The antihypertensive function of the kidney. Its elucidation by captopril plus unclipping

Changes in mean arterial pressure predict degranulation of renomedullary interstitial cells

1. Renomedullary interstitial cells (RMIC) are characterized by numerous intracellular granules thought to contain renal medullary antihypertensive substances. However, the nature of the trigger for RMIC degranulation remains to be elucidated. The present study examines the effects of acute alterations in mean arterial pressure (MAP) and medullary blood flow (MBF) on RMIC granulation. 2. Basal MAP and MBF in anaesthetized Sprague-Dawley rats (n = 4/group) were altered by intravenous infusions of vasoactive agents, including angiotensin II alone or with a nitric oxide (NO) synthase inhibitor (N-omega-nitro-l-arginine) or NO donor (sodium nitroprusside), noradrenaline and by carotid artery clamping. Following these treatments, kidneys were examined by electron microscopy and the absolute volume of granules in the renal medulla was calculated using unbiased stereological methods. 3. Acute increases in MAP, regardless of the treatment causing the increase, were associated with a reduction in the absolute volume of granules in the range of 42-67%. Regression analysis revealed that only increases in MAP, but not MBF, strongly predict RMIC degranulation. 4. Despite previous reports that changes in MBF activate renomedullary antihypertensive activity, we conclude that the change in MAP is an important determinant of the activity of the blood pressure-lowering mechanism of the renal medulla, with the assumption that the medullary lipids mediate the antihypertensive property of the renal medulla.

Long-term effects of angiotensin-converting enzyme inhibition on renal medullary neutral lipid in spontaneously hypertensive rats

Short-term treatment of young spontaneously hypertensive rats (SHR) with angiotensin-converting enzyme (ACE) inhibitors reduces systolic blood pressure. Renal medullary neutral lipids (RMNLs) have vasodilator properties that may explain the effects of ACE inhibition. We measured RMNL levels of SHR treated between 6 and 10 weeks of age with (1) vehicle, (2) ramipril 1 mg. kg-1. d-1, (3) the bradykinin B2 receptor antagonist icatibant 0.5 mg. kg-1. d-1, or (4) icatibant 0.5 mg. kg-1. d-1 plus ramipril 1 mg. kg-1. d-1. RMNLs were quantified by oil red O fluorescence at 10 and 20 weeks of age. Systolic blood pressure (BP) was measured by tail-cuff plethysmography. Ramipril reduced BP at 10 weeks of age and increased RMNLs compared with controls (0.99+/-0.07% versus 0.56+/-0. 06%, P<0.01). Icatibant alone had no significant effect on RMNLs (0.55+/-0.04%) but attenuated the increase in RMNLs by ramipril (0. 81+/-0.05%). In control SHR, the increase in BP between 10 and 20 weeks of age was associated with a significant increase in RMNLs (0.79+/-0.09%). SHR that had received ramipril had significantly lower BP than controls at 20 weeks of age, but RMNL was not significantly different (0.92+/-0.10%). Therefore, in young SHR, ACE inhibition increases RMNLs and reduces blood pressure, an effect that appears to depend on bradykinin. The changes in RMNLs at the age of 10 weeks paralleled long-term BP effects and may be involved in setting the BP track in SHR.

Effects of angiotensin-converting enzyme inhibition on vascular remodeling of resistance vessels in hypertensive patients

Essential hypertension is known to be associated with a decrease in the lumen diameter and an increase in the wall thickness to lumen diameter ratio of the resistance vessels. Recently, it has been clarified that this alteration does not necessarily involve vascular growth, but could be due to a rearrangement of the same amount of material, a phenomenon now termed "eutrophic remodeling." These changes are found both in human essential hypertension and in animal models of genetic hypertension. Antihypertensive treatment with angiotensin-converting enzyme (ACE) inhibitors causes a dose-dependent regression of the media to lumen ratio in rats. Clinical studies have now confirmed these findings, showing that when previously untreated essential hypertensive patients are treated with the ACE inhibitor perindopril (PE), the abnormal structure of resistance vessels regresses toward normal values; in contrast, treatment with a beta-blocker does not affect the abnormal vascular structure. The available evidence thus indicates that ACE inhibitors are able to normalize the abnormal resistance vessel structure in essential hypertension, and suggest that this effect may not only be dependent on their ability to reduce blood pressure. This review summarizes these findings, and discusses the extent to which this is desirable.

Evidence for a renomedullary vasodepressor hormone

1. Recent physiological experiments have established that increasing the perfusion pressure of the kidney causes the release of vasodepressor substance from the renal medulla. 2. The substance is not a platelet activating factor, a prostaglandin or nitric oxide and the vasodepressor response to increased renal perfusion is not due simply to inhibition of renin release. 3. The mechanisms by which the renomedullary vasodepressor substance lowers arterial pressure remain to be determined. Sympathoinhibition may account for part of the response, but the hypotension still occurs in autonomic ganglion blocked animals. 4. The source of substance appears to be the renomedullary interstitial cells, though the control of the production and release of the substance remain to be determined. 5. The substance may be a lipid but it is yet to be fully isolated and identified. 6. The threshold for release of the substance appears to be close to normal resting arterial blood pressure. 7. Despite strong evidence that the renal medulla releases a vasodepressor hormone in response to increased renal perfusion pressure, much is still to be determined regarding the physiology of this hormone and its involvement in the aetiology of hypertension.

Peripheral vasculature in essential hypertension

1. Haemodynamic evidence shows that in essential hypertension minimum vascular resistance and vascular pressor response are increased and that the vascular reserve is decreased. 2. The haemodynamic changes are most easily explained in terms of a generalized narrowing of the vasculature and an increase in the ratio between the thickness of the tunica media and the lumen diameter (media: lumen ratio), with no change in the functional properties of the smooth muscle itself. 3. Histological and in vitro studies of resistance vessels confirm these predictions. Moreover, the evidence indicates that these changes are associated mainly with remodelling (rearrangement of the same amount of material) of the vessels, rather than growth. 4. Although the alteration in small artery structure is usually appropriate to the actual blood pressure, the structure appears not only to be a secondary adaptation, but is also dependent on other factors, including neurohumoral factors. 5. The available evidence shows that normalization of the resistance vessel structure (by increasing lumen diameter and decreasing the media:lumen ratio) should be achieved not by inhibition of growth but by (reverse) remodelling. Recent evidence from clinical investigations shows that this can be achieved in essential hypertensive patients treated with the angiotensin-converting enzyme inhibitor perindopril. 6. The role of the resistance vasculature as a primary determinant of blood pressure remains unclear. It is suggested that the requirement for normalization of resistance vascular structure is due to a need to increase the vascular reserve.

Efferent renal nerve stimulation inhibits the antihypertensive function of the rat renal medulla when studied in a cross-circulation model

The aim of this study was to investigate the effects of renal nerve stimulation on the humoral renal antihypertensive system. An isolated kidney (IK) was perfused at normal or high arterial pressures from a normotensive assay rat by means of a perfusion pump. Perfusion pressure (PP) to the IK was 90 mmHg for a control period of 30 min. In three of five experimental groups PP was then increased to 175 mmHg. In two of the groups the renal nerves were stimulated at 2 (P-175(2Hz)) or 5 Hz (P-175(5Hz)) for 60 min. The remaining group served as a control (P-175C). In two groups IK pressure was maintained at 90 mmHg with 5 Hz nerve stimulation (P-90(5Hz) or without nerve stimulation (P-90C). MAP of the assay rat decreased by 22 and 27% (P < 0.001) in the P-175C and P-175(2Hz) groups, respectively during the 60 min period of nerve stimulation, but remained stable in P-175(5Hz). Renal blood flow increased in the IK when PP was increased in P-175C, but did not change significantly in P-175(2Hz) or P-175(5Hz). Blood pressure remained constant in the assay rat when the IK was perfused at 90 mmHg. The renal excretory functions of the IK decreased in a frequency dependent manner by 2 and 5 Hz renal nerve stimulation compared with P-175C. We conclude that 5 Hz renal nerve stimulation inhibits the pressure dependent release of humoral depressor substances from an IK perfused at 175 mmHg, whereas this is not seen when stimulating at 2 Hz. It is suggested that hte release of antihypertensive substances from the renal medulla requires an increased renomedullary blood flow.

Platelet activating factor and one-kidney, one clip hypertension

The reduction in blood pressure to normotensive levels within 3 hours of unclipping the one-kidney, one clip Goldblatt hypertensive rat has been attributed to the release of potent blood pressure-lowering lipids, one of which is thought to be identical to platelet activating factor. The specific platelet activating factor receptor antagonist WEB 2086 was infused intravenously into hypertensive one-kidney, one clip rats, and the mean arterial blood pressure changes after unclipping were examined. Before infusion, blocking doses of WEB 2086 were confirmed to effectively abolish the fall in blood pressure induced by exogenous platelet activating factor. Serotonin release in response to exogenous platelet activating factor was also inhibited in platelets preincubated with plasma from rats infused with the antagonist. Hypertensive rats were given a bolus blocking dose of WEB 2086 (5 mg/kg i.v.) and the same dose by infusion (5 mg/kg/hr i.v.) before they were unclipped. A control group was given a bolus volume of saline and infused with saline before unclipping. In WEB 2086-treated rats, blood pressure fell from a baseline mean of 181 +/- 13.0 to 125 +/- 23 mm Hg after 4 hours, a fall of 28%. Saline-treated rats fell from a mean of 194 +/- 23 to 127 +/- 25 mm Hg (33%). There was no significant difference in the blood pressure fall between the two groups. Therefore, platelet activating factor is unlikely to be responsible for the restoration of normal blood pressure after unclipping the Goldblatt hypertensive rat. We attribute the fall in blood pressure to other presently unidentified renomedullary lipids.

Renal and circulatory effects of medullipin I, as studied in the in-vivo cross-circulated isolated kidney and intact Wistar-Kyoto (WKY) rat

The renal medulla harbours powerful humoral antihypertensive mechanisms, as earlier explored in unclipping experiments on renal hypertensive rats or in normotensive isolated kidneys cross-circulated at increased perfusion pressures from 'donor rats', in which renal function also seemed to be affected. Injection of the renomedullary factor medullipin I (Med I; formerly ANRL) mimics these haemodynamic responses, and Med I seems to be one of the most important mediators of the depressor effects. The present study was performed to analyse further the haemodynamic and, particularly, the renal effects of Med I, using anaesthetized intact WKY rats and constant-pressure perfused (90 mmHg) isolated WKY kidneys, cross-circulated by these intact 'donor' rats. Mean arterial pressure (MAP), heart rate (HR) and renal function were followed for one 30-min period before and two 30-min periods after injection of 1 mg Med I (M; n = 7) or an equal volume of saline as control (C; n = 13). In the intact 'donor' WKY, MAP and HR remained largely constant in C during the three periods, being 126 +/- 5, 125 +/- 5, and 120 +/- 5 mmHg, while MAP fell in the M group after Med I, from 121 +/- 5 to 107 +/- 7 and 107 +/- 5 mmHg (P less than 0.05), and also HR tended to decrease in M. Renal resistance (RR) fell while renal plasma flow (RPF) and glomerular filtration rate (GFR) increased significantly (P less than 0.05) after Med I in the M donor rats despite their MAP reduction. However, in the constant-pressure perfused, cross-circulated kidneys the RR, RPF and GFR changes were clearly more pronounced (P less than 0.01) and also diuresis, natriuresis, osmolar excretion and osmolar clearance increased significantly after Med I (P less than 0.01). In conclusion, the present results support the view that Med I not only has important and long-lasting depressor effects but also affects renal function in important ways, inducing vasodilatation and increasing GFR, RPF, diuresis and sodium-osmolar excretion.

Alterations in blood pressure by derangement of the mechanisms that regulate sodium excretion

Understanding the sequence of events responsible for pressure-related natriuresis and their pathophysiologic alterations may be useful in distinguishing various types of essential hypertension of renal origin. The perturbation of a distal step in the sequence is likely to be reflected in a simple physiologic defect. For instance, pathophysiologic alterations in the medullary production of prostaglandin E2 might directly influence natriuresis and diuresis because of its modulatory effect on tubular reabsorption of sodium and water. Perturbation of more proximal steps in the sequence could influence all the distal events as well. For instance, prostaglandin I2 and endothelium-derived relaxing factor may be produced by the preglomerular vasculature in response to alterations in renal perfusion pressure and may modulate the release of renin from the juxtaglomerular cells. Thus, variations in the production of prostaglandin I2 or endothelium-derived relaxing factor may be reflected by various renal vascular, tubular, and systemic homeostatic events related to the renin-angiotensin system.

Activation of the humoral antihypertensive system of the kidney increases diuresis

Isolated kidneys taken from normotensive Wistar-Kyoto rats were cross-perfused extracorporeally by normotensive strain-matched donor rats. The extracorporeal perfusion circuit was arranged so that the perfusion pressure to the normotensive recipient kidney could be varied from 90 to 200 mm Hg without any change in total flow through this circuit. This setup avoided hemodynamic or mechanical interferences with reflexogenic circulatory control in the normotensive donor rat when the recipient kidney was manipulated. Diuresis and natriuresis were measured in the normotensive donor rat and the normotensive recipient kidney. A few minutes after normotensive recipient kidney perfusion pressure had been raised, mean arterial pressure (MAP) and heart rate started to decline rapidly in the normotensive donor rat, and circulatory collapse ensued within 15 to 100 minutes. During the control period at 90 mm Hg normotensive recipient kidney perfusion pressure, urinary flow, MAP and heart rate were stable in the normotensive donor rat and the normotensive recipient kidney. When perfusion pressure was raised to 200 mm Hg in the recipient kidney, the urinary flow in the donor rat increased 62% on average in the first 10 minutes over values recorded before the pressure rise (p less than 0.05) while MAP simultaneously fell by 16% and HR remained unchanged. During the subsequent period, the urinary flow of the donor rat declined together with MAP and heart rate. In the extracorporeally high-pressure perfused recipient kidneys, an eightfold to ninefold increase in diuresis and natriuresis occurred during the first 45 minutes.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanisms underlying pressure-related natriuresis: the role of the renin-angiotensin and prostaglandin systems. State of the art lecture

It has long been known that increments in renal perfusion pressure can induce an elevation of urine sodium excretion without changing renal blood flow or glomerular filtration rate. The mechanism underlying this pressure-related natriuresis remains undefined, although the interest in its elucidation has been stimulated by the notion that it may constitute the central phenomenon through which the kidney regulates blood volume and, thereby, blood pressure. Recently, the use of novel experimental techniques has disclosed some important clues about changes in renal hemodynamics that, along with changes in renal humoral regulators, allow us to visualize a possible sequence of events responsible for pressure-related natriuresis. According to this hypothesis, the autoregulatory responses responsible for maintaining glomerular filtration rate are elicited in preglomerular vasculature by changes in renal perfusion pressure. These myogenic responses are coupled through Ca2+ entry in juxtaglomerular cells with inversely related changes in the release of renin and, consequently, with the amount of angiotensin II generated in renal interstitium. The release of renin from juxtaglomerular cells is modulated by the synthesis of prostaglandin I2 from the adjacent endothelial cells. Interstitial angiotensin II could influence sodium tubular reabsorption directly by stimulating sodium transport in proximal renal tubules and indirectly by altering medullary blood flow and, thereby, medullary interstitial pressure. In the renal medulla, the effects of interstitial pressure on sodium reabsorption can be amplified by the release of prostaglandin E2 from interstitial cells. A deficient regulation of this relationship could result in a shift of the pressure-natriuresis curve, leading to hypertension.

Structure and function of the arteries in hypertension

Like tissues in other parts of the body, those of the heart and blood vessels can rapidly adapt their design. The principles of these differentiated structural changes in response to altered functional demands will be outlined in this report. With respect to arterial resistance vessels in hypertension, any sustained arterial pressure elevation leads to wall (w) hypertrophy, whereas the average inner radius (ri) decreases. The reverse occurs at sustained pressure reductions, and this process is aptly termed "structural autoregulation." By means of this structural autoregulation, wall tension per unit layer (T) remains largely constant when pressure (P) increases (decreases), according to Laplace's law: T = P X r/w. Furthermore, this structural w/ri increase, because it is a local vascular response although it is often considerably modified by neurohormonal "trophic" influences, results in a geometrically based vascular hyperreactivity affecting the systemic precapillary resistance vessels, whereas the structural ri reduction leads to an upward resetting of systemic resistance to flow which is present even at maximal vasodilation (Rmin). Because of this "structural amplifier" principle, an increased systemic resistance can be maintained even at normal vascular smooth muscle activity, and smooth muscle activations may then lead to exaggerated resistance elevations. Thus, a most important positive-feedback interaction is created between even mild functional "pressor" influences, if adequately sustained, and this normal process of structural adaptation. This positive-feedback interaction therefore gradually tends to accentuate the latter element until it entirely dominates the hemodynamics of established hypertension. As the process is induced early and established rapidly, and in primary hypertension often even seems to be genetically reinforced in various ways, it becomes of utmost pathogenetic significance. With respect to the aorta-large conduit arteries, their "Windkessel function" becomes reduced by the same process of adaptive wall thickening. This increases the pulse amplitude and thereby accentuates the systolic afterload of the left heart, the load of which is raised also because of the upward structural resetting of systemic precapillary resistance. Furthermore, the same type of structural adaptation also contributes to the upward resetting of the cardiac, arterial, and renal "barostat" mechanisms, as cardiac and arterial walls become thicker and stiffer, whereas renal preglomerular resistance vessels participate in the upward structural autoregulation.(ABSTRACT TRUNCATED AT 400 WORDS)

Inhibitory effect of frusemide on sympathetic vasoconstrictor responses: dependence on a renal hormone and the vascular endothelium

1. Mesenteric perfusion pressure was measured in the in situ mesentery perfused at a constant rate with blood drawn from the carotid artery of the same anaesthetized rat. Increases in perfusion pressure were produced by mesenteric periarterial electrical stimulation. These responses were measured before and 30 min after the administration of frusemide (5 mg/kg i.v.) to the rat. Loss of volume due to the frusemide-induced diuresis was prevented by a urinary bladder-venous extracorporeal circuit. 2. Responses to stimulation were reduced after frusemide and were not increased by the subsequent administration of indomethacin (2 mg/kg i.v.). This indomethacin treatment rapidly and completely prevented the fall in blood pressure produced by i.v. arachidonic acid. 3. In rats where the renal papilla had been ablated by treatment with bromoethylamine (200 mg/kg i.p.) 5 weeks previously, frusemide administration did not reduce sympathetic responses in the in situ blood-perfused mesentery. 4. A segment of rat tail artery, cannulated at both ends was mounted in an organ bath and perfused with blood withdrawn from, and returned to, an anaesthetized rat. Increases in perfusion pressure produced by periarterial electrical stimulation of this ex vivo blood perfused tail artery segment were reduced by frusemide administration to the anaesthetized rat. 5. When the endothelium was removed from the tail artery segment, frusemide administration did not lead to any reduction of vasoconstrictor responses. 6. Frusemide may lead to the release of a non-prostanoid hormone from the renal medulla which results in inhibition of peripheral sympathetic vasoconstrictor responses. The release of the hormone may involve intra-renal prostaglandins. The final antivasoconstrictor effect requires an intact vascular endothelium.

The reno-hepatic axis of blood pressure control: further studies

A reno-hepatic axis of blood pressure (BP) control has been proposed primarily by two developments: the lag period between the injection of the antihypertensive neutral renomedullary lipid (ANRL) into the vena cava and the beginning of the drop of the BP is significantly reduced by injection of this lipid into the portal vein; and a reno-portal shunt (RPS) accelerates markedly the drop of the BP following unclipping the Goldblatt hypertensive rat. In the present studies, three maneuvers were performed: perfusion of the isolated, blood-drained rat liver with plasma containing ANRL and delivery of the plasma into the jugular vein of a hypertensive rat; injection of captopril, establishment of a RPS of a hypertensive rat (one-kidney, one-clip) followed by unclipping; and establishment of a RPS in a normal rat. The blood-drained liver was capable of decreasing significantly the lag period of ANRL. Captopril did not potentiate the antihypertensive action of the kidney when RPS was coupled with unclipping. Following the RPS, the BP of the normal rat dropped modestly over a 3-hour period. These results further support the concept of reno-hepatic axis of BP control.

The hypotensive effect of captopril is not mediated by renal medullary lipids

The role of renomedullary lipids in the hypotensive effect of captopril was studied in spontaneously hypertensive rats, one-kidney, one clip hypertensive rats and two-kidney, one clip hypertensive rats using inhibitors of acetyl glyceryl ether phosphorylcholine and prostaglandins, and chemical medullectomy. Our experiments indicate that it is unlikely that renomedullary lipids contribute to the hypotensive action of captopril.

The liver converts the antihypertensive hormone of the kidney. The renohepatic axis

The antihypertensive neutral renomedullary lipid, derived from the renal papilla, causes a vasodepressor effect when injected into a peripheral vein, such as the inferior vena cava, after a lag period of 1 to 2 minutes. The blood pressure tracing is skewed (cuplike effect). The lag period is significantly reduced after injection of the antihypertensive lipid into the portal vein. The vasodepressor configuration (cuplike) is the same whether the lipid is injected into the vena cava or portal vein. Removal of the liver from the circulation prevents the depressor effect. Thus, passage through the liver is essential for antihypertensive lipid activity. Renoportal shunting of blood potentiates the antihypertensive function of the kidney after unclipping in the one-kidney, one clip hypertensive rat. Lack of a hepatic circulation prevents the antihypertensive function of the kidney after unclipping. Antihypertensive neutral renomedullary lipid and the renal venous effluent after unclipping have the same biological behavior. We conclude that antihypertensive neutral renomedullary lipid is a promolecule, the putative prohormone of the renal papilla and its renomedullary interstitial cells. The liver converts the antihypertensive prohormone of the kidney to an active antihypertensive substance. A renohepatic axis of blood pressure control appears to exist.