Tissue Hormones: Angiotensin, Bradykinin and the Regulation of Regional Blood Flows

Tissue Hormones: Angiotensin, Bradykinin and the Regulation of Regional Blood Flows JOHN C. McGIFF, M.D.*

Within the past decade, several polypeptides isolated from the blood were synthesized, thereby accelerating investigation of their physiological roles and possible therapeutic applications. Two of these polypeptides, bradykinin and angiotensin, though differing in many of their biological properties, share a sufficient number of properties to distinguish them as members of a group, the tissue hormones. 54 Tissue hormones - hormones not secreted by specialized glands - circulate in an inactive form. The precursor in plasma of angiotensin and bradykinin is activated by enzymes having selective distribution: renin and kallikrein for angiotensin and bradykinin, respectively. Angiotensin and bradykinin, which are vasoactive polypeptides, presumably have physiological actions at least partially dependent upon their effects on blood vessels. It is improbable that the active form of angiotensin, normally formed within the kidney, achieves systemic concentrations sufficient to constrict extrarenal blood vessels, except under the most extraordinary circumstances - for example, early in the evolution of renal vascular hypertension. Tissue hormones ideally would function in the regulation of regional circulation, or more importantly in the regulation of blood flow to the functional units within an organ, e.g., the nephron for angiotensin. A mechanism regulating glomerular filtration rate (GFR) and tubular reabsorption of sodium for each nephron would satisfy the conditions required for maintained glomerular-tubular balance with respect to proximal tubular sodium reabsorption. Glomerular-tubular balance From the Cardiovascular Section, Department of Internal Medicine, St. Louis University School of Medicine, St. Louis, Missouri The writing of this review and our own research discussed in it were underwritten by funds from American Heart Association, Grant 65 G 54, the Missouri Heart Association, and USPHS Grants 5-T1-HE-5239, HE-08805 and HE-l1041. *Associate Professor of Medicine and Director of Cardiovascular Section, St. Louis University School of Medicine. Established Investigator of the American Heart Association. Medical Clinics of North America- Vo!. 52, No. 2, March, 1968

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refers to the constancy of the fractional reabsorption of sodium by the proximal tubule in spite of variations in the filtered load of sodium. Angiotensin, which influences both GFR and tubular reabsorption of sodium, was proposed to be the determinant of glomerular-tubular balance (Fig. 1).35 Physiologically, angiotensin may have only an intrarenal action, and then only in response to a stimulus operative within a nephron.

ACTIVATION OF TISSUE HORMONES Angiotensin Renin substrate (angiotensinogen), the circulating precursor of angiotensin, is converted to angiotensin I, a decapeptide, by renin which is synthesized and stored in the juxtaglomerular apparatus of the kidney. The octapeptide, angiotensin n, is rapidly formed from angiotensin I by the converting enzyme of plasma. 5o The effects of renin are related to the action of angiotensin n.52 Angiotensin n shall be simply designated angiotensin, for its decapeptide precursor presumably does not exert any actions in vivo. 50 Since there is no reliable method for the determination of angiotensin, the level of activity of the renin-angiotensin system is usually determined by assaying renin activity in plasma. This is at best a rough measure of circulating angiotensin, which may vary independently of plasma renin. 58 THE JUXTAGLOMERULAR ApPARATUS. The critical event in the activation of the precursor of angiotensin is the release of renin. However, the physiological stimulus releasing renin is unsettled. The juxtaglomerular apparatus was proposed to function as a baroreceptor or stretch receptor, for which its location in the afferent glomerular arteriolar wall would be ideally suited. 59. 65 An alternative explanation invokes the macula dens a, one of the components of the juxtaglomerular apparatus. The macula dens a, which is an element of the distal tubule, makes contact with the vascular pole of its own glomerulus; it is attached to the juxtaglomerular cells of the afferent glomerular arteriole (Fig. 1). The macula densa may act as a sensing element to relate changes in release of renin to a distal renal tubular event, such as a change in sodium concentration or sodium load, bulk flow of tubular fluid or osmolality of tubular fluid. 22 • 68 Thurau, examining directly the relationship between distal tubular sodium content and glomerular perfusion, introduced increasing concentrations of sodium into the macula densa segment of distal tubules by micropuncture. 63 When sodium chloride in a concentration of 300 mM. per liter was introduced into the macula densa segment, Thurau observed collapse of the proximal tubules of the same nephron, which denoted cessation of glomerular filtration. Thurau related this to release of renin, which he did not measure, produced through the effect of sodium on the macula densa. Vander and Miller 67 have also concluded that renin release is controlled by the load of sodium presented to the macula densa. However, they proposed that renin is released in response to reduced, rather than increased, sodium load at the macula densa.

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Afferent Arteriolar Resistance

Efferent Arteriolar Resistance

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Figure l. Schematic representation of the glomerulus, its blood vessels and tlle juxtaglomerular apparatus. Angiotensin formed from renin is suggested by the arrows to have a primary effect on the postglomerular blood vessels (efferent arteriole) and perhaps on the peritubular (Le., contraluminal) side of the renal tubular cells. Administered (exogenous) angiotensin is suggested to have a primary effect on the larger preglomerular blood vessels (interlobar and interlobular).

The observations of both groups support the hypothesis that the GFR is determined by the extent of tubular reabsorption of sodium, rather than that alteration in the rate of sodium reabsorption by the proximal tubules is the consequence of a change in GFR. The observations of Thurau imply that angiotensin has its major action on the afferent glomerular arteriole (preglomerularly). However, evidence cited later suggests that a postglomerular (efferent arteriolar) action of angiotensin is probable, though not necessarily the exclusive site of action of angiotensin (Fig. 1).44

Uterine Renin The exclusive identification of the renin-angiotensin system with the kidney, in terms of the production and release of renin, must be revised in view of the isolation of renin, or a substance very similar to it (not presently distinguishable from it), from the gravid uterus 17, 19 and salivary glands. 70 Inasmuch as renal renin has not been purified, the pressor material isolated from the gravid uterus is best designated renin-like ("renin"). The "renin" of the gravid uterus was suggested to participate in the regulation of placental blood flow, since acute placental ischemia produced hypertension. 4 This material is (1) synthesized by the uterus; (2) not trapped from circulating renin of renal origin; (3) is not susceptible to stimuli which alter renal renin; and (4) achieves a concentration in the pregnant uterus equal to that of whole kidney. 17, 19 Whether "renin" released in the gravid uterus occurs only in response to placental ischemia, or whether it is involved in the regulation of placental blood flow, so as' to mediate fine adjustments in blood flow in response to the metabolic requirements of the gravid uterus, has not been determined. This consideration raises questions similar to those resulting from proposals for a physiological role of renal renin. Release

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of uterine renin in response to placental ischemia may be the determinant of toxemia of pregnancy. There are interesting parallels between the pressor system of the gravid uterus and the kidney, particularly in terms of the autonomic nervous system, which influences release of renin in both organs as well as influencing the vasoconstrictor action of angiotensin.

Bradykinin While activation of angiotensin precursor has been related to renin, an enzyme of limited distribution, bradykinin substrate appears to be susceptible to activation by several systems. 37 Kallikrein activity produces kallidin-la, which is the decapeptide precursor to kallidin-9 or bradykinin, a nonapeptide. This system has been designated the kallikrein-kallidin system. Several factors associated with blood clotting, clot lysis (plasmin) and increased capillary permeability (permeability factor) have been related to the conversion of bradykinin precursor to the active form.37 The key event in their participation in the formation of bradykinin may be through the activation of the Hageman factor.37 Production of bradykinin at this level- by the clotting and fibrinolysin (plasmin) systems-may bear some relationship to the proposed role of bradykinin in mediating in part the inflammatory response. 33 The distribution of kallikreins, which are the enzymes responsible for the activation of bradykinin and kallidin-la, is less selective than that of renin. Kallikreins, having different physiochemical properties, have been found in highest concentration in plasma, urine and exocrine glands. 13 Bradykinin has been proposed to regulate glandular blood flow in response to glandular secretory activity and may mediate functional vasodilation of exocrine glands. 26 ,34 The presence of a distinctive kallikrein in its active form in the urine suggested to Gill and associates a regulatory role of bradykinin for the renal circulation and for the excretion of salt. 18

INACTIVATION OF TISSUE HORMONES Angiotensin Increased or decreased activity of the renin-angiotensin system may theoretically occur in the absence of altered formation of angiotensin by decreasing or increasing its rate of degradation, respectively, by angiotensinases. Since several angiotensinases of unknown physiological importance have been identified, the term "angiotensinase activity" avoids a connotation of specificity in a physiological setting. Alteration of angiotensinase activity by genetic factors or disease states may have significance for (1) a consideration of hypertension as a hereditary disease and (2) an inquiry into the pathogenesis of the accelerated phase of hypertension in which renin and angiotensin are presumed to participate. Since angiotensin is comparatively slowly inactivated by circulating enzymes, and since one of the angiotensinases is present in very high concentrations only in the kidney, it is suggested that the kidney is predominantly concerned with the metabolism of angiotensin. 28 ,58 A reduction by 90 per cent of the angiotensinase activity of the kidney was

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a consequence of lesions destroying the proximal and distal convoluted tubules. 28 The close association of ang'iotensinases and the kidney supports the suggestion that the primary, if not exclusive, physiological locus of action for the renin-angiotensin system is the kidney. 56 An increased angiotensinase activity in hypertensive sJ1bjects was suggested to be the result of enzymatic adaptation-an adaptive increase in angiotensinases consequent to increased angiotensin. 25 The proposal of enzymatic adaptation was not confirmed by Itskovitz and associates,27 who did not find a correlation between plasma angiotensinase activity and renin levels in peripheral or renal venous blood in normotensive or hypertensive subjects.

Bradykinin The inactivation of bradykinin probably is a consequence of the activity of several enzymes, the kininases. Their physiological significance is less clear than that of the angiotensinases. Of the peptidases examined, a carboxypeptidase of plasma was demonstrated to destroy bradykinin most readily.14 The isolation of a renal peptidase, which inactivates bradykinin rapidly, is an interesting parallel development to the definition of renal angiotensinases, which also are most active in the kidney. IS

INTERACTION OF TISSUE HORMONES, NEUROTRANSMITTERS AND OTHER HORMONES The renin-angiotensin and kallikrein-bradykinin (kallidin) systems are not only capable of stimulating the release of catecholamines and the secretion of hormones, but may themselves be activated by circulating neurotransmitters or autonomic nervous activity. Angiotensin and Aldosterone The effects of angiotensin on other organs and systems complicate efforts to define the effect of angiotensin on a single event, such as renal excretion of sodium. Thus there are renal tubular, renal vascular and extrarenal actions of angiotensin which represent both direct and indirect effects of angiotensin. These actions have different time courses and are influenced in different ways by disease states, as well as by the experimental methods used by various investigators. Furthermore, the physiological blood levels of angiotensin have not been established. Until this is accomplished, assigning a primary role to angiotensin in the regulation of aldosterone secretion is premature. The absence of a differential threshold of angiotensin on aldosterone secretion and blood pressure endorses this cautionary attitude. I The effect of angiotensin on aldosterone secretion (physiological action?) has been reported to occur only with doses of angiotensin which are pressor (adventitious action?). Therefore the interpretation of experiments purporting to resolve the physiological action of angiotensin requires the utmost caution. While it is undeniable that angiotensin has the capacity to increase aldosterone secretion, at least as a pharmacological demonstration, it is by no means

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certain that angiotensin is the regulator of aldosterone secretion under physiological conditions. A number of stimuli have been suggested to influence aldosterone secretory rate, including plasma sodium and plasma potassium concentration, changes in blood volume and corticotropin, as well as angiotensin. 5 ,3l Whether the various stimuli to the secretion of aldosterone operate physiologically through a single agency-a proximate cause such as the renin-angiotensin system-is not now possible to answer. Release of Adrenal Medullary Catecholamines The effect of angiotensin and bradykinin on catecholamines of the adrenal medulla and possibly on the catecholamines of the vasculature may be a major determinant of the action of these polypeptides, at least in experimental, if not physiological, settings. Angiotensin and bradykinin were demonstrated by Feldberg and Lewis 16 to release catecholamines from the adrenal medulla. Angiotensin has a potency several hundred times that of bradykinin in releasing catecholamines from the adrenal medulla. 16 The action of angiotensin on promoting release of catecholamines assumes much larger significance when the effect of administered angiotensin on sodium excretion by the kidney is considered (see below). Carcinoid Tumors A compelling case has been made for the participation of bradykinin in some subjects having carcinoid tumorsY,48 A kallikrein which is present in carcinoid tumors may be released by several circulating amines, including epinephrine. Flushes were produced in these patients by giving epinephrine, which releases from the enterochromaffin Cells a kallikrein resulting in bradykinin formation. In 5 of 11 patients having carcinoid tumors, elevated concentrations of bradykinin in the hepatic venous blood were found during flushes. 48 Role of Calcium Bradykinin .injected into the carotid artery of the unanesthetized dog was demonstrated to release vasopressin. 55 This action, as indeed all the actions of vasoactive polypeptides, may be the result of a general effect of these agents on cell membranes, including those of secretory tissues. It is perhaps premature to relate the various effects of vasoactive polypeptides to their capacity to make calcium available ~o a critical site within the cell; however, there is increasing evidence that some of the activity of angiotensin is linked to the calcium ion. 11,30 For glands, this relationship between secretory activity and calcium effect on secretion has been termed excitation-secretion coupling. It is important to emphasize that the actions of vasoactive polypeptides under discussion may be only pharmacological actions. Physiologically, the tissue hormones may be rapidly destroyed after selectively affecting a functional unit of a single organ. Certain disease states may result in large increases in systemic concentrations of tissue hormones - anaphylactic shock perhaps for bradykinin, and renal vascular hypertension, at least initially in its course, for angiotensin. lo , 13

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A consideration of the renal action of angiotensin demonstrates some of the properties of tissue hormones, particularly as they may relate to regulation of blood flow to a tissue as a working hypothesis. The proposed physiological actions of angiotensin, the determinants of these actions and induction of the renin-angiotensin system in disease states will now be considered.

PROPOSED PHYSIOLOGICAL ACTIONS Angiotensin has been proposed to be the mediator of autoregulation of the renal circulation. 64 The regulation of salt excretion and glomerular filtration is implicit in any consideration of renal circulatory autoregulation, since the renal circulation subserves maintenance of glomerular filtration at a level consistent with preserving the excretory and regulatory function of the kidney. Control of sodium excretion by the renin-angiotensin system may occur not only indirectly as a consequence of regulation of GFR and renal blood flow (RBF), but also by a direct effect of angiotensin on sodium reabsorption in the proximal and distal convoluted tubules. 35

Regulation of Glomerular Filtration Rate The control of GFR is the most important aspect of that property of the renal circulation termed autoregulation, which is the capacity of the kidney to maintain its blood flow (and GFR) constant in the face of large alterations in perfusion pressure, in the range of 70 to 190 mm. Hg. A preglomerular site of the major vascular resistance changes (afferent glomerular arteriole) mediating autoregulation has been proposed by Thurau 64 (Fig. 1). Thus elevations in renal perfusion pressure from 90 to 190 mm. Hg did not produce changes in the pressure within peritubular (post glomerular) capillaries. 21 In the face of a constant GFR and RBF, this can be explained only by a reduction of the lumen of the preglomerular (afferent) arteriole. During hypotension, the maintenance of GFR would be more readily accomplished by an increased postglomerular resistance (centered in the efferent glomerular arteriole) than by a decrease in the preglomerular vascular resistance (Fig. 1). However, any consideration of the homeostatic adjustments of biological systems should allow for their incredible adaptibility, which suggests that autoregulation is the result of an interplay of preglomerular and postglomerular resistance sites; the dominance of either one is probably conditioned by other systems such as the level of circulating neurohumors. Furthermore, there is morphologic evidence ruling against an exclusively preglomerular localization of autoregulation (Fig. 2); viz., those nephrons (the juxtamedullary nephrons) which are incapable of autoregulation possess a post glomerular (efferent) arteriole which presumably is not a major site mediating increases in renal vascular resistance. Thus, their efferent arterioles lack well-developed smooth muscle envelopes and possess calibers conspicuously larger than those of cortical nephrons. 39 ,66 The juxtamedullary nephrons, comprising about 20 per cent of the population of nephrons in man, also lack a well-developed juxtaglomerular apparatus, which suggests

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a· relationship between autoregulation and the renin-angiotensin system 64 (Fig. 2). An interdependency of the renin-angiotensin system and renal autoregulation is probable, for: (1) a plasma factor (renin substrate?) is necessary for autoregulation;68 (2) the anatomical location of the juxtaglomerular apparatus (at the vascular pole of the glomerulus) ideally serves an autoregulatory (control of GFR) function (Fig. 1); (3) administration of antirenin to dogs results in a reduced ability to autoregulate GFR;56 (4) autoregulation is absent in the juxtamedullary nephrons, which also lack a well developed juxtaglomerular apparatus as indicated above;64 in support of this morphologic observation, Peart 53 reported a low content of renin in the inner cortex; (5) the effect of angiotensin given by intravenous infusion on GFR and RBF is in large part dependent on their control levels. Thus, when angiotensin is given during induced reductions of GFR and RBF, increments of both occur, whereas angiotensin given in the same dose when RBF and GFR are normal reduces each. 43 . 44

RENIN, ANGIOTENSIN AND RENAL ISCHEMIA Disease states may result in the fullest expression of the activity of a system, such as hyperaldosteronism of cirrhosis with ascites, which then permits definition of some of the properties of that biological system. The experimental production of renal ischemia mimics, at least, the incipience of renal arterial occlusive disease. Angiotensin has been suggested to increase renal blood flow during renal ischemia, secondarily to increasing systemic blood pressure. Several investigators

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Figure 3. Paradoxical effect of angiotensin during renal ischemia. Polygraphic recording of changes in left renal blood flow (upper trace) and mean aortic blood pressure (lower trace). The renal blood flow was restored almost to control levels after administration of angiotensin during renal ischemia. Immediately after release of the renal arterial constriction, angiotensin caused a reduction of renal blood flow. (Reprinted with the permission of the Journal of Clinical Investigation.)

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suggested a homeostatic role for angiotensin in renal arterial stenosis through restoration of the diminished RBF by its pressor effect.46, 57 This hypothesis fails to account for the marked renal vasoconstrictor effect of angiotensin.

Paradoxical Effect of Angiotensin on an Ischemic Kidney To test the above hypothesis, we studied the action of angiotensin on the renal vasculature during renal ischemia. 43 The vasoconstrictor action of angiotensin was abolished in the ischemic (clipped) kidney (Fig. 3). Vasoconstriction occurred in the uninvolved kidney simultaneously with an increased blood flow to the ischemic kidney in response to angiotensin (Fig. 4). A corollary of this observation is that renal ischemia, as an initial event in the production of renal hyperGFR23 (ml/minl L.RENAlBlOOD FlDW (ml/minl

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Figure 4. Effect of angiotensin on renal blood flows during renal arterial constriction. Polygraphic recording of left and right renal blood flows and mean aortic blood pressure. At the arrow, the right renal blood flow was constricted. The breaks in the trace represent 5-minute intervals. Angiotensin increased the blood flow to the ischemic kidney and simultaneously reduced the blood flow to the opposite kidney.

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tension, may be obscured by the homeostatic mechanism invoked by it, formation of angiotensin and restoration of RBF to control levels. This finding may reconcile investigators who consider renal ischemia the sine qua non of hypertension of renal origin 60 with those who maintain that there is no constant relationship between nephrogenic hypertension and renal ischemia. 49 Thus, the reported variable changes in RBF in hypertension associated with coarctation of the aorta,23 renal arterial disease 71 and experimental procedures designed to produce nephrogenic hypertension 10 do not exclude an initial renal ischemic stimulus.

The Effects of Renal Ischemia on the Contralateral Kidney Since renin, or other humors, may be released early during renal ischemia, it appears probable that effects would be registered on the contralateral kidney. The vascular changes in the kidney contralateral to the renal arterial lesion are of importance, for they may, if longstanding, determine the success of operative procedures designed to cure renal vascular hypertension. That is, the secondary vascular disease of the kidney, not having the arterial stenosis, may perpetuate the hypertension after correction of the renal arterial lesion. In dogs, reduction of blood flow to one kidney by 40 to 75 per cent of the control value elicited a mechanism, probably humoral, which restored blood flow of the ischemic kidney towards control, while the contralateral kidney demonstrated a simultaneous reduction (20 to 40 per cent) in its blood flow without any consistent changes in its GFR (Fig. 5). These compensatory renal vascular effects evoked by renal ischemia were reproduced by an angiotensin infusion (Figs. 4 and 6). Angiotensin given by intravenous infusion invariably restored GFR of the clipped kidney toward control levels while producing little or no change in the GPR of the opposite kidney (Fig. 4). The effects of angioGFR 20

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Figure 5. Effects of graded renal arterial constrictions on renal blood flow and glomerular filtration rate. Poly graphic recording of renal blood flows and mean aortic blood pressure. At the first arrow the right renal artery was constricted. At the second arrow, the constriction was increased. Note the restoration of the right renal blood flow to control between the first and second arrows and the simultaneous reduction of the left renal blood flow. The glomerular filtration rates (GFR) corresponding to the control, first constriction period, and second constriction period are placed from left to right.

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tensin on both GFR and RBF were seen after as little as 5 nanograms (0.005 f.Lg.) per kg. per minute intravenously. Release of the renal arterial occlusion restored blood flow to the ischemic kidney. With return of RBF to normal, angiotensin, given at rates which previously had increased GFR and RBF in the clipped kidney, resulted in bilateral reduction in RBF's and either unchanged or moderately reduced GFR's.44 This experimental model in dogs appears to reproduce rapidly the renal hemodynamic changes observed in man during renal vascular hypertension over a period of months or years. Determinants of Vascular Reactivity to Angiotensin The determinants of the attenuated vasoconstrictor activity of angiotensin in the ischemic kidney are undefined. Other vasopressor agents do not increase blood flow to a clipped kidney. On the contrary, norepinephrine or epinephrine will further reduce the already diminished blood flow to an ischemic kidney.43 Some of the possible determinants of the altered vasoconstrictor action of angiotensih on the ischemic kidney will be considered. (1) The activity of angiotensinases. Several angiotensinases have been identified, and at least one with a high degree of specificity achieves its highest concentration in the kidney.28 If angiotensinase activity was initiated or increased by renal ischemia, a reduction of the vasoconstrictor action of angiotensin within the ischemic kidney would be expected to occur. (2) The production of endogenous tachyphylaxis. Attenuation of the renal vasoconstrictor effect of angiotensin is readily demonstrated in response to giving angiotensin in excess of 0.05 f.Lg. per kg. per min. Reduction or loss of the vasoconstrictor action of angiotensin consequent to its previous administration is referred to as tachyphylaxis. Endogenous tachyphylaxis denotes loss of the vasoconstrictor action of administered (exogenous) angiotensin, due presumably to an increase in formation of angiotensin intrarenally (endogenously) that occurs in response to a stimulus, such as renal ischemia. Reduction of the vascular activity of angiotensin in some disease states, particularly those associated with secondary hyperaldosteronism, may be the con-

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sequence of endogenous tachyphylaxis to angiotensin. Thus, if one considers secondary hyperaldosteronism to be the result of increased blood levels of angiotensin, then diminished vascular response to angiotensin may be the expression of increased formation of angiotensin (endogenous tachyphylaxis). This interpretation is supported by the observation that in primary hyperaldosteronism, in which renin levels and presumably angiotensin formation are depressed, there is an increased vasoconstrictor response to angiotensin. 29 Kaplan and Silah proposed the diagnostic use of altered vascular activity of angiotensin to assist in detecting renal vascular lesions in hypertensive subjects. Thus, blunting of the pressor response to angiotensin in states associated with increased activity of the renin-angiotensin system, such as renal vascular disease, would be expected. (3) Renal innervation. Acute renal denervation abolished the vasoconstrictor activity of angiotensin given intravenously in the denervated kidney (Fig. 7).4:1 The abolition of the renal vasoconstrictor action of angiotensin by denervation of the kidney suggests a relationship between angiotensin and the autonomic nervous system. Investigation of the interactions of angiotensin and the autonomic nervous system has resulted in definition of one of the major determinants of the action of angiotensin on blood vessels. 42 In addition, adrenergic-angiotensin interactions may partially determine the variable effects of angiotensin on sodium excretion. 32 c

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Figure 7. Effect of renal denervation on the renal vasoconstrictor action of angiotensin. Polygraphic recording of left renal blood flow and mean aortic blood pressure. At the first arrow, angiotensin was given intravenously; at the second arrow, into the renal artery. Renal denervation was carried out by transection of the renal artery. Angiotensin given intravenously was without effect on renal blood flow after denervation; intraarterial angiotensin showed a reduced effect. (Reprinted with the permission of the Journal of Clinical Investigation.)

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The determinants, considered above, of the altered renal vascular response to angiotensin during renal ischemia could only account for a loss of the renal vasoconstrictor action of angiotensin. The increase in blood flow produced by angiotensin in the ischemic kidney remains unexplained. An increased blood flow to the ischemic kidney secondary to the pressor response produced by angiotensin appears to be an incomplete explanation of this event. 43

AUTONOMIC NERVOUS SYSTEM AND THE RENIN-ANGIOTENSIN SYSTEM The anatomical relationships of the renal nerves and the juxtaglomerular apparatus suggest an interaction between the two systems. Sympathetic postganglionic nerves infiltrate the juxtaglomerular apparatus. 2 • 12 The vascular pole of the glomerulus which contains an element of the juxtaglomerular apparatus is the site of the largest uptake of tritiated norepinephrine within the kidney.38 Renin was reported to be released by stimulation of the renal nerves and infusion of sympathomimetic amines. 7 Denervation of the kidney was noted to decrease the renin content of that kidney.62 Some of these observations made in animals have been extended to man. For example, an individual with severe autonomic insufficiency showed a retarded and diminished secretion of renin and aldosterone to known potent stimuli of release of renin: sodium deprivation and positional changes sufficient to effect a large fall in blood pressure. 20 This report suggests an autonomic nervous dependency of renin and aldosterone secretion in response to physiological stimuli such as postural changes. 9 The release of renin by the kidney as well as the renal vascular action of angiotensin are partially determined by autonomic nervous activity.42 Since renal denervation attenuated the renal vascular action of angiotensin, blockade of the renal vascular effects of angiotensin was attempted by autonomic blocking agents. 42 The ganglionic blocking agent, hexamethonium, and the adrenergic alpha receptor blocking agent, phentolamine CRegitine), did not alter the renal vasoconstrictor action of angiotensin. However, guanethidine, bretylium and hydralazine, which produce adrenergic blockade by interfering with the events leading to adrenergic neurotransmitter release, reduced the renal vasoconstrictor action of angiotensin given intravenously. Reserpine succeeded in achieving this effect only in very large doses. Spinal anesthesia and cord section were also shown to attenuate the renal vasoconstrictor action of angiotensin. Parenthetically, guanethidine and hydralazine are effective in the treatment of hypertension secondary to renal vascular lesions. The release of the pressor principle of the gravid uterus, which is probably renin, is also subject to autonomic nervous influences. 4 Thus, the pressor response in pregnant rabbits produced by placental ischemia was prevented by spinal cord section or spinal anesthesia. The applications of these findings to the treatment or prevention of toxemia of pregnancy await clinical investigation.

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A more precise definition of angiotensin-autonomic nervous interactions may be made only conditionally. Thus, the vascular action of angiotensin may have a cholinergic determinant, since angiotensin was shown to release acetylcholine from preganglionic and postganglionic nerve endings. 51 A cholinergic link for sympathetic nervous activity has been advanced and derives additional support from a recent study on a cholinergic determinant of the renal vasoconstriction induced by stimulation of the sympathetic nerves to the kidneyY That is, after invasion of the sympathetic nerve ending by the nerve impulse, acetylcholine is required for release of the sympathetic neurotransmitter, norepinephrine. Guanethidine may have a primary anticholinergic action which is the basis for its reduction of adrenergic nervous activity, if a cholinergic link in sympathetic postganglionic nerves is assumed. 8 The partial definition of autonomic-angiotensin interactions suggested that some of the effects of angiotensin on salt excretion might be determined by the level of autonomic nervous activity.

THE EFFECT OF ANGIOTENSIN AND BRADYKININ ON THE EXCRETION OF SALT The Effect of Angiotensin in Normal Subjects Contrasted to Those Having Cirrhosis or Severe Hypertension In normal subjects, intravenous administration of angiotensin, in a dose which produces a moderate pressor response results in reduced sodium excretion, a large reduction in renal plasma flow and a lesser reduction in GFR.45 The sodium-retaining (antinatriuretic) action of angiotensin in normal subjects was not dependent on antidiuretic hormone or aldosterone, for patients with diabetes insipidus and adrenalectomized individuals demonstrated similar changes. 61 Furthermore, it was not explained by a reduction in the filtered load of sodium (GFR times plasma sodium), for changes in the filtered load were a variable effect of angiotensin. In contrast, in subjects with severe hypertension 6 and cirrhosis with ascites,32 angiotensin produces a natriuresis not accounted for by an increased filtered load of sodium. In subjects having cirrhosis with ascites, the natriuresis is particularly striking when viewed in contrast with their very low control rate of sodium excretion. Some of the subjects have shown an increase in urinary excretion of sodium from a control level of 2 to 20 J1,Eq. per min. to levels in excess of 1000 ILEq. per min. in response to an angiotensin infusion. A renal tubular effect of angiotensin which promotes sodium excretion was proposed by Leyssac 36 and other investigators 3. 24 to explain the effects of angiotensin on the renal excretion of salt in several species of mammals.

The Effect of Angiotensin as Determined by Route of Administration The definition of the mechanism of action of angiotensin on salt

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excretion derived from observations on the effect of administered angiotensin is confounded by the route of administration. 40 Intravenous or renal intraarterial administration of angiotensin does not reproduce the effect of endogenous angiotensin, in terms of either localization of action to cortical blood vessels or the sequence of vascular and tubular elements stimulated. Thus, endogenous angiotensin may have no preglomerular vascular actions under physiological conditions and probably does not affect directly juxtamedullary or medullary blood vessels, for renin is primarily located in association with the cortical nephrons (Fig. 2). If endogenous angiotensin has a preglomerular action (as postulated for autoregulation of the renal circulation), then this action is limited to the afferent arteriole (Fig. 1). In contrast, administered (exogenous) angiotensin makes contact with all the preglomerular and postglomerular vascular elements, large and small; cortical, juxtamedullary and medullary (Fig. 1). The failure to demonstrate a natriuretic effect in normal individuals or animals on giving angiotensin in no way dismisses a possible physiological action of angiotensin on the renal tubules, such as regulation of proximal tubular sodium reabsorption, as proposed by Leyssac. 35 Thus, the antinatriuretic effect produced by angiotensin in normal subjects may have been due to the release of catecholamines or other neurohumors from the preglomerular arterial elements and the adrenal medulla.

Adrenergic Blockade and Angiotensin Natriuresis Angiotensin-autonomic interactions were examined as possible determinants of the effect of angiotensin on sodium excretion. The renal vascular and excretory responses to angiotensin were determined in dogs before and during adrenergic blockade. 40 A graded change occurred in sodium excretion in response to angiotensin after the successive administration to the same dog of reserpine and guanethidine (Fig. 8). Before reserpine, angiotensin produced sodium retention. Reserpine prevented the antinatriuresis and guanethidine permitted the demonstration of a natriuresis on giving angiotensin. Inasmuch as the natriuresis produced by angiotensin after sympathetic blockade was not primarily determined by elevated blood pressure, increased filtered load of sodium or decreased renal vasoconstrictor action of angiotensin, a renal tubular action of angiotensin presumably determined the natriuresis. These experiments strongly suggest that the variable effect of angiotensin on urinary excretion of sodium is determined in large measure by interaction with the autonomic nervous system, attendant upon the mode of administration of angiotensin. In extending these relationships, an examination of the interactions of angiotensin and the autonomic nervous system on the zona glomerulosa of the adrenal cortex may result in a comprehensive definition of the determinants of the secretion of aldosterone.

278

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ml/min.

GFR

C.

MCGIFF

IlEq/min. 400

OL-----------~L-------------

+

C

H

AC

+

A

o

t

C

H

AC

t

A

Figure 8. Effect of intravenously administered angiotensin (0.0375 /Lg. per kg. per min.) on glomerular filtration rate (GFR in m!. per min.) and sodium excretion (U ,,, V in /LEq. per min.) in untreated dogs and reserpine pretreated dogs before and after guanethidine administration. On the left-hand side of each line plot are the control values (C), on the righthand side the values produced by intravenous infusions of angiotensin (A). The mean values and the standard errors are plotted for two groups of dogs: control dogs (5 experiments) and reserpine-pretreated dogs before and after guanethidine (10 mg. per kg.) (8 experiments). The values are corrected to 1.0 square meter of body surface area.

The Effect of Bradykinin on Salt Excretion The interactions of bradykinin and the autonomic nervous system are less well defined than angiotensin-autonomic interactions. The intrarenal infusion of kallidin-l 0, the immediate precursor of bradykinin, has an effect on renal circulatory and excretory events indistinguishable from acetylcholine: increased urinary excretion of sodium, potassium and chloride associated with an increased RBF and variable changes in GFR.69 Acetylcholine may release kallikrein which then activates bradykinin. Webster and Gilmore suggested that the renal effects of acetylcholine result from activation of the kallikrein-kallidin system, a relationship which at this moment can be considered of interest, but unsubstantiated. 69 A final observation on the parallel activity of tissue hormones is the blockade by guanethidine of the natriuretic action produced by low doses of bradykinin. 18

SUMMARY The vasoactive polypeptides, angiotensin and bradykinin (kallidin-g), are tissue hormones - that is they circulate in an inactive form, rather than being secreted by specialized glands. The selective distribution of their activating enzymes, renin and kallikrein, would permit their regulation of blood flow to the functional units of an organ, such as the nephron. The effect of angiotensin on renal blood flow and glomerular filtration rate is determined partially by the control level of

279

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these values. Thus, in the presence of renal ischemia, angiotensin, a potent renal vasoconstrictor, will increase RBF and GFR. Renal denervation also will attenuate the vasoconstrictor action of angiotensin on the denervated kidney. Exploration of the interactions of angiotensin and the autonomic nervous system revealed that those adrenergic blocking drugs which interfere with the release of the adrenergic neurotransmitter-those agents having their primary action at the nerve endings - will reduce the vasoconstrictor action of angiotensin. The antinatriuretic effect of angiotensin in normal subjects may also be related to an adrenergic interaction, for after autonomic blockade angiotensin invariably produced a natriuresis.

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