Pressure natriuresis and control of arterial pressure during chronic norepinephrine infusion

Salt Sensitivity: Causes, Consequences, and Recent Advances

Salt (sodium chloride) is an essential nutrient required to maintain physiological functions. However, for most people, daily salt intake far exceeds their physiological need and is habitually greater than recommended upper thresholds. Excess salt intake leads to elevation in blood pressure which drives cardiovascular morbidity and mortality. Indeed, excessive salt intake is estimated to be responsible for ≈5 million deaths per year globally. For approximately one-third of otherwise healthy individuals (and >50% of those with hypertension), the effect of salt intake on blood pressure elevation is exaggerated; such people are categorized as salt sensitive and salt sensitivity of blood pressure is considered an independent risk factor for cardiovascular disease and death. The prevalence of salt sensitivity is higher in women than in men and, in both, increases with age. This narrative review considers the foundational concepts of salt sensitivity and the underlying effector systems that cause salt sensitivity. We also consider recent updates in preclinical and clinical research that are revealing new modifying factors that determine the blood pressure response to high salt intake.

Partial vasopressin 1a receptor agonism reduces portal hypertension and hyperaldosteronism and induces a powerful diuretic and natriuretic effect in rats with cirrhosis and ascites

The vasopressin system has emerged as a therapeutic focus for lowering portal hypertension and reducing splanchnic vasodilation in patients with refractory ascites. Clinically available vasopressin agonists are limited by preferential selectivity for V1 receptors that also have steep concentration-response curves with potential risks of excess vasoconstriction and/or complete antidiuretic effects. OCE-205 is a novel, selective, partial V1a receptor agonist with mixed agonist/antagonist activity and no V2 receptor activation at therapeutic doses. We carried out two studies assessing the in vivo effects of OCE-205 in different rat models of cirrhosis and ascites. In a carbon tetrachloride rat cirrhosis model, OCE-205 administration produced a marked reduction in portal hypertension and hyperaldosteronism, along with robust diuretic and natriuretic effects. These effects were accompanied by marked decreases in ascites volume, with three of five animals experiencing total mobilization of ascites. There was no evidence of fluid overload or sodium or water retention, confirming OCE-205's lack of V2 receptor activity. In a second, corroborative study using a bile duct ligation rat model of ascites, OCE-205 produced significant decreases in ascites volume and body weight and a significant increase in urine volume versus vehicle. Urine sodium excretion increased significantly after the first administration of OCE-205 relative to vehicle; however, repeat administration over 5 days did not lead to hyponatremia. Thus, in separate in vivo models, the mixed agonist/antagonist OCE-205 demonstrated relevant and expected endpoint findings consistent with its known mechanism of action and in vitro pharmacology without apparent unwanted effects or nonspecific toxicities.

Renal sympathetic activity: A key modulator of pressure natriuresis in hypertension

Hypertension is a complex disorder ensuing necessarily from alterations in the pressure-natriuresis relationship, the main determinant of long-term control of blood pressure. This mechanism sets natriuresis to the level of blood pressure, so that increasing pressure translates into higher osmotically driven diuresis to reduce volemia and control blood pressure. External factors affecting the renal handling of sodium regulate the pressure-natriuresis relationship so that more or less natriuresis is attained for each level of blood pressure. Hypertension can thus only develop following primary alterations in the pressure to natriuresis balance, or by abnormal activity of the regulation network. On the other hand, increased sympathetic tone is a very frequent finding in most forms of hypertension, long regarded as a key element in the pathophysiological scenario. In this article, we critically analyze the interplay of the renal component of the sympathetic nervous system and the pressure-natriuresis mechanism in the development of hypertension. A special focus is placed on discussing recent findings supporting a role of baroreceptors as a component, along with the afference of reno-renal reflex, of the input to the nucleus tractus solitarius, the central structure governing the long-term regulation of renal sympathetic efferent tone.

Vasopressin 1a receptor partial agonism increases sodium excretion and reduces portal hypertension and ascites in cirrhotic rats

Patients and rats with cirrhosis and ascites have portal hypertension and circulatory dysfunction. Synthetic arginine vasopressin (AVP) receptor agonists able to induce systemic and mesenteric vasoconstriction have shown their usefulness in reducing portal pressure (PP) in this condition. We assessed the potential therapeutic value of a new V1 a -AVP receptor partial agonist with a preferential splanchnic vasoconstrictor effect (FE 204038) in rats with cirrhosis and ascites. The hemodynamic effects of cumulative intravenous doses of FE 204038, terlipressin, or vehicle were investigated. Mean arterial pressure and PP were continuously recorded and cardiac output and systemic vascular resistance (SVR) assessed at 30-minute intervals for 90 minutes. Urine volume, urine osmolality, and urinary excretion of sodium and creatinine were measured in basal conditions and following twice-daily subcutaneous doses of FE 204038 or vehicle. PP, mean arterial pressure, cardiac output, SVR, and ascites volume were also measured after 6 days. The expression of an array of vasoactive genes was assessed in the thoracic aorta and the mesenteric circulation of control rats and rats with cirrhosis and ascites. FE 204038 dose-dependently decreased PP, did not modify mean arterial pressure, and increased SVR. The effect of the V1a -AVP receptor partial agonist on PP was associated with an improvement in urine volume and urinary excretion of sodium during the first day of treatment. SVR was higher and cardiac output and ascites volume were lower in rats with cirrhosis and ascites treated with FE 204038. V1a -AVP receptor expression in rats with cirrhosis and ascites was markedly enhanced in the mesenteric circulation compared to the thoracic aorta.

CONCLUSION

FE 204038 increases sodium excretion and reduces portal hypertension and ascites in experimental cirrhosis. V1a -AVP receptor partial agonism could be a useful pharmacological treatment in decompensated patients with cirrhosis.

Role of the kidney in the pathogenesis of hypertension: time for a neo-Guytonian paradigm or a paradigm shift?

The "Guytonian paradigm" places the direct effect of arterial pressure, on renal excretion of salt and water, at the center of long-term control of blood pressure, and thus the pathogenesis of hypertension. It originated in the sixties and remains influential within the field of hypertension research. However, the concept of one central long-term feedback loop, through which arterial pressure is maintained by its influence on renal function, has been questioned. Furthermore, some concepts in the paradigm are undermined by experimental observations. For example, volume retention and increased cardiac output induced by high salt intake do not necessarily lead to increased arterial pressure. Indeed, in multiple models of salt-sensitive hypertension the major abnormality appears to be failure of the vasodilator response to increased cardiac output, seen in salt-resistant animals, rather than an increase in cardiac output itself. There is also evidence that renal control of extracellular fluid volume is driven chiefly by volume-dependent neurohumoral control mechanisms rather than through direct or indirect effects of changes in arterial pressure, compatible with the concept that renal sodium excretion is controlled by parallel actions of different feedback systems, including hormones, reflexes, and renal arterial pressure. Moreover, we still do not fully understand the sequence of events underlying the phenomenon of "whole body autoregulation." Thus the events by which volume retention may develop to hypertension characterized by increased peripheral resistance remain enigmatic. Finally, by definition, animal models of hypertension are not "essential hypertension;" progress in our understanding of essential hypertension depends on new results on system functions in patients.

Pressure natriuresis and the renal control of arterial blood pressure

The regulation of extracellular fluid volume by renal sodium excretion lies at the centre of blood pressure homeostasis. Renal perfusion pressure can directly regulate sodium reabsorption in the proximal tubule. This acute pressure natriuresis response is a uniquely powerful means of stabilizing long-term blood pressure around a set point. By logical extension, deviation from the set point can only be sustained if the pressure natriuresis mechanism is impaired, suggesting that hypertension is caused or sustained by a defect in the relationship between renal perfusion pressure and sodium excretion. Here we describe the role of pressure natriuresis in blood pressure control and outline the cascade of biophysical and paracrine events in the renal medulla that integrate the vascular and tubular response to altered perfusion pressure. Pressure natriuresis is impaired in hypertension and mechanistic insight into dysfunction comes from genetic analysis of blood pressure disorders. Transplantation studies in rats show that blood pressure is determined by the genotype of the kidney and Mendelian hypertension indicates that the distal nephron influences the overall natriuretic efficiency. These approaches and the outcomes of genome-wide-association studies broaden our view of blood pressure control, suggesting that renal sympathetic nerve activity and local inflammation can impair pressure natriuresis to cause hypertension. Understanding how these systems interact is necessary to tackle the global burden of hypertension.

Hypertension: physiology and pathophysiology

Despite major advances in understanding the pathophysiology of hypertension and availability of effective and safe antihypertensive drugs, suboptimal blood pressure (BP) control is still the most important risk factor for cardiovascular mortality and is globally responsible for more than 7 million deaths annually. Short-term and long-term BP regulation involve the integrated actions of multiple cardiovascular, renal, neural, endocrine, and local tissue control systems. Clinical and experimental observations strongly support a central role for the kidneys in the long-term regulation of BP, and abnormal renal-pressure natriuresis is present in all forms of chronic hypertension. Impaired renal-pressure natriuresis and chronic hypertension can be caused by intrarenal or extrarenal factors that reduce glomerular filtration rate or increase renal tubular reabsorption of salt and water; these factors include excessive activation of the renin-angiotensin-aldosterone and sympathetic nervous systems, increased formation of reactive oxygen species, endothelin, and inflammatory cytokines, or decreased synthesis of nitric oxide and various natriuretic factors. In human primary (essential) hypertension, the precise causes of impaired renal function are not completely understood, although excessive weight gain and dietary factors appear to play a major role since hypertension is rare in nonobese hunter-gathers living in nonindustrialized societies. Recent advances in genetics offer opportunities to discover gene-environment interactions that may also contribute to hypertension, although success thus far has been limited mainly to identification of rare monogenic forms of hypertension.

Chronic blood pressure control

Chronic blood pressure is maintained within very narrow limits around an average value. However, the multitude of physiologic processes that participate in blood pressure control present a bewildering array of possibilities to explain how such tight control of arterial pressure is achieved. Guyton and Coleman and colleagues addressed this challenge by creating a mathematical model that integrated the short- and long-term control systems for overall regulation of the circulation. The hub is the renal-body fluid feedback control system, which links cardiac function and vascular resistance and capacitance with fluid volume homeostasis as the foundation for chronic blood pressure control. The cornerstone of that system is renal sodium excretory capability, which is defined by the direct effect of blood pressure on urinary sodium excretion, that is, "pressure natriuresis." Steady-state blood pressure is the pressure at which pressure natriuresis balances sodium intake over time; therefore, renal sodium excretory capability is the set point for chronic blood pressure. However, this often is misinterpreted as dismissing, or minimizing, the importance of nonrenal mechanisms in chronic blood pressure control. This article explains the renal basis for the blood pressure set point by focusing on the absolute dependence of our survival on the maintenance of sodium balance. Two principal threats to sodium balance are discussed: (1) a change in sodium intake or renal excretory capability and (2) a change in blood pressure. In both instances, circulatory homeostasis is maintained because the sodium balance blood pressure set point is reached.

ACUTE PRESSURE?NATRIURESIS RELATIONSHIP FOLLOWING WITHDRAWAL OF CHRONIC NORADRENALINE INFUSION

1. Pathological changes to the kidney, such as vascular remodelling, have been found in several models of hypertension and may contribute to the maintenance of hypertension or confer susceptibility to redeveloping hypertension after the original prohypertensive stimulus is withdrawn. 2. To investigate whether noradrenaline-induced hypertension induces persistent, functionally important changes to the kidney, the acute pressure-natriuresis relationship was characterized in anaesthetized rats under controlled neural and hormonal conditions following chronic (14 days) intravenous infusion of noradrenaline (48 microg/kg per h) or vehicle (0.04 mg/mL ascorbic acid and 0.156 mg/mL NaH2PO4 2 H2O in 10 IU/mL heparinized saline). 3. Conscious mean arterial pressure was significantly elevated by infusion of noradrenaline at 48 microg/kg per h (+10 +/- 2 mmHg at Day 14; P < 0.01 vs vehicle group). The acute relationships between arterial pressure and renal blood flow, glomerular filtration rate, Na+ excretion and urine flow were not significantly different between the noradrenaline- and vehicle-infused rats immediately after termination of noradrenaline infusion. 4. In summary, chronic intravenous noradrenaline infusion did not cause persistent changes in renal function, indicating that, in contrast with many models of hypertension, this model does not induce underlying prohypertensive changes to the kidney.

Hemodynamic consequences of chronic parasympathetic blockade with a peripheral muscarinic antagonist

Whereas the sympathetic nervous system has a well-established role in blood pressure (BP) regulation, it is not clear whether long-term levels of BP are affected by parasympathetic function or dysfunction. We tested the hypothesis that chronic blockade of the parasympathetic nervous system has sustained effects on BP, heart rate (HR), and BP variability (BPV). Sprague-Dawley rats were instrumented for monitoring of BP 22-h per day by telemetry and housed in metabolic cages. After the rats healed from surgery and a baseline control period, scopolamine methyl bromide (SMB), a peripheral muscarinic antagonist, was infused intravenously for 12 days. This was followed by a 10-day recovery period. SMB induced a rapid increase in mean BP from 98 ± 2 mmHg to a peak value of 108 ± 2 mmHg on day 2 of the SMB infusion and then stabilized at a plateau value of +3 ± 1 mmHg above control ( P < 0.05). After cessation of the infusion, the mean BP fell by 6 ± 1 mmHg. There was an immediate elevation in HR that remained significantly above control on the last day of SMB infusion. SMB also induced a decrease in short-term (within 30-min periods) HR variability and an increase in both short-term and long-term (between 30-min periods) BPV. The data suggest that chronic peripheral muscarinic blockade leads to modest, but sustained, increases in BP, HR, and BPV, which are known risk factors for cardiovascular morbidity.

Role of endothelin in noradrenaline-induced hypertension in rats

OBJECTIVE

To investigate the role of endothelin in noradrenaline-induced hypertension in rats.

DESIGN

The dose-response relationship of chronic noradrenaline infusion on arterial pressure was characterized to identify a dose that would produce sustained hypertension, and the effect of combined endothelin ETA and ETB receptor blockade (TAK-044) on the response to this dose was then examined.

METHODS AND RESULTS

Noradrenaline (or vehicle) was infused intravenously at 1 (subpressor acutely), 24 or 48 microg/kg per h (acute pressor response of 9 +/- 1 and 11 +/- 1 mmHg, respectively) for a 14-day infusion, and blood pressure was measured by radiotelemetry. Noradrenaline infusion at 1 microg/kg per h did not produce a 'slow pressor' rise in blood pressure. During noradrenaline infusions at 24 and 48 microg/kg per h, mean arterial pressure peaked initially on days 2-3 (+10 +/- 1 and 14 +/- 2 mmHg, respectively; P < 0.01), fell towards basal levels after day 3, and then began to rise again at days 5-6 only with 48 microg/kg per h, being 10 +/- 1 mmHg above control levels at days 13-14 (P < 0.05). TAK-044 treatment did not alter the magnitude of the initial (13 +/- 1 mmHg) or eventual (12 +/- 2 mmHg) rise in blood pressure achieved in response to 14 days' infusion of noradrenaline at 48 microg/kg per h, but abolished the transient fall.

CONCLUSION

Chronic noradrenaline infusion at acutely pressor doses leads either to a transient blood pressure elevation at a moderate dose, or to a triphasic but sustained hypertension at a higher dose, with a temporary escape from the hypertension apparently mediated by endothelin.

The kidney, hypertension, and obesity

This paper provides a personal perspective of the role of abnormal renal-pressure natriuresis in the pathogenesis of hypertension. Direct support for a major role of renal-pressure natriuresis in long-term control of arterial pressure and sodium balance comes from studies demonstrating that (1) pressure natriuresis is impaired in all forms of chronic hypertension and (2) prevention of pressure natriuresis from operating, by servo-control of renal perfusion pressure, also prevents the maintenance of sodium balance hypertension. Although the precise mechanisms of impaired pressure natriuresis in essential hypertension have remained elusive, recent evidence suggests that obesity and overweight may play a major role. Obesity increases renal sodium reabsorption and impairs pressure natriuresis by activation of the renin-angiotensin and sympathetic nervous systems and by altered intrarenal physical forces. Chronic obesity also causes marked structural changes in the kidneys that eventually lead to a loss of nephron function, further increases in arterial pressure, and severe renal injury in some cases. Although there are many unanswered questions about the mechanisms of obesity hypertension and renal disease, this is one of the most promising areas for future research, especially in view of the growing, worldwide "epidemic" of obesity.

Mechanisms of pressure natriuresis

A central component of the feedback system for long-term control of arterial pressure is the pressure-natriuresis mechanism, whereby increases in renal perfusion pressure lead to decreases in sodium reabsorption and increases in sodium excretion. The specific intrarenal mechanism for the decrease in tubular reabsorption in response to increases in renal perfusion pressure appears to be related to increases in hemodynamic factors such as medullary blood flow and renal interstitial hydrostatic pressure (RIHP), and renal autocoids such as nitric oxide, prostaglandins, kinins, and angiotensin II. Increases in renal perfusion pressure are associated with significant increases in RIHP, nitric oxide, prostaglandin E2, and kinins, and decreases in angiotensin II. The mechanism whereby RIHP increases in the absence of discernible changes in whole kidney renal blood flow and peritubular capillary hydrostatic and/or oncotic pressures may be related to increases in renal medullary flow as a result of nitric oxide-induced reductions in renal medullary vascular resistance. Several lines of investigation support an important quantitative role for RIHP in mediating pressure natriuresis. Preventing RIHP from increasing in response to increases in renal perfusion pressure markedly attenuates pressure natriuresis. Furthermore, direct increases in RIHP, comparable to increases measured in response to increases in renal perfusion pressure, have been shown to significantly decrease tubular reabsorption of sodium in the proximal tubule and increase sodium excretion. The exact mechanism whereby RIHP influences tubular reabsorption is unknown, but may be related to alterations in tight junctional permeability to sodium in proximal tubules, redistribution of apical sodium transporters, and/or release of renal autacoids such as prostaglandin E2.

State-dependent expression of pressure diuresis in conscious rats

In 1967, Guyton and Coleman modeled pressure diuresis as the underlying, essential, long-term mechanism that regulates arterial pressure when sodium intake changes. Other mechanisms that influence renal function interact with pressure diuresis to achieve sodium balance and determine the blood pressure. Increases in sodium intake suppress sodium conserving mechanisms and activate natriuretic mechanisms; decreases in sodium intake have the opposite effect. If the Guyton-Coleman model is correct, then pressure diuresis should be more readily detected in animals on a high-salt diet than in animals on a low-salt diet. We measured spontaneous changes in arterial pressure and urine flow in conscious rats fed low-salt (0. 4% NaCl) and high-salt (8.0% NaCl) chow. For 10 rats fed a high-salt diet, arterial pressure and urine flow were positively correlated in 19 of 32 (59%) trials. In 10 rats fed a low-salt diet, a positive correlation was observed in 10 of 33 (30%) trials. Chi-square analysis revealed that differences in Na+ content of the diet were significantly associated with the probability of a positive relationship between blood pressure and urine flow. These results support the hypothesis that the expression of pressure diuresis across time is dependent on the state of sodium balance.

Renal denervation supersensitivity revisited

To determine whether the chronically denervated kidney is supersensitive to either physiological or pathophysiological plasma levels of norepinephrine (NE), studies were conducted in conscious dogs subjected to unilateral renal denervation and surgical division of the urinary bladder into hemibladders to allow separate 24-h urine collection from denervated and innervated kidneys. Plasma NE concentration was increased by chronic infusion of NE (4-5 days) at rates of 25, 100, and 200 ng . kg-1 . min-1. Twenty-four-hour control values for mean arterial pressure (MAP), plasma NE concentration, and ratios for urinary sodium and potassium excretion from denervated and innervated kidneys (Den/Inn) were 94 +/- 4 mmHg, 145 +/- 24 pg/ml, 1.05 +/- 0.05, and 0.97 +/- 0.07, respectively. With infusions of NE producing plasma levels of NE of up to approximately 3,000 pg/ml or plasma concentrations of NE at least threefold greater than present under most pathophysiological conditions and during acute activation of the sympathetic nervous system, there were no significant long-term changes in MAP or relative excretion rates of sodium and potassium from denervated and innervated kidneys. In marked contrast, pharmacological plasma levels of NE ( approximately 7,000 pg/ml) produced chronic increases in MAP (to 116 +/- 2% of control) and sustained reductions in Den/Inn for urinary sodium and potassium excretion to 57 +/- 4 and 68 +/- 5% of control, respectively, indicating a lower excretion rate of these electrolytes from denervated vs. innervated kidneys. We conclude that the chronically denervated kidney does not exhibit an exaggerated antinatriuretic response to either physiological or pathophysiological levels of circulating NE. It is therefore unlikely that renal denervation supersensitivity is a confounding issue in studies employing chronic renal denervation to elucidate the role of the renal nerves in the regulation of sodium excretion.

Spontaneous changes in arterial blood pressure and renal interstitial hydrostatic pressure in conscious rats

1. Previous work has demonstrated a positive relationship between experimentally induced changes in arterial pressure (AP) and renal interstitial hydrostatic pressure (RIHP). The purpose of the present study was to test the hypothesis that RIHP is positively correlated with the normal changes in AP that occur spontaneously in conscious rats. 2. Rats were chronically instrumented for the recording of AP (via an aortic catheter) and RIHP. RIHP was measured by implanting a Millar microtransducer, whose tip had been encapsulated in a 35 microns pore polyethylene matrix (5 mm long, 2 mm o.d.), approximately 5 mm below the renal cortical surface. 3. A total of 56 h of simultaneous analog recording of AP and RIHP was obtained from ten rats. Each 1 h segment was digitized and evaluated at frequencies of 1, 0.1, 0.02 and 0.01 Hz. 4. In forty-nine out of fifty-six of these 1 h recordings taken at 1 Hz, there were significant positive linear correlations between AP and RIHP (mean r = 0.32) with a mean slope of 0.11 mmHg RIHP/1 mmHg AP. Low-pass filtering to 0.01 Hz significantly increased the r value to 0.48. 5. These results demonstrate that spontaneous changes in AP and RIHP are positively correlated. The spontaneous coupling of AP and RIHP may be of importance in the regulation of salt and water excretion by the pressure diuresis mechanism.

Louis K. Dahl Memorial Lecture. Renal and cardiovascular mechanisms of hypertension in obesity

In all forms of hypertension, including human essential hypertension, pressure natriuresis is reset to higher blood pressures. Because human essential hypertension is a heterogeneous disease, it is likely that there are multiple neurohumoral and intrarenal causes of abnormal pressure natriuresis and increased blood pressure. Weight gain is recognized to be an important contributor to essential hypertension, although the mechanisms that link obesity with altered renal function and high blood pressure have not been fully elucidated. In obese dogs and humans, the shift of pressure natriuresis to higher blood pressures appears to be due mainly to increased tubular reabsorption, as glomerular filtration rate and renal plasma flow are increased compared with normal. Multiple causes of increased tubular reabsorption and hypertension in obesity have been postulated, including insulin resistance and hyperinsulinemia, activation of the sympathetic nervous and renin-angiotensin systems, and physical changes within the kidney itself. Support for the insulin resistance-hyperinsulinemia link between obesity and hypertension has been inferred mainly from acute and epidemiologic studies showing a correlation between insulin and blood pressure. Recent studies suggest that chronic hyperinsulinemia, comparable to that found in obesity, cannot account for obesity hypertension in dogs or humans. Activation of the sympathetic nervous system may play a role in obesity-induced hypertension, and there is evidence for a role of altered intrarenal physical forces caused by histological changes within the renal medulla. The quantitative importance of each of these abnormalities in altering renal function and raising blood pressure in obesity remains to be determined but is an important area of research for understanding human essential hypertension.

The role of the kidney in canine blood pressure control: direct assessment of the closed-loop gain

1. The feedback control of arterial blood pressure by the kidney in the range of hours was investigated in resting, conscious foxhounds. 2. A servo-control device (connected to an aortic occlusive cuff implanted above both renal arteries) was used to maintain a constant pressure difference of 20 mmHg between aortic pressure measured proximal (mean arterial blood pressure: MAP) and distal (renal artery pressure: RAP) to the aortic cuff. 3. Protocol 1 (n = 6) served as a 4 h time control without intervention, protocol 2 (n = 6) consisted of three periods: after a control of 20 min duration, the servo-control device was activated for 180 min; this was followed by a recovery period of 40 min. Protocol 3 (n = 6) was as protocol 2, but during converting-enzyme inhibition. 4. Servo-control increased plasma renin activity (PRA) transiently from 0.5 ng angiotensin I (AI) ml-1 h-1 to a peak value of 2.4 ng AI ml-1 h-1, subsequently both RAP and MAP rose to reach a new steady state. During this increase in RAP, PRA declined to 1.4 ng AI ml-1 h-1. 5. On average, the compensation of the pressure decrease sensed by the kidney amounted to 63% of the error signal (closed-loop gain of 0.63 +/- 0.1). 6. Converting-enzyme inhibition reduced this closed-loop gain significantly (protocol 2 vs. protocol 3, 0.63 +/- 0.1 vs. 0.15 +/- 0.1; P < 0.05). 7. It is concluded, that the kidney plays an important role in medium-term blood pressure regulation, most probably via the renin-angiotensin system.

Chronic inhibition of nitric oxide synthesis. A new model of arterial hypertension

Recent studies have indicated that acute inhibition of nitric oxide biosynthesis in the rat promotes arterial hypertension and renal vasoconstriction. We evaluated the renal and systemic effects of 4-6 weeks of nitric oxide blockade in Munich-Wistar rats receiving the nitric oxide inhibitor nitro-L-arginine orally. Age-matched untreated rats were used as controls. In an additional seven rats, nitric oxide blockade was carried out in conjunction with oral administration of the novel angiotensin II antagonist losartan potassium. Tail-cuff pressure rose progressively in nitro-L-arginine-treated rats, reaching 164 +/- 6 mm Hg at 4-6 weeks, compared with 108 +/- 3 mm Hg in controls. In rats concomitantly receiving losartan, tail-cuff pressure reached 125 +/- 6 mm Hg, still elevated compared with rats receiving losartan alone (98 +/- 3 mm Hg). Nitro-L-arginine-treated rats presented marked renal vasoconstriction and hypoperfusion, as well as a 30% fall in glomerular filtration rate and a 39% increase in filtration fraction. Treatment with Losartan normalized glomerular filtration rate, but not filtration fraction or renal vascular resistance. Plasma renin activity was elevated after nitro-L-arginine treatment. Renal histological examination revealed widespread arteriolar narrowing, focal arteriolar obliteration, and segmental fibrinoid necrosis in the glomeruli. In a separate group of rats, nitro-L-arginine administered for 1 week induced hypertension that was partially reversed by acute L-arginine, but not D-arginine or L-glycine, infusions. We conclude that chronic nitric oxide blockade may constitute a new model of severe arterial hypertension.(ABSTRACT TRUNCATED AT 250 WORDS)

Control of blood pressure by the renin-angiotensin-aldosterone system

Modification of the renin-angiotensin system, part of a powerful feedback system for long-term control of arterial pressure and volume homeostasis, through use of angiotensin-converting enzyme (ACE) inhibitors, offers a powerful means of reducing blood pressure in many hypertensive patients. There is considerable evidence to suggest that the chronic renal and blood pressure actions of ACE inhibitors are mediated mainly by blockade of angiotensin II formation, rather than by other effects such as increased levels of kinins or prostaglandins. The long-term actions of angiotensin II and aldosterone on blood pressure are closely intertwined with their effects on volume homeostasis and the renal pressure natriuresis mechanism. In most instances, changes in angiotensin II and aldosterone act to amplify the effectiveness of pressure natriuresis and minimize changes in blood pressure needed to maintain sodium balance. When angiotensin II or aldosterone levels are inappropriately elevated, the antinatriuretic effects of these hormones shift pressure natriuresis to higher levels, thereby necessitating increased blood pressure to maintain sodium balance. Control of renal excretory function and modulation of pressure natriuresis by angiotensin II is mediated by intrarenal and extrarenal effects, including stimulation of aldosterone secretion. Current evidence indicates that the intrarenal effects of angiotensin II are quantitatively more important than changes in aldosterone in regulating renal excretion and arterial pressure. The intrarenal actions of angiotensin II include a direct effect on tubular sodium transport as well as a potent constrictor action on efferent arterioles, which increases reabsorption by altering peritubular capillary forces. The constrictor effect of angiotensin II on efferent arterioles also helps to stabilize glomerular filtration rate and therefore excretion of metabolic waste products, an action that may be particularly important when renal perfusion is impaired (e.g., in renal artery stenosis or heart failure).

Abnormal pressure natriuresis. A cause or a consequence of hypertension?

In all forms of chronic hypertension, the renal-pressure natriuresis mechanism is abnormal because sodium excretion is the same as in normotension despite the increased blood pressure. However, the importance of this resetting of pressure natriuresis as a cause of hypertension is controversial. Theoretically, a resetting of pressure natriuresis could necessitate increased blood pressure to maintain sodium balance or it could occur secondarily to hypertension. Recent studies indicate that, in several models of experimental hypertension (including angiotensin II, aldosterone, adrenocorticotrophic hormone, and norepinephrine hypertension), a primary shift of renal-pressure natriuresis necessitates increased arterial pressure to maintain sodium and water balance. In genetic animal models of hypertension, there also appears to be a resetting of pressure natriuresis before the development of hypertension. Likewise, essential hypertensive patients exhibit abnormal pressure natriuresis, although the precise cause of this defect is not clear. It is likely that multiple renal defects contribute to resetting of pressure natriuresis in essential hypertensive patients. With long-standing hypertension, pathological changes that occur secondary to hypertension must also be considered. By analyzing the characteristics of pressure natriuresis in hypertensive patients and by comparing these curves to those observed in various forms of experimental hypertension of known origin, it is possible to gain insight into the etiology of this disease.