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

Endotoxin-induced atrial natriuretic factor release: in vivo and in vitro studies

OBJECTIVE

We evaluated the effects of coliform endotoxin on the circulating levels of atrial natriuretic factor and renal function. To understand the direct effects of endotoxin in the release of atrial natriuretic factor by cardiac tissue, studies in isolated rat atria were performed.

STUDY DESIGN

In vivo studies were used. Anesthetized dogs were studied, with one group receiving isotonic saline solution (n = 6) and the other group receiving 50 microg/kg of coliform endotoxin (n = 7) as a continuous infusion over a 4-hour period. Cardiovascular parameters, renal function, and circulating levels of atrial natriuretic factor were measured at specified time intervals. In another set of experiments with in vitro studies left atria from Sprague-Dawley rats were isolated and perfused. In the control group (n = 9) the standard Krebs perfusate was used. In the endotoxin group (n = 9) coliform endotoxin was added at a concentration of 250 microg/mL to the standard perfusate. Atrial pressure was used as an index of stretch, and atrial natriuretic factor was measured from the perfusate.

RESULTS

Administration of endotoxin resulted in decreased blood pressure (P < .05) with a concomitant increase in heart rate. Renal artery flow, however, showed an increase (P < .05) initially followed by a return to its baseline value, with a sustained increase occurring in the saline solution control group. A significant (P < .05) and sustained increase in the circulating levels of atrial natriuretic factor after endotoxin infusion did not prevent the decrease in fractional sodium excretion (P < .05) and creatinine clearance despite an increase in the urinary output. Serum sodium, serum potassium, and osmolalities, however, remained relatively stable. The study pertaining to isolated atria showed that in the presence of low atrial pressures, addition of endotoxin had no significant effect on the release of atrial natriuretic factor. With the increase in atrial pressure atrial natriuretic factor release was significantly higher in the group directly exposed to endotoxin compared with the control group.

CONCLUSIONS

These studies demonstrate that the slow infusion of coliform endotoxin results in increased circulating levels of atrial natriuretic factor. This increase is in part due to the direct effect of endotoxin on the heart as indicated by studies in isolated atria. Our data suggest that atrial natriuretic factor in endotoxemia acts in an integrative manner with other hormones on a variety of target organs to modulate cardiovascular function and fluid balance.

Role of nitric oxide and prostaglandins in the long-term control of renal function

Previous studies have reported evidence of an important interaction between nitric oxide (NO) and prostaglandins in the acute regulation of renal function. The objective of this study was to determine in conscious dogs whether the renal effects of the prolonged administration of a cyclooxygenase inhibitor are enhanced when NO synthesis is reduced. Meclofenamate infusion (5 microg x kg(-1) x min(-1)) during 4 consecutive days (n=8) elicited a continuous decrease (P<0.05) in renal blood flow and plasma renin activity and a transitory decrease in sodium excretion. NG-Nitro-L-arginine methyl ester (L-NAME) infusion (5 microg x kg(-1) x min(-1)) during 6 days (n=8) produced a significant increase in arterial pressure and a transitory decrease (P<0.05) in both renal blood flow and plasma renin activity. The simultaneous inhibition of NO and prostaglandin synthesis (n=7) led to an increase in arterial pressure and a decrease in renal blood flow similar to those observed during the administration of either L-NAME or meclofenamate. In contrast, this simultaneous inhibition produced a decrease in glomerular filtration rate, which was not observed in the previous groups, and also induced an increase in renal vascular resistance and a decrease in sodium excretion greater (P<0.05) than those found during the inhibition of either NO or prostaglandins. Only a transitory decrease in plasma renin activity was found during meclofenamate infusion in this group. The results of this study present new evidence that the renal vasoconstrictor and antinatriuretic effects induced by the prolonged infusion of a cyclooxygenase inhibitor are significantly enhanced when NO synthesis is reduced. These results suggest that renal function may be more sensitive to the prolonged administration of a cyclooxygenase inhibitor in situations where NO production is reduced.

Renal changes induced by nitric oxide and prostaglandin synthesis reduction: effects of trandolapril and verapamil

The benefits of the simultaneous administration of low doses of a calcium antagonist and a converting enzyme inhibitor in the treatment of hypertension and renal vasoconstriction are well established. The objective of this study was to evaluate whether the administration of low doses of a calcium antagonist and a converting-enzyme inhibitor have beneficial effects in treating the renal alterations induced by the acute administration of a cyclooxygenase inhibitor when nitric oxide synthesis is reduced. These effects were examined in anesthetized dogs before and during an acute sodium load. It was found that the intrarenal infusion of meclofenamate (5 microg x kg[-1] x min[-1]), simultaneously with a low dose of NG-nitro-L-arginine methyl ester (1 microg x kg[-1] x min[-1]), produced a 40% decrease of renal blood flow and glomerular filtration rate and a reduction in the renal excretory response to the sodium load. In a second group of dogs, intrarenal verapamil (0.5 microg x kg[-1] x min[-1]) was effective in blocking the effects of nitric oxide and prostaglandin synthesis inhibition on sodium excretion and glomerular filtration rate but did not modify the effects on renal blood flow. An intrarenal infusion of trandolapril (0.3 microg x kg[-1] x min[-1]) was effective in a third group of dogs in reducing the renal hemodynamic effects but not in preventing the antinatriuretic effect observed in the first group. Finally, in a fourth group, the simultaneous administration of verapamil and trandolapril was effective in treating all the renal changes induced by the cyclooxygenase inhibitor when nitric oxide synthesis was reduced. These results suggest that the combination of low doses of trandolapril and verapamil has additive effects in treating the renal vasoconstriction and antinatriuresis induced by the acute administration of a cyclooxygenase inhibitor, when nitric oxide synthesis is reduced.

Systemic inhibition of nitric oxide and prostaglandins in volume-induced natriuresis and hypertension

Nitric oxide (NO) synthesis inhibition with NG-nitro-L-arginine methyl ester (L-NAME) (10 micrograms.kg-1.min-1 i.v.), cyclooxygenase inhibition with meclofenamate (Meclo; 5 mg/kg i.v. bolus), and combination of drugs (L-NAME + Meclo) were used to investigate the roles of NO and prostaglandins (PG) in the hemodynamic and natriuretic responses to isotonic saline volume expansion (VE; 5% body wt over 60 min) in anesthetized dogs. Before VE, L-NAME (n = 6), Meclo (n = 6), and L-NAME + Meclo (n = 6) produced significant increments in mean arterial pressure (MAP) of 12 +/- 2, 15 +/- 3, and 17 +/- 3 mmHg, respectively. VE did not change MAP in Meclo-treated dogs, but produced a significant elevation in the control dogs (14 +/- 6 mmHg), in L-NAME-treated dogs (17 +/- 6 mmHg), and in dogs pretreated with L-NAME + Meclo (12 +/- 5 mmHg). VE alone induced marked natriuretic responses in the control (38 +/- 9 to 562 +/- 86 mumol/min), L-NAME (31 +/- 9 to 664 +/- 65 mumol/min), and Meclo groups (41 +/- 10 to 699 +/- 51 mumol/min). However, this natriuretic response was attenuated in dogs pretreated with L-NAME + Meclo (12 +/- 4 to 185 +/- 52 mumol/ min). These results indicate that 1) blockade of both NO and PGs has significant diminishing effects on volume-induced natriuresis, 2) NO blockade alone impairs volume-induced natriuresis in a manner that requires further increases in MAP to restore the natriuresis, and 3) PG blockade alone does not curtail volume-induced natriuresis.

Hypothesis: the role of acquired tubulointerstitial disease in the pathogenesis of salt-dependent hypertension

We present a new hypothesis to explain the development of salt-dependent hypertension in humans. We propose that hypertension has two phases: an early phase in which elevations in blood pressure (BP) are mainly episodic and are mediated by a hyperactive sympathetic nervous or renin-angiotensin system, and a second phase in which BP is persistently elevated and that is primarily mediated by an impaired ability of the kidney to excrete salt (NaCl). We propose that the transition from the first phase to the second occurs as a consequence of catecholamine-induced elevations in BP that preferentially damage regions of the kidney (juxtamedullary and medullary regions) that do not autoregulate well to changes in renal perfusion pressure. The catecholamine response is associated with both an increase in peritubular capillary pressure and a reduction in peritubular capillary plasma flow, resulting in injury to the peritubular capillaries with ischemia to the tubules and interstitium. The local injury triggers the release or activation (angiotensin II, adenosine, renal sympathetic nerves) or inhibition (nitric oxide, prostaglandins, dopamine) of vasoactive mediators that further augment ischemia and result in abnormal tubuloglomerular feedback and enhanced NaCl reabsorption. The peritubular capillary injury with rarefaction simultaneously blunts the pressure natriuresis mechanism. The combined effect of enhanced tubuloglomerular feedback and impaired pressure natriuresis results in a defect in NaCl excretion which, on the exposure to salt, results in the development of persistent hypertension. Evidence is provided to suggest that this may be the major mechanism for the development of salt-dependent hypertension, and particularly for the hypertension associated with blacks, aging and obesity. Thus, essential hypertension may be a type of acquired tubulointerstitial renal disease.

Perfusion pressure dependency of in vivo renal tubular dynamics

To examine whether changes in renal perfusion pressure (RPP) within the range of autoregulation induce detectable changes in tubular dynamics in an entire nephron population of the intact kidney, we measured, using electron beam computed tomography (EBCT), transit times (TT, s) and intratubular concentration (%) of filterable contrast media in various nephron segments simultaneously with renal regional perfusion. In seven dogs (group A) this was performed at the upper and lower limits of autoregulation (RPP = 130 and 95 mmHg, respectively) while group B (n = 5) served as control. In group A alone, a decrease in RPP led to an increase in TT by 40%, 68%, and 32% in the proximal tubules, ascending limb of Henle's loop, and distal tubules, respectively, in association with an increase in intratubular concentration (+ 50%, 80%, and 42%, respectively). Papillary perfusion decreased, whereas perfusion of the adjacent, outlying inner medulla increased. The decrease in papillary perfusion correlated positively with the concurrent change in sodium excretion (R = 0.81). This study demonstrates that changes in RPP within the autoregulatory range elicit changes of tubular sodium reabsorption mainly in proximal, distal, and ascending tubules, in which most of the nephrons participate. These tubular changes are associated with an alteration of perfusion circumscribed to two areas of the inner renal medulla.

The kidney in blood pressure regulation and development of hypertension

Systemic arterial pressure is a dynamic and responsive physiologic parameter that can be influenced by many different factors. In particular, short-term changes in arterial pressure are caused by a myriad of mechanisms that affect cardiac output, total peripheral resistance, and cardiovascular capacitance. In the long run, however, most of these actions can be buffered or compensated by appropriate renal adjustments of sodium balance, ECFV, and blood volume. As long as the mechanisms regulating sodium excretion can maintain sodium balance by appropriately modulating the sensitivity of the pressure-natriuresis relationship, normal arterial pressure can be sustained. Derangements that compromise the ability of the kidneys to maintain sodium balance, however, can result in the kidney's need for an elevated arterial pressure to reestablish net salt and water balance.

Erythrocyte sodium-lithium countertransport in non-modulating offspring and essential hypertensive individuals: response to enalapril

Non-modulators are a subset of essential hypertensive individuals in whom renal hemodynamic and adrenal aldosterone responses to angiotensin II fail to modulate appropriately during high dietary salt intake. The main aim of this study was to investigate the familial aggregation of non-modulation and several erythrocyte Na+ transport systems in normotensive and hypertensive individuals as well as offspring of hypertensive parents. An additional aim was to evaluate the effect of treatment with enalapril on erythrocyte Na+ transport. We studied 15 normotensive subjects (6 males, 27+/-6 years), 14 untreated modulating essential hypertensive subjects (7 males, 38+/-7 years), 12 untreated non-modulating essential hypertensive subjects (7 males, 38+/-6 years), 14 modulating offspring of hypertensive parents (8 males, 25+/-6 years), and 14 non-modulating offspring of hypertensive parents (8 males, 26+/-4 years). Blood pressure was recorded with an oscillometric device and renal plasma flow and glomerular filtration rate by clearances of para-aminohippurate and inulin, respectively. Non-modulating subjects were identified as individuals who failed to increase effective renal plasma flow by 30% and decrease filtration fraction by at least 30% 10 days after changing from a low (20 mmol/d) to a high (250 mmol/d) sodium intake. Erythrocyte Na+ transport was characterized by measurements of the Na+-K+ pump, Na+-Li+ countertransport, Na+-K+-Cl- cotransport, passive Na+ permeability, and Na+ content. After the initial studies, hypertensive individuals were treated with enalapril (20 mg/d P.O.) for 6 months, after which erythrocyte Na+ transport measurements were again made. The main findings were that Na+-Li+ countertransport is increased in non-modulating hypertensive subjects and non-modulating offspring of hypertensive parents, that the increase in blood pressure in response to high salt intake is greater in non-modulating than modulating hypertensive subjects, and that enalapril decreases Na+-Li+ countertransport activity to normal in non-modulating hypertensive subjects. These findings provide support for a possible genetic role in the development of salt sensitivity and suggest that Na+-Li+ countertransport and non-modulation are related phenotypes.

Pressure natriuresis and renal medullary blood flow in dogs

In the present study, we evaluated the effects of changes in arterial pressure on regional renal blood flows and sodium excretion in anesthetized dogs during control conditions and after 5% volume expansion with isotonic saline. Medullary and cortical blood flow responses were determined with laser-Doppler needle flow probes inserted into the midmedullary and midcortical regions, and whole-kidney blood flow was assessed with an electromagnetic flow probe. Volume expansion in six dogs caused marked increases in urine flow (20.2 +/- 5.5 to 82.5 +/- 22.7 microL.min-1.g-1) and sodium excretion (3.2 +/- 0.5 to 11.1 +/- 2.7 mumol.min-1.g-1), with slight increases in glomerular filtration rate (0.92 +/- 0.03 to 1.01 +/- 0.02 mL.min-1.g-1) but no significant changes in total renal blood flow (4.7 +/- 0.3 to 5.2 +/- 0.6 mL.min-1.g-1), medullary blood flow (+6 +/- 9%), or cortical blood flow (+12 +/- 10%). During stepwise reductions in renal arterial pressure (150 to 75 mm Hg) elicited with a renal arterial occluder, both before and after volume expansion, medullary, cortical, and total renal blood flows as well as glomerular filtration rate exhibited efficient autoregulation, with slopes not significantly different from zero over this range of arterial pressure. Ther were marked increases in the slopes of the relationships between arterial pressure and urine flow (0.18 +/- 0.05 to 0.78 +/- 0.27 microL.min-1.g-1.mm Hg-1) as well as sodium excretion (0.03 +/- 0.004 to 0.10 +/- 0.03 mumol.min-1.g-1.mm Hg-1) during volume expansion. These data demonstrate that medullary blood flow is efficiently autoregulated in dogs during control and volume-expanded states and indicate that the mechanism responsible for the arterial pressure-induced changes in sodium excretion does not depend on coincident alterations in medullary blood flow.

New insights into the pathophysiology of renovascular hypertension

In recent years, the pathophysiology of renovascular hypertension has been reviewed, and the classic concept that activation of the renin-angiotensin system is solely responsible for the development and maintenance of renovascular hypertension has been challenged. In fact, experimental evidence indicates that other systems, such as the lipoxygenase pathway, may have a more critical role in the long-term maintenance of high blood pressure after renal artery stenosis. Herein we discuss the intrarenal mechanisms that control pressure-induced natriuresis under physiologic conditions and the role of the kidney in the pathophysiology of renovascular hypertension.

Pressure natriuresis and autoregulation of inner medullary blood flow in canine kidney

We have evaluated the responses to changes in arterial pressure on regional blood flows in the renal medulla and sodium excretion simultaneously in denervated kidneys of six anesthetized sodium-replete dogs. Renal regional blood flow responses were determined using laser-Doppler needle flow probes and whole-kidney blood flow was assessed using an electromagnetic flow probe. The responses to stepwise reductions in renal arterial pressure (140 to 70 mm Hg) were examined first with a laser-Doppler needle probe inserted in the outer medulla and then repeated after advancing the same probe in the inner medulla. There were no differences in the control values of total renal blood flow (4.4 +/- 0.7 to 4.5 +/- 0.5 mL.min-1.g-1), glomerular filtration rate (0.89 +/- 0.7 to 0.94 +/- 0.9 mL.min-1.g-1), sodium excretion (3.6 +/- 0.6 to 3.4 +/- 0.5 mumol.min-1.g-1), and urinary excretion rate of nitric oxide metabolites (nitrate/nitrite, 1.6 +/- 0.2 to 1.5 +/- 0.2 nmol.min-1.g-1) at the start of both experimental periods. During changes in renal arterial pressure, inner medullary blood flow exhibited efficient autoregulation similar to that in outer medullary blood flow. Usual excretory responses to reductions in renal arterial pressure as well as autoregulation of cortical and whole-kidney blood flows and glomerular filtration rate were observed in these dogs. The slopes of the relationship between arterial pressure and sodium excretion (0.046 +/- 0.007 to 0.044 +/- 0.009 mumol.min-1.g-1.mm Hg-1) or nitrate/nitrite excretion (0.014 +/- 0.003 to 0.013 +/- 0.003 nmol.min-1.g-1.mm Hg-1) were similar in both experimental periods. These data indicate that blood flow to the inner medulla is efficiently autoregulated as in outer medulla and cortex of the kidney in anesthetized dogs and demonstrate further that the arterial pressure-induced natriuretic responses do not require associated changes in medullary blood flow.

Neuropeptide Y-enhanced diuresis and natriuresis in anaesthetized rats is independent of renal blood flow reduction

1. Neuropeptide Y (NPY) has been reported to enhance diuresis and natriuresis in anaesthetized rats although it is a potent renal vasoconstrictor in vitro in vivo in several species. Therefore, we have investigated anaesthetized rats to see whether reduction in renal blood flow (RBF) and enhancement of diuresis and natriuresis can occur concomitantly, and how diuresis and natriuresis might be enhanced despite reduced RBF. 2. Systemic or intrarenal NPY infusion (0.03-3 micrograms kg-1 min-1) had only a small effect on mean arterial pressure (maximal increase 15-20 mmHg) and heart rate (maximal decrease 30 beats min-1) but dose-dependently reduced RBF (maximal peak reduction 3 ml min-1) Endogenous creatinine clearance was not significantly altered. 3. In anaesthetized rats systemic infusion of 1 or 3 micrograms kg-1 min-1 NPY enhanced urine formation and sodium and calcium excretion by a maximum of 110, 110 and 45%, respectively, but did not alter potassium excretion. Enhancement of diuresis was also detectable in conscious rats. 4. The diuretic and natriuretic effects of systemically infused NPY were at least partly maintained in rats with decapsulated kidneys and in rats where NPY-induced increase of renal perfusion pressure was excluded mechanically by an adjustable clamp placed on the abdominal aorta. 5. Intrarenal infusion of 0.3 or 1 microgram kg-1 min-1 NPY reduced RBF to a greater extent than systemic infusion (maximal peak reduction 4 ml min-1) but caused a smaller enhancement or even a reduction of urine formation and sodium excretion. 6. We conclude that systemic infusion of NPY reduces RBF by a direct effect on the renal vasculature. Systemic NPY infusion enhances urine formation and sodium and calcium excretion. This occurs independently (at least in part) of pressure natriuresis by formation and/or release of an extrarenal factor which might act on distal tubules and/or collecting ducts.

Renal function reserve and sodium sensitivity in essential hypertension

It has been postulated that glomerular capillary pressure is elevated in sodium-sensitive types of hypertension. In addition, the presence or absence of renal function reserve, in response to a chronic protein load, is thought to be useful in predicting the existence of glomerular hypertension. Intrarenal hemodynamic parameters in the sodium-sensitive type of essential hypertension were therefore calculated by analyzing the pressure-natriuresis curve, and the preservation of renal function reserve was evaluated. Fifteen patients with essential hypertension were maintained on a normal sodium diet for 1 week and a low-sodium diet for a second week in study 1. This protocol was repeated for low and high protein intake in 8 patients in study 2. Subjects in study 1 whose mean arterial pressure was reduced by more than 10% by sodium restriction were considered sodium sensitive (n = 7), with the remaining patients classified as non-sodium sensitive (n = 8). There were no significant differences in mean arterial pressure (125 +/- 2 mm Hg), glomerular filtration rate (80 +/- 3 ml/min), or renal plasma flow rate (355 +/- 24 ml/min) on the normal sodium diet between sodium-sensitive and non-sodium-sensitive patients. Glomerular capillary pressure (59 +/- 2 mm Hg vs 47 +/- 1 mm Hg, p < 0.0002) was estimated to be elevated in sodium-sensitive patients relative to that in non-sodium-sensitive patients, whereas the whole kidney ultrafiltration coefficient of glomerular capillary walls (0.068 +/- 0.009 (ml/sec)/mm Hg vs 0.221 +/- 0.042 (ml/sec)/mm Hg, p < 0.005) was decreased. Chronic protein loading increased both glomerular filtration and renal plasma flow rates in study 2. Although the sodium sensitivity of blood pressure showed no significant correlation with the increase in either glomerular filtration or renal plasma flow rate, it showed a weak negative correlation with the increase in filtration fraction (r = -0.69, p < 0.06), which is the ratio of the two rates. Taken together, these results suggest that glomerular capillary pressure is elevated and renal function reserve is impaired in patients with sodium-sensitive essential hypertension.

Reproducibility of human kidney perfusion and volume determinations with electron beam computed tomography

RATIONALE AND OBJECTIVES

Alterations in whole kidney, cortical, and medullary perfusion and volume play a pivotal role in various physiologic and pathophysiologic processes. Electron-beam computed tomography (EBCT) provides accurate measurements of these traits in animals, but their reproducibility in humans has not been established.

METHODS

Perfusion, volume, and flow measurements were obtained by EBCT in eight healthy human volunteers under controlled conditions on two consecutive days.

RESULTS

Mean values for whole kidney, cortical, and medullary perfusion and volume obtained with EBCT were similar in scan 1 and scan 2 (P > 0.1), and correlated highly. Coefficients of variation for the repeated measurements usually were less than 10%. Values obtained for renal regional perfusion and volume agreed with previously reported values.

CONCLUSIONS

Electron-beam computed tomography estimates of single whole kidney, cortical, and medullary perfusions and volumes are highly reproducible in normal humans, and may be useful to advance understanding of renal involvement in human disease.

Importance of nitric oxide and prostaglandins in the control of rat renal papillary blood flow

The role of nitric oxide and prostaglandins in the control of rat renal papillary blood flow has been studied in anesthetized Munich-Wistar rats by use of laser Doppler flowmeter. Acute administration of N omega-nitro-L-arginine methyl ester (L-NAME) 10 mg/kg IV (n=8) increased mean arterial pressure by 27.8 +/- 3.6%, decreased papillary blood flow by 39.4 +/- 3.8%, and decreased renal blood flow by 47.4 +/- 1.9%. The subsequent administration of indomethacin (7.5 mg/kg IV) further decreased papillary blood flow (35.2 +/- 2.5%) without significant changes in mean arterial pressure or renal blood flow. In a second group (n = 6), administration of indomethacin before L-NAME decreased papillary blood flow by 39.6 +/- 2.1% without significantly altering mean arterial ressure or renal blood flow. The subsequent injection of L-NAME further decreased papillary blood flow (32.9 +/- 1.8%) and renal blood flow (49.8 +/- 6.6%) while increasing mean arterial pressure to a level not significantly different from that found in the first group. Autoregulation studies showed that L-NAME but not indomethacin reduced the renal perfusion pressure-renal blood flow relationship without altering autoregulation. However, both nitric oxide and prostaglandins importantly affected the renal perfusion pressure-papillary blood flow relationship because L-NAME and indomethacin significantly decreased this relationship in an additive fashion. Although both drugs reduced the sensitivity of the pressure-papillary flow relationship, only L-NAME affected autoregulation so that papillary blood flow was autoregulated at higher renal perfusion pressures. Thus, the present results indicate that both nitric oxide and prostaglandins control a similar percentage of rat renal papillary blood flow, but nitric oxide seems to be more important than prostaglandins as a mediator of the pressure-blood flow relationship. In contrast, only nitric oxide modifies the renal blood flow level, although it does not disturb whole-kidney blood flow autoregulation.

Involvement of renin-angiotensin system in the reduced pressure natriuresis response of hyperthyroid rats

Previous studies have indicated that the pressure diuresis and natriuresis (PDN) response is greatly impaired in thyroxine-treated hypertensive rats. In the present study, we have examined the role of the renin angiotensin system (RAS) as a mediator of these alterations by characterizing the relationships between renal perfusion pressure and urine flow and sodium excretion in hyperthyroid rats acutely treated with a converting-enzyme inhibitor (captopril, 2 mg/kg) or an AT1 angiotensin II receptor blocker (losartan, 10 mg/kg). In the control animals, captopril did not change mean arterial pressure (MAP) or renal blood flow (RBF) but significantly decreased MAP and increased RBF and glomerular filtration rate in the hyperthyroid rats. Captopril did not change the PDN response of the control animals but improved significantly that of the hyperthyroid rats, although it was not completely normalized. Losartan also significantly improved renal hemodynamics and excretion in hyperthyroid rats. These results indicate that an increased intrarenal activity of the RAS is partly responsible for the blunted renal PDN mechanism of the hyperthyroid rats.

Relation between pressure natriuresis and urinary excretion of nitrate/nitrite in anesthetized dogs

Alterations in intrarenal nitric oxide (NO) formation during changes in renal arterial pressure (RAP) have been suggested as a mechanism mediating pressure natriuresis. To test this hypothesis further, we examined the relation between RAP and the urinary excretion rate of nitrate/nitrite (NO3-/NO2-; NO metabolites) in anesthetized sodium-replete dogs before (n = 9) and during (n = 6) intrarenal infusion of the NO synthesis inhibitor nitro-L-arginine (NLA; 50 micrograms.kg-1.min-1). Urinary NO3-/NO2- concentrations were measured with the Griess reaction and spectrophotometry methods after enzymatic reduction of NO3- to NO2- in the samples. During control conditions, there were decreases in the urinary NO3-/NO2- excretion rate in response to reductions in RAP (150 to 75 mm Hg; slope, 0.04 +/- 0.01 nmol.min-1.g-1.mm Hg-1) in association with decreases in urinary sodium excretion (UNaV). There was a positive correlation between changes in NO3-/NO2- excretion rate and changes in RAP (r = .48; P < .005) or UNaV (r = .59; P < .001). NLA infusion resulted in decreases in NO3-/NO2- excretion rate (4.8 +/- 1.4 to 1.0 +/- 0.3 nmol.min-1.g-1) in association with reductions in UNaV (4.3 +/- 0.3 to 0.7 +/- 0.2 microL.min-1.g-1), fractional excretion of sodium (2.9 +/- 0.2% to 0.5 +/- 0.1%), and renal blood flow (4.8 +/- 0.3 to 3.3 +/- 0.2 mL.min-1.g-1), without changes in glomerular filtration rate. Furthermore, there was a marked attenuation of the NO3-/NO2- and sodium excretory responses to alterations in RAP during NO synthesis inhibition.(ABSTRACT TRUNCATED AT 250 WORDS)

Furosemide dynamics in conscious rabbits: modulation by angiotensin II

The aim of this study was to investigate the effects of an infusion of angiotensin II (50 ng/kg/min) on furosemide pharmacodynamics and kinetics in the conscious rabbit. The protocol included a 90-minute phase to estimate the glomerular filtration rate and the renal plasma flow, followed by a 60-minute phase where 5 mg/kg (n = 12) or 10 mg/kg (n = 9) of furosemide were administered. During the pre-furosemide phase, compared to control rabbits, angiotensin II increased natriuresis and diuresis. In the presence of angiotensin II, the furosemide-induced natriuresis decreased, that is, it was 174 +/- 14 versus 95 +/- 25 mumol/min (p < 0.05) and 187 +/- 17 versus 89 +/- 21 mumol/min (p < 0.05) for the 5 and the 10 mg/kg doses, respectively. The infusion of angiotensin II decreased renal plasma flow without modifying the glomerular filtration rate, thus the filtration fraction was increased. Angiotensin II increased the area under the furosemide plasma concentrations as a function of time since it decreased its systemic clearance. However, furosemide urinary excretion rate was not altered and its renal clearance decreased slightly without reaching statistical significance. It is concluded that angiotensin II decreases the response to furosemide and the mechanism underlying this effect is related to the pharmacodynamics rather than the kinetics of the diuretic.

Effects of NG-nitro-L-arginine on pressure natriuresis in anaesthetized rabbits

1. We tested the effects of blockade of nitric oxide synthesis on renal function under conditions of controlled renal artery pressure. Our hypothesis was that endogenous nitric oxides plays a role in the natriuresis that accompanies increased renal perfusion pressure. We used a novel technique which employed an extracorporeal circuit to produce step changes over a wide range of renal artery pressures in pentobarbitone-anaesthetized rabbits. 2. Rabbits were treated with either NG-nitro-L-arginine (NOLA, 20 mg/kg, i.v.; n = 8) or its vehicle (n = 8). Renal artery pressure was set (by adjusting the extracorporeal circuit) at 65, 80, 95, 110 and then 130 mmHg respectively, at the beginning of each of five 30 min experimental periods. 3. NOLA treatment caused profound renal vasoconstriction that was largely independent of the level of renal artery pressure, renal blood flow being 35-43% lower in NOLA-treated than in vehicle-treated rabbits across the range of renal artery pressures tested (P = 0.002). NOLA treatment increased filtration fraction (P = 0.02), and tended to reduce glomerular filtration rate (P = 0.09). 4. NOLA-treatment affected sodium excretion in a manner dependent on the legel of renal artery pressure, with the slope of the relationship between sodium excretion and renal artery pressure being lower in NOLA-treated than in vehicle-treated rabbits (P = 0.006). 5. These data provide direct evidence that in anaesthetized rabbits endogenous nitric oxide (i) tonically dilates the renal vasculature across a wide range of renal perfusion pressures, and (ii) enhances sodium excretion to a progressively greater degree as renal artery pressure is increased.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Perfusion pressure and volume status determine the microvascular response of the rat kidney to NG-monomethyl-L-arginine

This study investigated the role of volume status and perfusion pressure on the hemodynamic response of cortical and medullary renal capillaries to systemic inhibition of nitric oxide. NG-Monomethyl-L-arginine (L-NMMA) was infused intravenously (15-mg/kg bolus and 500-micrograms.min-1.kg-1 infusion), and blood flow in cortical capillaries (QCC) and in descending (QDVR) and ascending vasa recta (QAVR) was measured by fluorescence videomicroscopy in euvolemic and volume-expanded anesthetized Munich-Wistar rats. L-NMMA in euvolemic rats decreased vasa recta blood flow (delta QDVR, 3.97 +/- 0.80 nL/min [P < .01]; delta QAVR, 1.90 +/- 0.39 nL/min [P < .01]; n = 6) and QCC (delta QCC, 0.57 +/- 0.15 nL/min [P < .01]; n = 7) despite increases in renal perfusion pressure (RPP). Fractional excretion of sodium (FENa) remained unchanged. In volume-expanded rats, L-NMMA decreased vasa recta blood flow when RPP increased (delta QDVR, 1.42 +/- 0.79 nL/min [P = .05]; delta QAVR, 1.95 +/- 0.34 nL/min [P < .001]; n = 9) or was held constant by partial aortic occlusion (delta QDVR, 1.19 +/- 0.45 nL/min [P < .05]; delta QAVR, 1.44 +/- 0.40 nL/min [P < .01]; n = 8). QCC was unchanged by L-NMMA when RPP increased (delta QCC, 0.27 +/- 0.20 nL/min; n = 8) but decreased significantly by 0.61 +/- 0.11 nL/min (P < .01, n = 8) when increases in RPP were prevented. FENa increased when RPP increased (delta FENa, 2.47 +/- 0.51%; P < .001) and was held constant (delta FENa, 2.64 +/- 0.46%; P < .001).(ABSTRACT TRUNCATED AT 250 WORDS)

How much does the kidney participate in the origin of primary hypertension?

The existence of a relationship between the kidney and arterial hypertension has long been known. Renal participation in the development of arterial hypertension has been clearly shown in different animal models that mimic human essential hypertension. Different theories have tried to explain the mechanism(s) underlying the renal participation in human hypertension. According to the data contained in the literature renal vasoconstriction is present in the kidney since the very early stages of the hypertensive disease and could constitute the mechanism facilitating the development of arterial hypertension.

Blockade of distal nephron sodium transport attenuates pressure natriuresis in dogs

The sodium excretory responses (UNaV) to acute changes in renal arterial pressure (RAP) during blockade of distal nephron sodium transport were evaluated in seven sodium-replete anesthetized dogs. The major distal sodium entry pathways were blocked by intrarenal infusion of amiloride (AM, 10(-5) mol/L) and bendroflumethiazide (BZ, 10(-6) mol/L). Infusion of AM plus BZ caused slight increases in renal blood flow (RBF, 4.1 +/- 0.5 to 4.6 +/- 0.4 mL.min-1.g-1; P < .001) but no changes in glomerular filtration rate (GFR, 0.96 +/- 0.05 to 1.01 +/- 0.07 mL.min-1.g-1; P = NS) or autoregulatory efficiency of RBF and GFR. There were significant increases in UNaV (2.7 +/- 0.7 to 5.2 +/- 0.6 mumol.min-1.g-1) and fractional excretion of sodium (FENa, 1.8 +/- 0.4% to 3.5 +/- 0.3%) and decreases in potassium excretion (0.59 +/- 0.10 to 0.35 +/- 0.06 mumol.min-1.g-1) during AM plus BZ infusion. During the control period and during repeat measurements in time control studies, decreases in RAP (150 to 100 mm Hg) elicited the usual decreases in UNaV (slope, 0.022 +/- 0.007 mumol.min-1.g-1.mm Hg-1; P < .01). After administration of AM plus BZ, there was a marked attenuation of the pressure-natriuretic responses, and the slopes of the RAP versus UNaV and RAP versus FENa relations at RAP levels above 100 mm Hg were not significantly different from zero. However, the pressure-natriuresis response was maintained at arterial pressure between 75 and 100 mm Hg.(ABSTRACT TRUNCATED AT 250 WORDS)

Decreased sensitivity to renal interstitial hydrostatic pressure in Dahl salt-sensitive rats

The ability of Dahl salt-sensitive (DS) rats to excrete a sodium load is significantly lower than Dahl salt-resistant (DR) rats. Because renal interstitial hydrostatic pressure (RIHP) is a major mediator of natriuresis in response to a sodium load, we proposed that the renal tubules of DS rats are less responsive to increases in RIHP than those of DR rats. To test this hypothesis, we determined the effect of direct increases in RIHP on renal excretory function in prehypertensive DS and DR rats. RIHP was directly increased by renal interstitial volume expansion via injection of 50 microL of a 2% albumin and saline solution into the renal interstitium through a chronically implanted renal interstitial catheter. RIHP, mean arterial pressure, glomerular filtration rate, urine flow rate, urinary sodium excretion, and fractional excretions of sodium, potassium, and lithium (an indicator of proximal tubule sodium handling) were measured before and after direct increases in RIHP in DS (n = 8) and DR (n = 8) rats. Baseline urine flow rate; urinary sodium excretion; fractional excretions of sodium, potassium, and lithium; RIHP; mean arterial pressure; and glomerular filtration rate were not different between DS and DR rats. Renal interstitial volume expansion in DS rats significantly increased RIHP (delta 4.7 +/- 0.8 mm Hg), urine flow rate (delta 14.5 +/- 3.4 microL/min), urinary sodium excretion (delta 2.62 +/- 0.62 mumol/min), and fractional excretions of sodium (delta 1.54 +/- 0.37%), potassium (delta 17.84 +/- 2.90%), and lithium (delta 19.68 +/- 3.52%).(ABSTRACT TRUNCATED AT 250 WORDS)

Are renal hemodynamics a key factor in the development and maintenance of arterial hypertension in humans?

The kidney plays a key role in the control of body fluids and blood pressure. Evidence has shown that impairment of renal function can lead to the development of arterial hypertension. The regulation of renal blood flow appears to be a key element in the pathophysiology of the hypertensive process, because multiple evidence suggests the existence of a functional enhancement of renal vascular tone in this disorder. The existence of renal vasoconstriction and of an inherited defect in the regulation of renal blood flow has been proposed in the prehypertensive stage. The mechanisms responsible for this alteration include a lack of modulation of the renal vasculature to angiotensin II, increased sympathetic activity, or suppressed renal dopaminergic activity. Established hypertension is characterized by elevated renal vascular resistance, decreased renal blood flow, sustained glomerular filtration rate, and increased filtration fraction. The increase in renal vascular resistance is initially due to elevations in renal vascular tone and is reversible, whereas later it becomes irreversible because of structural changes involved in nephrosclerosis. Antihypertensive drugs are able to decrease blood pressure and to prevent the development of further renal vascular damage independently of variable effects on renal hemodynamics.

Sulfhydryl group donors potentiate the hypotensive effect of acetylcholine in rats

Nitric oxide mediates the vasodilator and hypotensive responses of acetylcholine infusion. It has been reported that nitric oxide could be protected from free radical destruction by forming an S-nitrosothiol compound. Furthermore, sulfhydryl donors such as N-acetylcysteine or thiosalicylic acid enhance nitric oxide production from nitroglycerin. Consequently, the hypotensive effect of intravenous acetylcholine infusion might be potentiated during the simultaneous administration of sulfhydryl donors. The objective of the present study was to test in Okamoto spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats (1) whether the hypotensive effect of acetylcholine (10 micrograms/kg per minute) was affected by the simultaneous administration of N-acetylcysteine (10 micrograms/kg per minute) or thiosalicylic acid (10 micrograms/kg per minute), and (2) whether NG-nitro-L-arginine-methyl ester (100 micrograms/kg per minute) administration was able to reverse the changes induced by acetylcholine plus N-acetylcysteine or acetylcholine plus thiosalicylic acid. The administration of acetylcholine reduced (P < .05) mean arterial pressure in WKY rats (13 +/- 2%) and SHR (14 +/- 2%) without affecting urine flow rate, urinary sodium excretion, and glomerular filtration rate. In the presence of N-acetylcysteine, the acetylcholine-induced reduction in mean arterial pressure was potentiated (P < .05) in WKY rats (24 +/- 4%) and SHR (20 +/- 2%). These changes in mean arterial pressure were accompanied by significant reductions in urine flow rate and urinary sodium excretion in WKY rats, as well as in glomerular filtration rate in SHR.2

Role of pressure natriuresis in long-term control of renal electrolyte excretion

If pressure natriuresis is to play an important role in arterial pressure control, renal perfusion pressure must have a long-term effect on urinary sodium excretion. The aim of this study was to quantitate the importance of renal perfusion pressure per se in controlling renal hemodynamics and electrolyte excretion chronically. Female mongrel dogs (n = 6) were instrumented with bilateral renal artery catheters for measurement of renal perfusion pressure and occluders on both renal arteries for servo-control of renal perfusion pressure at different levels; the urinary bladder was split for determination of renal clearances and electrolyte excretion from each kidney separately. Because both kidneys were exposed to the same neurohumoral influences, any changes in renal function could be attributed to differences in renal perfusion pressure between the two kidneys. After 5 days of control, renal perfusion pressure to one kidney was reduced from 86.7 +/- 0.2 to 74.2 +/- 0.6 mm Hg for 12 days, and pressure in the contralateral kidney increased to 91.5 +/- 0.4 mm Hg. Sodium excretion decreased from 41 +/- 2 to 25 +/- 1 mmol/d in the servo-controlled kidney and increased from 41 +/- 1 to 55 +/- 1 mmol/d in the contralateral kidney during 12 days of servo-control. Urine volume, chloride excretion, and potassium excretion exhibited similar patterns during servo-control. In addition, autoregulation of effective renal plasma flow and glomerular filtration rate was relatively well maintained; however, in the low-pressure kidney, glomerular filtration rate was slightly but significantly lower (approximately 8%) than in the contralateral kidney.(ABSTRACT TRUNCATED AT 250 WORDS)

Alterations in renal endothelin-1 production in the spontaneously hypertensive rat

Endothelin-1 inhibits sodium and water transport systems in the inner medullary collecting duct. Endothelin-1 levels are reduced in the medulla of spontaneously hypertensive rats (SHR), raising the possibility that decreased inner medullary collecting duct production of endothelin-1 could contribute to inappropriate sodium and water retention. In the current study, immunoreactive endothelin-1 was measured in the urine, blood, and eluates from cortex and outer and inner medulla of SHR before (age 3-4 weeks) and after (age 8-9 weeks) the development of hypertension and in age-matched Wistar-Kyoto (WKY) controls. There was no difference in endothelin-1 levels between prehypertensive SHR and WKY rats. In contrast, 8-9-week-old SHR had significantly reduced endothelin-1 in the urine and outer and inner medulla, but not in the cortex or serum compared with those of WKY controls. Furthermore, inner medullary collecting duct cells from 8-9-week-old SHR, either acutely isolated or cultured, released less endothelin-1 than did those from WKY rats. Finally, the level of endothelin-1 messenger RNA was only reduced in the inner medulla and in inner medullary collecting duct cells from 8-9-week-old SHR. In summary, renal medullary, and in particular terminal collecting duct, endothelin-1 production is reduced in SHR only after the development of hypertension. Such decreases in inner medullary collecting duct endothelin-1 production may contribute to the hypertensive state in SHR.

Role of nitric oxide in mediating renal response to volume expansion

The objective of the present study was to determine the role of endothelium-derived nitric oxide in mediating the renal response to extracellular volume expansion with isotonic saline (5% body weight). In anesthetized dogs (n = 7) and before volume expansion, nitric oxide synthesis was inhibited in the right kidney by continuous intrarenal infusion of NG-nitro-L-arginine-methyl ester (1 microgram/kg/min). Arterial pressure and renal hemodynamics of both kidneys did not change significantly either during nitric oxide synthesis inhibition or during 5% volume expansion. However, in response to extracellular volume expansion, increases in natriuresis, diuresis, and fractional excretion of lithium (an index of proximal sodium reabsorption) were inhibited in the right kidney by 27%, 28%, and 41%, respectively, when compared with the contralateral kidney. Increases of renal interstitial hydrostatic pressure during 5% volume expansion were not statistically different between both kidneys. In another group of dogs (n = 4), the administration of L-arginine (0.5 mg/kg/min) into the right renal artery prevented the renal effects induced by the nitric oxide synthesis inhibitor during volume expansion. The findings in this study suggest that nitric oxide production plays an important role in regulating the renal response to extracellular volume expansion. The proximal tubule seems to be involved in the reduced renal excretory response to volume expansion during nitric oxide synthesis inhibition.

Renal effects of angiotensin II inhibition during increases in renal venous pressure

Increases in renal venous pressure have been shown to consistently increase renal interstitial pressure; however, not until renal interstitial pressure is increased threefold is a natriuresis noted in normal animals. Since the intrarenal angiotensin II (Ang II) concentration has been postulated to increase with increasing renal venous pressure, the antinatriuretic action of Ang II could override the natriuretic effect of increased renal interstitial pressure. Therefore, the role of Ang II in the natriuretic response to increased renal venous pressure was examined in 10 pentobarbital-anesthetized dogs. Mean arterial pressure, renal blood flow, renal interstitial pressure, glomerular filtration rate, urinary sodium excretion, plasma renin activity, and prostaglandin E2 excretion were measured at renal venous pressures of 3, 15, and 30 mm Hg. The measurements were repeated after the administration of captopril (1 mg/kg i.v. bolus, n = 5) or [Sar1,Ile8]Ang II (50 micrograms/kg i.v. bolus + 50 micrograms/kg/hr infusion, n = 5). Under control conditions, mean arterial pressure, renal blood flow, plasma renin activity, and prostaglandin E2 excretion remained unchanged when renal venous pressure was increased. The elevations in renal venous pressure increased renal interstitial pressure from 7 +/- 2 to 12 +/- 2 and 22 +/- 4 mm Hg, while sodium excretion remained unchanged until renal venous pressure was 30 mm Hg. In the captopril-treated group, increasing renal venous pressure increased renal interstitial pressure as under control conditions; however, sodium excretion (23 +/- 4, 19 +/- 4, and 27 +/- 6 mueq/min) was not significantly increased even at the highest renal venous pressure.(ABSTRACT TRUNCATED AT 250 WORDS)

Urinary PGE2 in rats with chronic partial unilateral ureteral obstruction

Urinary prostaglandin E2 (PGE2) was measured in Munich-Wistar rats with surgically created chronic partial unilateral ureteral obstruction (UUO). Mean values of bladder urine PGE2 were higher in sham than in UUO (24.5 +/- 14.4 vs 12.9 +/- 8.2 ng/mg creatinine, respectively, P less than 0.05). Following diuresis, both ureters were cannulated and urine was collected. PGE2 excretion was increased in sham (66.5 +/- 34.4 and 70.1 +/- 44.5 ng/mg creatinine, left and right, respectively). But in UUO, the obstructed kidney excreted less PGE2 than the contralateral kidney (32.1 +/- 6.0 vs 62.3 +/- 40.4 ng/mg creatinine, obstructed vs contralateral, respectively, P = 0.08). PGE2 synthesis was then determined in separated renal medullary and cortical slices. Renal medullary slices from kidneys with severe obstruction synthesized less PGE2 than the contralateral unobstructed side (3.30 +/- 1.22 vs 10.52 +/- 3.23 ng/mg wet wt-30 min, respectively, P less than 0.05) and failed to respond to arachidonic acid stimulation with any significant increase in PGE2 synthesis (3.30 +/- 1.22 vs 4.47 +/- 1.04 ng/mg wet wt-30 min, baseline vs stimulated). In contrast, contralateral unobstructed kidney slices responded with a significant increase in PGE2 synthesis (10.52 +/- 3.23 vs 21.10 +/- 2.50 ng/mg wet wt-30 min, baseline vs stimulated, P less than 0.05). We conclude that chronic partial UUO in the Munich-Wistar rats resulted in significantly less PGE2 elaboration.

Impaired response to angiotensin II in type 1 (insulin-dependent) diabetes mellitus. Role of prostaglandins and sodium-lithium countertransport activity

The pathogenesis of diabetic nephropathy remains elusive. A role for renal prostaglandins in antagonizing the hormonal effects of renin-angiotensin II has been postulated as a putative factor leading to hyperfiltration in patients with Type 1 (insulin-dependent) diabetes mellitus. Our aim was to elucidate the effects of angiotensin II on kidney haemodynamics and on blood pressure in eight normal subjects, in nine normotensive, in nine hypertensive with normal sodium-lithium countertransport activity in erythrocytes, in seven hypertensive without and in eight hypertensive Type 1 diabetic patients with microalbuminuria and with high sodium-lithium countertransport activity in erythrocytes. Angiotensin II infusion (4 ng.kg-1.min-1 for 60 min) decreased the glomerular filtration rate to a greater extent in normal subjects (-20%), than in normotensive patients (-5% p less than 0.01), in hypertensive patients with normal sodium-lithium countertransport activity in erythrocytes (-8% p less than 0.01) in hypertensive patients with high sodium-lithium countertransport (-6% p less than 0.01) and in hypertensive microalbuminuric patients (-5% p less than 0.01) with Type 1 diabetes. The urinary excretion rate of vasodilatory prostaglandins was two-three fold higher in all patients than in normal subjects. Acute indomethacin treatment restored a normal response to angiotensin II infusion in normotensive patients, but did not change the renal haemodynamic response in normal subjects. With regard to hypertensive patients with and without microalbuminuria indomethacin treatment restored a normal response to angiotensin II in some but not all patients.(ABSTRACT TRUNCATED AT 250 WORDS)

ACE inhibitors in renal disease

Recent experimental studies suggest that the resistance state of the preglomerular and postglomerular capillary arterioles may determine if a particular class of antihypertensive agents will protect the kidney from hemodynamically mediated glomerular injury. This review discusses (1) the effects of angiotensin II on the renal microcirculation, (2) the pathophysiology of essential hypertensive renal disease, (3) the renal pharmacology of angiotensin-converting enzyme (ACE) inhibitors, and (4) the hypothesis that renal protection is dependent on control of systemic and glomerular hypertension.

Effect of enalapril treatment on the pressure-natriuresis curve in spontaneously hypertensive rats

The effect of chronic angiotensin I converting enzyme inhibition on the pressure-natriuresis relation was studied in Wistar-Kyoto and spontaneously hypertensive rats. Enalapril maleate (25 mg.kg-1.day-1 in drinking water) was started at 4-5 weeks of age. At 7-9 weeks of age, the pressure-natriuresis relation was studied while the rats were under Inactin anesthesia 1 week after the right kidney and adrenal gland were removed. Neural and hormonal influences on the remaining kidney were fixed by surgical renal denervation, adrenalectomy, and infusion of a hormone cocktail (330 microliters.kg-1.min-1) containing high levels of aldosterone, arginine vasopressin, hydrocortisone, and norepinephrine dissolved in 0.9% NaCl containing 1% albumin. Changes in renal function resulting from alterations in renal artery pressure were compared between enalapril-treated and control rats. Mean arterial pressure (+/- SEM) under anesthesia was 118 +/- 5, 94 +/- 4, 175 +/- 3, and 124 +/- 2 mm Hg for control Wistar-Kyoto (n = 10), enalapril-treated Wistar-Kyoto (n = 10), control spontaneously hypertensive (n = 9), and enalapril-treated spontaneously hypertensive (n = 9) rats, respectively. When renal artery pressure was set at values above approximately 125 mm Hg, control spontaneously hypertensive rats excreted less sodium and water than control Wistar-Kyoto rats. Enalapril treatment resulted in a significant and similar shift to the left of the pressure-natriuresis relation in both strains of rats so that a lower renal artery pressure was required to excrete a similar amount of sodium when compared with their respective untreated controls.(ABSTRACT TRUNCATED AT 250 WORDS)

Plasma concentrations of atrial natriuretic factor and hemodynamics in pregnancy-induced hypertension

Plasma values of atrial natriuretic factor (ANF) were evaluated in 31 women with pregnancy-induced hypertension (PIH) and 31 normal pregnant women at the same age of gestation. In 27 women with PIH and 27 normal pregnant women forearm venous tone (FVT) was evaluated by Strain Gauge Plethysmography. Forearm vascular resistance (FVR) was measured as the ratio of mean blood pressure (MBP) to forearm blood flow. In addition Cardiac Index (CI) by means of transthoracic electrical bioimpedance and total peripheral vascular resistance (TPR) (with the Frank Equation) were also measured. In comparison with the normal pregnant women, the women with PIH had similar values of hematocrit (as an index of plasma volume) and significantly higher levels of FVR and TPR, while ANF plasma values did not differ significantly. Subdividing the women with PIH in relation to the presence of proteinuria (greater than or equal to 0.3 g/l), those with proteinuria, in addition to significantly higher levels of FVR and TPR, had significantly higher levels of FVT than normal pregnant women, while ANF plasma values were higher even though the difference was only near the level of significance. Hypertensive women with proteinuria also had higher values of FVT than hypertensive women without proteinuria. By means of multiple regression ANF did not show any significant correlations with hematocrit or sodium excretion. Hypertension with proteinuria seems to represent a more severe form of the disease in which, in addition to the probable influence of other factors such as the renin-angiotensin and prostaglandin systems, a greater increase in peripheral sympathetic tone than in hypertension alone appears to be present, causing a reduction in venous compliance in addition to the elevation in FVR and TPR, with increase in central blood volume and atrial stretch. This may explain the higher ANF plasma levels in these patients in comparison with normal pregnant women, even though the absence of a significant correlation of ANF with hematocrit and the fact that ANF increase was only near the level of significance may suggest a change in the relation between ANF secretion and atrial volume receptors in pregnancy either normal or complicated by hypertension. ANF does not seem to play an important role in water and sodium excretion in PIH probably because of the presence of very high plasma levels of hormones such as aldosterone, progesterone and oestriol which, together with renal prostaglandins, seem to be involved in diuresis and natriuresis in pregnancy.

Response of superficial proximal convoluted tubule to decreased and increased renal perfusion pressure. In vivo microperfusion study in rats

Although urinary sodium excretion is positively influenced by acute changes in renal perfusion pressure, micropuncture studies show highly conflicting results concerning the response of superficial proximal tubule sodium reabsorption to changes in renal perfusion pressure. In the present study, the changes of superficial proximal reabsorption to decreased and increased renal perfusion pressure were determined in rats by an in vivo microperfusion method. In vivo microperfusion was selected as the method to determine the proximal sodium reabsorption because this method made it possible to deliver a constant fluid and electrolyte load to the proximal tubule without the influence of possible changes of glomerular filtration rate. Renal perfusion pressure was decreased from normal pressure by inflating a suprarenal aortic cuff and was increased from the normal level by the occlusion of celiac and mesenteric arteries and the infrarenal aorta. Although fractional excretion of sodium (FENa) in the urine was decreased from 1.24 +/- 0.23% to 0.45 +/- 0.11% (n = 7, p less than 0.01) when renal perfusion pressure was decreased from 125 +/- 6 to 99 +/- 3 mm Hg, absolute tubular reabsorption by the superficial proximal convoluted tubules was not increased (from 4.4 +/- 0.5 to 4.2 +/- 0.3 nl/min/mm, n = 22). When the renal perfusion pressure was elevated from 126 +/- 4 to 149 +/- 4 mm Hg, tubular reabsorption by the superficial proximal tubules was decreased from 4.1 +/- 0.3 to 2.5 +/- 0.3 nl/min/mm (n = 36, p less than 0.01) with an accompanying increase in FENa (from 1.28 +/- 0.24% to 2.29 +/- 0.37%, n = 9, p less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Studies of renal autoregulation in pancreatectomized and streptozotocin diabetic rats

We studied renal autoregulation in pancreatectomized Munich-Wistar diabetic rats and in their sham-operated controls. In a second experiment we studied renal autoregulation in untreated and insulin treated streptozotocin diabetic Munich-Wistar rats and their nondiabetic controls. In the first experiment the diabetic rats had higher baseline renal blood flows (RBF). There was a fall in renal vascular resistance (RVR) and sustained RBF in both diabetic and control rats as renal perfusion pressures (RPP) was reduced from 130 and 110 mm Hg. As RPP was reduced from 110 and 80 mm Hg, there was no significant Change in RVR in control rats and RBF began to fall. Below RPP of 80 mm Hg RVR rose and RBF fell sharply in these rats. In contrast, there was a progressive fall in RVR as RPP was lowered to 60 mm Hg in the diabetic rats and, thus, RBF was much better sustained in these animals. In the second experiment the plasma glucose level was 502 +/- 52 mg/dl (X +/- SD) in the untreated diabetic rats and only modestly reduced to 411 +/- 49 mg/dl in the insulin treated animals. Untreated streptozotocin diabetic rats had moderately reduced and insulin-treated diabetic rats had mildly reduced baseline RVR and RBF. However, in these animals as in the pancreatectomized rats the increases in RVR noted in control rats at subautoregulatory RPPs were not seen. Thus, regardless of whether baseline RBFs were increased or decreased, diabetic rats sustained RBF at markedly reduced RPPs far more efficiently than did nondiabetic rats. The pathogenesis of these abnormalities in diabetic rats was not learned in these studies.(ABSTRACT TRUNCATED AT 250 WORDS)

Acute and chronic effects of angiotensin converting enzyme inhibitors on the essential hypertensive kidney

The natural course of essential hypertensive renal disease is characterized by a slowly progressive impairment of renal function. Initially, the changes are functional and reversible; however, structural changes gradually occur, leading to hypertensive nephrosclerosis. Similarities exist between the early functional hemodynamic changes observed in the essential hypertensive kidney and the physiologic renal effects of angiotensin II. To the degree that the initial functional changes are the result of excessive endogenous production of angiotensin II, interruption of the integrity of this humoral system could be expected to reverse the pathophysiologic sequence of events leading to hypertensive nephrosclerosis. This review focuses on the pathophysiology of the essential hypertensive kidney, the intrarenal effects of angiotensin II, and the acute and chronic effects of angiotensin converting enzyme (ACE) inhibition therapy on the essential hypertensive kidney. The data reviewed suggest that ACE inhibition therapy does reverse the initial functional hemodynamic changes observed in the essential hypertensive kidney and may protect the glomerulus from hemodynamically mediated injury.

Influence of prostaglandins on papillary blood flow and pressure-natriuretic response

The present study examined whether renal prostaglandins influence the pressure-natriuretic response by altering medullary hemodynamics or renal interstitial pressure. The diuretic and natriuretic responses to changes in renal perfusion pressure were compared in control rats (n = 15) and in rats receiving either meclofenamate (2 mg/kg, n = 9) or indomethacin (2 mg/kg, n = 4). In control rats, urine flow and sodium excretion increased from 10 +/- 2 to 118 +/- 10 microliters/min/g kidney wt and from 1.8 +/- 0.3 to 21.0 +/- 1.5 mueq/min/g kidney wt, respectively, when renal perfusion pressure was increased from 109 to 167 mm Hg. Urinary excretion of prostaglandin E2 and thromboxane B2 increased significantly by 152% and 190%, respectively. Meclofenamate lowered thromboxane B2 and prostaglandin E2 excretion and prevented the increase in eicosanoid excretion produced by elevations in perfusion pressure. The pressure-diuretic and pressure-natriuretic responses of rats given meclofenamate or indomethacin were approximately half of those observed in the control rats. Papillary blood flow increased 21% and renal interstitial pressure rose from 5.0 +/- 0.7 to 8.2 +/- 0.7 mm Hg in control rats when pressure was elevated from 100 to 150 mm Hg. Meclofenamate and indomethacin lowered papillary blood flow and renal interstitial pressure and blunted the increases in these values produced by elevations in perfusion pressure. These results support the view that renal prostaglandins modulate the pressure-natriuresis relation by altering renal medullary hemodynamics and suggest that an intact renal prostaglandin system is necessary for the full expression of the medullary hemodynamic and natriuretic responses to increases in renal perfusion pressure.

Intrarenal mechanisms that regulate sodium excretion in relationship to changes in blood pressure

Because pressure-related natriuresis may be central to the regulatory role of the kidney on blood pressure, it is important to understand the relationship of humoral systems involved in the control of renal hemodynamics and tubular function. The preglomerular endothelial synthesis of prostaglandin I2 and endothelium-derived relaxing factor seem to modulate autoregulatory control by the afferent arterioles and the release of renin by the juxtaglomerular apparatus. The release of renin is followed by an increase in angiotensin II in the renal interstitium, which is responsible for adjusting the vascular tone of the efferent arterioles and vasa recta and for stimulating proximal tubular reabsorption of sodium. Variations in medullary circulation induced by angiotensin II could alter medullary interstitial pressure and the medullary production of prostaglandins E2 and I2 and, ultimately, could modulate sodium reabsorption in the medullary thick ascending limbs and the collecting ducts.

Control of blood and extracellular volume

Blood and extracellular fluid volume are maintained within narrow limits despite considerable daily variations in the intake in salt and water. As summarized schematically in Figure 15, the urinary excretion of salt and water responds to changes in blood volume and arterial pressure. Volume-sensitive receptors located predominantly in the cardiac atria and arterial tree sense acute changes in the filling of the blood volume compartment, and urinary sodium excretion is adjusted in response to these detector mechanisms by virtue of alterations in both glomerular filtration rate and tubular sodium reabsorption. The reabsorption of sodium by the tubule responds to changes in extracellular fluid volume as well as to changes in filtered sodium load. Glomerular filtration rate and tubular reabsorption of sodium are influenced importantly by physical properties of the plasma in glomerular and peritubular capillaries and by the composition of the tubular fluid. The renal arterial perfusion pressure is a major factor regulating tubular reabsorption of sodium and water as signalled via changes in renal interstitial hydrostatic fluid pressure. Renal nerves and a variety of systemic and local hormones also influence tubular reabsorption of sodium and water directly by effects on transepithelial sodium transport and/or indirectly by altering renal medullary haemodynamics and the pressure-natriuresis-diuresis relationships. Thus, utilizing a variety of overlapping effector mechanisms that influence renal sodium and water excretion, mammalian organisms have achieved a high degree of stability of body fluid volumes. The fundamental relationship between arterial pressure and renal excretion appears to be the major mechanism which provides for the long-term control of body fluid volume. The sensitivity of the pressure-natriuresis-diuresis relationship is modified by the efferent pathways of the rapid-acting reflex and mechanoreceptor detectors of volume. Working together, these mechanisms provide a remarkable degree of rapid and long-term extracellular and blood volume stability.

Comparison of the effects of calcium antagonists and converting enzyme inhibitors on renal function under normal and hypertensive conditions

Calcium antagonists decrease the ability of the kidney to autoregulate renal blood flow (RBF) and glomerular filtration rate (GFR). Therefore, when afferent renovascular resistance is elevated, as in essential hypertension, there is a resultant increase in RBF and GFR with the administration of calcium antagonists. These agents also induce a marked natriuresis because of direct tubular action through unknown mechanisms. The natriuresis can be dissociated from renal and systemic hemodynamic actions, indicating that the decreased sodium reabsorption could override other compensatory mechanisms explaining the absence of sodium retention during the treatment. The renal effects of converting enzyme inhibitors (CEIs) can be explained by the reduction of intrarenal formation in angiotensin II. Because the activation of the renin-angiotensin system is mainly responsible for inducing sodium retention during a decrease in systemic blood pressure, CEIs could have a protecting effect without disturbing other homeostatic mechanisms. CEIs decrease efferent glomerular resistance, reducing capillary pressure and thereby reducing GFR. This effect is not translated in sodium retention because the reduction of GFR is mild during captopril administration in kidneys with normal or increased renal perfusion pressure. At low renal perfusion pressure, the reduced glomerular afferent vasoconstriction can compromise GFR, leading to renal insufficiency. Although these situations are not likely to be encountered during the treatment of uncomplicated essential hypertension, in severe hypertension with hypertrophy of pre-glomerular vessels, glomerular perfusion may decrease. Combination therapy of calcium antagonists and CEIs has been reported to be an effective treatment of severe hypertension. Currently, little information is available on the manner in which renal function is affected by simultaneous administration of both drugs.