Angiotensins I and II, active and inactive renin, renin substrate, renin activity, and angiotensinase in human liquor amnii and plasma

Active plasma renin concentration throughout healthy and complicated pregnancy: a systematic review and meta-analysis

BACKGROUND

Pregnancy is characterized by profound circulatory changes and compensatory adjustments in the renin-angiotensin-aldosterone system (RAAS). Differences in regulatory response may antedate or accompany vascular complicated pregnancy. We performed a systematic review and meta-analysis to delineate the trajectory of active plasma renin concentration (APRC) in healthy pregnancy and compare this to complicated pregnancy.

METHODS

We performed a systematic review and meta-analysis on APRC during normotensive and hypertensive pregnancies, using PubMed (NCBI) and Embase (Ovid) databases. We included only studies reporting measurements during pregnancy together with a nonpregnant reference group measurement. Risk of bias was assessed with QUIPS. Ratio of the mean (ROM) and 95% confidence intervals (CI) of APRC values between pregnant and nonpregnant women were estimated for predefined intervals of gestational age using a random-effects model. Meta-regression was used to analyze APRC over time.

RESULTS

In total, we included 18 studies. As compared to nonpregnant, APRC significantly increased as early as the first weeks of healthy pregnancy and stayed increased throughout the whole pregnancy (ROM 2.77; 95% CI 2.26-3.39). APRC in hypertensive complicated pregnancy was not significantly different from nonpregnancy (ROM 1.32; 95% CI 0.97-1.80).

CONCLUSION

Healthy pregnancy is accompanied by a profound rise in APRC in the first trimester that is maintained until term. In hypertensive complicated pregnancy, this increase in APRC is not observed.

Effects of Fetal Sex on Expression of the (Pro)renin Receptor and Genes Influenced by its Interaction With Prorenin in Human Amnion

Males are more likely to be born preterm than females. The causes are unknown, but it is suggested that intrauterine tissues regulate fetal growth and survival in a sex-specific manner. We postulated that prorenin binding to its prorenin/renin receptor receptor (ATP6AP2) would act in a fetal sex-specific manner in human amnion to regulate the expression of promyelocytic zinc finger, a negative regulator of ATP6AP2 expression as well as 2 pathways that might influence the onset of labor, namely transforming growth factor β1 (TGFB1) and prostaglandin endoperoxide synthase 2 (PTGS2). Our findings demonstrate that there are strong interactions between prorenin, ATP6AP2, and TGFB1 and that this system has a greater capacity in female amnion to stimulate profibrotic pathways, thus maintaining the integrity of the fetal membranes. In contrast, glucocorticoids or other factors independent of the prorenin/prorenin receptor pathway may be important regulators of PTGS2 in human pregnancy.

Effects of verapamil in hypertensive patients

In fifteen patients with uncomplicated essential hypertension verapamil reduced systolic as well as diastolic blood pressure by virtue of a decrease in total peripheral vascular resistance. Renal vascular resistance also fell, but renal perfusion was well maintained. Although levels of renin and angiotensin II remained unchanged, aldosterone levels fell significantly. Noradrenaline levels rose slightly, but changes were not significant. On the other hand adrenaline was significantly reduced. Renal sodium excretion was markedly enhanced, perhaps secondary to aldosterone suppression. Despite sodium loss, plasma volume increased suggesting that verapamil not only lowers arterial but also venous resistance.

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.

Characterization of renin mRNA expression and enzyme activity in rat and mouse mesangial cells

Renin is an enzyme involved in the stepwise generation of angiotensin II. Juxtaglomerular cells are the main source of plasma renin, but renin activity has been detected in other cell types. In the present study we evaluated the presence of renin mRNA in adult male Wistar rat and mouse (C-57 Black/6) mesangial cells (MC) and their ability to process, store and release both the active and inactive forms of the enzyme. Active renin and total renin content obtained after trypsin treatment were estimated by angiotensinogen consumption analyzed by SDS-PAGE electrophoresis and quantified by angiotensin I generation by HPLC. Renin mRNA, detected by RT-PCR, was present in both rat and mouse MC under basal conditions. Active renin was significantly higher (P<0.05) in the cell lysate (43.5 +/- 5.7 ng h-1 10(6) cells) than in the culture medium (12.5 +/- 2.5 ng h-1 10(6) cells). Inactive prorenin content was similar for the intra- and extracellular compartments (9.7 +/- 3.1 and 3.9 +/- 0.9 ng h-1 10(6) cells). Free active renin was the predominant form found in both cell compartments. These results indicate that MC in culture are able to synthesize and translate renin mRNA probably as inactive prorenin which is mostly processed to active renin inside the cell. MC secrete both forms of the enzyme but at a lower level compared with intracellular content, suggesting that the main role of renin synthesized by MC may be the intracellular generation of angiotensin II.

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.

In vivo enzymatic assay reveals catalytic activity of the human renin precursor in tissues

The aspartyl protease renin is secreted into the circulation of mammals in 2 forms: the proteolytically processed active form of the enzyme and the precursor form, prorenin. Prorenin has no detectable enzymatic activity in the circulation, but it is the exclusive form of the enzyme produced by several tissues that also produce the other components of the renin enzymatic cascade (renin-angiotensin system). To test whether prorenin might be enzymatically active in these tissues, transgenic mice expressing the human renin substrate (angiotensinogen) exclusively in the pituitary gland were mated to mice expressing either active human renin or prorenin in the same tissue. Measurement of in vivo product formation in pituitary glands of double-transgenic mice revealed that human prorenin was enzymatically active, and Western blot analysis demonstrated that this prorenin was in the precursor form with its prosegment attached. This in vivo enzymatic assay demonstrates for the first time that human prorenin can be activated within tissues by nonproteolytic means, where it could contribute to the activity of a localized renin-angiotensin system.

The human placental renin-angiotensin system

The human placenta and related tissues are considered to be examples of the recently accepted local renin-angiotensin systems (RAS). The brain is another example of a system that is thought to be regulated independently of the kidney and the role of angiotensin within the CNS as a neural mediator has drawn considerable attention. It has been known for a long time that many of the neuroendocrine mediators and receptors are expressed in the placenta and it has been suggested that there are many parallels between the classical neuroendocrine system and the placental one. The present review summarizes information that components of the RAS are expressed in uteroplacental tissues, are regulated by endogenous substances, and have important biological functions within this reproductive system. A comparison of similarities and differences between the classical and the placental RAS may provide clues to functions in other endocrine and neuroendocrine systems. The major components of the placental RAS that are considered are renin, prorenin, angiotensin I, angiotensin II, angiotensin converting enzyme (ACE), angiotensin receptors, and angiotensinogen (renin substrate). The factors that regulate these components at the cellular and the nuclear level are described. It is concluded that prorenin via angiotensin-dependent and angiotensin-independent mechanisms influences functions within uteroplacental tissues. Some of these actions are direct and others are mediated by the release of different signalling molecules. These features are similar to many neuroendocrine systems and utilize some of the same messengers.

Renin stimulates decidual prostaglandin production via a novel mechanism that is independent of angiotensin II formation

Renin is a proteolytic enzyme that has been considered to have only one function which is to cleave angiotensinogen between the 10th and 11th amino acids to form angiotensin-1. This is then converted to angiotensin-II, a potent vasoconstrictor, antinatriuretic and antidiuretic by angiotensin-converting enzyme. We have investigated the action of renin to stimulate prostaglandin production by decidual cells and in so doing have generated data that challenge the prevailing dogma. Renin stimulates decidual prostaglandin production in a concentration-related fashion that is unaffected by saralasin treatment. This stimulatory action of renin is enhanced rather than reduced by arachidonic acid treatment but abolished by treatment with cycloheximide or actinomycin D. Renin caused a more rapid recovery of decidual prostaglandin biosynthesis from acetylsalicylic acid treatment than did control media. Moreover, renin treatment of both decidual and amnion cells induced increased levels of PGHS-2 within 2 h. Collectively, these results indicate that renin can act directly, separately from the generation of angiotensin-I and II. In this case renin can induce PGHS expression.

First-trimester villous placenta has high prorenin and active renin concentrations

OBJECTIVE

Term villous placental concentrations of prorenin are known to be very low, whereas those of decidua and fetal membranes are high. It has been demonstrated that prorenin synthesis is modulated by hormones in other reproductive tissues, thus suggesting a means for paracrine regulation in the placenta. This study was performed to test the hypothesis that placental tissue prorenin concentrations may be influenced by gestational age and are temporally related to alterations in the hormonal milieu.

STUDY DESIGN

Decidua and villous placental tissue were obtained from term and first-trimester human pregnancies, and concentrations of prorenin, active renin, prolactin, and human chorionic gonadotropin were measured. Values were compared between gestational periods, and relationships between renin and hormone values were analyzed.

RESULTS

Prorenin concentrations in first-trimester placenta were nearly 200-fold higher than at term. The proportion of active renin was higher with early gestation. Decidual prorenin and active renin concentrations were similar in both groups. Placental prorenin correlated with chorionic gonadotropin but not prolactin in both groups.

CONCLUSIONS

This study demonstrates large differences in placenta prorenin and active renin in villous placental tissue between first-trimester and term gestation tissues, yet these differences were not observed in decidual tissues. The contrast in placental prorenin values observed at the extremes of pregnancy parallel those of placental human chorionic gonadotropin.

Control of vascular resistance in the maternal and feto-placental arterial beds

This review aims to provide a comprehensive summary of the mechanisms involved in the physiological adaptation of the vasculature to pregnancy. Profound changes occur both systemically and in discrete circulations in the mother, but it is debatable which factors are responsible. Similarly, whilst the feto-placental circulation must be substantially controlled by humoral mechanisms, the exact role of each potential contributor is not known. In view of the hitherto unappreciated and very important role of the endothelium-derived vasodilator, nitric oxide, in the control of peripheral vascular resistance, considerable emphasis will be placed on the many recent investigations in this area.

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.

Measurement and identification of prorenin and renin in ovarian follicular fluid from cattle and pig

1. In previous studies we have demonstrated and solved several methodological problems in relation to the measurement of prorenin by trypsin activation in rat, bovine, hog and horse plasma. 2. The aim of the present study was to develop a method for the measurement of prorenin in bovine and porcine ovarian follicular fluid. 3. Trypsin activation of follicular fluid generated angiotensin I immunoreactive material (AI IM) in both species. 4. The AI IM interfered with the renin assay, but could be completely removed by a cation exchange resin in a batch-wise technique. 5. The enzymatic activity of trypsin-activated prorenin and pre-existing active renin was completely inhibited by a specific inhibitor of renin. 6. The reactions were optimized and an accurate measurement of prorenin in ovarian follicular fluid was developed. 7. The existence of prorenin and renin in bovine ovarian follicular fluid was established. Prorenin and renin in porcine ovarian follicular fluid was demonstrated for the first time. 8. The ratio between ovarian follicular fluid and plasma was 43 for prorenin and 19 for active renin in cattle. The same ratios in pigs were 1.3 and 0.4, respectively. These findings indicate a species difference with respect to the amount of prorenin or active renin present in ovarian follicular fluid.

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

1. Stimulation of adult female rats with pregnant mare serum gonadotropin (PMSG) and human chorion gonadotropin (hCG) increased active plasma renin about two-fold, but caused only a slight increase of plasma prorenin. The concentrations of active renin and prorenin in the ovaries, and active renin in the uterus all increased about two-fold 2 days after stimulation with PMSG. The prorenin in the uterus was below detection in unstimulated rats and did not change consistently after PMSG. 2. Active renin and prorenin in plasma were unchanged in relation to pregnancy, except for a slight decrease of prorenin in the third trimester. In the first and third trimester the concentration in the ovaries of active renin and prorenin was decreased to about one-third of that in normal female rats. In contrast active renin in the uterus was increased about two-fold in the first trimester, whereas prorenin did not change consistently. 3. Our results confirm that gonadotropins and pregnancy affect the renin-angiotensin system in rats. However, the changes in the plasma seem to be much smaller than those previously reported in humans. Accordingly, our results do not support a systemic role of prorenin for reproduction in the rat.

Measurement of renin and prorenin in cattle, hog and horse

1. Species specific problems complicating the measurement of prorenin and renin concentrations were studied in bovine, hog and horse plasma. 2. In contrast to horse renin, bovine and hog renin reacted with rat angiotensinogen, allowing measurement of the plasma renin concentration in cattle and hog with rat angiotensinogen as exogenous substrate. 3. Trypsin treatment of plasma in order to activate prorenin generated an interfering angiotensin I immunoreactive material in all three species, most extensively in horse plasma. 4. This material could be removed in bovine and hog plasma by a cation-exchange resin, allowing an assay of the plasma prorenin concentration to be constructed in these species. 5. Another strategy has to be followed in order to measure prorenin and renin concentrations in horse plasma.

Sexual dimorphism of inactive renin in rat plasma

The plasma concentration of inactive renin was higher (p less than 0.002) in male (0.48 Goldblatt units (GU)/l, range 0.28-0.67 GU/l; n = 20) than in female rats (0.19 GU/l, range 0.09-0.30 GU/l; n = 20). This new finding indicates a sexual dimorphism of inactive plasma renin similar to that recently reported in mice and humans. Castration of male rats (n = 10) decreased (p less than 0.01) inactive plasma renin from 0.45 GU/l (range 0.22-0.65 GU/l) to 0.26 GU/l (range 0.15-0.37 GU/l) over a period of seven weeks. Similarly, inactive renin decreased (p less than 0.01) from 0.50 GU/l (range 0.28-0.67 GU/l) to 0.30 GU/l (range 0.19-0.47 GU/l) during treatment with the anti-androgen flutamide (n = 10). In castrated male rats (n = 10), inactive plasma renin was unaffected of substitution with testosterone. In intact female rats (n = 10), inactive renin increased (p less than 0.05) from 0.18 GU/l (range 0.12-0.30 GU/l) to 0.27 GU/l (range 0.15-0.40 GU/L) during testosterone treatment. Our findings indicate that androgens may be involved in the regulation of the plasma concentration of inactive renin in rats, but also suggest that other factors may be of importance.

Renin substrate in human amniotic fluid

Purified human amniotic fluid renin substrate (RS) was compared to purified plasma RS. RS in plasma and amniotic fluid were similar in molecular weight, isoelectric point and immunological properties. Immunoreactivity of radio-iodinated amniotic fluid RS was lower than that of plasma RS. Measured by direct radioimmunoassay, RS-levels were only 10-22% of those obtained with indirect assay in 22 amniotic fluid samples. This difference suggests that amniotic fluid RS is less immunoreactive than plasma RS, possibly due to biochemical alteration or complex formation. No such difference in immunoreactivity was noticed in RS of decidual and placental cytosolic fraction.

The ovarian prorenin-angiotensin system. Lessons from IVF

Recent evidence suggests that prorenin and other active components of the renin-angiotensin system may be linked to ovarian physiology. Prorenin, the inactive form of the enzymatically active renin, is present in the fluid of mature human ovarian follicles in concentrations more than 10 times higher than that found in the plasma of women undergoing ovarian stimulation. Only 1% of the renin in follicular fluid is in active form. Concentrations of prorenin in fluid of immature follicles are lower than levels detected in concomitantly aspirated mature follicles. Study of prorenin levels disclosed a positive correlation to testosterone and E2 in fluids of mature follicles. Plasma prorenin increases about twofold at midcycle at the time of the LH surge; the peak of prorenin is sustained for about 40 hours. In patients undergoing ovarian stimulation for IVF, hCG administration results in much higher plasma prorenin levels and the height of prorenin response is directly related to the number of mature follicles. It peaked 4 to 6 days after hCG injection and then fell, close to baseline, by about 12 days after hCG administration. Examination of the time course of hormonal changes in response to hCG revealed a temporal relationship between prorenin and both plasma E2 and progesterone. In women who conceived, prorenin began to rise again on days 8 to 12 after embryo transfer when endogenous hCG was detected in the blood. No such changes in prorenin occurred in women with ovarian failure who conceived after transfer of a donor egg. These findings indicate that prorenin is produced by the mature follicle and the corpus luteum in response to LH/hCG. Since angiotensin II affects intracellular calcium and phospholipase activity, there are many potential roles for prorenin via angiotensin II action. Putative actions of the ovarian renin system may include control of oocyte maturation, ovulation, ovarian blood flow, and ovarian steroid biosynthesis. Future work to elucidate the function of this new renin system may have relevance to many basic and clinical aspects of human reproduction.

Plasma prorenin response to human chorionic gonadotropin in ovarian-hyperstimulated women: correlation with the number of ovarian follicles and steroid hormone concentrations

Plasma prorenin and active renin were measured before and after human chorionic gonadotropin (hCG) administration in two groups of patients undergoing ovarian stimulation for 4-6 days with follicle-stimulating hormone alone or in combination with luteinizing hormone, for in vitro fertilization. Baseline total plasma renin (prorenin plus active renin; n = 12) averaged 25 +/- 8 ng/ml per hr (mean +/- SD). Total renin did not change during ovarian stimulation but it increased to 46 +/- 16 ng/ml per hr (P less than 0.05) 1 or 2 days later, just before hCG administration. Thirty-six hours after hCG administration, just before laparoscopy and egg retrieval, total renin was 123 +/- 97 ng/ml per hr; a peak of 182 +/- 143 ng/ml per hr occurred 2-6 days later--i.e., during the luteal phase of the menstrual cycle. In eight of the patients who did not conceive, total renin returned to baseline 14 days after hCG administration. In four who conceived, a nadir was reached (57 +/- 13 ng/ml per hr) 8-12 days after hCG administration and then total renin increased again as the plasma beta hCG measurement began to rise. By day 16 it averaged 225 +/- 157 ng/ml per hr. In a second group of five patients active renin and prorenin were measured separately. Active renin comprised less than 20% of the total renin at all times. It was unchanged until day 4 after hCG administration and then increased significantly only when plasma progesterone was high. Thus, the initial response to hCG was entirely due to an increase in prorenin. A highly significant correlation was observed between the number of follicles and the total renin increases on the day of aspiration (r = 0.93, P less than 0.001) and at the peak (r = 0.89, P less than 0.001). After hCG administration, a temporal relationship was observed between the rise in total renin and plasma estradiol and progesterone levels. These results demonstrate that plasma prorenin increases markedly after administration of hCG and that the rise is directly related to the number of ovarian follicles and to plasma estrogen and progesterone levels. The findings suggest that prorenin is produced by the mature ovarian follicle and by the corpus luteum in response to gonadotropin stimulation.

The inhibitory role of 12- and 15-lipoxygenase products on renin release

Release of arachidonic acid from membrane phospholipids is a limiting step in the synthesis of both cyclooxygenase products and lipoxygenase products. The direct effects of prostacyclin and some lipoxygenase products on renin release were studied using rat renal cortical slices. Prostacyclin, at concentrations of 10(-5) M, stimulated renin secretion, but this effect was short-lived. Leukotrienes or their precursor, 5-hydroperoxyeicosatetraenoic acid, did not affect basal renin release. In contrast, 10(-9) M 12-hydroperoxyeicosatetraenoic acid and 10(-8) M 12-hydroxyeicosatetraenoic acid were potent inhibitors of renin secretion. Similarly, 15-hydroperoxyeicosatetraenoic acid and its hydroxy derivative, 15-hydroxyeicosatetraenoic acid, at somewhat higher molar concentrations (10(-6) M) also reduced basal renin. These studies confirm prostacyclin as a potential renin secretagogue; however, its action in vitro is transient, probably because of its rapid degradation. Our studies provide new evidence that products of the 12-lipoxygenase and 15-lipoxygenase pathways, reported to be present in renal vascular tissue, are potent inhibitors of renin secretion and much more active on a molar basis on renin secretion than is prostacyclin. These studies suggest the potential presence of a dual system of stimulation and suppression that may regulate renin secretion in normal and clinical states.

Studies on the effects of angiotensin II on human chorionic plate arteries and their modification by a calcium antagonist, nitrendipine

1 Six-point concentration-response curves were initially established for the contractile effect of angiotensin II (AII), at final concentrations of 10(-11) to 10(-6) M, on helically-cut strips of human chorionic plate arteries. The strips were then randomly allocated to one of four treatment groups: a control group and three treatment groups (n = 10 in each group) which were exposed to carrier or nitrendipine 10(-11), 10(-10) or 10(-9) M final concentration respectively, before the establishment of second concentration-response curves. 2 The initial responses of the tissues to AII were very similar in the four treatment groups with median EC50s ranging between 2.51 X 10(-10) and 4.04 X 10(-10) M (P greater than 0.8). The addition of nitrendipine was associated with significant (P less than 0.005) dose-related inhibition of AII-induced contractions. The median EC50 of the control group during the second half of the experiment was 1.77 X 10(-7) M, while that of the 10(-11) M nitrendipine treatment group was 2.16 X 10(-7) M. The EC50 for the other treatment groups could not be calculated from the curves due to the markedly blunted responses. 3 It is thus suggested that nitrendipine might be effective in vivo in the relief of vasospasm of the placental vascular system in pregnancy-induced hypertension.

Role of alpha 1- and alpha 2-adrenergic receptors in the human hypertensive kidney

Since it is not known for certain which alpha-adrenergic receptors mediate renal vasoconstriction in human essential hypertension, we infused either doxazosin (n = 7) or yohimbine (n = 7) into the renal arteries of hypertensive subjects immediately prior to diagnostic angiography. Both agents caused an increment in renal blood flow as assessed with the xenon-washout technique. Doxazosin increased renal flow from 342 +/- 36 to 360 +/- 55 ml/min per 100 g (0.05 less than p less than 0.10). Yohimbine enhanced flow from 380 +/- 41 to 485 +/- 63 ml/min per 100 g (p less than 0.01). The effect of yohimbine was significantly greater than that of doxazosin. In a control group (n = 7) receiving only saline, no changes in renal blood flow occurred. Doxazosin enhanced renin secretion in the kidney by 10 +/- 4% over levels in controls (0.05 less than p less than 0.10), whereas yohimbine increased renin release by 80 +/- 23% (p less than 0.01). The latter increase was apparently not due to alterations in flow alone, since the arteriovenous gradient for renin also widened. We conclude that in resting conditions, neurogenic vascular tone in the kidney depends mainly upon activation of alpha 2-adrenergic receptors. Moreover, these receptors exert a tonic inhibitory influence on renin release.

Postural suppression of plasma atrial natriuretic polypeptide concentrations in man

The effects of sequential changes in posture, from recumbency, to sitting and then to the upright position, each for 60 min, respectively, on the levels of plasma immunoreactive atrial natriuretic polypeptide (ANP) in healthy human subjects were studied using a radioimmunoassay (RIA) method. At the end of each change in posture, plasma ANP levels were respectively 150 +/- 16.4 pg/ml (recumbent), 103 +/- 11.2 pg/ml (sitting), and 78.1 +/- 7.90 pg/ml (upright). In contrast, plasma renin concentration (PRC) and plasma aldosterone concentration (PAC) determined concomitantly with ANP showed a significant increase in response to the sitting and upright postures. Plasma ANP levels determined in normal subjects who remained in the recumbent posture for the same period did not show any significant change. This suggests that ANP is involved in the maintenance of haemodynamic homeostasis under physiological conditions and emphasizes that postural factors must be taken into account and controlled in order to evaluate plasma ANP levels properly, as well as those of PRC and PAC.

Direct determination of plasma renin concentration during pregnancy: comparison with renin enzymatic activities

A solid phase sandwich assay for human renin was used to determine the total renin concentration in plasma throughout the course of a normal pregnancy. These values were compared with the enzymatic activity of renin, with and without trypsin treatment. Plasma concentration of immunoreactive renin rapidly increased four fold from 214 +/- 33 pg/ml (in non-pregnant subjects) within a few weeks after conception and remained high up to the third trimester. Total renin concentration, estimated by trypsin-activated enzyme activity showed similar changes, both qualitatively and quantitatively. A positive correlation (r = 0.63, p less than 0.001) between immunoreactive renin concentration and its enzymatic activity was noted whereas the active renin concentration did not positively correlate with the immunoreactive renin concentration. These data indicate that solid phase sandwich assay for human renin should be a pertinent tool to elucidate the role of the renin-angiotensin system in pregnancy.

Inactive renin in peritoneal fluid of the cat: responses to acute stimuli which alter plasma renin

Active and inactive renin in plasma and peritoneal dialysate were studied in pentobarbitone anesthetized cats during 8 hours of peritoneal dialysis. In control cats, despite a significant rise in plasma active renin from 1.3 +/- 0.22 to 4.4 +/- 0.76 pmoles AI/ml/hr over 8 hours, plasma inactive renin was unchanged. Four-fold rises in plasma active renin after hemorrhage (15 ml/kg) and captopril (1 mg/Kg I.V.) were unaccompanied by significant change in inactive renin. Bilateral nephrectomy resulted in undetectable plasma active renin after 3 hours but plasma inactive renin was 25-30% of initial levels 6 hours post-nephrectomy. In peritoneal dialysate, active renin remained low in control cats but there was a ten-fold rise of inactive renin from 0.06 +/- 0.02 to 0.60 +/- 0.07 pmoles AI/ml/hr. After hemorrhage there was a slight rise of active renin after 3 and 4 hours. Inactive renin was lower than control 6 hours after nephrectomy and captopril, but higher than control 1 and 2 hours after hemorrhage. Dialysate inactive renin accumulation slopes were not different among treatments. Acute changes in plasma active renin have little effect on peritoneal dialysate renin.

Ovarian prorenin

We review here recent evidence that the ovaries synthesize and secrete prorenin and we explore the possible reasons why prorenin, and not active renin, is formed almost exclusively in this extra-renal site. Very high concentrations of prorenin are present in the human ovary in the fluid inside mature follicles. This ovarian prorenin appears to be secreted into the circulation since plasma prorenin increases in normal women for two to three days at mid-menstrual cycle, at the time of ovulation. No change in plasma active renin occurs at this time. Plasma prorenin increases much more at mid-cycle in women whose ovaries have been hyperstimulated with gonadotropins. Their mid-cycle increment in plasma prorenin (after hCG) is directly related to the number of ovarian follicles. Plasma prorenin also increases markedly (10-fold) in pregnant women within two weeks after conception, in parallel with the rise in endogenous hCG. The ovaries are the apparent source of the increase in plasma prorenin during pregnancy since no such increase occurred in a woman with ovarian failure who conceived after receiving a donor egg. These results suggest that the ovaries synthesize and secrete prorenin in response to stimulation by gonadotropic hormones. Future studies will investigate the potential role of ovarian prorenin in human reproductive function. We postulate the existence of a prorenin receptor which activates prorenin and, in consequence, activates a local renin-angiotensin system. The functioning of this system may be regulated by changes in prorenin and its receptor.

Prorenin as a reproductive hormone. New form of the renin system

Prorenin, the biosynthetic precursor of renin, is synthesized by the kidneys. Herein is reviewed recent evidence that the ovaries also secrete prorenin. It was found that prorenin is present in mature human ovarian follicular fluid in extremely high concentrations and that plasma prorenin levels increase transiently in blood during the menstrual cycle at the time of ovulation. No change in plasma active renin levels occurs at this time. Plasma prorenin level also increases 10-fold in pregnant women very soon after conception. The ovaries are apparently the source of this rise, since plasma prorenin levels did not increase in a pregnant woman with ovarian failure who received a donor egg. All of these changes in plasma prorenin levels appear to be caused by gonadotropic hormones. These results suggest a role for ovarian prorenin in human reproductive function. They may have relevance to studies of female infertility, birth control, and toxemia of pregnancy. They also suggest that a renin system exists that is regulated by changes in prorenin.

Prorenin in high concentrations in human ovarian follicular fluid

Although the kidney is a major source of prorenin, the precursor of renin, there are extrarenal sources for plasma prorenin that have not been identified. The selective increase in plasma prorenin at the time of ovulation suggested that one of these sources might be the ovary. Prorenin was therefore measured in fluid aspirated from 18 ovarian follicles and in plasma collected from three women who were undergoing in vitro fertilization. The follicular fluid contained high concentrations of prorenin that were approximately 12 times higher than plasma prorenin. The prorenin from follicular fluid was immunochemically identical to kidney and plasma prorenin. Thus, the ovary is a likely source for the ovulatory peak of plasma prorenin.

High molecular weight angiotensinogen: a pregnancy associated protein

Although it was previously recognized that human amniotic fluid contained appreciable quantities of angiotensinogen, it remained unknown what fraction of the total is high molecular weight angiotensinogen. Fractionation of human amniotic fluid showed that high molecular weight angiotensinogen represents the major component of total angiotensinogen; 58% for 11 normotensive pregnant women and 67% for 12 hypertensive pregnant women. In contrast to plasma where high molecular weight angiotensinogen is elevated in hypertensive pregnant women as compared to normotensive pregnant women, no such difference exists for amniotic high molecular weight angiotensinogen. Serum and amniotic fluid high molecular weight angiotensinogen were compared in six subjects, and no significant correlation was found. In fetal cord blood, high molecular weight angiotensinogen represented only 5.8% of the total angiotensinogen. The site of synthesis of high molecular weight angiotensinogen remains unknown but these data suggest that it is produced in the placenta or in the maternal uterine tissue. Therefore, we propose that human high molecular weight angiotensinogen should be classified as a pregnancy-associated protein.

Extraction of plasma prorenin by human heart

Arterial plasma prorenin underwent 58% extraction during one passage through the coronary circulation in eleven patients undergoing myocardial metabolic studies. Active renin and renin substrate were unaffected. Changes in renin and renin substrate concentrations were not detected across the enterohepatic, femoral, or renal vascular beds. This high degree of selective extraction of prorenin by the heart accounts for its proposed half-life. Prorenin could be a cardiotropic hormone.

Acute renal effects of doxazosin in man

To evaluate the role of alpha 1-adrenoceptors in the regulation of renal blood flow and renin secretion, we infused doxazosin in incremental doses (group I, n = 5) or at a fixed rate of 1 microgram kg-1 min-1 (group II, n = 6) into the renal artery of hypertensive patients just prior to diagnostic renal angiography. In group I, stepwise increasing doses of doxazosin were associated with increases in renal perfusion and, at doses above 1 microgram kg-1 min-1, also with a fall in blood pressure. Compared with observations in control subjects, doxazosin enhanced renal renin release at rest but did not modify the renin response to isometric exercise in group II. alpha 1-Adrenoceptors are involved in causing renal vasoconstriction and inhibition of renin secretion. However, the latter is only of importance in resting subjects.

Cyclical secretion of prorenin during the menstrual cycle: synchronization with luteinizing hormone and progesterone

Plasma prorenin, a high molecular weight precursor form of renin, (renin, EC 3.4.23.15; old number, EC 3.4.99.19), was measured three times weekly in normal young women during the menstrual cycle and was related to changes in luteinizing hormone, estradiol, and progesterone. In all subjects a stable baseline level of prorenin occurred during the follicular phase. Then, simultaneously or soon after the luteinizing hormone peak, plasma prorenin consistently increased about 2-fold. Baseline prorenin ranged from 18 to 40 ng per ml per hr, and peak prorenin ranged from 35 to 65 ng per ml per hr. The maximum increase in prorenin averaged 80%. Prorenin remained elevated during the mid-luteal phase of the menstrual cycle and returned to baseline during the late-luteal phase in coordination with the decrease in progesterone. The changes in prorenin were not synchronized with changes in active renin which was significantly increased only during the mid-luteal phase. These findings suggest that prorenin may be involved in reproductive physiology.

Plasma prorenin in first-trimester pregnancy: relationship to changes in human chorionic gonadotropin

Prorenin and human chorionic gonadotropin are both synthesized in chorionic cells. The relationship of changes in maternal plasma prorenin to changes in human chorionic gonadotropin were therefore evaluated during the first trimester. In samples submitted to the routine chemistry laboratory for detection of pregnancy a positive relationship was observed between prorenin and beta human chorionic gonadotropin during the 5 weeks following conception. Subsequently human chorionic gonadotropin continued to rise but prorenin had reached a plateau. Serial studies in one subject demonstrated that prorenin had increased to 65% of maximum by the thirteenth day following conception whereas human chorionic gonadotropin had risen to only 0.2% of maximum. By 3 to 5 days post partum, beta human chorionic gonadotropin had fallen by 98% but prorenin had fallen by only 50%. The early rise in prorenin following conception and the relatively slow fall post partum suggest that pregnancy-related changes in maternal plasma prorenin are of maternal, not fetal, origin.

Active and acid-activable inactive renin during inhibition by indomethacin of prostaglandin synthesis in sodium-replete man

The effect of inhibition of prostaglandin synthesis by indomethacin on active renin and on acid-activable inactive renin was studied in nine healthy, sodium-replete men, both at rest and exercise. These volunteers were investigated after pretreatment with placebo or indomethacin, 150 mg daily for 3 days. Indomethacin induced a decrease in active (P = 0.004), total (P less than 0.001) and inactive (P = 0.02) renin at rest recumbent and at rest, sitting. Inhibition of prostaglandins with indomethacin reduced (P less than 0.001) active and total renin at each level of work load but not (P = 0.32) inactive renin. However the exercise-induced stimulation (P less than 0.05) of active and of total renin still occur during indomethacin. Indomethacin reduced (P less than 0.001) at rest, sitting and at maximal exercise the plasma concentrations of immunoreactive prostaglandins E2, prostaglandin F2 alpha and 13, 14-dihydro-15-keto-prostaglandin F alpha; the urinary excretion of immunoreactive prostaglandin E2 and F2 alpha were also reduced.

Effect of sleep on blood pressure and its correlates

The aim of the present study was to investigate changes in blood pressure, heart rate, renal plasma flow, catecholamines, renin and angiotensin in fifteen patients with uncomplicated essential hypertension. During sleep a progressive fall in blood pressure and heart rate was found. Renal plasma flow and calculated renal vascular resistance were reduced as well. Noradrenaline and adrenaline decreased, but renin and angiotensin rose. The rise in renin was inversely related to age. Blood pressure was more variable during REM-sleep but otherwise there were no differences in the level of blood pressure and catecholamines during REM-sleep or during sleep stage IV.

Characterization of inactive renin ("prorenin") from renin-secreting tumors of nonrenal origin. Similarity to inactive renin from kidney and normal plasma

Inactive renin comprises well over half the total renin in normal human plasma. There is a direct relationship between active and inactive renin levels in normal and hypertensive populations, but the proportion of inactive renin varies inversely with the active renin level; as much as 98% of plasma renin is inactive in patients with low renin, whereas the proportion is consistently lower (usually 20-60%) in high-renin states. Two hypertensive patients with proven renin-secreting carcinomas of non-renal origin (pancreas and ovary) had high plasma active renin (119 and 138 ng/h per ml) and the highest inactive renin levels we have ever observed (5,200 and 14,300 ng/h per ml; normal range 3-50). The proportion of inactive renin (98-99%) far exceeded that found in other patients with high active renin levels. A third hypertensive patient with a probable renin-secreting ovarian carcinoma exhibited a similar pattern. Inactive renins isolated from plasma and tumors of these patients were biochemically similar to semipurified inactive renins from normal plasma or cadaver kidney. All were bound by Cibacron Blue-agarose, were not retained by pepstatin-Sepharose, and had greater apparent molecular weights (Mr) than the corresponding active forms. Plasma and tumor inactive renins from the three patients were similar in size (Mr 52,000-54,000), whereas normal plasma inactive renin had a slightly larger Mr than that from kidney (56,000 vs. 50,000). Inactive renin from each source was activated irreversibly by trypsin and reversibly by dialysis to pH 3.3 at 4 degrees C; the reversal process followed the kinetics of a first-order reaction in each instance. The trypsin-activated inactive renins were all identical to semipurified active renal renin in terms of pH optimum (pH 5.5-6.0) and kinetics with homologous angiotensinogen (Michaelis constants, 0.8-1.3 microM) and inhibition by pepstatin or by serial dilutions of renin-specific antibody. These results indicate that a markedly elevated plasma inactive renin level distinguishes patients with ectopic renin production from other high-renin hypertensive states. The co-production of inactive and active renin by extrarenal neoplasms provides strong presumptive evidence that inactive renin is a biosynthetic precursor of active renin. The unusually high proportion of inactive renin in plasma and tumor extracts from such patients is consistent with ineffective precursor processing by neoplastic tissue, suggesting that if activation of "prorenin" is involved in the normal regulation of active renin levels it more likely occurs in the tissue of origin (e.g., kidney) than in the circulation.

Influence of augmented Hageman factor (Factor XII) titers on the cryoactivation of plasma prorenin in women using oral contraceptive agents

Prolonged cold storage of plasma may induce the conversion of plasma prorenin (inactive renin) to renin. This phenomenon is exaggerated in oral contraceptive (OC) users; the titer of Hageman factor (HF, Factor XII) in OC users is higher than in nonusers. The present study relates these observations. The increment in plasma renin activity (PRA) during cold storage, as measured by generation of angiotensin I, correlated strongly with the initial plasma titer of HF. Increasing the HF titer of nonusers to that observed in OC users by addition of purified HF increased cold-induced PRA at least twofold, while reducing the plasma HF titer of OC users correspondingly decreased cold-induced PRA. Thus, in OC users, the enhanced conversion of plasma prorenin to renin during cold storage reflects the elevated plasma titer of HF.

Generation of angiotensin I by human chorion-decidua in vitro

An angiotensin I-like substance has been detected in media after incubation of human chorion-decidua in vitro. The substance was identified as angiotensin I by gel filtration, ion exchange chromatography, and a specific antiserum to angiotensin I. The presence of angiotensin I in these incubates indicates that renin substrate may be either stored or synthesized by the tissue. The levels of angiotensin I produced during incubation were greater than could be explained by the action of renin on plasma substrate since the major part of the chorion-decidua is avascular. Previously chorion-decidua has been shown to contain large amounts of renin and to be capable of synthesizing both active and inactive forms of the enzyme in vitro; however, renin substrate has not been described in this tissue.

Generation of angiotensin I by tissues of the human female genital tract

Tissues from the fetoplacental unit were incubated in medium E 199 at 0 degrees C and following removal of the tissue were incubated at 37 degrees C. Compared to control tissues held at 0 degrees C, large quantities of angiotensin I, measured by means of specific radioimmunoassay, were detected in the media from both amnion and chorion-decidua while only small amounts were found in the media from placenta and umbilical cord. Following prior incubation with antirenin, the generation of angiotensin I was abolished in all tissues. These results indicate that both renin and its substrate are released into the media during incubation at 0 degrees C and suggest the existence of a locally functioning renin-angiotensin system in certain tissues of the human female genital tract.

Characterization of inactive renin from human kidney and plasma. Evidence of a renal source of circulating inactive renin

An inactive form of renin has been isolated from human plasma. It has been suggested that this may represent renin precursor secreted from the kidney. However, early studies failed to isolate inactive renin from human renal tissue. In this investigation, rapid processing of human kidney cortex at temperatures below 4 degrees C in the presence of protease inhibitors followed by cibacron-blue affinity chromatography allowed us to extract a totally inactive form of renal renin. Furthermore, we found that in kidney inactive renin constituted from 10 to as much as 50% of the total renin concentration. Biochemical characterization of the inactive renin from plasma and from kidney indicates that they are structural homologues and, when activated, have enzymatic properties that resemble active renal renin. Renal and plasma inactive renin were found to have the following properties in common: (a) a pH optimum of activation of 3.3; (b) reversible activation by acid dialysis on return to pH 7.4, 37 degrees C; (c) pH optima of enzyme activity of 7.8 with sheep angiotensinogen and 5.5 and 6.7 (biphasic) with human angiotensinogen; (d) Michaelis-Menten constants, Km, of 0.29-0.34 microM with sheep angiotensinogen, and 0.99-1.25 microM with human angiotensinogen; (e) an antibody to human renal renin mean inhibitory titer of 1:30,000 with 1 X 10(-4) Goldblatt units of activated renal or plasma inactive renin; (f) gel filtration profiles consisting of two peaks with apparent molecular weights of 56,000 +/- 1,500 and 49,200 +/- 1,000. Activation of plasma and kidney inactive renin by acid plus renal kallikrein was not accompanied by a change in gel filtration elution patterns. To determine whether inactive renin is released by the kidney, we measured inactive renin in samples obtained simultaneously from both the renal veins and inferior vena cava below the origin of the renal veins. In eight consecutive patients, inactive renin concentration was significantly higher in renal venous blood than in inferior vena caval blood. These data indicate that human kidney contains and secretes significant quantities of inactive renin. Thus, the kidney appears to be a major source of inactive renin in human plasma.