Effects of renin-angiotensin system blockade in guinea pigs

Losartan metabolite EXP3179 is a unique blood pressure-lowering AT1R antagonist with direct, rapid endothelium-dependent vasoactive properties

BACKGROUND AND PURPOSE

Losartan is an anti-hypertensive angiotensin II (ANGII) type 1 receptor (AT1R) blocker (ARB) with many unexpected therapeutic properties, even in non-blood pressure (BP)-related diseases. Administered as a prodrug, losartan undergoes serial metabolism into EXP3179, a metabolite alleged to lack AT1R-blocking properties, and EXP3174, the dominant AT1R antagonist. Having observed that losartan can decrease vascular tone in mice with low AT1R expression and inhibit Marfan aortic widening at very high doses, we investigated whether EXP3179 may have unique, AT1R-independent effects on vascular tone and endothelial function.

EXPERIMENTAL APPROACH

We compared the AT1R blocking capabilities of EXP3179 and EXP3174 using AT1R-expressing cell lines. Their BP lowering and vasoactive properties were studied in normal, hypertensive and transgenic rodents, and ex vivo wire myography.

KEY RESULTS

We observed that both EXP3179 and EXP3174 can fully block (100%) AT1R signaling in vitro and significantly decrease BP in normotensive and spontaneously hypertensive rats. Only EXP3179 prevented PE-induced contraction by up to 65% (p < 0.01) in L-NAME and endothelium removal-sensitive fashion. Use of transgenic mice revealed that these effects involve the eNOS/caveolin-1 axis and the endothelium-dependent hyperpolarization factor (EDHF).

CONCLUSION AND IMPLICATIONS

We provide direct structure-activity evidence that EXP3179 is a BP-lowering AT1R blocker with unique endothelial function-enhancing properties not shared with losartan or EXP3174. The major pharmacological effects of losartan in patients are therefore likely more complex than simple blockade of AT1R by EXP3174, which helps rationalize its therapeutic and prophylactic properties, especially at very high doses. Reports relying on EXP3179 as an AT1R-independent losartan analogue may require careful re-evaluation.

Pleiotropic activation of endothelial function by angiotensin II receptor blockers is crucial to their protective anti-vascular remodeling effects

There are no therapeutics that directly enhance chronic endothelial nitric oxide (NO) release, which is typically associated with vascular homeostasis. In contrast, angiotensin II (AngII) receptor type 1 (AT1R) blockers (ARBs) can attenuate AngII-mediated oxidative stress, which often leads to increased endothelial NO bioavailability. Herein, we investigate the potential presence of direct, AngII/AT1R-independent ARB class effects on endothelial NO release and how this may result in enhanced aortic wall homeostasis and endothelial NO-specific transcriptome changes. Treatment of mice with four different ARBs induced sustained, long-term inhibition of vascular contractility by up to 82% at 16 weeks and 63% at 2 weeks, an effect reversed by L-NAME and absent in endothelial NO synthase (eNOS) KO mice or angiotensin converting enzyme inhibitor captopril-treated animals. In absence of AngII or in tissues with blunted AT1R expression or incubated with an AT2R blocker, telmisartan reduced vascular tone, supporting AngII/AT1R-independent pleiotropism. Finally, telmisartan was able to inhibit aging- and Marfan syndrome (MFS)-associated aortic root widening in NO-sensitive, BP-independent fashions, and correct aberrant TGF-β signaling. RNAseq analyses of aortic tissues identified early eNOS-specific transcriptome reprogramming of the aortic wall in response to telmisartan. This study suggests that ARBs are capable of major class effects on vasodilatory NO release in fashions that may not involve blockade of the AngII/AT1R pathway. Broader prophylactic use of ARBs along with identification of non-AngII/AT1R pathways activated by telmisartan should be investigated.

High plasma renin activity is combined with elevated urinary albumin excretion in essential hypertensive patients

BACKGROUND

Several studies suggest that the hyperactivity of the circulating renin-angiotensin system might favor the progression of renal disease in essential hypertension. To elucidate this aspect, we investigated the relationship between plasma renin activity (PRA) and the urinary albumin excretion rate (UAER), an early marker of hypertension-related renal changes, in human essential hypertension.

METHODS

Ninety nonobese, nondiabetic, nonhyperlipidemic patients with mild-to-moderate essential hypertension (67 males, 23 females; mean age 51.4 +/- 6.2 years) were divided into low renin (LR), normal renin (NR), and high renin (HR) subgroups according to individual PRA while they were on a constant NaCl intake (120 mmol NaCl/day). The UAER was assessed during the same NaCl intake.

RESULTS

Data showed significantly higher UAER (31.3 +/- 12.9 microg/min) in HR (N = 30) than NR (N = 30, 22.7 +/- 14.4 microg/min, P < 0.02) and LR patients (N = 30, 21.7 +/- 10.8 microg/min, P < 0. 01).

CONCLUSIONS

Our study demonstrates that the UAER is elevated in HR essential hypertensive patients, suggesting that high PRA accelerates the onset of early renal changes in human essential hypertension.

Role of nitric oxide in responses to renin-angiotensin system inhibition in sodium-depleted guinea pig and rat

Previous reports have suggested that NO is an important mediator of the antihypertensive effects of renin-angiotensin system (RAS) inhibition. We examined the effects of the NO synthase inhibitor L-NNA on the hypotensive effects of captopril, the Ang II antagonist EXP 3174, or the renin inhibitor terlakiren. In sodium-depleted guinea pigs (GPs), L-NNA (3 mg/kg) increased MAP by 15-21% for at least 5 hours. L-NNA partially blocked the hypotensive effects of captopril (1 mg/kg, iv), but not those of EXP 3174 (1 mg/kg, iv) or terlakiren (3 mg/kg). In sodium-depleted rats, 10 mg/kg L-NNA (iv) increased MAP by 16-22%, and partially or fully blocked the hypotensive effect of EXP 3174 (1 mg/kg, iv) or captopril (3 mg/kg, iv), respectively. Thus, in contrast to the rat, NO in GPs appears to participate only in the hypotensive action of ACE inhibition and does not appear to be strongly involved in the hypotensive action of AII antagonism or renin inhibition. The involvement of NO in the hypotensive effects of RAS antagonists other than ACE inhibitors may be species-dependent.

A novel renal hypertensive guinea pig model for comparing different inhibitors of the renin-angiotensin system

The present study describes a novel renal hypertensive guinea pig model for comparing different inhibitors of the renin-angiotensin system (RAS). Renal hypertension was induced by a two-step procedure consisting of ligation of the left caudal renal artery and right nephrectomy. Sham-operated animals were used as controls. Arterial blood pressure and heart rate were monitored in conscious animals. Left caudal renal artery ligation and subsequent right nephrectomy led to a significant increase (32% over sham-operated controls, p < .05) in mean arterial blood pressure (MABP), 3 to 4 weeks following surgery. Renal hypertensive animals had increased urine production (from 63 +/- 8 mL/kg per day to 143 +/- 29 mL/kg per day, p < .05) and an increased incidence of proteinuria (11/13 animals had urine protein levels higher than 20 mg/kg per day). Five of the 13 renal hypertensive animals also had hematuria. On autopsy, an 83% increase in the left kidney/body weight ratio and a 37% increase in the heart/body weight ratio were observed in the renal hypertensive animals, compared to the sham-operated controls. Changes in blood pressure and heart rate were assessed before and after an intravenous bolus injection of the drug to be tested. Captopril reduced MABP in both sham-operated and renal hypertensive animals with equal efficacy (up to a maximum of 42%). In contrast, BILA 2157 BS, one of our human renin inhibitors, produced a similar maximum MABP decrease but only in renal hypertensive animals. This selective antihypertensive effect was also observed with enalkiren, another renin inhibitor. These results indicate that the renal hypertensive guinea pig is an useful model for comparing and contrasting different RAS inhibitors.

Distribution of remikiren, a potent orally active inhibitor of human renin, in laboratory animals

1. Whole-body autoradiography was used to compare the distribution of remikiren in the squirrel monkey, in which the compound is a potent inhibitor of renin, with the rat and the guinea-pig in which it is less active. 2. Following intravenous administration, drug-related material was rapidly and extensively taken up by the tissues of all three species. Consistent with rapid biliary elimination, high levels of radioactivity were found in the bile duct/gall bladder/intestinal contents. Of the remaining organs, the kidney consistently showed the highest concentrations of drug-related material. 3. Radio-hplc analysis of the kidney samples demonstrated that the majority of the retained material was present as intact remikiren, even at 24 h after administration. A similar degree of retention by the kidney was also found after oral dosing. 4. Uptake of remikiren by the kidney may act as a reservoir for the drug, resulting in the prolonged duration of pharmacological activity, which, despite the high plasma clearance of the drug, has previously been observed in primates.

Comparative studies on differential inhibition of the renin - angiotensin system in the anesthetized guinea pig

The present study compares the hemodynamic effects and mechanisms of action of angiotensin II (AngII) antagonists, angiotensin converting enzyme (ACE) inhibitors, and renin inhibitors in the guinea pig, an animal with high similarity to primates in terms of in vitro and in vivo responses to several human renin inhibitors. Animals were anesthetized with urethane and ketamine. The carotid artery was catheterized for monitoring blood pressure and heart rate. After 30 min stabilization, drug (or vehicle) effects were monitored for 1 h following each increasing dose (i.v. bolus injection). Drugs tested include losartan, an AngII receptor antagonist; two renin inhibitors, BILA 2157 BS and PD-134672; and captopril, an ACE inhibitor. All drugs dose dependently decreased blood pressure. Diastolic blood pressure was reduced more than systolic blood pressure, suggestive of vasodilation. The maximum decrease (32 +/- 6%, p < 0.05 vs. vehicle) in mean arterial blood pressure (MABP) by losartan was achieved with a dose of 1 mg/kg. A similar decrease in MABP was observed with renin inhibitors at a dose of 3 mg/kg, without affecting heart rate. A further increase in the dose of renin inhibitors (6 mg/kg) decreased not only blood pressure but also heart rate. Captopril decreased MABP with a maximum of 48 +/- 3% (p < 0.05 vs. vehicle, losartan, and PD-134672). In the presence of HOE-140, a bradykinin antagonist, the MABP decrease by captopril was only 35 +/- 4%, (p < 0.05 vs. captopril alone). Bilateral nephrectomy reduced the peak MABP effect of PD-134672 by 67%, while the effects of captopril on MABP were affected to a lesser degree (57%). Therefore, captopril remains more effective in reducing MABP (p < 0.05 vs. that of PD-134672). These results suggest that renin inhibitors and AngII antagonists act more specifically on the renin - angiotensin system cascade, while captopril acts partially by a bradykinin-dependent mechanism. The small animal model described provides a novel tool for the comparative pharmacologic assessment of different renin - angiotensin system inhibitors.

Renal circulation and blockade of the renin-angiotensin system. Is angiotensin-converting enzyme inhibition the last word?

The mechanism by which angiotensin-converting enzyme (ACE) inhibition influences renal perfusion and function has assumed growing importance as alternatives for blocking the system have emerged. Neither renin inhibitors nor angiotensin II (Ang II) antagonists are likely to trigger responses similar to ACE inhibitor-induced involvement of kinins, prostaglandins, or nitric oxide. Several observations suggest species variation in the contribution of these pathways to the renal response to ACE inhibition. In humans, recent investigation suggests that virtually all of the renal response is due to a fall in Ang II formation. Perhaps most persuasive is the surprising observation that the renal hemodynamic response to renin inhibitors exceeds by more than 50% the response to ACE inhibition in healthy humans. To the extent that kinins or prostaglandins contribute to the renal response to ACE inhibition, one would anticipate a smaller response to renin inhibition. Possible explanations include an unanticipated additional action of renin inhibitors, better tissue penetration of these highly lipophilic agents, or more effective blockade of Ang II formation through an action at the rate-limiting step or non-ACE-dependent Ang II generation. Substantial evidence favors the latter two possibilities. Whatever the explanation, these observations raise the intriguing possibility that the undoubted therapeutic efficacy of ACE inhibition in renal injury, documented most rigorously for type I diabetes mellitus, might be exceeded with the newer classes of agent.

Effects of angiotensin II generated by an angiotensin converting enzyme-independent pathway on left ventricular performance in the conscious baboon

Human chymase is a serine proteinase that converts angiotensin (Ang) I to Ang II independent of angiotensin converting enzyme (ACE) in vitro. The effects of chymase on systemic hemodynamics and left ventricular function in vivo were studied in nine conscious baboons instrumented with a LV micromanometer and LV minor axis and wall thickness sonomicrometer crystal pairs. Measurements were made at baseline and after [Pro11DAla12] Ang I, a specific substrate for human chymase, was given in consecutive fashion as a 0.1 mg bolus, an hour-long intravenous infusion of 5 mg, a 3 mg bolus, and after 5 mg of an Ang II receptor antagonist. [Pro11DAla12]Ang I significantly increased LV systolic and diastolic pressure, LV end-diastolic and end systolic dimensions and the time constant of LV relaxation and significantly decreased LV fractional shortening and wall thickening. Administration of a specific Ang II receptor antagonist reversed all the hemodynamic changes. In separate studies, similar results were obtained in six of the baboons with ACE blockade (20 mg, intravenous captopril). Post-mortem studies indicated that chymase-like activity was widely distributed in multiple tissues. Thus, in primates, Ang I is converted into Ang II by an enzyme with chymase-like activity. This study provides the first in vivo evidence of an ACE-independent pathway for Ang II production.

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.

In vitro incubation of bovine ovarian follicles: indications for an active and regulated renin-angiotensin system

1. The ovarian follicular renin-angiotensin system was investigated by using an in vitro incubation method, based on tissue incubation of individual bovine follicles. 2. Very high and varying concentrations of active renin (median 18.0 GU/kg, range 2.1-107 GU/kg; n = 101) and prorenin (11.7 GU/kg, range < 2.6-142 GU/kg; n = 101) existed in unincubated ovarian follicular tissue. 3. Active renin and prorenin increased 35 and 959%, respectively, in follicular wall tissue and incubation medium during 72 h of in vitro incubation. The protein synthesis inhibitor cyclohexamide inhibited active renin and prorenin formation. No activation of prorenin occurred in incubation medium or follicular fluid in vitro. Active renin was degraded during incubation. 4. Primarily prorenin but also active renin were secreted into the incubation medium. Secretion was directed both to the internal and external surface of the follicular wall. 5. Follicle-stimulating hormone (FSH) slightly stimulated the synthesis and secretion of prorenin in vitro. 6. The synthesis and secretion rates of active renin and prorenin varied markedly between individual follicles. 7. The finding of very high and varying concentrations of active renin in follicular wall tissues together with the formation and secretion of active renin during in vitro incubation provide further evidence for an active and regulated renin-angiotensin system in ovarian follicles.

Angiotensin II receptor density in bovine ovarian follicles relates to tissue renin and follicular size

1. The purpose of this study was to investigate angiotensin II (AII) receptors in isolated bovine ovarian follicles and the relationship of their density to follicular concentrations of prorenin, active renin, oestradiol and progesterone. 2. Displacement of [125I]-[Sar1-Ile5-Ile8]-AII binding by the AII receptor antagonists PD 123319 and Losartan (DuP 753) confirmed that follicular AII receptors are of subtype 2 (AT2 receptor). 3. The dissociation constant (Kd) for [Ile5]-AII (human AII) was 0.84 (range 0.51-1.47) nmol/L. The receptor density varied between 90 and 5990 (mean 1640) fmol/mg membrane protein. 4. The follicular AII receptor density correlated positively with follicular diameter (Spearman's rho = 0.518; P < 0.003) and tissue weight (Spearman's rho = 0.636; P < 0.0001), and negatively with the active renin concentration in the follicular wall (Spearman's rho = -0.399; P < 0.02). The AII receptor density did not correlate with the follicular fluid concentrations of prorenin, active renin, oestradiol (E2), progesterone (P4) or the E2/P4 ratio. The follicular fluid concentrations of prorenin correlated negatively with the E2/P4 ratio (Spearman's rho = -0.716; P < 0.0001). 5. The inverse relationship between AII receptor density and the high active renin concentrations in the follicular wall suggests an active regulated tissue renin-angiotensin system. A high AII receptor density is a general feature of large bovine ovarian follicles.

Vascular renin in the guinea pig. Suppression by the renin inhibitor remikiren

Angiotensin I and II are generated by the vascular wall. Whether this generation depends on renin or on other enzymes is debated. We tested the hypothesis that remikiren, a highly specific inhibitor of human and guinea pig renin, may inhibit the vascular renin-angiotensin system. Isolated hindquarters from guinea pigs were perfused with an artificial medium, and angiotensin I and II release was measured by high-performance liquid chromatography and radioimmunoassay. Guinea pig hindquarters released angiotensin I (23.8 +/- 5.6 fmol/30 min; n = 13) and angiotensin II (95.2 +/- 19 fmol/30 min; n = 13) spontaneously. Inhibition of the angiotensin I-converting enzyme by captopril (10 nmol/mL) suppressed angiotensin II by 85% and increased angiotensin I by 352% (n = 5, P < .05). Infusion of remikiren (1.6 nmol/mL) in addition to captopril decreased angiotensin I release by 68% (P < .05 versus captopril alone, n = 5 each). We conclude that renin generates angiotensin I in an isolated guinea pig resistance vessel bed. Our study demonstrates that renin rather than nonrenin enzymes is responsible for the major part of vascular angiotensin formation.

Chronic monitoring of cardiovascular function in the conscious guinea pig using radio-telemetry

An implantable radio-telemetry device for chronic monitoring of arterial pressure and heart rate in the conscious guinea pig was validated against measurements using an exteriorized, indwelling catheter. There were no significant differences between simultaneous measurements in animals instrumented with both the telemetry system and the conventional catheter (implanted 24 hrs prior to comparisons) in response to a variety of vasoactive agents. The device was shown to be accurate up to 3 weeks after implantation (longest time point tested). Resting pressures and heart rates in the telemetered guinea pig were stable in 100% of the animals tested. In contrast, animals instrumented with only exteriorized catheters showed a significant decline in pressure by 8 days after surgery and a 39% attrition rate due to loss of catheter patency. Performance of the telemetric device was examined in both normal and sodium-deficient animals, since the latter is a useful normotensive model in which blood pressure is rendered highly renin-dependent for evaluating the efficacy of potential antihypertensive agents that target the renin-angiotensin system. The telemetered guinea pig is an appropriate model for assessing responses to chronic exposure of cardiovascular agents.

Is the antiproteinuric effect of ACE inhibition mediated by interference in the renin-angiotensin system?

Angiotensin converting enzyme (ACE) inhibition causes specific renal effects, such as a rise in effective renal plasma flow, a fall in filtration fraction and a lowering of proteinuria. The mechanism of these renal effects is still debated. Recent animal studies suggest that non-angiotensin (Ang) II related actions of ACE inhibition, such as bradykinin accumulation, may have a role. We therefore investigated the effects of specific intervention in the renin-angiotensin system with the Ang II receptor antagonist losartan, and compared these effects to those obtained with ACE inhibition, as this comparison might resolve the question whether or not the effects of ACE inhibition are Ang II related. The effects of losartan and enalapril were studied in eleven patients with non-diabetic proteinuria and hypertension. The protocol consisted of seven periods, each lasting one month, in which patients received once daily placebo, 50 mg losartan, 100 mg losartan, placebo, 10 mg enalapril, 20 mg enalapril, and placebo, respectively. At the end of each study period proteinuria, blood pressure, and renal function were determined. On both doses of losartan and enalapril proteinuria and blood pressure fell, whereas ERPF increased and GFR remained stable. The fall in urinary protein excretion was similar for both drugs: 46.3% (28.3% to 63.1%) on 100 mg losartan versus 51.6% (37.0% to 69.2%) on 20 mg enalapril (expressed as Wilcoxon-based estimated median with 95% CI).(ABSTRACT TRUNCATED AT 250 WORDS)

Endothelin plays a role in the maintenance of blood pressure in normotensive guinea pigs

The objective of the present study was to evaluate if endothelin plays a role in the maintenance of arterial blood pressure in normotensive guinea pigs. For this purpose, the effects of a new mixed (ETA + ETB) endothelin receptor antagonist, Ro 47-0203 (bosentan), were evaluated in vitro on aortic rings and in anesthetized and conscious guinea pigs. In vitro, bosentan was a potent (pA2 = 7.5) and competitive endothelin receptor antagonist as shown by the parallel rightward shift of the concentration-response curve for endothelin-1 on guinea pig aortic rings in presence of increasing concentrations of bosentan. In vivo, bosentan significantly decreased arterial blood pressure of both anesthetized and conscious guinea pigs. This effect was similar to the effect of BQ-123, a selective ETA receptor antagonist. No additional effect was observed when bosentan was given on top of BQ-123. Neither inhibition of the renin angiotensin system with remikiren, cyclooxygenase inhibition with indomethacin, bradykinin antagonism with Hoe 140, ganglionic blockade with chlorisondamine, parasympathetic inhibition with atropine nor nitric oxide synthase blockade with L-NAME altered the effect of bosentan. In conclusion, the present results show that endothelin contributes to the maintenance of arterial blood pressure in normal normotensive guinea pigs most likely through stimulation of ETA receptors.

Major role of the renin angiotensin system in the neointima formation after vascular injury in guinea pigs

ACE inhibition has been shown to prevent neointima formation after vascular injury. However, it is not known if this effect is due to a specific inhibition of the renin angiotensin system or to another mechanism such as the accumulation of bradykinin. In order to answer this question we compared the effects of maximal effective doses of cilazapril, an angiotensin converting enzyme (ACE) inhibitor, and ciprokiren, a new renin inhibitor, in guinea pigs. Vascular injury was induced by endothelial denudation of the right carotid artery of guinea pigs treated either by saline (control group), cilazapril (30 mg/kg/day) or ciprokiren (24 mg/kg/day). Twelve days after the ballooning, the guinea pigs were sacrificed, the carotid arteries were perfused fixed and neointima formation was evaluated by quantitative morphometry. Both, ciprokiren and cilazapril prevented neointima formation to the same extent (inhibition by 42 and 49%, respectively, p < 0.05). These results suggest that, in guinea pigs, renin inhibition prevents neointima formation to a similar extent as ACE inhibition. Therefore, ACE inhibitors seem to act in this model by inhibiting the renin angiotensin system and not by other effects such as accumulation of bradykinin.

Effect of different endothelin receptor antagonists and of the novel non-peptide antagonist Ro 46-2005 on endothelin levels in rat plasma

The goal of our study was to evaluate and compare the effects of receptor blockade with different endothelin (ET) receptor antagonists on plasma concentrations of ET-1, big ET-1 and ET-3 in conscious rats. Ro 46-2005 10 mg/kg, i.v.), a novel non-peptide antagonist of both ETA and ETB receptors, increased the concentrations of ET-1 in plasma to 200 +/- 13% of basal levels (P < 0.001). This effect was dose- and time-dependent and reached a maximum at 15 min. Ro 46-2005 had no effect on plasma concentrations of big ET-1 and only a minor effect on those of ET-3. In contrast to Ro 46-2005, the selective peptide ETA antagonists BQ-123 and FR-139317 had no effect on plasma ET-1 concentrations. The increase in plasma ET-1 concentrations by Ro 46-2005 was most likely not due to de novo synthesis, since big ET-1 levels were not increased and peak levels were reached early after compound injection, but perhaps to displacement of ET-1 from the ETB receptors.

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.