intake; the tissue renin-angiotensin systems in the brain, the anterior pituitary, and the ovaries and testes, that is, in all the components of the hypothalamo-pituitary-gonadal axis; the presence of some components of the renin-angiotensin system in the uterus and the fetoplacental unit; and the possible relation of renin and angiotensin to ovulation and fetal well-being. These interactions are described and their significance considered in this short review.

Ovarian derived prorenin-angiotensin cascade in human reproduction

OBJECTIVE

To review the available literature concerning the renin-angiotensin system of the human and animal ovary and to outline the clinical relevance of this system.

DESIGN

The location, function, and regulation of the components of the ovarian prorenin cascade are described. The possible functions of this system as well as its association with common gynecologic problems are also given.

CONCLUSIONS

The ovary contains a complete cascade whose end product is the formation of angiotensin II. Angiotensin II may have a role in steroid synthesis, oocyte maturation, ovulation, and corpus luteum formation. Further, aberrations in this system are associated with ovarian tumors, ectopic pregnancy, pre-eclampsia, and ovarian hyperstimulation syndrome.

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

1. The aim was to analyse the in vivo variations with time of prorenin and active renin and their relationship to steroid hormones in ovarian follicular fluid during follicular growth in heifers. 2. Thirty one beef heifers were assigned to two groups after oestrous synchronization: an unstimulated and a follicle-stimulating hormone (FSH)-treated (superovulated) group. Within each group, animals were slaughtered at different times of the follicular phase of the oestrous cycle. Ovarian follicular fluids were aspirated and analysed for the concentrations of active renin, prorenin, oestradiol-17 beta (E2) and progesterone (P4). 3. Prorenin and active renin concentrations in follicular fluid remained constant until the luteinizing hormone (LH) peak, after which time they increased four- and two-fold, respectively, in superovulated heifers. 4. In follicular fluid, prorenin and active renin correlated negatively with oestradiol and E2/P4 ratio but positively with progesterone during follicular growth in superovulated heifers. Prorenin also correlated negatively with oestradiol and E2/P4 ratio in unstimulated heifers. 5. The increase of renin concentrations in ovarian follicles after the LH peak and the correlations to steroid hormones suggest an important role of the ovarian renin-angiotensin system in bovine follicular growth and maturation.

Inactivation of renin substrate by soybean trypsin inhibitors: implications for measurement of circulating inactive renin

Semipurified soybean trypsin inhibitor added to rat and human plasma leads to a concentration dependent decrease in the rate of angiotensin I generation. This inhibition is due to binding of renin substrate to the inhibitor. Renin substrate present in nephrectomized rat plasma was more susceptible to binding than substrate of the normal rat suggesting structural differences in the substrate generated following nephrectomy. Because trypsin inhibition is necessary for measurement of active and inactive renin, we examined several alternate trypsin inhibitors. The Bowman-Birk inhibitor from soybean had similar actions as purified soybean trypsin inhibitor while trypsin inhibitors from lima bean and chicken did not depress renin substrate, but did have variable effects on the measured levels of active and total plasma renin. Surprisingly, crude soybean trypsin inhibitor did not suppress renin substrate and actually increased angiotensin I generation during PRA and PRC measurements. Since the crude preparation did not suppress renin substrate, changes in the specificity of the inhibitor may occur during its purification. The augmentation of PRA and PRC may be related to angiotensinase inhibitory actions.

Renin and prorenin in reproductive tissues during gestation in pigs and cattle

1. High concentrations of prorenin and active renin were previously found in ovarian follicular fluid from cattle but not from pigs. In the present study female reproductive tissues and fluids from cattle and pigs during gestation were investigated to clarify a possible species difference in active renin and prorenin concentrations. 2. Very high concentrations of active renin but no prorenin were found in corpus luteum from both species. 3. Relatively low concentrations of active renin, in the same order as in maternal blood plasma, were found in myometrium, endometrium, placenta and fetal membranes from both species. Prorenin was undetectable in these tissues except for bovine myometrium and porcine endometrium in some animals. 4. The concentrations of active renin and prorenin in amnionic fluid from both species were below the maternal plasma values. In allantoic fluid the concentrations were higher than in amnionic fluid. 5. The plasma concentrations of active renin and prorenin did not change during gestation in pigs. This finding is in contrast to the observations in humans and does not support a systemic effect of prorenin during gestation. 6. The presence of renin in the reproductive tissues, especially the very high concentrations in the corpus luteum, indicates a local function of the renin-angiotensin system during gestation.