Intrarenal conversion of angiotensin I to angiotensin II in the dog

Diabetic Nephropathy Induced by Increased Ace Gene Dosage Is Associated with High Renal Levels of Angiotensin (1-7) and Bradykinin

Population studies have shown an association between diabetic nephropathy (DN) and insertion/deletion (I/D) polymorphism of the angiotensin-converting enzyme (ACE) gene (ACE in humans, Ace in mice). The aim was to evaluate the modulation of Ace copies number and diabetes mellitus (DM) on renal RAS and correlate it with indicators of kidney function. Increased number of copies of the Ace gene, associated with DM, induces renal dysfunction. The susceptibility to the development of DN in 3 copies of animals is associated with an imbalance in activity of RAS enzymes leading to increased synthesis of Ang II and Ang-(1-7). Increased concentration of renal Ang-(1-7) appears to potentiate the deleterious effects triggered by Ang II on kidney structure and function. Results also show increased bradykinin concentration in 3 copies diabetic group. Taken together, results indicate that the deleterious effects described in 3 copies diabetic group are, at least in part, due to a combination of factors not usually described in the literature. Thus, the data presented here show up innovative and contribute to understanding the complex mechanisms involved in the development of DN, in order to optimize the treatment of patients with this complication.

Sex differences in angiotensin-converting enzyme modulation of Ang (1-7) levels in normotensive WKY rats

BACKGROUND

Levels of the vasodilatory peptide angiotensin (Ang) (1-7) have been reported to be greater in females than in males, although the molecular mechanism responsible for this is unknown. Angiotensin-converting enzyme (ACE), ACE2, and neprilysin are key enzymes regulating Ang (1-7) formation. We conducted a study to determine the effect of sex on the activities of ACE, ACE2, and neprilysin in the kidneys of normotensive rats. We hypothesized that greater ACE2 or neprilysin activity in females would result in enhanced Ang (1-7) formation as compared with that in males.

METHODS

We measured the enzymatic activities of ACE, ACE2, and neprilysin in the renal cortex and medulla of 12-week-old male and female WKY rats. We treated additional rats with vehicle or enalapril (10 mg/kg/day in drinking water) for 14 days, and measured their Ang II and Ang (1-7) levels.

RESULTS

Renal cortical activity of ACE was greater in female than in male WKY rats (P < 0.05), but the activity of ACE in the renal medulla was not significantly different in the two sexes. Renal cortical and medullary ACE2 and neprilysin activities were comparable in male and female WKY rats. Treatment with enalapril significantly decreased Ang II levels in the renal cortex and medulla of male and female WKY rats as compared with those in vehicle-treated controls (P < 0.05); enalapril did not change the plasma levels of Ang II. Cortical levels of Ang (1-7) were higher in vehicle-treated females than in vehicle-treated males (P < 0.05), and treatment with enalapril decreased Ang (1-7) levels only in females (P < 0.05).

CONCLUSIONS

Our data supports a role for ACE in the formation of renal cortical Ang (1-7) in female WKY rats that is absent in males.

Angiotensin I is largely converted to angiotensin (1-7) and angiotensin (2-10) by isolated rat glomeruli

Intraglomerular renin-angiotensin system enzyme activities have been examined previously using glomerular lysates and immune-based assays. However, preparation of glomerular extracts compromises the integrity of their anatomic architecture. In addition, antibody-based assays focus on angiotensin (Ang) II detection, ignoring the generation of other Ang I-derived metabolites, some of which may cross-react with Ang II. Therefore, our aim was to examine the metabolism of Ang I in freshly isolated intact glomeruli using matrix-assisted laser desorption ionization time of flight mass spectrometry as an analytic method. Glomeruli from male Sprague-Dawley rats were isolated by sieving and incubated in Krebs buffer in the presence of 1 micromol/L of Ang I for 15 to 90 minutes, with or without various peptidase inhibitors. Peptide sequences were confirmed by matrix-assisted laser desorption ionization time of flight tandem mass spectrometry or linear-trap-quadrupole mass spectrometry. Peaks were quantified using customized valine-(13)C(.15)N-labeled peptides as standards. The most prominent peaks resulting from Ang I cleavage were 899 and 1181 m/z, corresponding with Ang (1-7) and Ang (2-10), respectively. Smaller peaks for Ang II, Ang (1-9), and Ang (3-10) also were detected. The disappearance of Ang I was significantly reduced during inhibition of aminopeptidase A or neprilysin. In contrast, captopril did not alter Ang I degradation. Furthermore, during simultaneous inhibition of aminopeptidase A and neprilysin, the disappearance of Ang I was markedly attenuated compared with all of the other conditions. These results suggest that there is prominent intraglomerular conversion of Ang I to Ang (2-10) and Ang (1-7), mediated by aminopeptidase A and neprilysin, respectively. Formation of these alternative Ang peptides may be critical to counterbalance the local actions of Ang II. Enhancement of these enzymatic activities may constitute potential therapeutic targets for Ang II-mediated glomerular diseases.

Quantification of conversion and degradation of circulating angiotensin in rats

The aim of the present study was to quantify with a uniform technique the rates of conversion of ANG I to ANG II in the lung and kidney and the degradation of both peptides to biologically inactive products in the pulmonary, renal, and systemic circulation. We infused the peptides intravenously, into the left ventricle, and into the left renal artery of rats and compared their effects on renal blood flow. The measured change in renal blood flow was used as a bioassay parameter to estimate the concentration of circulating ANG II. Mathematical analysis of our data allowed us to calculate conversion and degradation rates. Furthermore, the role of aminopeptidases A (EC 3.4.11.7) and N (EC 3.4.11.2) in the degradation of the peptides in the kidney was investigated by intrarenal infusion of the inhibitor amastatin. Our results show that the conversion rate of ANG I is 75% in the pulmonary and 21% in the renal circulation. Both peptides are degraded by 5% in the pulmonary, by 67% in the systemic, and by 93% in the renal circulation. Amastatin prevented 60% of the renal degradation of the peptides to inactive products, and this effect could be attributed to inhibition of aminopeptidase N. The results indicate that the converting capacity of the kidney is of minor importance for endocrine generation of ANG II but could be useful for the paracrine production.

Pathways for angiotensin II generation in intact human tissue: evidence from comparative pharmacological interruption of the renin system

Multiple lines of evidence have suggested that alternative pathways to the angiotensin-converting enzyme (ACE) exists for angiotensin II (Ang II) generation in the heart, large arteries, and the kidney. In vitro studies in intact tissues, homogenates, or membrane isolates from the heart and large arteries have repeatedly demonstrated such pathways, but the issue remains unresolved because the approaches used have not made it possible to extrapolate from the in vitro to the in vivo situation. For our in vivo model, we studied young and healthy human volunteers, for the most part white and male; when these subjects achieved balance on a low salt diet to activate the renin system, the response of renal perfusion to pharmacological interruption of the renin system was studied. With this approach, we studied the renal vasodilator response to 3 ACE inhibitors, 2 renin inhibitors, and 2 Ang II antagonists at the top of their respective dose-response relationships. When these studies were initiated, our premise was that a kinin-dependent mechanism contributed to the renal hemodynamic response to ACE inhibition; therefore, the renal vasodilator response to ACE inhibition would exceed the alternatives. To our surprise, both renin inhibitors and both Ang II antagonists that were studied induced a renal vasodilator response of 140 to 150 mL/min/1.73 m2, approximately 50% larger than the maximal renal hemodynamic response to ACE inhibition, which was 90 to 100 mL/min/1.73 m2. In light of the data from in vitro systems, our findings indicate that in the intact human kidney, virtually all Ang II generation is renin-dependent but at least 40% of Ang I is converted to Ang II by pathways other than ACE, presumably a chymase, although other enzyme pathways exist. Preliminary data indicate that the non-ACE pathway may be substantially larger in disease states such as diabetes mellitus. One implication of the studies is that at the tissue level, Ang II antagonists have much greater potential for blocking the renin-angiotensin system than does ACE inhibition-with implications for therapeutics.

Tissue renin-angiotensin system: a site of drug action?

In this review, we present background material that provides partial support for a tissue renin-angiotensin system (RAS). Evidence for the existence of this system relied in part on the use of drugs, which has entailed using low doses or concentrations of angiotensin-converting enzyme inhibitors, renin inhibitors, and angiotensin antagonists to block the RAS in vascular beds and in isolated arteries or organs. Other evidence for a tissue RAS has depended upon measurements of the components of the system, i.e. enzymes, substrates, and mRNAs for these proteins. All of these components were first believed to be present in the heart and blood vessels; however, it is now known that renin in the circulating blood derived from the kidney is used for the local synthesis of angiotensins. The main emphasis of the review is on the renal RAS because it is believed that the local RAS is most prominent in this organ. The renal RAS is probably involved in the long-rather than short-term regulation of renal vascular resistance and maintenance of normal blood pressure through the regulation of sodium reabsorption.

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.

Spironolactone in congestive heart failure refractory to high-dose loop diuretic and low-dose angiotensin-converting enzyme inhibitor

Patients with severe congestive heart failure (New York Heart Association [NYHA] functional classes III-IV) often can tolerate only low doses of angiotensin-converting enzyme (ACE) inhibitors because pronounced hypotension caused by additional ACE inhibitor increments may decrease renal perfusion. The use of high-dose loop diuretics is currently advised to overcome diuretic resistance in refractory congestive heart failure (CHF). In a baseline controlled study, we evaluated 21 patients with diuretic resistance and evident fluid retention for the responses to 5 days of double drug therapy consisting of high-dose loop diuretic (10 mg oral bumetanide) in combination with the maximum tolerable dose of an ACE inhibitor (individualized to blood pressure and kidney function). Five patients (24%) showed a gross natriuresis and reduction in excess weight > 25% in response to this therapy. The remaining 16 patients (76%) with insufficient responses (i.e., < 25% reduction in excess weight) subsequently received 100 mg spironolactone once a day for 7 days in addition to the double therapy. Spironolactone coadministration was highly effective in 13 of 16 patients (81%). Marked natriuresis and diuresis were achieved within the next week of treatment, and CHF symptoms regressed or disappeared. The clinical course was similar in the bumetanide-ACE inhibitor and the bumetanide-ACE inhibitor-spironolactone treatment (triple therapy) groups. Plasma aldosterone was significantly higher (p < 0.05) in the patients who needed spironolactone. The 3 patients who were considered refractory to triple therapy exhibited the highest baseline plasma aldosterone concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)

Distribution and metabolism of angiotensin I and II in the blood vessel wall

The demonstration of all components of the renin-angiotensin system in vascular tissue has raised questions as to the precise location of the local angiotensin II generation within the vascular wall. We investigated the metabolism of angiotensin I to angiotensin II in the vascular wall in the isolated rabbit thoracic aorta. Angiotensin I (3 x 10(-9) M) applied into the aortic lumen was partially converted to angiotensin II (14% after 60 minutes), but most of the luminal angiotensin I was degraded to peptide fragments or diffused as intact angiotensin I, peptide fragments, or both, into the vessel wall. Incubation studies with [3H]angiotensin I revealed that angiotensin I or angiotensin I fragments mainly diffused into the medial layer of the aorta and to a lesser degree into the adventitia and the endothelium. After removal of the endothelium, angiotensin II generation could no longer be detected. Addition of the angiotensin converting enzyme inhibitor ramiprilat (10(-7) M) to the incubation medium led to a complete blockade of angiotensin II generation by endothelial angiotensin converting enzyme. Our results underline the importance of the endothelium for conversion of angiotensin I to angiotensin II and provide evidence that conversion of angiotensin I to angiotensin II is predominantly achieved by endothelial cells. They also support the concept of an endocrine versus autocrine/paracrine renin-angiotensin system where the endothelium of the vasculature is the critical target site for angiotensin II production by both systems and, thus, the most important site for the actions of angiotensin converting enzyme inhibitors.

Efficacy of low-dose captopril in addition to furosemide and spironolactone in patients with decompensated liver disease during blunted diuresis

The renin-angiotensin-aldosterone system is activated by diuretics and involved in the diuretic resistance of cirrhotic patients with ascites and oedema. In previous studies relatively high doses of captopril (25-400 mg daily) were unsuccessful in promoting diuresis and natriuresis in these patients. We analyzed the efficacy of a low dose of captopril in eight patients with massive ascites resistant to therapy of salt/fluid restriction and increasing doses of spironolactone and furosemide. Mean duration of diuretic use was 73 days (range 7-240 days). After at least 3 days of observation on 80 mg furosemide and 100 mg spironolactone only, captopril was added. Four out of eight patients responded with an increase in natriuresis and diuresis; daily dose of captopril was 20.6 mg in responders and 26.5 mg in non-responders. After the addition of captopril the mean weight change was -7.5 kg in responders and +0.25 kg in non-responders. Mean urinary sodium output in responders increased from 72.8 (S.D. = 35.2) to 128.5 (63.5) mmol within 10 days. Increased diuresis in responders made diuretic reduction necessary: mean furosemide from 80 to 53.3 mg, and mean spironolactone from 100 to 68.1 mg. Creatinine clearances remained stable. High levels of plasma renin activity, plasma aldosterone and angiotensin-II were found in all patients. Non-responders showed more severe hyponatremia and higher vasopressin levels. Natriuretic atrial factor (NAF) was in the upper-normal range or slightly elevated in both groups. In non-responders we noticed low levels of cGMP in 24-h urine, compared with responders.(ABSTRACT TRUNCATED AT 250 WORDS)

Receptors for angiotensins I and II: their relevance to renal haemodynamics, blood pressure control and hind-limb blood flow

The well established differential pulmonary handling of angiotensins I and II indicates the possibility that vascular receptors for the deca- and octa-peptides do not necessarily involve common sites in the renal vasculature either. Experimental findings involving haemodynamic changes within the kidney in anaesthetised and conscious sheep, with utilization of the angiotensins, and also of noradrenaline, are briefly presented; the implications of the intra-renal water and creatinine transfers are discussed, especially as they concern the possible location of angiotensin receptors in the renal blood vessels. Other aspects of the relationships between the peptides are also taken into account particularly with regard to a postulated angiotensin I [NaCl] dependent peritubular capillary antidiuretic action, angiotensin converting enzyme inhibition, Goldblatt clamp induced hypertension and blood flow through the hind-limbs.

Renal perfusion and function. The implications of converting enzyme inhibition

Because the kidney is both the source of circulating renin and the final determinant of the state of sodium balance, which in turn defines responsiveness to angiotensin II, one might have anticipated substantial interest in the impact of converting enzyme inhibitors on the kidney when these agents were developed. The lessons learned about the role of the renin-angiotensin system in normal renal perfusion and function, and possible disorders of control that contribute to disease in patients with essential hypertension, renovascular hypertension, and chronic progressive renal parenchymal disease are reviewed. In each case, the lessons here have important implications for the clinical application of converting enzyme inhibition.

Biochemistry of human converting enzyme

Angiotensin converting enzyme (ACE, EC 3.4.15.1) was purified to homogeneity from human kidney and its specific antibody was raised in the rabbit. The antibody cross-reacted equally with human enzymes from kidney, lung, intestine, plasma and urine. Immunofluorescent and immunoelectron microscopic observation indicated that the enzyme was located on the plasma membrane and micropinocytic vesicles at the luminal site of vascular endothelium in the lung. It was also present on the brush border, intercellular and basal infolding membranes of proximal tubular epithelium, but was not detected on the distal tubular epithelium or vascular endothelium in the kidney. ACE was demonstrated immunocytologically in human alveolar macrophages and renal carcinoma tissues. The carcinoma tissue contained a possible isoenzyme of ACE differing in part immunologically from the enzyme of normal kidney.

Preclinical studies on angiotensin converting enzyme inhibitors

The identification of the renin-angiotensin-aldosterone system in the control of blood pressure, and the preclinical development of the angiotensin converting enzyme inhibitors for therapeutic use are reviewed. The properties of these compounds are discussed with respect to their in vitro enzyme inhibitory potency; prevention of the pharmacological effects of angiotensin I; potentiation of those of bradykinin; tissue enzyme inhibition; mechanism of effect on blood pressure both alone and in combination with other antihypertensive agents; and effect on cardiac parameters.

Mechanism of the pressor response to tetradecapeptide renin substrate in the rat

The synthetic tetradecapeptide renin substrate (TDP; Asp-arg-val-tyr-ile-his-pro-phe-his-leu-leu-val-tyr-ser) has been employed frequently to elucidate the enzymatic action of renin in vitro and, to a lesser extent, in vivo. We assessed the utility of TDP as a renin substrate in vivo using conscious spontaneously hypertensive rats. Intravenous injection of TDP (1 and 3 micrograms/kg) increased diastolic pressure by 45 +r2 and 67 +/- 2 mmHg, respectively. The pressor response to TDP was significantly inhibited by captopril (3 mg/kg, po), indicating its dependence on conversion by ACE to some active molecule. Pressor responses to TDP also were less in animals subjected to bilateral nephrectomy 18-24 hr before study. However, responses to angiotensin I and II also were reduced, implying a non-specific effect of nephrectomy. Intravenous infusion of the renin inhibitor pepstatin (200 micrograms/min) inhibited pressor responses to hog renin by approximately 60%, but did not affect those to TDP. Intravenous infusion of the water soluble renin inhibitor, pepstatinyl-arginine-o-methyl ester (500 micrograms/min), also inhibited pressor responses to renin (approx. 80%) and did not affect those of TDP. Incubation to TDP (5 microM) with rabbit lung ACE resulted in generation of AI that was blocked by captopril (1 microM). These data suggest that TDP is a substrate for ACE and that the increase in blood pressure produced by TDP is due to its sequential cleavage by ACE to AII and can be independent of renin.

The localization of converting enzyme in kidney vessels of the rat

An antibody against pure rabbit lung converting enzyme (CE) showing cross-reaction with CE from other species was used for immunocytochemical studies in the kidney of rats. Using the indirect labelling PAP-technique, specific immunostaining was found in the endothelial layer of all arteries and arterioles of kidney cortex and in some descending vasa recta. CE-positive reactions were also seen in most glomeruli, the reaction product being confined to only a few capillary loops in connection with the glomerular stalk. A few immunostained capillaries in the cortical labyrinth were suspected to belong to the first ramifications of the efferent arteriole. The bulk of all other of the glomerular and peritubula capillaries as well as all veins of the kidney showed no obvious immunostaining. The functional significance of this specific localization pattern of CE in the endothelium of kidney vessels is discussed with respect to the actions of the systemic and the local, intrarenal renin-angiotensin-system on kidney functions.

Mechanism of angiotensin I converting enzyme inhibition by SQ20,881 (less than Glu-Trp-Pro-Arg-Pro-Gln-Ile-Pro-Pro) in vivo. Further evidence for extrapulmonary conversion

The mechanism by which the angiotensin I (AI) converting enzyme inhibitor SQ20,881 (less than Glu-Trp-Pro-Arg-Pro-Glu-Ile-Pro-Pro) blocks the pressor response to exogenous AI was studied in vivo in the intact anesthetized dog. When administered as a single dose 250 times that of injected AI (250 nmoles/kg) into either the pulmonary or systemic circulation, SQ20,881 produced inhibition of pulmonary conversion of exogenous AI to AII that lasted for more than 6 hours as judged by the absence of immunoreactive or labeled AII in the pulmonary venous effluent. In contrast, the pressor response to exogenous AI began to reappear within 1 hour of SQ20,881 administration. Six hours following SQ20,881, the pressor response to AI had nearly returned to normal, still in the absence of demonstrable intrapulmonary conversion and without release of detectable amounts of AII into the pulmonary venous effluent. These experiments demonstrated that AI has a pressor effect in the presence of SQ20,881 that is independent of pulmonary conversion. Studies with (Des-Asp) AII and (Des-Asp, Arg) AII showed that the delayed pressor response to AI following SQ20,881 administration could not be accounted for by circulating peptide metabolites of AI or AII. A competitive inhibitor of AII, (D-Asp, Ile) AII completely blocked the returning pressor response, suggesting that extrapulmonary generation of AII was responsible. The data strongly suggest that the systemic vascular bed taken as a whole contains large amounts of AI converting enzyme that is capable of rapid generation of AII without releasing the peptide into circulation. The extrapulmonary enzyme is more resistant to long-lasting blockade by SQ20,881 than pulmonary converting enzyme. The physiological role of extrapulmonary conversion systemic and local circulatory homeotasis remains to be assessed.

Conversion of angiotensin I to angiotensin II in sheep

1. The methodology for measurement of angiotensin I in whole blood is described. 2. Angiotensin I was measured in arterial and venous blood samples from sodium-loaded, sodium-replete and sodium-depleted sheep. Venous blood concentrations were higher than arterial angiotensin I concentrations. Arterial angiotensin I concentrations increased with increasing sodium deficiency. 3. Angiotensin II was infused into six sodium-replete sheep, the blood concentrations of angiotensin II measured and the metabolic clearance rates calculated. The average value was 121 +/- 21 1/h. 4. Angiotensin I was then infused into these sheep and arterial and venous blood concentrations of angiotensins I and II determined. From this data and the metabolic clearance rate of angiotensin II, the degree of conversion of angiotensin I to II was calculated. The values varied in different sheep from 54 +/- 2% to 105 +/- 6% (n = 6).

Renal vascular response to interruption of the renin-angiotensin system in normal man

We assessed the role of the renin-angiotensin system in the response of the renal circulation to restriction of sodium intake in 38 normal patients. Both saralasin (10 to 30 ng/kg/min), an angiotensin antagonist, and SQ 20881 (30 to 300microgram/kg), a converting enzyme inhibitor, induced a dose-related increase in renal blood flow (xenon 133 washout) only when the resin-angiotension system was activated by restriction of sodium intake to 10 MEq/day. Increasing doses of saralasin (100 to 1,000 ng/kg/min) reduced renal blood flow, presumably due to the angiotensin-like action of this partial agonist. The renal vascular response to SQ 20881 paralleled the endocrine response: An identical threshold dose (30 microgram/kg) increased renal blood flow and reduced plasma angiotensin II concentration, which fell despite a progressive rise of plasma renin activity. Plasma bradykinin concentration did not change in response to SQ 20881, which also blocks kininase II. Both agents also induced a small but consistent and statistically significant reduction in arterial blood pressure, which will be important in assessing the pathogenetic significance of a blood pressure reduction in patients with hypertension. This study indicates that angiotensin mediates the renal vascular response to restriction of salt intake in normal man and provides an approach to assessing the role played by angiotensin in the pathogenesis of functional renal disease.

Converting enzyme activity in endothelial cells isolated from pig pulmonary artery and aorta

Converting enzyme activity was studied in endothelial cells, isolated from pig pulmonary arteries and aorta by exposure to collagenase. The measure was based on the release of His-Leu from Z-Phe-His-Leu was related to the DNA content of the cellular suspension. The same activity was found in the two types of endothelium: 1 nmol His-Leu/mug DNA per 30 min. Subendothelial cells showed a very low activity, amounting to 10% of the value found for the endothelium. The enzyme activity was 2inhibited by the nonapeptide SQ 20881, EDTA, and the lack of Cl- in the same fashion for the two types of endothelium. The presence of another enzyme hydrolyzing His-Leu was detected in both endothelial populations. Isolated fragments of plasma membrane, however, exhibited only converting enzyme activity. It can be concluded that endothelial cells isolated from large vessels of the pulmonary and the systemic circulations have similar properties when dipeptidyl carboxypeptidase activity is measured.

Conversion of angiotensin I to angiotensin II

The angiotensin I converting enzyme has two important functions: it inactivates bradykinin and converts angiotensin I to angiotensin II. Inhibition of the enzyme blocks the renin-angiotensin system and decreases systemic blood pressure if the pressure is maintained or increased by renin. The enzyme occurs in a variety of tissues and cell forms. The vascular endothelial cells of the lung and of peripheral blood vessels, and the epithelial cells of the kidney tubules are major sources of the enzyme. In addition to inactivating hypotensive peptides and activating a hypertensive one in the systemic circulation, the enzyme may affect organ functions by hydrolyzing peptides that are formed and released locally.

The renal circulation in hypertensive disease

The pivotal role of the kidney in sustaining hypertension from any source or etiology is becoming increasingly clear. The possibility that the renal vasculature participates not only in the pathogenesis of renal vascular hypertension, but also in that of essential hypertension, has been the subject of continuing interest for 40 years. Evidence that a functional abnormality resulting in increased renal vascular tone is present in about two-thirds of patients with uncomplicated essential hypertension is reviewed, along with more circumstantial evidence that sympathetic nervous system activity operating on the renal vasculature is responsible. Two additional groups of patients in whom a characteristic abnormality of the renal vasculature is present have also been identified. In one group there is severe hypertension which is resistant to most forms of antihypertensive therapy but which is especially responsive to propranolol. In these patients renal blood flow and glomerular filtration rate are reduced, renin secretion rate is increased and the renal vessels are resistant to vasodilators, suggesting the presence of advanced organic arteriolonephrosclerosis, as a complication of long-standing, severe hypertension. The renal lesion, in turn, contributes to the increasing severity of the process. In a second group of patients, generally young and with uncomplicated hypertension, renal blood flow is inappropriately increased. In these patients a number of observations on their renal vasculature, renin and aldosterone responses to a volume challenge suggest an abnormality in the perception of extracellular fluid volume. A perfectly normal renal arterial tree, free of organic abnormality or an increase in tone due to active vasoconstriction, is distinctly unusual in essential hypertension.

Regulation of renin release and intrarenal formation of angiotensin. Studies in the isolated perfused rat kidney

1. Isolated rat kidneys were perfused at a constant pressure of 90 mmHg in a single-pass system with either a cell-free medium or a suspension of washed bovine red blood cells, free of the components of the renin-angiotensin system. In red blood cell perfused kidneys renal haemodynamics and sodium reabsorption corresponded closer to values observed in the intact rat than in cell-free perfused kidneys. 2. In red blood cell-perfused kidneys in the absence of plasma renin substrate autoregulation of renal blood flow was almost complete at pressures above 90 mmHg, provided that perfusion pressure was changed rapidly. 3. Renin release varied inversely with perfusion pressure within a pressure range from 50 to 150 mmHg; the greatest changes of renin release occurred, when perfusion pressure was reduced from 90 to 70 mmHg; maximal stimulation of renin release was observed at 50 mmHg. After reduction of perfusion pressure, renin release immediately started to rise and reached a new level within 5 min. Local reduction of perfusion pressure in small arteries and arterioles by the injection of microspheres induced a short-lasting decrease in renal plasma flow and a transient stimulation of renin release. 4. High concentrations of furosemide stimulated renin release by a direct intrarenal mechanism. 5. Isoproterenol stimulated renin release in low concentrations without a concomitant vasodilation, whereas high concentrations induced an increase in both renal plasma flow and renin release. The effects of isoproterenol were completely blocked by propranolol. 6. Sodium nitroprusside induced similar increases in renal plasma flow, as did high concentrations of isoproterenol, but only a small and slow increase in renin release was observed. 7. Angiotensin II (AII) suppressed renin release in concentrations corresponding to plasma levels measured in the intact rat independently of its vasoconstrictor effects, whereas vasopressin in antidiuretic concentrations did not affect renin release. 8. AII, AI, synthetic tetradecapeptide renin substrate (TDP), crude and purified rat plasma renin substrate induced a dose-dependent reduction in renal plasma flow. SQ 20 881, a competitive inhibitor of converting enzyme, and low doses of 1-Sar-8-Ala-AII (saralasin), a competitive antagonist of AII, did not change renal plasma flow, whereas high concentrations of saralasin had a vasoconstrictor effect on their own. 9. Saralasin inhibited the vasoconstrictor effects of AII and TDP to a similar degree. SQ 20 881 inhibited the vasoconstrictor effects of AI and purified renin substrate, but did not influence the actions of TDP and the crude renin substrate preparation. 10. From these data it is concluded, that AI is converted into AII within the kidney at a rate of 1-2%. The vasoconstriction induced by the crude renin substrate probably does not involve the AII receptors. TDP may act by itself on the AII receptors or via the direct intrarenal formation of AII...

Role of extrapulmonary conversion in mediating the systemic pressor activity of angiotensin I

The effect of antibodies against angiotensin II (AII) on systemic pressor responses to intravenously injected AII and angiotensin I (AI) was studied in a group of bioassay rats. AII antibody was only 29% as effective in neutralizing AI given intravenously as it was in neutralizing AII injected by the same route. Control plasma caused no change in the relative potencies of AI and AII. In a further series of experiments, AII antibody was significantly less effective in blocking intra-arterial AI than in blocking intravenous AI. The potency of intra-arterial AI, initially less than that of intravenous AI, became nearly twice that of intravenous AI after antibody administration, a result which could not occur if AI were inactive before lung transit. Thus, AI can elicit systemic pressor activity independently of pulmonary conversion to AII. However, since the intra-arterial AI responses were abolished by an inhibitor of angiotensin-converting enzyme, the activity would appear to be mediated by peripheral conversion to AII rather than by an intrinsic action of the decapeptide. Both series of experiments suggest that the efficacy of AII antibody in abolishing the systemic pressor activity of AI is highly dependent on the site of conversion of the AI to AII. The occurrence of localized intramural conversion of AI to AII near arteriolar receptors in vivo may so minimize exposure of the liberated AII to circulating antibody as to render AII immunization an inefficient means of blocking endogenous pressor activity of the renin-angiotensin system.

Effects of angiotensin I and antiotensin II on canine hepatic vascular resistance

The effects of angiotensin I (0.2-3.2 µg) and angiotensin II (0.1-1.6 µg) injections into the pump-perfused arterial supply of the liver were studied in dogs anesthetized with sodium pentobarbital. Marked increases in hepatic artery perfusion pressure (10-50%), reflecting directionally similar changes in resistance to blood flow, were caused by either angiotensin I or angiotensin II. Resistance increases produced by angiotensin I were significantly attenuated by the synthetic nonapeptide SQ 20881 (Pyr-Trp-Pro-Arg-Pro-Gln-Ile-Pro-Pro, 50 µg/kg, iv) that inhibits enzymatic conversion of angiotensin I to angiotensin II. In contrast, responses caused by angiotensin II were unaltered by SQ 20881. However, resistance increases caused by either angiotensin I or angiotensin II were blocked by 1-Sar-8-Ala-angiotensin II (100 µg/kg min -1 , ia), a specific angiotensin II antagonist. These findings parallel the finding that responses to angiotensin I in the vasculature supplied by the hepatic artery are largely caused by local enzymatic conversion of angiotensin I to angiotensin II. Such conversion appears to occur to the extent of about 46%.

Inhibition of converting enzyme of the renin-angiotensin system in kidneys and hindlegs of dogs

The activity of angiotensin-converting enzyme in hindlegs and kidneys was compared in anesthetized dogs. Intra-arterial injections of angiotensin I or angiotensin II to one kidney or one hindleg caused dose-dependent decreases in blood flow in that vascular bed. An inhibitor of angiotensin-converting enzyme activity did not affect the vasoconstrictor activity of angiotensin II but substantially reduced that of angiotensin I. The reduction in vasoconstrictor activity of angiotensin I during enzyme inhibition was used to calculate conversion of angiotensin I to angiotensin II. There was 40% conversion in hindleg but only 2.1% in kidney. After intra-arterial injection of angiotensin I, bioassay of angiotensin II in the renal or iliac venous blood indicated that 60-90% of freshly formed angiotensin II was inactivated before leaving the kidney or hindleg. The results show that converting enzyme activity is much greater in hindlegs than in kidneys and that the endogenously formed angiotensin II following intra-arterial injection of angiotensin I is destroyed to the same extent as intra-arterially injected angiotensin II. Intra-arterial infusion of the enzyme inhibitor increased hindleg blood flow in half of the dogs; renal blood flow increased in only one dog. The inhibitor did not affect vascular responses produced by norepinephrine, histamine, vasopressin, or prostaglandin F 2α but the effects of bradykinin were potentiated.