The Renin-Angiotensin System in the Year 2000

Home , Angiotensin, History of hypertension

MG Nicholls1 and JIS Robertson2 1Department of Medicine, Christchurch Hospital, Christchurch 1, New Zealand; 2Elmbank, Manse Road, Bowling, Glasgow, UK

Keywords: renin-angiotensin system

Introduction demonstrating almost certainly, as was later confirmed, that renin is a protein. Their work further The beginning of the third millennium falls very suggested that renin was released into renal venous close to the centenary of the discovery of the renin- blood so as to cause its pressor effect; it is now angiotensin system. In 1898 Tigerstedt and 1 known that renin is indeed secreted into both the Bergman reported that they had obtained, in both renal vein and renal lymphatics.2 watery and alcoholic extracts of the renal cortex of rabbits, material which had a pressor effect on intravenous injection. They named this substance ‘renin’. Goldblatt and renovascular hypertension A century later, the renin-angiotensin system is Little further work was performed on renin in the revealed as possessing remarkable biochemical and early years of the 20th century. Then Goldblatt et pharmacological complexity, aspects which have al,5 in 1934, showed that it was possible to produce profound evolutionary implications, as shall be dis- sustained hypertension by applying constricting cussed herein. This system is widely and deeply clamps to the renal arteries. Surprisingly, although involved in diverse physiological and pathological Goldblatt speculated that a humoral substance might processes, such that drugs designed to influence its be involved in the pathogenesis of this form of different aspects have extensive, and still increasing, experimental hypertension, he did not at first con- application. Whilst, just over a century following its sider renin to be a contender. Nevertheless, others discovery, the full physiological and pathological did, and once more renin was successfully extracted significance of the renin-angiotensin system remains from kidneys and partially purified.6 Even so, the even now in several respects imperfectly compre- role of the renin-angiotensin system in the causation hended, its importance cannot be in doubt. of renovascular hypertension eventuated as being remarkably complex, and more than 60 years of Historical review extensive study, and some fierce controversy, were required to clarify the situation, which in certain A historical review2 of advancing knowledge and respects remains still unresolved. These matters will understanding of the renin-angiotensin system was be considered in more detail later. proferred in a chapter of the two-volume study of the system that we edited and which was published in 1993;3 further details on various aspects appear Renin as an enzyme in relevant individual chapters therein. Additional perspectives are given in a celebratory centenary The possibility that renin was an enzyme was raised 7 8 volume.4 The following abbreviated account is larg- by Kohlstaedt et al and by Munoz et al in the late ely derived from those several sources. 1930s. Extensive studies consequently confirmed that renin itself did not have a direct pressor action, but that it acted upon a plasma substrate (now called The discovery of renin angiotensinogen) to form ultimately the vasoactive Tigerstedt and Bergman1 achieved remarkably material subsequently identified as the peptide detailed characterisation in their initial studies, angiotensin II (Figure 1). The comparatively slow and prolonged rise in arterial pressure when renin is injected intravenously is because of the requirement for the generation of angiotensin II in vivo; this Correspondence: Dr MG Nicholls, Department of Medicine, Christchurch Hospital, Private Bag 4710, Christchurch, New contrasts with the more rapid and briefer rise in Zealand. E-mail: barbara.griffinȰchmeds.ac.nz pressure when preformed angiotensin II itself is Received and accepted 10 April 2000 given. The renin-angiotensin system in the year 2000 MG Nicholls and JIS Robertson 650

Figure 1 Outline of the biochemistry of the renin-angiotensin system (equine renin substrate is indicated, with the differences in composition in man immediately below). In man, the enzyme, renin, cleaves its substrate (angiotensinogen) between leucine and valine in positions 10 and 11 to form the decapeptide, Ang I. In several other species, both positions 10 and 11 of angiotensinogen are occupied by leucine. Converting enzyme removes residues 9 and 10 (histidine and leucine) to form the active octapeptide, Ang II. Removal of aspartic acid from position 1 forms the heptapeptide, Ang III. In the rat, the 2–10 nonapeptide is present in blood, and Ang III can be formed directly from this nonapeptide by the action of converting enzyme. (From Robertson2,3 with permission.)

The renal juxtaglomerular cells renin splits off first a decapeptide, the inactive angiotensin I. Angiotensin I is then cleaved by The Belgian histologist and physiologist Goormagh- angiotensin-converting enzyme (ACE) to form the tigh9 in 1945 recognised the juxtaglomerular cells on main active component of the sytem, the octapep- the renal afferent arteriole to be the principal source tide angiotensin II (Figure 1). The amino acid of renin, as has since been thoroughly confirmed. sequences of bovine and equine angiotensin were elucidated respectively by Elliott and Peart,14 and by Long-term pressor effect of renin Skeggs et al.15 Angiotensin was shortly afterwards 16 Pickering perceived that renin was well suited to synthesised by Schwyzer and colleagues. serve as a long-term regulator of the circulation, and he and his colleagues10 executed experiments of Renin and aldosterone considerable difficulty given the apparatus then 17 available in 1950, demonstrating that intravenous Gross in 1958 formulated the hypothesis that infusion of renin for up to 18 days’ duration could renin, via angiotensin II, was an important stimulus initiate and then sustain hypertension throughout and hence regulator of adrenocortical aldosterone that period. secretion. This notion owed much to the aforementioned histological observations of Goormaghtigh,9 since distinct changes in the prominence and con- The slow pressor effect of angiotensin II figuration of the juxtaglomerular cells were found to Thus far, the pressor action had been studied at take place with alterations in sodium balance. doses sufficient to raise arterial pressure immedi- Gross’s suggestion was thoroughly vindicated by ately, presumably via direct vasoconstriction. In several groups proceeding roughly simultaneously, 1963 Dickinson and Lawrence11 observed that the including those of Davis,18 Genest,19 Laragh,20 and intravenous infusion of angiotensin II at low doses, Mulrow and Ganong.21 having no immediate effect on arterial pressure This discovery gave much impetus to the study of would, over the course of several days, result in a many aspects of the renin-angiotensin system. One gradual, and eventually very marked, rise in press- demerit, however, was that the more important ure. Angiotensin II could thus be shown to elevate range of direct actions of angiotensin II on renal its own acute pressure dose-response curve, such function22 tended for a time to be neglected. that a direct response could be superimposed on a slow rise that had reached a plateau, the combined Renin, angiotensin, and sodium balance effect exceeding that of the maximum simple initial direct response.12 Following closely on the recognition of the renin- angiotensin-aldosterone connection, it was demon- strated23 that sodium restriction caused a rise in Structure of angiotensin plasma renin and angiotensin II, while, conversely, Skeggs et al13 found that the essential component sodium loading depressed plasma renin. of angiotensinogen is a tetradecapeptide from which The physiological relevance of this observation

Journal of Human Hypertension The renin-angiotensin system in the year 2000 MG Nicholls and JIS Robertson 651 was then further emphasised by the finding24,25 that (and often remain today) rarely standardised, quan- sodium restriction enhanced the stimulant effect of titative comparison of results over time, or between angiotensin II on aldosterone secretion, while simul- laboratories, was hardly ever possible. taneously depressing the pressor effect, both A different problem arose with some early phenomena obviously helping to promote sodium methods for the assay of plasma renin concentration conservation at a time of its deficiency. (PRC).32,33 During extraction, inactive prorenin could be activated, and such procedures thus gave a measure of total, rather than of active, renin con- Renin and plasma sodium concentration centration. An inverse association between plasma sodium con- While studies employing those early PRA and centration within the last week of life and sub- PRC assays provided much valuable information, sequent granularity of the juxtaglomerular cells at their limitations have inevitably become more post mortem was demonstrated in 24 patients with obtrusive. diverse medical conditions.26 Subsequent workers showed that plasma levels of renin and sodium were Circulating inactive renin (prorenin) related inversely in patients with hypertension and heart failure.27 These associations pointed to func- As mentioned in the foregoing paragraphs, it became tional interactions between activity of the renin- evident that a substantial proportion of circulating angiotensin system and water balance in particular, renin was in an inactive form (prorenin). Appropri- which could be accounted for in part by actions of ate assays were hence developed for the separate angiotensin II on thirst, antidiuretic hormone release assessment of prorenin.34–38 The relative proportions and water retention by the kidney (Figure 2). of active renin and prorenin vary very considerably in different circumstances and diseases (Figure 3). Renin assays Angiotensin assays The early 1960s saw the introduction of assays sufficiently sensitive to detect renin in peripheral Practicable and reliably accurate methods for the blood.28 assay of angiotensin II in the circulation were In many early methods,29–31 a plasma sample was developed from the early 1970s on.39 (Assay of cir- incubated in vitro, and the angiotensin generated culating angiotensin I, though it was also from the interaction of endogenous active renin with accomplished,39 was less often needed.) Initially, angiotensinogen was then measured (‘plasma renin and, largely because of the technical demands, parti- activity’; PRA). It will be apparent that PRA cularly during the administration of ACE inhibitors, depended on the respective concentrations of both only a few laboratories undertook these procedures. active renin and of angiotensinogen in the sample. They provided however, two major investigational While such PRA measurements should reflect advantages. First, results could be more readily broadly the capacity of the plasma sample to gener- quantified and compared than was the case with ate angiotensin II, no distinction between the separ- renin assay. Second, because pure synthetic angio- ate concentrations of angiotensinogen and active tensin II was available and suitable for adminis- renin could be made with this approach. Unfortu- tration to man and to laboratory animals, dose- nately also, because several such PRA assays were response curves could be determined, thus permit- ting quantitative analysis of the renin-angiotensin

Figure 2 Outline of the renin-angiotensin system, showing some of the principal actions of angiotensin II. Also shown are sites of action of drugs antagonising the system. (From Robertson JIS and Figure 3 The relation between the percentage of enzymically Ball SG. Hypertension for the Clinician. WB Saunders, 1994, active renin and the total renin (prorenin plus active renin) con- p 43 – with permission.) centration in plasma. (From Skinner79 with permission.)

Journal of Human Hypertension The renin-angiotensin system in the year 2000 MG Nicholls and JIS Robertson 652 system in various physiological and pathophysiol- ation of renin itself in pure and stable form; determi- ogical circumstances. Such a numerate approach is nation of its three-dimensional structure; and delin- of course crucial to a proper evaluation of a hor- eation of its active site.60 monal system.40 The renin-angiotensin system: present Renin in extrarenal tissues cognizance Bing and his colleagues pioneered the study of renin The foregoing historical account is by no means in extrarenal tissues, early work from their Danish comprehensive, but is intended to provide a brief laboratory reporting the presence of renin in uterus 41,42 survey of some of the major progressions in the and salivary glands. understanding of the system. Our present knowl- Renin is now recognised as being widely distrib- 43 edge can be summarised here. uted, and is detectable in, for example, also heart, The basic biochemistry and principal actions of blood vessels,43 adrenal cortex,44 ovary,45 amniotic 46 47 47 48 49 the renin-angiotensin system are shown in Figure 2. fluid, testis, epididymis, eye, and brain. Angiotensin II can be seen to have a range of physiological and pathophysiological effects. It is vasocon- Antagonists and inhibitors of the renin- strictor, raises blood pressure by both an immediate angiotensin system and a slower mechanism, stimulates the secretion of The introduction and deployment of drugs acting aldosterone and vasopressin, facilitates the sym- against the renin-angiotensin system began in the pathetic nervous system at several levels, inhibits 1970s. From that time the specificity, and hence the vagal tone, promotes thirst and sodium appetite, and relevance, of these agents has progressively been produces a range of direct renal responses. Inter- refined. action with the atrial natriuretic peptide system is 61 Early studies employed beta-adrenergic blockers. complex. Infusion of atrial natriuretic peptide While these drugs do, inter alia, lower plasma renin, (ANP) lowers renin and angiotensin II, while there their antihypertensive effect, for example, cannot be are reports of angiotensin II administration stimulat- 62 fully explained by suppression of renin release,50 ing ANP release. and thus evaluation of their influence in terms of The control of renin secretion has turned out to the renin-angiotensin system has to be cautious. be more complex than was realised. Whereas it was Although ACE inhibitors are more specific to the clear that a renal afferent arteriolar baroreceptor, the renin-angiotensin system than are beta-blockers, at macula densa, sympathetic innervation of the kid- least part of their overall effect could reflect ney and various hormones (angiotensin II, anti- enhancement of bradykinin.51 diuretic hormone and prostaglandins) provided 63 Angiotensin II antagonists52 were developed important input signals to renin release, it is now initially as variants of the octapeptide angiotensin II understood that additional factors such as nitric 64 65–68 69 molecule capable of blocking angiotensin II recep- oxide, the natriuretic peptides, endothelin, 70 tors. While their experimental application provided and adrenomedulin, for example, modulate renin a wealth of information concerning the physiology53 release. and pathophysiology54 of the renin-angiotensin sys- Two principal angiotensin II receptor subtypes 71 tem, their need for parenteral administration restric- have been identified, AT1 and AT2. The AT1 recep- ted clinical application. Further, several of such tor mediates those effects enumerated above. Some 72 analogues of angiotensin II possessed some agonist possible physiological roles of the AT2 receptor are activity which could hinder ready interpretation of shown in Table 1. It also seems likely that the AT2 results. For example, one of the most widely used receptor is involved in the development of the kid- 73–75 of these agents, saralasin, was sufficiently agonistic ney and urinary tract. to induce the slow pressor effect when administered The ACE gene is very widely expressed, perhaps 76 over several days to rats.55 ubiquitously, through the body. The structurally quite distinct orally-active antag- While the primary sites of production of renin and of angiotensinogen are respectively kidney and onists of the angiotensin II AT1 receptor represented a major advance,52,56,57 enabling indefinite clinical liver, renin and angiotensinogen genes are exten- inhibition of the renin-angiotensin system more sively manifested in other tissues, many of which, specifically than that achieved by ACE inhibitors, as was stated in the preceding section, have been and inter alia avoiding the troublesome cough shown to contain renin. Thus possible local which can accompany use of the latter. autocrine and paracrine renin-angiotensin systems Numerous effective inhibitors of the enzyme renin assume potential physiological, pathological, and itself have also been developed, and their efficacy perhaps therapeutic, importance. Thus far however, and biochemical sequelae clearly elucidated.58 Thus such local tissue renin-angiotensin systems remain, far however, their clinical application has been lim- by comparison with circulatory aspects, but spar- ited by their modest oral absorption and rather brief sely explored. duration of action.59 Phylogenetic and evolutionary aspects Renin: purification and structural elucidation The foregoing sections have considered exclusively Progressive biochemical developments from the mammals, and especially man. In a detailed 1960s on permitted the isolation and characteris- review,77 Henderson and Deacon have described the

Journal of Human Hypertension The renin-angiotensin system in the year 2000 MG Nicholls and JIS Robertson 653 Table 1 Physiological roles of AT2 receptor in cardiovascular cal and pathophysiological requirements in a wide system range of tissues, organs, environments, and syndromes. Action Cell or tissue These considerations can help to rationalise the present evident complexity of the renin-angiotensin Growth inhibition VSMC Endothelial cell system as observed in man (Figures 1 and 2). If we Cardiomyocyte restrict our attention just to the circulation, we Cardiac ﬁbroblast observe, in peripheral plasma, the active enzyme Proapoptosis VSMC renin, the concentration of which is delicately regu- Endothelial cell Cardiomyocyte lated. The concentration of its substrate, angiotensi-

AT1 receptor nogen, is controlled largely, but not entirely, by AT2 receptor other, independent mechanisms. Concurrently, sub- (Neuronal cell, R3T3 fibroblast) stantial quantities of inactive prorenin, widely vari- Differentiation VSMC able in its proportion to active renin in different cir- (Neuronal cell) Decrease in cellular matrix Heart cumstances, also circulate (Figure 3). The possible Decrease in blood pressure functions of prorenin remain imperfectly defined, Vasodilation Glomerular afferent arteriole albeit fiercely argued.79,80 The principal product of NO production Kidney the reaction of renin with angiotensinogen, angio- Coronary artery and microvessel tensin I, is inactive; two amino acid residues require Aorta to be removed by the action of ACE before the active Improvement of cardiac Heart octapeptide angiotensin II is formed. Further trunc- function (LVEDV, LVESV, ation forms the heptapeptide angiotensin III,81–83 EF) with generally more muted, but still discernable, Decrease in chronotrophic Heart effect physiological actions. Angiotensin III has been pro- posed as, inter alia, having a distinct depressor 82 VSMC, vascular smooth muscle cell; LVEDV, left ventricular end- action mediated via AT1 receptors. Additionally, diastolic volume; LVESV, left ventricular end-systolic volume; there is evidence of pathways alternative to those EF, ejection fraction. (From Horiuchi et al72 with permission.) described above for the generation of angiotensin II and angiotensin III.83 Such elegant biochemical complexity denotes the expression and actions of different components of requirement, adequately fulfilled, to deliver appro- the renin-angiotensin system also in other vertebrate priate, but often very different, concentrations of classes. Despite inevitable gaps in the record, the active peptide at several tissues and organs, thus renin-angiotensin system has been identified, in achieving regulated physiological function while varying complexity, in Agnatha (Cyclostomata); minimising (but not always circumventing) the risks cartilaginous fishes; bony fishes; amphibians; rep- of forming or delivering toxic concentrations at any tiles; and birds. It was in that review speculated that given site. Such biochemical virtuosity can be illus- phylogenetically some of the earliest functions of trated by considering in some detail the involve- the renin-angiotensin system involved, broadly, the ment of the renin-angiotensin system at different changing requirements to obtain, retain, or excrete stages of the pathogenesis of just two conditions, water; and to retain or excrete sodium. The regu- renovascular hypertension and cardiac failure, as lation of arterial pressure, locally and/or sys- shall be presented shortly. Moreover, such dis- temically, could thus readily be envisaged as an cussion is concerned exclusively with circulatory early necessary acquisition to the range of actions. aspects; to these must be added the presently much A major event in vertebrate phylogeny, with obvious less clearly comprehended local paracrine and and weighty implications for the renin-angiotensin autocrine functions of the renin-angiotensin system. system, was the adoption of a terrestrial habitat. Henderson and Deacon77 regard the present con- Physiological and pathophysiological spicuous interplay between the renin-angiotensin involvement of the renin-angiotensin system, adrenal cortex, atrial natriuretic peptide, and the kidney as indicating convergent evolution, system given that the demands of salt economy amongst It will be apparent from the foregoing account that contemporary vertebrates are so diverse. As they the renin-angiotensin system is intricately involved note, it is remarkable that such relatively uniform in a wide diversity of physiological and pathophysi- statements concerning the physiological spectrum of ological circumstances. The system is patho- actions of angiotensin II can be sustained. They genically responsible wholly or partly for some syn- further speculate that the renin-angiotensin system dromes, such as renovascular hypertension, and is may not be unique to vertebrates, but could be more probably concerned with others. Table 2 provides widely represented and influential in the animal a list of conditions in which the renin-angiotensin kingdom. system is variously stimulated or depressed. A Thus, as we ourselves remarked in more detail detailed appraisal of all of these lies beyond the elsewhere,78 the renin-angiotensin system appears scope of the present review. Just two syndromes, to have undergone repeated modification and diver- renovascular hypertension and cardiac failure, will sification, being adapted time and again through the be considered here in some detail so as to indicate course of evolution to mediate different physiologi- the complexity of the participation, and the thera-

Journal of Human Hypertension The renin-angiotensin system in the year 2000 MG Nicholls and JIS Robertson 654 Table 2 The renin system secretion from the contralateral normal kidney is suppressed. Continued exposure to elevated plasma Stimulated angiotensin II, even if the rise is only modest, results in an upward shift of the angiotensin II/pressor 1. Certainly or probably 2. Possibly involved dose-response curve. Thus in the established clini- involved physiologically, pathophysiologically and/or pathophysiologically, and/or pathogenically. cal phase of hypertension, there may be only modest pathogenically. elevation of peripheral levels of renin and angioten- Normal pregnancy. Early polycystic kidney sin II. Indeed in some cases values may be in the Renin-secreting tumour. disease. upper part of the normal range. In this form of Angiotensinogen-secreting Nephrotic syndrome. hypertension, blood pressure elevation is depen- tumour. Aortic coarctation. dent, in both the phases described, on activation of Renovascular Phaeochromocytoma. the renin-angiotensin system. Hypertension can hypertension. Connective tissue diseases. Malignant phase Raynaud’s phenomenon. develop in the absence of changes in sodium bal- hypertension (some cases). Cancerous proliferation. ance, and is not prevented by sodium restriction. Haemorrhage. Indeed, with severe unilateral renal artery stenosis Acute circulatory renal or occlusion, frank and progressive sodium failure. Multiple organ failure. depletion can develop, with consequent ever-higher Hepatic cirrhosis. plasma renin and angiotensin II concentrations, and Addison’s disease and worsening hypertension. This is the hyponatraemic related disorders. hypertensive syndrome, which can often evolve into Anorexia nervosa, bulimia frank malignant hypertension. This form of hyper- nervosa, diuretic abuse, purgative abuse. tension usually responds to reconstruction of the Bartter’s syndrome and affected renal artery, to unilateral nephrectomy, or related disorders. to the administration of antagonists of the renin sys- Vascular and cardiac tem, such as ACE inhibitors. However, ACE inhi- remodelling. bition will nearly always worsen the already Mycardial infarction. Chronic renal failure with impaired function of the affected kidney. hypertension. Hypertension resulting from narrowing of the Diabetes insipidus; artery supplying a sole remaining kidney is not usu- especially nephrogenic ally dependent on an initial rise in renin, even forms. Suppressed though this can be shown usually to occur transi- ently. In the established phase there is often modest Sodium and fluid excess. expansion of body sodium content, accompanying Mineralocorticoid hypertension. normal or low plasma levels of renin and angioten- Gordon’s syndrome. sin II. There may later, with marked renal artery nar- Liddle’s syndrome. rowing, be progression to a terminal phase of severe Syndrome of apparent mineralocorticoid excess. Essential hypertension (majority of cases). hypertension and uraemia, with then an increase in Primary renin deficiency. plasma renin and angiotensin II. Surgical relief of Pregnancy hypertension (ie, relative to normal the renal artery stenosis can be curative. Combined pregnancy). sodium deprivation and pharmacological antagonism of the renin system will lower blood pressure, but at the expense of further impairing renal function. Administration of an antagonist of the renin- peutic relevance of drugs antagonistic to, the renin- angiotensin system in the terminal phase of severe angiotensin system. hypertension and uraemia will cause profound hypotension, worsening uraemia, and early death. The usual causes of clinical renovascular hyper- Renovascular hypertension tension are arterial atheroma (atherosclerosis) and, Renovascular hypertension84,85 occurs in two patho- less often, fibromuscular dysplasia. The latter has a physiologically distinct forms. Unilateral renal familial predisposition. Several commentators have artery stenosis, the opposite kidney and renal artery warned that renovascular lesions often pass unre- remaining intact, corresponds to the Goldblatt two- cognised clinically, with the therapeutic dangers kidney one-clip experimental model. Less often indicated herein if, for example, ACE inhibitors are encountered is hypertension due to stenosis of the administered. Even so, it must be emphasised that artery to a sole remaining kidney, corresponding to the coexistence of a renal artery stenosis with hyper- the Goldblatt one-kidney one-clip model. Bilateral tension does not establish cause and effect, and is renal artery stenosis with hypertension has as its alone insufficient for the diagnosis of true hyperten- experimental counterpart Goldblatt two-kidney two- sion of renovascular origin. clip hypertension; pathogenesis in this variety is similar to that in the one-clip one-kidney model. Cardiac failure In two-kidney one-clip hypertension, initiation of hypertension is due to increased renin release from The exigencies of clinical practice have obviously the clipped kidney and thus to the immediate direct restricted studies of the renin-angiotensin system in pressor effect of the elevated peripheral plasma angi- untreated heart failure in man, and there is for equ- otensin II concentration. Concurrently, renin ally obvious reasons little or no information on

Journal of Human Hypertension The renin-angiotensin system in the year 2000 MG Nicholls and JIS Robertson 655 changes in renin or angiotensin II as the disease tensin II.61 This inverse relation is not merely a evolves.86,87 One early report88 on 21 patients with result of passive responses to opposing forces; untreated heart failure found plasma renin concen- increases in atrial natriuretic peptide within the tration to be subnormal in three, within the normal physiological range reinforce that inverse corre- range in twelve, and high in six subjects. Most lation by inhibiting renin secretion. The superven- importantly, plasma sodium concentration was tion of cardiac failure requires however that atrial closely correlated inversely with plasma renin in natriuretic peptide and renin be recruited together; these untreated patients (r =−0.87; P Ͻ 0.001); with atrial natriuretic peptide to limit the abnormal tend- therapy that significant inverse relationship was ency to salt and water retention, renin to help sus- maintained, while becoming less close (r =−0.55). tain renal function despite the reduction in renal Later studies from the same department89 concern- blood flow. The concomitant increase in both atrial ing treated heart failure showed a highly significant natriuretic peptide and renin is contrived because inverse correlation between plasma active renin the normal inhibitory action of the former on renin concentration and both plasma potassium and total secretion is lost or markedly diminished when renal body potassium. perfusion is impaired,61 as it is from the onset in Experimental animal studies have shown that cardiac failure. Thus in heart failure, atrial natriur- broadly,86,87 and with some inconsistencies, there is etic peptide and renin are both secreted in enhanced in cardiac failure an initial rise in circulating renin, amounts, and their respective concentrations in per- often to very high levels, with a parallel increase in ipheral blood are, quite abnormally, positively cor- plasma aldosterone. This early rise in renin is related. Moreover, in these circumstances their usu- almost certainly limited, and to a very varied extent, ally conflicting actions are modified so that the two by the inhibitory action of an antecedent or concur- systems become mutually reinforcing. In cardiac rent increase in atrial natriuretic peptide, which failure, atrial natriuretic peptide minimises some of restrains renin secretion. With progression of heart the unwanted peripheral effects of the renin-angio- failure, and the achievement of a new steady state tensin system, such as stimulation of aldosterone of sodium and fluid retention, plasma renin values secretion, while enhancing its beneficial renal attain a plateau or may even decline. actions. Activation of the renin-angiotensin system in cardiac failure is thus seen as central to its pathophysi- Currently active topics ology. Such enhanced renin secretion is crucially concerned with modifications of renal and cardiac The range of actions of the renin-angiotensin system function; in central and peripheral haemodynamics; shown in Figure 2, and the wide variety of physio- aldosterone secretion; sodium and water balance; logical and pathophysiological processes enumer- total body potassium and plasma potassium; and ated in Table 2 will indicate that because so many activation of the sympathetic nervous system. and diverse topics are concerned, and under con- Almost certainly, and via several of these mech- tinuous exploration, only a few selected aspects can anisms, the renin system is a potent arrhythmog- be mentioned here. enic influence. It is scarcely surprising therefore that inhibition Antihypertensive and cognate therapy of the renin-angiotensin system in clinical heart failure, employing ACE inhibitors or orally-active The ACE inhibitors are well-established as a major angiotensin II antagonists, almost always on a back- class of antihypertensive drug.94 They are as effec- ground therapy comprising digitalis plus diuretics, tive as other drug types, with the advantage of a rela- has shown major benefit, distinct from, and superior tively low burden of side effects, of which to, other forms of vasodilator treatment.90–92 Such occasional troublesome cough is the most promi- renin-angiotensin antagonism has consistently been nent.95 There are good reasons to explore possible revealed as relieving symptoms and increasing exer- additional benefits of ACE inhibitor therapy over cise capacity, slowing the progression of left ven- that employing alternative agents in limiting cardio- tricular dysfunction, and extending life expectancy. vascular complications. Administered angiotensin II Similar benefits have been found with ACE inhi- has been repeatedly shown to cause arterial, cardiac bition given to patients suffering myocardial infarc- and renal lesions.96 These aspects are also discussed tion and with but modest evidence of cardiac func- in more detail later. Angiotensin II has also been tional impairment.87,93 implicated as a trophic factor in promoting Despite the inevitable lacunae in our knowledge, remodelling of cardiac and resistance arterial the involvement of the renin-angiotensin system in musculature,97 the therapeutic reversal of which cardiac failure provides an elegant example of pur- should prove especially beneficial. Neither the poseful physiological evolution.78 In normal man, CAPPP study98 nor the Swedish STOP-2 trial99 the renin-angiotensin system and the atrial natriur- revealed any unequivocal differential benefit of ACE etic peptide system subserve opposing functions: inhibitor, or ACE inhibitor based therapy versus that the former to protect, inter alia, against sodium and with longer-established drug classes. However, the volume depletion, the effects of haemorrhage, and HOPE study,100 which recruited subjects at high risk hypotension; the latter to prevent or limit salt and of cardiovascular complications, of whom 47% were water overload. Thus in normal physiology there is regarded as ‘hypertensive’, did clearly indicate that a consistent inverse correlation between circulating ACE inhibitor treatment was especially protective atrial natriuretic peptide and both renin and angio- against cardiovascular complications. All the fore-

Journal of Human Hypertension The renin-angiotensin system in the year 2000 MG Nicholls and JIS Robertson 656 going trials98–100 were prospective. To complicate jects, with or without diabetes mellitus,116–123 ACE matters further, whereas STOP-299 showed no differ- inhibitors have been shown to be highly effective, ential benefit between ACE inhibitor and calcium and often superior to earlier approaches, notably antagonist therapy, a retrospective evaluation of beta-blockade and calcium antagonist therapy in hypertensive patients suggested that ACE inhibitors limiting micro- and macro-albuminuria, and in conferred better survival, and calcium antagonists a slowing a decline in renal function. Captopril was poorer outcome.101 more effective than older drugs in preventing the Another retrospective survey suggested that ACE onset of diabetes mellitus in hypertension.98 Irres- inhibitors, used in the treatment of hypertension, pective of hypertension, ramipril limited cardio- could incidentally protect against the development vascular and renal morbidity in diabetic patients.124 of malignant disease.102 There are plausible reasons More limited data indicate that angiotensin II antag- why this might be so, since angiotensin II can pro- onists possess similar properties.125 mote cellular replication and enhance angiogenesis, It is stressed that the benefits of ACE inhibitors in both effects being counteracted by ACE inhibitors. diabetes are probably independent of the antihyper- In theory, at least, angiotensin II antagonists (which tensive action.124 block the AT1 receptor but stimulate the AT2 receptor) might prove even more efficient than ACE Genetic aspects inhibitors in preventing development of neoplastic disease.103 One of the most exciting fields of development link- A further observation104 is that ACE inhibitors can ing the renin-angiotensin system with hypertension reduce the risk of pneumonia in elderly patients. concerns genetics.76,126–132 The genetics of four prin- All of these aspects require clarification or sub- cipal components of the system, renin, angiotensin- stantiation, and are indeed under further detailed ogen, ACE, and angiotensin II receptors, as well as scrutiny. of more peripheral, but related aspects, have been As was described earlier, ACE is not specific for studied. Requiring emphasis is that the association the conversion of inactive angiotensin I to active of a form of hypertension with a genetic locus does angiotensin II, but is responsible also for the break- not necessarily imply that there exists in that down of bradykinin into inactive fragments. Thus element a functional alteration which is responsible ACE inhibition will potentiate the action of such for the hypertension; the relevant modification kinins, and some of its consequences, including might be near, but outside, the particular gene. Not possibly the troublesome cough, could have that ori- surprisingly, information is less extensive and less gin.95 The more recently introduced orally-active consistent concerning hypertension in man than in antagonists of angiotensin II which block the AT1 various animal models. receptor are more specific to the renin-angiotensin Many human studies of renin gene polymorphism system.52,105 Clinically, they appear thus far to pos- in relation to hypertension have been nega- sess the therapeutic attributes of ACE inhibitors, tive.76,131,133 However, an association between renin while circumventing some disadvantages, most gene restriction fragment length polymorphism, eth- obviously cough. nicity, and hypertension has been reported,134 albeit Another aspect of the use of selective AT1 block- without accompanying elevation of plasma renin ing drugs is that unopposed angiotensin II actions activity.135 via AT2 receptors should be exposed, thereby clari- There have been numerous reports of associations, fying the physiological functions of the latter.106–110 possibly causal, between molecular variants of the There is some evidence that such AT2 stimulation is gene for angiotensinogen and essential hypertension additionally vasodilator, and, as mentioned above, and its complications in man.136–154 Ranged against might inhibit neoplasia.103 However, at present it these is a series of contrary findings.155–164 Possible remains uncertain whether clinically such AT2 reasons for these major discrepancies have been stimulation would be beneficial, adverse, or debated at length.139,153,162,165,166 Critics have, with neutral.106 obtrusive disapprobation, considered some of the early positive surveys to have lacked statistical power, to have incurred selection bias and con- ACE inhibition and renal protection founding, and even to have been subject to basic lab- Macroalbuminuria is almost invariably indicative of oratory errors in genetic analysis. renal disease.111 Nevertheless the full pathological Association of polymorphism of the ACE and prognostic significance of microalbuminuria gene127,167–172 with human essential hypertension remains uncertain. There have been sugges- has been reported by some173–177 but by no means tions111,112 that it predicts cardiovascular risk and all,159,164,171,178–185 workers. Irrespective of hyperten- renal deterioration in hypertension although this sion, there is extensive, but also controversial, evi- has not been confirmed by all.113 While some work- dence of variations in the ACE genotype being ers114 have queried the matter, microalbuminuria linked to cardiovascular and renal mor- may perhaps identify patients with high blood bidity.151,169,170,186–195 Other investigators have pressure in whom renal function may deteriorate failed152,196–200 to confirm the described associations, especially rapidly.112 Irrespective of hypertension, and several cautionary criticisms200,201 have been microalbuminuria is a feature of diabetes mel- directed to the positive reports. 112,115 litus. Genetic polymorphism of the angiotensin II AT1 Both in hypertensive and non-hypertensive sub- receptor has been recognised,126 and there have been

Journal of Human Hypertension The renin-angiotensin system in the year 2000 MG Nicholls and JIS Robertson 657 suggestions of associations of AT1 receptor vari- plasma renin and/or angiotensin II has been diffi- ations severally with plasma renin activity,202 cult. hypertension,202 and coronary artery disease.203 The findings of Meade et al219,220 have tended to Once again, others163,204,205 have failed to confirm refute such an association. They found that low, the suggestions and have questioned the value of rather than high, plasma renin activity was likely to these approaches. be associated with increased organ damage in hyper- Bianchi and his colleagues have made detailed tension; in normotensive men moreover, they saw and extensive studies206–210 of adducin, a heterodim- no relationship between plasma renin activity and eric protein which probably regulates cell-signal either myocardial infarction or sudden death from transduction via changes in the actin cytoskeleton likely coronary artery disease. and hence renal tubular sodium reabsorption. They By contrast, a series of epidemiological surveys by found evidence that ␣-adducin gene polymorphism Alderman et al221–227 indicated that low urinary is associated with some cases of essential hyperten- sodium and either observed or presumed elevated sion in man, with plasma renin activity being lower plasma renin activity was associated with a greater in the predisposed subjects.209 The findings have risk of myocardial infarction and both cardiovascu- been supported by some,211,212 but not all,213–216 lar and all-cause mortality. These observations led other workers. Alderman et al to question the safety of dietary The foregoing is necessarily a very abbreviated sodium restriction in the treatment of hypertension. account of a complex and rapidly expanding field of The aforementioned HOPE trial,100 and the retro- investigation. This shows promise of becoming one spective studies from the Glasgow Blood Pressure of the most fiercely controversial areas of an already Clinic,101,228 in which ACE inhibitors limited highly fractious topic. One clearly commendable cardiovascular morbidity, accord with the specu- accompaniment is that the interest in individual lations of Alderman et al. Nevertheless, the various components of the system, for example renin, angi- papers by Alderman et al were, not surprisingly, 229–236 otensinogen, and angiotensin II AT1 receptors, has subjected to vigorous attack, not least from become focused. These entities now require to be advocates of dietary salt restriction. Even were diet- addressed by specific assays and/or characterisation. ary sodium restriction confirmed as predisposing to myocardial infarction, elevation of plasma renin and The future hence of angiotensin II would not necessarily be the sole, or even principal cause of the problem. The Speculation on likely impending developments con- highly significant increases in plasma total and low cerning the renin-angiotensin system is certain to density lipoprotein (LDL) cholesterol which are also involve errors. As the old Irish saying has it, predic- induced by salt restriction237 would require to be tion is hazardous, especially in relation to the taken into account. future. Nevertheless, some topics are proferred here, This is an area of controversy which will surely several, necessarily, deriving from the preceding receive much investigational attention in the section. immediate future.

Human pregnancy Endothelium It has been well-established for over 30 years, that Burgeoning interest especially over the ﬁnal years of plasma renin, angiotensinogen, angiotensin II, and the 1990s in endothelium-derived vasodilators such aldosterone are markedly increased, to a very vari- as nitric oxide and vasoconstrictors such as endo- able degree, from an early stage and throughout thelin has generated a wealth of data concerning the human pregnancy.217 With pregnancy-induced endothelium in relation to hypertension and arterial hypertension, the increases are relatively less.218 disease. Interactions with the renin-angiotensin sys- Despite extensive investigation, proper comprehen- tem are prominent herein. The endothelium synthe- sion of the physiological implications remains elus- sizes renin locally, while ACE, which both generates ive. A steady, but possibly still protracted, advance angiotensin II and inactivates bradykinin, is abun- can be predicted. dantly expressed at the surface of endothelial cells.238–240 Endothelin-1 has been shown to aug- ment the pressor action of angiotensin II,241 whereas Renin and cardiovascular risk blockade of endothelin receptors reportedly pre- There are sound experimental reasons for supposing vents the pressor and renal (but not dipsogenic) that high levels of circulating renin and hence of actions of angiotensin II.242 Inhibition of nitric oxide angiotensin II could predispose to a range of cardio- synthesis with L-NAME activates vascular ACE and vascular lesions in hypertension. The experimental causes oxidative stress.243 Angiotensin II can stimu- infusion of angiotensin II can cause necrotizing late nitric oxide and oxygen radical release simul- arterial lesions, multifocal myocardial lesions, and taneously.244 An ACE inhibitor, captopril, in con- renal tubular necrosis.96 At the lower rates of admin- trast to the calcium antagonist nifedipine, has been istration myocardial necroses have been seen with shown to improve endothelium-dependent vaso- plasma concentrations of angiotensin II likely to be dilation in hypertensive patients.245 The adminis- encountered clinically. However, translating this tration of ACE inhibitors has been demonstrated to extensive experimental information into clinical protect endothelial vasomotor function by inducing evidence of increased morbidity with elevated the local accumulation of bradykinin, stimulating

Journal of Human Hypertension The renin-angiotensin system in the year 2000 MG Nicholls and JIS Robertson 658 autacoid formation, and to inhibit secretion of endo- been shown that in patients with cardiac failure, the thelin-1,246,247 although not all reports have been contractile response in resistance arteries to angio- positive.248 Local generation of angiotensin II pro- tensin I is blunted in the presence of the ACE inhibi- baby facilitates potentially harmful neointimal pro- tor enalaprilat, suggesting that chymase cannot fully liferation after injury, thus offering another thera- compensate for and hence override ACE inhi- peutic target for ACE inhibition.249 bition.257 These and like subjects will remain under Such notions are not exclusive to the peripheral close future scrutiny. arteries; coronary kinin generation has been 250 reported as mediating nitric oxide release. Overexpression of the AT receptor Somewhat inconsistent ﬁndings have emerged 1 258 concerning the interrelation of angiotensin II with It was shown by Hein et al that overexpression nitric oxide release. Hennington et al251 reported of the AT1 receptor in mice was associated with an that angiotensin II stimulated nitric oxide synthase. increase in atrial size, hyperplasia of myocytes However, long-term ACE inhibition evidently within the atria and heart block. The implications enhanced the availability of nitric oxide;246 brady- of this observation for human disease are presently kinin accumulation might of course have contrib- unclear. uted to this latter observation.

A somewhat separate, but very pertinent report, Blockade of AT1 receptors especially in relation to the aforementioned effect of Drugs blocking AT receptors104 are characterised by angiotensin II in facilitating cardiac remodelling and 1 a particular lack of significant side effects which causing myocardial lesions, was published by Takiz- might translate into a comparatively superior com- awa et al.252 These workers found that nitric oxide pliance rate259—a vital factor in preventing the com- modulated the proliferation of fibroblasts with angi- plications of hypertension. Whether, for compliant otensin II-induced cardiac fibrosis. patients, the angiotensin II receptor blockers prove Although data are short of definitive, there is sus- more or less effective in preventing complications picion that angiotensin II, either circulating or remains to be determined. Likewise, whether these locally within blood vessels might contribute to the drugs are more or less effective than the ACE inhibi- development of atherosclerotic lesions. Indeed angi- tors after acute myocardial infarction should become otensin II has many actions which might promote clear from the results of ongoing studies. In estab- atheroma formation. Furthermore, expression of lished heart failure, ELITE I suggested that losartan angiotensin II in the intima is inhanced in atheroma- might be associated with a lower mortality than cap- tous plaques,253 chymase dependent angiotensin II topril in elderly heart failure patients.260 However, formation is increased in human atherosclerotic an appropriately powered study with mortality as aorta254 and antagonism of AT receptors reduces 1 its end point showed no such difference between endothelial dysfunction and intimal thickening in losartan and captopril, although the former had the atherosclerotic rabbits.255 Warnholtz et al256 suggest superior side effect profile.261 In theory, the combi- that angiotensin II mediated superoxide production nation of an ACE inhibitor and an angiotensin II might play a pivotal role in the early stages of receptor blocker might prove superior to either drug atherosclerosis. alone in the above disorders, and formal trials These brief comments should serve to justify the indeed are underway. confident prediction that this broad field will in the near future be further investigated, intensively and profitably, concerning both pathophysiological and The angiotensin II AT2 receptor therapeutic understanding. The AT1 receptor for angiotensin II mediates those responses shown in Figure 2. The functions of the Local renin-angiotensin systems AT2 receptor hitherto have by contrast remained comparatively obscure. The recent increasing use of The preceding section on the endothelium will have orally-active AT1 receptor blocking agents, which at least partly served to emphasise the importance will expose AT2 receptors to stimulation, has simul- of local renin-angiotensin generation, and the inter- taneously provided a means to elucidate the func- action with other systems having physiological and tions of the latter and to evaluate resultant benefits pathological roles. or disadvantages.105–110 The previous understandable emphasis on the cir- Such matters urgently require, and will receive, culatory aspects of renin and angiotensin will attention. Early results, which were mentioned henceforward be leavened by increased attention to briefly above,105–110 indicate that unopposed stimu- local renin-angiotensin systems and their autocrine lation of AT2 receptors by angiotensin II is a vaso- and paracrine functions. dilator, and hence probably additionally antihypertensive. Other likely actions mediated via the AT2 Alternative pathways of angiotensin formation receptor are shown in Table 1. These aspects will undoubtedly soon be investigated in detail. There is longstanding,83 and continuing, interest in routes alternative to those depicted in Figure 1 for the generation of active angiotensins. For example, Renin inhibitors in human resistance arteries angiotensin II can be We predicted in 19933 that effective orally-active formed via either ACE or chymase. It has however inhibitors of the enzyme renin itself would be

Journal of Human Hypertension The renin-angiotensin system in the year 2000 MG Nicholls and JIS Robertson 659 developed and introduced into clinical therapeutics. ney, together with a method for its biological assay. Though a range of renin inhibitors has appeared,58,59 Clin Sci 1938; 3: 211–227. none so far has been sufficiently well absorbed by 7 Kohlstaedt KG, Helmer OM, Page IH. Activation of mouth or long-acting for such application. Mean- renin by blood colloids. Proc Soc Exper Biol Med 1938; 39: 214–215. while angiotensin II AT1 receptor blocking drugs 105 8 Munoz JM, Braun-Menendez E, Fasciolo JC, Leloir LF. have become well-established. Hypertensin: The substance causing renal hyperten- Nevertheless, there remains a need and an oppor- sion. Nature 1939; 144: 980. tunity for improved renin inhibitors. The very diver- 9 Goormaghtigh N. La fonction endocrine des arte´rioles sity and complexity of the renin-angiotensin system reńales: Son roˆle dans la pathogenie de l’hypertension calls for a range of different agents capable of inter- arte`rielle. Revue Belge des Sciences Medicales 1945; rupting that system at several sites. Continued 16: 65–155. development of renin inhibitors should thus still 10 Blacket RB et al. Hypertension produced in the rabbit be pursued. by long-continued infusion of renin. Clin Sci 1950; 9: 223–245. 11 Dickinson CJ, Lawrence JR. A slowly developing Standardisation pressor response to small concentrations of angiotensin. Lancet 1963; I: 1354–1356. A long-standing and much repeated plea,262–265 12 Lever AF. The fast and the slowly developing pressor almost as often ignored, is for the general adoption effect of angiotensin II. In: Robertson JIS, Nicholls MG of a renin standard, enabling results to be expressed (eds). The Renin-Angiotensin System. Gower Medical in common, quantitative, terms. It is truly shameful Publishing: London, 1993, pp 28.1–28.9. that we remain largely unable to compare the results 13 Skeggs LT, Lentz KE, Hochstrasser H, Kahn JR. The of renin assays from time to time, or from one lab- chemistry of renin substrate. Canadian Med Assoc J 1964; 90: 185–189. oratory to another. 14 Elliott DF, Peart WS. Amino acid sequence in a hypert- We here express the hope, but not regrettably the ensin. Nature 1956; 177: 527–528. prediction, that such standardisation will be 15 Skeggs LT et al. The amino acid sequence of hyperten- accepted, and implemented, before the second cen- sin II. J Exper Med 1956; 104: 193–197. tury of research into the renin-angiotensin system 16 Schwyzer R, Sieber P. New synthesis in the peptide is ended. field. Chimia 1956; 10: 265. 17 Gross F. Renin und Hypertensin: Physiologische oder pathologische Wirkstoffe? Klinische Wochenschrift Conclusions 1958; 36: 693–706. 18 Davis JO. 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