Local renin-angiotensin systems: the unanswered questions

The concept of local renin-angiotensin systems has been introduced almost 20 years ago to explain the beneficial blood pressure-independent effects of ACE inhibitors and AT(1) receptor antagonists in cardiovascular diseases. In the past decade, research has focussed on the local effects of angiotensin II rather than on the mechanism(s) of its local generation. This review addresses several of the unanswered questions with regard to tissue angiotensin II generation, focussing in particular on the heart and vascular wall: (1) what is the origin of the renin that is required to generate angiotensin II locally, (2) where does tissue angiotensin generation occur (intra- versus extracellular), (3) what is the importance of alternative (non-renin, non-ACE) angiotensin-generating enzymes, (4) do ACE inhibitors and AT(1) receptor antagonists exert local effects that are renin-angiotensin system independent (thereby incorrectly leading to the conclusion that they interfere with the local generation or effects of angiotensin II), and (5) to what degree do differences in tissue angiotensin generation underlie the association between cardiovascular diseases and renin-angiotensin system gene polymorphisms?     

Peptide inhibitors of converting enzyme.

Human plasma and atypical lung converting enzyme, and porcine plasma converting enzyme are substantially inhibited by other components of the renin-angiotensin system, and by angiotensin II and its analogues. Des-Asp¹ angiotensin II (angiotensin III) 0.1 mM and tridecapeptide renin substrate 0.1 mM are both effective inhibitors of human lung, plasma and porcine plasma converting enzymes. Des-Asp¹-Arg² angiotensin II also was an effective inhibitor of plasma enzymes. Bradykininase activity (kininase II) of the converting enzymes was also inhibited by angiotensin I, angiotensin III, tetradecapeptide renin substrate and tridecapeptide renin substrate. The substantial kininase and converting enzyme inhibitory effects of components of the renin-angiotensin system, suggest a potential close physiologic relationship between the kallikrein-kinin system and the renin-angiotensin system.     

Local renin-angiotensin systems

The existence of a local cardiovascular renin-angiotensin system (RAS) is often invoked to explain the long-term beneficial effects of RAS inhibitors in heart failure and hypertension. The implicit assumption is that all components of the RAS are synthesized in situ, so that local angiotensin II formation may occur independently of the circulating RAS. Evidence for this assumption however is lacking. The angiotensin release from isolated perfused rat hearts or hindlimbs depends on the presence of renal renin. When calculating the in vivo angiotensin production at tissue sites in humans and pigs, taking into account the extensive regional angiotensin clearance by infusing radiolabeled angiotensin I or II, it was found that angiotensin production correlated closely with plasma renin activity. Moreover, in pigs the cardiac tissue levels of renin and angiotensin were directly correlated with their respective plasma levels, and both in tissue and plasma the levels were undetectably low after nephrectomy. Similarly, rat vascular renin and angiotensin decrease to low or undetectable levels within 48 h after nephrectomy. Aortic renin has a longer half life than plasma renin, suggesting that renin may be bound by the vessel wall. In support of this assumption, both renin receptors and renin-binding proteins have been described. Like ACE, renin was enriched in a purified membrane fraction prepared from cardiac tissue. Binding of renin to cardiac or vascular membranes may therefore be part of a mechanism by which renin is taken up from plasma. It appears that the concept of a local RAS needs to be reassessed. Local angiotensin formation in heart and vessel wall does occur, but depends, at least under normal circumstances, on the uptake of renal renin from the circulation. Tissues may regulate their local angiotensin concentrations by varying the number of renin receptors and/or renin-binding proteins, the ACE level, the amount of metabolizing enzymes and the angiotensin receptor density.Abbreviations RAS renin-angiotensin system - ANG angiotensin - ACE angiotensin-converting enzyme - PRA plasma renin activity     

Adrenal renin: a possible local regulator of aldosterone production

Extrarenal renin has been identified in a number of tissues, including the brain, the submaxillary gland, uterus, ovary, vascular endothelium, testes, pituitary gland, and the adrenal cortex. In some tissues, including the adrenal cortex, all of the components of the renin-angiotensin system have been identified; however, no specific physiologic role has been clearly demonstrated for these extrarenal renin-angiotensin systems. We have studied the role of the renin-angiotensin system in the adrenal cortex of the rat and have found that renin is localized and synthesized in the zona glomerulosa cells. Its production can be influenced by alterations in electrolyte balance, as well as the genetic background of the rat. In adrenal capsular explant cultures, a converting enzyme inhibitor can lower angiotensin II production and reduce the stimulation of aldosterone by potassium, suggesting that this system is involved in the aldosterone response to potassium. In addition to rat adrenals, renin has been identified in human adrenal tissue and human adrenal tumors, including aldosteronomas, and a patient with hypertension has been reported to have an adrenal tumor that appeared to be secreting renin into the circulation.     

Renin activity and angiotensin I concentration in genetically selective inbred line of hypertensive mice

Blood pressure lowering kallikrein-kinin and blood pressure raising renin-angiotensin systems play a major role in the maintenance of normal blood pressure. In a previous study, we have shown that a kallikrein-like prorenin converting enzyme (PRCE C) is elevated in the submandibular gland tissue of a mouse line (BPH) that was genetically selected and inbred for high blood pressure in comparison to normotensive line (BPN) that was derived from the ancestors of BPH line. In the present investigation we wanted to find out if elevated levels of PRCE C were involved in the modulation of tissue (local) renin-angiotensin system in the submandibular gland tissue. Results indicate significantly high renin activity but low angiotensin I level in the tissue of BPH mouse model. These results tend to suggest PRCE C's involvement in tissue (local) renin-angiotensin system.     

The renin and iso-renin-angiotensin system in rats with experimental pitutary tumors (38585).

Renin, iso-renin, angiotensin I. angiotensin-converting enzyme, and angiotensinases were measured in plasma and in various extrarenal tissues of rats. Despite complete suppression of plasma renin in rats bearing pituitary tumors iso-renin and all other components of the renin-angiotensin system were found to be at or above control concentrations. The results strongly suggest that there is local synthesis of iso-renin in extrarenal tissues.     

Presence of renin, angiotensinogen, angiotensin II in the lamb anterior pituitary gland: immunocytochemical study after cryoultramicrotomy.

The presence of renin, angiotensin I-converting enzyme and angiotensin II detected by immunocytochemistry in the adult male rat anterior pituitary has suggested the existence of a pituitary renin-angiotensin system. To establish another mammalian experimental model we have investigated the presence of renin, angiotensinogen, angiotensin I-converting enzyme, and angiotensin II II in five normal lamb anterior pituitaries by immunocytochemistry after cryoultramicrotomy. Renin, angiotensinogen and angiotensin II immunoreactivities were observed only in cytoplasmic granules of lactotrophs, and the three proteins were found co-localized with prolactin in the same granules by double immunolabelling. No immunoreactive angiotensin I-converting enzyme was observed. These results suggest an activation of renin in the cytoplasmic granules of lactotrophs leading to a local synthesis of angiotensin II. Thus, the lamb anterior pituitary may provide a good experimental model for investigating the possible autocrine action of a local renin-angiotensin system on prolactin release in the human pituitary.     

Presence of renin,angiotensinogen, angiotensin II in the lamb anterior pituitary gland: immunocytochemical study after cryoultramicrotomy

Summary The presence of renin, angiotensin I-converting enzyme and angiotensin II detected by immunocytochemistry in the adult male rat anterior pituitary has suggested the existence of a pituitary renin-angiotensin system. To establish another mammalian experimental model we have investigated the presence of renin, angiotensinogen, angiotensin I-converting enzyme, and angiotensin II in five normal lamb anterior pituitaries by immunocytochemistry after cryoultramicrotomy. Renin, angiotensinogen and angiotensin II immunoreactivities were observed only in cytoplasmic granules of lactotrophs, and the three proteins were found co-localized with prolactin in the same granules by double immunolabelling. No immunoreactive angiotensin I-converting enzyme was observed. These results suggest an activation of renin in the cytoplasmic granules of lactotrophs leading to a local synthesis of angiotensin II. Thus, the lamb anterior pituitary may provide a good experimental model for investigating the possible autocrine action of a local renin-angiotensin system on prolactin release in the human pituitary.     

Renin-angiotensin system and vascular remodelling

The renin-angiotensin system (RAS) is compartmented between circulating blood and tissue pericellular space. Whereas renin and its substrate diffuse easily from one compartment to another, the angiotensin peptides act in the compartment where there are generated: blood or pericellular space. Renin is trapped in tissues by low and high affinity receptors. In the target cells, angiotensin II/AT1 receptor interaction generates different signals including an immediate functional calcium-dependent response, secondary hypertrophy and a late proinflammatory and procoagulant response. These late pathological effects are mediated by NADPH oxydase-generated free oxygen radicals and NFkappaB activation. In vivo, the tissue binding of renin and the induction of converting enzyme are the main determinants of the involvement of the RAS in vascular remodeling. The target cells of interstitial angiotensin II are mainly the vascular smooth muscle cells and fibroblasts, whereas the endothelial cells and circulating leukocytes are the main targets of circulating angiotensin II. In vivo, angiotensin II participates in the vascular wall hypertrophy associated with hypertension. In diabetes, as in other localized fibrotic cardiovascular diseases, the tissue effects of angiotensin II are mainly dependent on its ability to induce TGF-beta expression. In experimental atherosclerosis, angiotensin II infusion induces aneurysm formation mediated by activation of circulating leucocytes. In these models, the administration of angiotensin II antagonists has beneficial effects on pathological remodeling. Such beneficial effects of angiotensin II antagonists in localized pathological remodeling have not yet been demonstrated in humans.     

Multiple defects in the renin-angiotensin system in alloxan-diabetic kidney

A defect in the renin-angiotensin system has been shown in diabetic patients and experimental animals, in particular with nephropathy or autonomic neuropathy. The mechanism for this low plasma renin activity (PRA) is poorly understood. In order to clarify this defect, the renin-angiotensin system was studied in alloxan-induced diabetic and age-match control mice. In diabetic animals, kidney renin activity (KRA) was significantly lower than that of the controls, while plasma renin substrate (PRS) concentration was slightly higher and PRA was normal. The amount of injected radiolabeled renin extracted by the kidney was normal, but the amount extracted by the liver was significantly decreased in diabetic animals. On the other hand, the degradation of the extracted renin by both the kidney and the liver was elevated as compared to the controls. This high degradation rate was accompanied by a slight increase in lysosomal protease activity in the kidneys. In in vivo studies, isoproterenol-induced PRA was 20-fold in control animals. In diabetics, isoproterenol-induced PRA was attenuated and rose only four- to fivefold over basal level. The angiotensin converting enzyme (ACE) activity in the kidney was significantly decreased in the diabetic state. It is concluded that there were multiple defects in the renin-angiotensin system in this diabetic model, namely, a depletion of renin storage with subsequent loss of maximal responsiveness to the adrenergic agonist in renin release, an elevation of intrarenal renin degradation together with a deficiency in ACE which would possibly lead to a decrease in intrarenal formation of angiotensin II.     

Regulation of renin angiotensins by gonadotropic hormones in cultured murine Leydig tumor cells. Release of angiotensin but not renin

Renin and angiotensins coexist in various tissues. The mode of control of the extrarenal renin-angiotensin system is not clear. Whether it is renin or angiotensin that is secreted has not been identified. We have investigated gonadotropin-dependent synthesis and subsequent release of the components of the intracellular renin-angiotensin system in a cloned and cultured mouse Leydig tumor cell line (MA-10). Treatment of cultured Leydig cells with bovine luteinizing hormone (bLH, 100 ng/ml) or human chorionic gonadotropin (hCG, 25 ng/ml) resulted in greater than 150- and 40- fold increased formation of angiotensin I and angiotensin II. In cells incubated with bLH or hCG, the majority of AII (up to 90%) was found in the culture medium while most of angiotensin I (greater than 85%) was in the cell lysate. Treatment with gonadotropic hormones (bLH/ hCG) increased renin 35- to 40-fold. Renin activity was confined mainly in the cell lysate even after the stimulation by gonadotropins, and only 1-2% of the total renin activity was detectable in culture medium. These results were interpreted that, in these transformed cells, hormonally-induced renin functions to generate angiotensin I within the Leydig cell and it is the angiotensins which are secreted.     

Substrate-dependent angiotensin II formation in the peripheral circulation.

An alternative angiotensin II-forming system distinct from the vascular renin-angiotensin system was demonstrated using a rat hindlimb perfusion system and synthetic substrates. This pathway was resistant to captopril and aprotinin, but was highly sensitive to chymostatin. Moreover, angiotensin II formation was substrate-dependent, i.e. angiotensin II formation from tridecapeptide human renin substrate in the presence of captopril was more than twice than that from an equimolar amount of angiotensin I. Both pathways may play a role in regulating the peripheral circulation.     

Effects of angiotensin II on adipose conversion and expression of genes of the renin-angiotensin system in human preadipocytes.

A complete functional renin-angiotensin system exists in human adipose tissue, but its regulation and the effects of angiotensin II on cells from this tissue are only beginning to be understood. In this study, we examined the effects of angiotensin II on changes in lipid accumulation, specific glycerol-3-phosphate dehydrogenase activity, and the expression of five genes of the renin-angiotensin system during the adipose conversion of human primary cultured preadipocytes. Angiotensin II leads to a distinct reduction in insulin-induced differentiation, but only has a marginal effect on the adipose conversion of cells stimulated with insulin, cortisol, and isobutyl methyl xanthine. During differentiation, angiotensinogen mRNA levels rise, renin mRNA levels decline, whereas renin-binding protein and angiotensin-converting enzyme levels are unaffected. Angiotensin II downregulates angiotensinogen and renin gene expression, but it does not affect renin-binding protein and angiotensin-converting enzyme levels. Angiotensin II thus prevents the development of adipocytes in contact with high insulin levels, while not inhibiting differentiation, which is further stimulated. Therefore, angiotensin II could be a protective factor against uncontrolled expansion of adipose tissue. Further studies are needed to find out whether the effects of angiotensin II on the renin-angiotensin system are direct feedback loops or secondary to changes in the differentiation program.     

The cardiac renin-angiotensin system: novel signaling mechanisms related to cardiac growth and function

Angiotensin II, the effector peptide of the renin-angiotensin system, has been demonstrated to be involved in the regulation of cellular growth of several tissues in response to developmental, physiological, and pathological processes. The recent identification of renin-angiotensin system components and localization of angiotensin II receptors in cardiac tissue suggests that locally synthesized Ang II can modulate functional and growth responses in cardiac tissue. In this review, regulation of the cardiac RAS is discussed, with an emphasis on growth-related Ang II signal transduction systems.     

Endothelin activates the vascular renin-angiotensin system in rat mesenteric arteries

The effects of endothelin on the vascular renin-angiotensin system were examined in isolated perfused rat mesenteric arteries by measuring vascular renin activity and angiotensin II released into the perfusate. Infusion of endothelin (10(-9)M and 10(-11)M) increased the vascular renin activity and angiotensin II release. Pretreatment with nicardipine (10(-6)M), a calcium channel blocker, significantly suppressed these effects of endothelin. These results suggest that endothelin activates the vascular renin-angiotensin system via intracellular calcium metabolism. Vascular angiotensin II produced by endothelin may modulate the local effect of endothelin on the resistance vessels.     

Interrelations between vasopressin and the renin-angiotensin system.

The interrelationships between vasopressin and the renin-angiotensin system are reviewed. Vasopressin can inhibit the release of renin by the kidney. This effect can occur at physiological plasma concentrations of vasopressin. Centrally administered angiotensin II can stimulate the release of vasopressin, a response that may be partially mediated by brain prostaglandins. The significance of this action of angiotensin II depends on whether there is an effective brain renin-angiotensin system and on whether peripherally generated or administered angiotensin can reach sites in the brain where it can act on vasopressin release. Peripherally administered angiotensin II can under certain, but not all, conditions stimulate vasopressin release. Peripheral angiotensin II can also potentiate the vasopressin response to an osmotic stimulus and to dehydration, but has little effect the release of vasopressin and renin, there is a failure to demonstrate any correlation between the two. Blockade of the renin-angiotensin system fails to modify the vasopressin response to a reduction in blood volume. In conclusion, the physiological significance of the interactions between the vasopressin and the renin-angiotensin system is not as yet clearly established.     

The adipose tissue renin-angiotensin system and metabolic disorders: a review of molecular mechanisms

The renin-angiotensin system (RAS) is classically known for its role in regulation of blood pressure, fluid and electrolyte balance. In this system, angiotensinogen (Agt), the obligate precursor of all bioactive angiotensin peptides, undergoes two enzymatic cleavages by renin and angiotensin converting enzyme (ACE) to produce angiotensin I (Ang I) and angiotensin II (Ang II), respectively. The contemporary view of RAS has become more complex with the discovery of additional angiotensin degradation pathways such as ACE2. All components of the RAS are expressed in and have independent regulation of adipose tissue. This local adipose RAS exerts important auto/paracrine functions in modulating lipogenesis, lipolysis, adipogenesis as well as systemic and adipose tissue inflammation. Mice with adipose-specific Agt overproduction have a 30% increase in plasma Agt levels and develop hypertension and insulin resistance, while mice with adipose-specific Agt knockout have a 25% reduction in Agt plasma levels, demonstrating endocrine actions of adipose RAS. Emerging evidence also points towards a role of RAS in regulation of energy balance. Because adipose RAS is overactivated in many obesity conditions, it is considered a potential candidate linking obesity to hypertension, insulin resistance and other metabolic derangements.     

The adipose tissue renin-angiotensin system and metabolic disorders: a review of molecular mechanisms

The renin-angiotensin system (RAS) is classically known for its role in regulation of blood pressure, fluid and electrolyte balance. In this system, angiotensinogen (Agt), the obligate precursor of all bioactive angiotensin peptides, undergoes two enzymatic cleavages by renin and angiotensin converting enzyme (ACE) to produce angiotensin I (Ang I) and angiotensin II (Ang II), respectively. The contemporary view of RAS has become more complex with the discovery of additional angiotensin degradation pathways such as ACE2. All components of the RAS are expressed in and have independent regulation of adipose tissue. This local adipose RAS exerts important auto/paracrine functions in modulating lipogenesis, lipolysis, adipogenesis as well as systemic and adipose tissue inflammation. Mice with adipose-specific Agt overproduction have a 30% increase in plasma Agt levels and develop hypertension and insulin resistance, while mice with adipose-specific Agt knockout have a 25% reduction in Agt plasma levels, demonstrating endocrine actions of adipose RAS. Emerging evidence also points towards a role of RAS in regulation of energy balance. Because adipose RAS is overactivated in many obesity conditions, it is considered a potential candidate linking obesity to hypertension, insulin resistance and other metabolic derangements.     

Rat epididymal fat tissue express all components of the renin-angiotensin system

In the present study the gene expression of components of the renin-angiotensin system was investigated in fat tissue of rats. mRNAs for angiotensinogen, renin, angiotensin-converting enzyme and type I (AT1) angiotensin II receptor were detected in the stromal-vascular fraction of the fat tissue and the same mRNAs, with the exception of the angiotesin-converting enzyme, in the adipocyte fraction. Renin and angiotensin-converting enzyme activity was measured. The main source of renin activity was found in adipocytes and some minor activity in the stromal-vascular fraction, while the majority of the angiotensin-converting enzyme activity was in the stromal-vascular fraction. The present data provide evidence for the presence of the active renin-angiotensin system in rat adipose tissue.