Intrarenal angiotensin III is the predominant agonist for proximal tubule angiotensin type 2 receptors

In angiotensin type 1 receptor-blocked rats, renal interstitial (RI) administration of des-aspartyl(1)-angiotensin II (Ang III) but not angiotensin II induces natriuresis via activation of angiotensin type 2 receptors. In the present study, renal function was documented during systemic angiotensin type 1 receptor blockade with candesartan in Sprague-Dawley rats receiving unilateral RI infusion of Ang III. Ang III increased urine sodium excretion, fractional sodium, and lithium excretion. RI coinfusion of specific angiotensin type 2 receptor antagonist PD-123319 abolished Ang III-induced natriuresis. The natriuretic response observed with RI Ang III was not reproducible with RI angiotensin (1-7) alone or together with angiotensin-converting enzyme inhibition. Similarly, neither RI angiotensin II alone or in the presence of aminopeptidase A inhibitor increased urine sodium excretion. In the absence of systemic angiotensin type 1 receptor blockade, Ang III alone did not increase urine sodium excretion, but natriuresis was enabled by the coinfusion of aminopeptidase N inhibitor and subsequently blocked by PD-123319. In angiotensin type 1 receptor-blocked rats, RI administration of aminopeptidase N inhibitor alone also induced natriuresis that was abolished by PD-123319. Ang III-induced natriuresis was accompanied by increased RI cGMP levels and was abolished by inhibition of soluble guanylyl cyclase. RI and renal tissue Ang III levels increased in response to Ang III infusion and were augmented by aminopeptidase N inhibition. These data demonstrate that endogenous intrarenal Ang III but not angiotensin II or angiotensin (1-7) induces natriuresis via activation of angiotensin type 2 receptors in the proximal tubule via a cGMP-dependent mechanism and suggest aminopeptidase N inhibition as a potential therapeutic target in hypertension.     

Conversion of renal angiotensin II to angiotensin III is critical for AT2 receptor-mediated natriuresis in rats

In the kidney, angiotensin II (Ang II) is metabolized to angiotensin III (Ang III) by aminopeptidase A (APA). In turn, Ang III is metabolized to angiotensin IV by aminopeptidase N (APN). Renal interstitial (RI) infusion of Ang III, but not Ang II, results in angiotensin type-2 receptor (AT(2)R)-mediated natriuresis. This response is augmented by coinfusion of PC-18, a specific inhibitor of APN. The present study addresses the hypotheses that Ang II conversion to Ang III is critical for the natriuretic response. Sprague-Dawley rats received systemic angiotensin type-1 receptor (AT(1)R) blockade with candesartan (CAND; 0.01 mg/kg/min) for 24 hours before and during the experiment. After a control period, rats received either RI infusion of Ang II or Ang II+PC-18. The contralateral kidney received a RI infusion of vehicle in all rats. Mean arterial pressure (MAP) was monitored, and urinary sodium excretion rate (U(Na)V) was calculated separately from experimental and control kidneys for each period. In contrast to Ang II-infused kidneys, U(Na)V from Ang II+PC-18-infused kidneys increased from a baseline of 0.03+/-0.01 to 0.09+/-0.02 micromol/min (P<0.05). MAP was unchanged by either infusion. RI addition of PD-123319, an AT(2)R antagonist, inhibited the natriuretic response. Furthermore, RI addition of EC-33, a selective APA inhibitor, abolished the natriuretic response to Ang II+PC-18. These data demonstrate that RI addition of PC-18 to Ang II enables natriuresis mediated by the AT(2)R, and that conversion of Ang II to Ang III is critical for this response.     

Renal angiotensin type 2 receptors mediate natriuresis via angiotensin III in the angiotensin II type 1 receptor-blocked rat

Whereas angiotensin (Ang) II is the major effector peptide of the renin-angiotensin system, its metabolite, des-aspartyl1-Ang II (Ang III), may also have biologic activity. We investigated the effects of renal interstitial (RI) administration of candesartan (CAND), a specific Ang II type 1 receptor (AT1) blocker, with and without coinfusion of PD-123319 (PD), a specific Ang II type 2 receptor (AT2) blocker, on Na+ excretion (UNaV) in uninephrectomized rats. We also studied the effects of unilateral RI infusion of Ang II or Ang III on UNaV with and without systemic infusion of CAND with the noninfused kidney as control. In rats receiving normal Na+ intake, RI CAND increased UNaV from 0.07+/-0.08 to 0.82+/-0.17 micromol/min (P<0.01); this response was abolished by PD. During Na+ restriction, CAND increased UNaV from 0.06+/-0.02 to 0.1+/-0.02 micromol/min (P<0.05); this response also was blocked by PD. In rats with both kidneys intact, in the absence of CAND, unilateral RI infusion of Ang III did not significantly alter UNaV. However, with systemic CAND infusion, RI Ang III increased U(Na)V from 0.08+/-0.01 micromol/min to 0.18+/-0.04 micromol/min (P<0.01) at 3.5 nmol/kg per minute, and UNaV remained elevated throughout the infusion; this response was abolished by PD. However, RI infusion of Ang II did not significantly alter UNaV at any infusion rate (3.5 to 80 nmol/kg per minute) with or without systemic CAND infusion. These results suggest that intrarenal AT1 receptor blockade engenders natriuresis by activation of AT2 receptors. AT2 receptor activation via Ang III, but not via Ang II, mediates the natriuretic response in the presence of systemic AT1 receptor blockade.     

Mechanisms of dopamine D(1) and angiotensin type 2 receptor interaction in natriuresis

Renal dopamine D(1)-like receptors (D(1)Rs) and angiotensin type 2 receptors (AT(2)Rs) are important natriuretic receptors counterbalancing angiotensin type 1 receptor-mediated tubular sodium reabsorption. Here we explore the mechanisms of D(1)R and AT(2)R interactions in natriuresis. In uninephrectomized, sodium-loaded Sprague-Dawley rats, direct renal interstitial infusion of the highly selective D(1)R agonist fenoldopam induced a natriuretic response that was abolished by the AT(2)R-specific antagonist PD-123319 or by microtubule polymerization inhibitor nocodazole but not by actin polymerization inhibitor cytochalasin D. By confocal microscopy and immunoelectron microscopy, fenoldopam translocated AT(2)Rs from intracellular sites to the apical plasma membranes of renal proximal tubule cells, and this translocation was abolished by nocodazole. Because D(1)R activation induces natriuresis via an adenylyl cyclase/cAMP signaling pathway, we explored whether this pathway is responsible for AT(2)R recruitment and AT(2)R-mediated natriuresis. Renal interstitial coinfusion of the adenylyl cyclase activator forskolin and 3-isobutly-1-methylxanthine induced natriuresis that was abolished either by PD-123319 or nocodazole but was unaffected by specific the D(1)R antagonist SCH-23390. Coadministration of forskolin and 3-isobutly-1-methylxanthine also translocated AT(2)Rs to the apical plasma membranes of renal proximal tubule cells; this translocation was abolished by nocodazole but was unaffected by SCH-23390. The results demonstrate that D(1)R-induced natriuresis requires AT(2)R recruitment to the apical plasma membranes of renal proximal tubule cells in a microtubule-dependent manner involving an adenylyl cyclase/cAMP signaling pathway. These studies provide novel insights regarding the mechanisms whereby renal D(1)Rs and AT(2)Rs act in concert to promote sodium excretion in vivo.     

Intrarenal dopamine D1-like receptor stimulation induces natriuresis via an angiotensin type-2 receptor mechanism

We explored the effects of direct renal interstitial stimulation of dopamine D(1)-like receptors with fenoldopam, a selective D(1)-like receptor agonist, on renal sodium excretion and angiotensin type-2 (AT(2)) receptor expression and cellular distribution in rats on a high-sodium intake. In contrast to vehicle-infused rats, sodium excretion increased in fenoldopam-infused rats during each of three 1-hour experimental periods (<0.001). Blood pressure was unaffected by vehicle or fenoldopam. In plasma membranes of renal cortical cells, fenoldopam increased D(1) receptor expression by 38% (P<0.05) and AT(2) receptor expression by 69% (P<0.01). In plasma membranes of renal proximal tubule cells, fenoldopam increased AT(2) receptor expression by 108% (P<0.01). In outer apical membranes of proximal tubule cells, fenoldopam increased AT(2) receptor expression by 59% (P<0.01). No significant change in total AT(2) receptor protein expression was detectable in response to fenoldopam. Fenoldopam-induced natriuresis was abolished when either PD-123319, a specific AT(2) receptor antagonist, or SCH-23390, a potent D(1)-like receptor antagonist, was coinfused with F (P<0.001). In summary, direct renal D(1)-like receptor activation increased urinary sodium excretion and the plasma membrane expression of AT(2) receptors in renal cortical and proximal tubule cells. D(1)-like receptor-induced natriuresis was abolished by intrarenal AT(2) receptor inhibition. These findings suggest that dopaminergic regulation of sodium excretion involves recruitment of AT(2) receptors to the outer plasma membranes of renal proximal tubule cells and that dopamine-induced natriuresis requires AT(2) receptor activation.     

Afferent arteriolar reactivity to angiotensin II is enhanced during the early phase of angiotensin II hypertension

Increased renal microvascular reactivity may contribute to the blunted pressure natriuretic response and increase in blood pressure during the development of angiotensin II hypertension. The current studies were performed to determine renal microvascular reactivity during the early phases of angiotensin II-infused hypertension. Male-Sprague Dawley rats received angiotensin II (60 ng/min) or vehicle via an osmotic minipump. Normotensive and angiotensin II hypertensive rats were studied 1 and 2 weeks after implantation of the minipump. Systolic blood pressure averaged 117 +/- 4 mm Hg (n = 31) before pump implantation. Angiotensin II infusion increased systolic blood pressure to 149 +/- 3 and 187 +/- 5 mm Hg on infusion days 6 and 12, respectively. Renal microvascular responses to angiotensin II and norepinephrine at renal perfusion pressures of 100 and 150 mm Hg were observed using the in vitro juxtamedullary nephron preparation. Afferent arteriolar diameters of 1-week normotensive animals averaged 22 +/- 1 microm and after 2 weeks of vehicle infusion averaged 21 +/- 1 microm at a perfusion pressure of 100 mm Hg. In animals infused with angiotensin II for 1 or 2 weeks, diameters of the afferent arterioles perfused at a pressure of 100 mm Hg were 20% and 9% smaller, respectively. Additionally, 1- and 2-week hypertensive animals had an enhanced responsiveness of the renal microvasculature to angiotensin II. At a perfusion pressure of 100 mm Hg, angiotensin II (10 nmol/L) decreased afferent arteriolar diameter by 26 +/- 5% and 22 +/- 3% in the 1- and 2-week angiotensin II hypertensive rats, respectively. In 1- and 2-week normotensive animals, angiotensin II (10 nmol/L) decreased afferent arteriolar diameter by 18 +/- 2% and 15 +/- 2%, respectively, at a perfusion pressure of 100 mm Hg. In contrast, the afferent arteriolar response to norepinephrine was not altered in angiotensin II hypertensive rats. These data demonstrate an elevated renal microvascular resistance and enhanced vascular reactivity that is selective for angiotensin II in the early phases of hypertension development after infusion of angiotensin II. Thus, an alteration in renal microvascular function contributes to the blunted pressure natriuretic response and progressive development of hypertension.     

Brain and peripheral angiotensin II type 1 receptors mediate renal vasoconstrictor and blood pressure responses to angiotensin IV in the rat

OBJECTIVES: Angiotensin (Ang) IV was reported to increase renal cortical blood flow (CBF) via putative angiotensin IV receptor (AT4) stimulation but reduce total renal blood flow (RBF) via angiotensin II type 1 (AT1) receptors. We investigated the effect of Ang IV on simultaneously measured mean arterial pressure (MAP), RBF, and CBF. The possible involvement of AT1 or AT4 receptors, the possible natriuretic effect, and responses to central administration were also explored. METHODS AND RESULTS: Intravenous injections of Ang IV dose dependently increased MAP and decreased CBF and RBF; these effects were abolished by AT1 receptor blockade. These reductions in CBF and RBF highly correlated as did renal vascular responses to Ang II and fenoldopam. Ang IV did not induce renal vasodilation even following AT1 receptor blockade. Intrarenal Ang IV infusion reduced CBF and RBF but had no natriuretic effect. Central Ang IV administration induced an AT1-mediated immediate increase in MAP and renal vascular resistance and a secondary increase in RBF. AT4 selective ligands, LVV-hemorphin-7 and AT4-16 (intravenous, intrarenal or intracerebroventricular), had no effects on MAP, RBF or urinary sodium excretion. Additional in-vitro experiments indicated that the majority of the Ang IV-sensitive aminopeptidase activity in kidney membranes is attributed to aminopeptidase-N. CONCLUSION: Insulin-regulated aminopeptidase (IRAP)/AT4 receptors are involved in neither the regulation of RBF or CBF nor in the handling of renal sodium. Ang IV increases MAP and induces renal vasoconstriction via stimulation of brain and peripheral AT1 receptors and may be involved in the regulation of renal blood flow and blood pressure.     

Sex-specific influence of angiotensin type 2 receptor stimulation on renal function: a novel therapeutic target for hypertension

The renin-angiotensin system is a powerful regulator of arterial pressure and body fluid volume. Increasing evidence suggests that the angiotensin type 2 receptor (AT(2)R), which mediates the vasodilatory and natriuretic actions of angiotensin peptides, is enhanced in females and may, therefore, represent an innovative therapeutic target. We investigated the therapeutic potential of direct AT(2)R stimulation on renal function in 11- to 12-week-old anesthetized male and female Sprague-Dawley rats. Renal blood flow was examined in response to a graded infusion of the highly selective, nonpeptide AT(2)R agonist, compound 21 (100, 200, and 300 ng/kg per minute), in the presence and absence of AT(2)R blockade (PD123319; 1 mg/kg per hour). Direct AT(2)R stimulation significantly increased renal blood flow in both males and females, without influencing arterial pressure. This was dose dependent in females only and occurred to a greater extent in females at the highest dose of compound 21 administered (males: 13.1±2.4% versus females: 23.0±3.2% change in renal blood flow at 300 ng/kg per minute versus baseline; P<0.01). In addition, AT(2)R stimulation significantly increased sodium and water excretion to a similar extent in males and females (P(Group)=0.05 and 0.005). However, there was no significant change in glomerular filtration rate in either sex, suggesting that altered tubular function may be responsible for AT(2)R-induced natriuresis rather than hemodynamic effects. Taken together, this study provides evidence that direct AT(2)R stimulation produces vasodilatory and natriuretic effects in the male and female kidney. The AT(2)R may, therefore, represent a valuable therapeutic target for the treatment of renal and cardiovascular diseases in both men and women.     

Renal angiotensin II type-2 receptors are upregulated and mediate the candesartan-induced natriuresis/diuresis in obese Zucker rats

Recently, there has been a growing interest in studying the role of angiotensin II type-2 (AT(2)) receptor in renal/cardiovascular function in pathological conditions. The present study was designed to determine the functional role of the AT(2) receptors on natriuresis/diuresis and compare the level of the tubular AT(2) receptor expression in obese and lean Zucker rats (12 weeks old). Under anesthesia, candesartan (angiotensin II type 1 [AT(1)]-specific antagonist; 100 microg/kg bolus) produced natriuresis/diuresis to a greater degree in obese than in lean rats. The specific AT(2) antagonist PD123319 (50 microg/kg per minute) after candesartan administration abolished the natriuretic/diuretic effects of candesartan in obese rats but not in lean rats. Infusion of AT(2) receptor agonist, CGP-42112A (1 microg/kg per minute), produced greater increase in sodium and urine excretion over basal in obese than in lean rats. The presence of the AT(2) receptor expression in the brush-border and basolateral membranes was confirmed by Western blotting using specific antibody and antigen-blocking peptide. Densitometric analysis of the bands revealed approximately 1.5- to 2.0-fold increase in the AT(2) receptor proteins in both membranes of obese compared with lean rats. Our results suggest upregulation of the AT(2) receptors, which play a role in mediating the natriuretic/diuretic effects of AT(1) receptor blockers in obese Zucker rats. We speculate that AT(2) receptors, by promoting sodium excretion, may protect obese Zucker rats against blood pressure increase associated with sodium and water retention.     

Depletion of tissue angiotensin-converting enzyme differentially influences the intrarenal and urinary expression of angiotensin peptides

The relative contribution of circulating versus tissue renin-angiotensin systems to the tissue expression of angiotensin peptides in the kidney remains unresolved. To address this issue, intrarenal and urinary levels of the peptide products of the renin-angiotensin system were assessed in a tissue angiotensin-converting enzyme knockout (tisACE-/-) mouse model. Systolic blood pressure was significantly lower (64.6+/-3.6 versus 81.4+/-4.5 mm Hg; P<0.02) and urinary volume was increased (7.25+/-0.86 versus 2.86+/-0.48 mL/d; P<0.001) in tisACE-/- mice compared with wild-type mice. Intrarenal angiotensin II was 80% lower in tisACE-/- mice compared with wild-type mice (5.17+/-0.60 versus 25.5+/-2.4 fmol/mg protein; P<0.001). Intrarenal angiotensin I levels also declined by a comparable extent (73%) in the tisACE-/- mice (P<0.01). Intrarenal angiotensin-(1-7) concentrations were similar between the strains, but the ratio of intrarenal angiotensin-(1-7) to angiotensin II and angiotensin I in tisACE-/- mice increased 470% and 355%, respectively, compared with wild-type mice. Urinary excretion of angiotensin II and angiotensin-(1-7) were not different, but the excretion of angiotensin I increased 270% in tisACE-/- mice (P<0.01). These studies suggest 2 potential mechanisms for the reduction of intrarenal angiotensin II in tisACE-/- mice: (1) an attenuated capacity to form angiotensin II by renal angiotensin-converting enzyme and (2) significant depletion of its direct precursor angiotensin I in renal tissue. Sustained intrarenal levels of angiotensin-(1-7) may contribute to chronic hypotension and polyuria in tisACE-/- mice, particularly in the context of depleted angiotensin II in the kidney.     

Enhanced angiotensin II type 2 receptor mechanisms mediate decreases in arterial pressure attributable to chronic low-dose angiotensin II in female rats

The renin-angiotensin system is a far more complex enzymatic cascade than realized previously. Mounting evidence suggests sex-specific differences in the regulation of the renin-angiotensin system and arterial pressure. We examined the hemodynamic responses, angiotensin II receptor subtypes, and angiotensin-converting enzyme 2 gene expression levels after graded doses of angiotensin II in males and females. Mean arterial pressure was measured via telemetry in male and female rats in response to a 2-week infusion of vehicle, low-dose (50 ng/kg per minute SC) or high-dose (400 ng/kg per minute SC) angiotensin II. The effect of concurrent infusion of the angiotensin II type 2 receptor (AT(2)R) blocker (PD123319) was also examined. The arterial pressure response to high-dose angiotensin II was attenuated in females compared with males (24+/-8 mm Hg versus 42+/-5 mm Hg; P for the interaction between sex and treatment <0.002). Remarkably, low-dose angiotensin II decreased arterial pressure (11+/-4 mm Hg; P for the interaction between sex and treatment <0.02) at a dose that did not have an effect in males. This decrease in arterial pressure in females was abolished by AT(2)R blockade. Renal AT(2)R, angiotensin-converting enzyme 2, and left ventricular AT(2)R mRNA gene expressions were markedly greater in females than in males with a renal angiotensin II type 1a receptor:AT(2)R ratio of approximately 1 in females. Angiotensin II infusion did not affect renal AT(2)R mRNA expression but resulted in significantly less left ventricular mRNA expression. Renal angiotensin-converting enzyme 2 mRNA expression levels were greater in females than in males treated with high-dose angiotensin II (approximately 2.5 fold; P for the interaction between sex and treatment <0.05). In females, enhancement of the vasodilatory arm of the renin-angiotensin system, in particular, AT(2)R and angiotensin-converting enzyme 2 mRNA expression, may contribute to the sex-specific differences in response to renin-angiotensin system activation.     

Interaction of angiotensin converting enzyme inhibition and atrial natriuretic factor

The interaction of angiotensin converting enzyme (ACE) inhibition and atrial natriuretic factor (ANF) was investigated in six supine, sodium-replete, normal volunteers who received captopril (10 mg i.v. bolus followed by 10 mg/hr constant infusion) or vehicle superimposed on background 3-hour, constant, low-dose (1.5 pmol/kg/min) infusions of human ANF (99-126). Plasma converting enzyme activity was significantly inhibited but this had no effect on endogenous plasma ANF concentrations. ANF infusions, with or without captopril, caused similar increases in plasma ANF concentrations, and calculated metabolic clearance rates for ANF were unchanged. Similarly, blood pressure, heart rate, renal blood flow, glomerular filtration rate, and renal electrolyte excretion, including ANF-induced natriuresis, were unaffected by captopril. The combination of ANF plus captopril produced a significant increase in plasma aldosterone (79 +/- 8 vs. 60 +/- 6 pmol/l, p less than 0.05), cortisol (406 +/- 52 vs. 265 +/- 29 nmol/l, p less than 0.01), adrenaline (119 +/- 21 vs. 76 +/- 10 pg/ml, p less than 0.05), and noradrenaline (319 +/- 49 vs. 215 +/- 38 pg/ml, p less than 0.05) compared with time-matched placebo data. Converting enzyme inhibition, in the absence of major changes in blood pressure or renal blood flow, has little effect on ANF metabolism or renal bioactivity. However, ACE inhibition and ANF combined may interact to increase activity of the hypothalamo-pituitary-adrenal axis and sympathetic nervous system by unknown mechanisms.     

Renal effects of atrial natriuretic factor during control of the renin-angiotensin system in anesthetized dogs

The contribution of the renin-angiotensin system to the natriuretic responses to intrarenal infusions of 1, 5, 25, and 125 pmol/kg/min synthetic rat atrial natriuretic peptide 101-126 was determined in one-kidney anesthetized dogs. In vehicle-treated dogs, atrial natriuretic peptide 101-126 increased fractional sodium excretion from 1.8 +/- 0.6% to a peak response of 5.1 +/- 0.9% during infusion of 25 pmol/kg/min. The peptide progressively decreased mean arterial pressure from 110 +/- 5 to 94 +/- 4 mm Hg, renal vascular resistance from 0.40 +/- 0.02 to 0.30 +/- 0.02 mm Hg/ml/min, and arterial plasma renin activity from 4.3 +/- 1.6 to 3.1 +/- 0.8 ng/ml/hr. When the renin-angiotensin system was blocked by 3 mg/kg i.v. enalaprilat, baseline pressure fell to 86 +/- 4 mm Hg, and subsequent infusions of atrial natriuretic peptide 101-126 did not affect fractional sodium excretion. The decreases in blood pressure (from 86 +/- 4 to 76 +/- 4 mm Hg) and in renal vascular resistance (from 0.27 +/- 0.03 to 0.23 +/- 0.02 mm Hg/ml/min) were also ameliorated compared with the control responses. Intravenous infusion of 2.5 ng/kg/min angiotensin II restored mean arterial pressure and potentiated the natriuretic and renal vascular responses to atrial natriuretic peptide 101-126. In two additional groups of anesthetized dogs, enalaprilat did not produce the profound hypotension and did not affect the natriuretic responses to atrial natriuretic peptide 101-126. When renal vascular resistance was elevated by intrarenal infusion of angiotensin II in enalaprilat-treated dogs, the natriuretic response was improved.(ABSTRACT TRUNCATED AT 250 WORDS)     

Role of prostaglandins in mediating the renal effects of atrial natriuretic factor

The natriuretic response to the intrarenal administration of atrial natriuretic factor (ANF) is accompanied by an increase in the synthesis of prostaglandins and by a redistribution of renal blood flow from the superficial to the deep cortex. This study was undertaken to define whether prostaglandins mediate the ANF-induced redistribution of renal blood flow and if prostaglandins and renal blood flow redistribution contribute to the natriuretic actions of ANF. In anesthetized dogs, the intrarenal administration of indomethacin (10 micrograms/kg/min) or the intravenous administration of meclofenamate (5 mg/kg) completely prevented the sixfold and twofold increments in urinary prostaglandin E2 and 6-keto-prostaglandin F1 alpha excretion, respectively; it also abolished the redistribution of renal blood flow to the deep cortex. However, ANF induced a similar natriuresis before (from 53 +/- 17 to 281 +/- 48 microEq/min) and after (from 45 +/- 13 to 273 +/- 60 microEq/min) the administration of prostaglandin synthesis inhibitors. It is concluded that the ANF-induced redistribution of renal blood flow to the deep cortex is prostaglandin-mediated but that neither redistribution nor increased prostaglandin synthesis is an important mediator of ANF's natriuretic action.     

Effect of intrarenal angiotensin II blockade on renal function in conscious dogs.

The effects of intrarenal infusion of 1-sar-8-ala angiotension II (P 113) and a converting enzyme inhibitor, SQ 20881, at doses that did not affect systemic blood pressure (2.0 mug/kg per min) were studied in conscious, uninephrectomized dogs. In dogs receiving approximately equal to 5 mEq/day of sodium, intrarenal infusion of P 113 increased renal blood flow (RBF) from 219.8 +/- 32.3 to 282.7 +/- 20.0 ml/min (P less than 0.004), and with intrarenal SQ 20881 infusion from 215.3 +/- 14.2 to 278.0 +/- 22.2 ml/min (p less than 0.005). In sodium-restricted dogs (approximately equal to 5 mEq/day), glomerular filtration rate (GFR) also increased with intrarenal P 113 infusion from 57.9 +/- 5.7 to 66.3 +/- 6.6 ml/min (P less than 0.05), and with SQ 20881 infusion from 43.1 +/- 2.1 to 55.7 +/- 4.5 ml/min (P less than 0.01). Dogs on approximately equal to 5 mEq/day of sodium showed significant increases in plasma renin activity (PRA) with intrarenal infusion of the peptides, unmasking a negative feedback inhibition of renin release by angiotensin II. No increases in RBF, GFR, or PRA were seen with infusion without inhibitors, or in dogs give P 113 or SQ 20881 while on approximately equal to 80 mEq/day of sodium. In addition, angiotensin II inhibition increased sodium excretion during sodium restriction. These findings suggest that intrarenal angiotensin II is intimately involved in renal responses to sodium restriction which result in conservation of sodium and water.     

Impaired intrarenal dopamine production following intravenous sodium chloride infusion in Type 1 (insulin-dependent) diabetes mellitus

Summary Type 1 (insulin-dependent) diabetes mellitus is characterized by impaired sodium excretion following NaCl infusion. To investigate the possible role of dopamine in the impaired natriuresis in diabetes, intrarenal sodium handling, sodium excretion and urinary dopamine output, reflecting intrarenal dopamine formation, were studied following a 2 h 0.9% NaCl infusion (25 ml/kg) in eight diabetic patients and nine control subjects. The increase in sodium excretion in response to NaCl infusion was significantly (p<0.01) reduced in diabetic patients (19±7%) as compared with control subjects (46±8%). Fractional proximal tubular sodium reabsorption (determined by lithium clearance) decreased in the control group (p<0.001) following NaCl infusion but not in the diabetic group. Fractional distal tubular reabsorption decreased similarly in both groups. In response to NaCl urinary dopamine excretion increased by approximately 15% (p<0.01) in the control group but did not change in the diabetic group. The mean urinary dopamine excretion above basal was significantly greater in the control group (8.4±2.1 nmol/h) than in the diabetic group (–2.2±2.1 nmol/h; p<0.01). The urinary sodium/dopamine excretion ratio did not differ significantly between the two groups in the basal state or following NaCl. Baseline plasma levels of atrial natriuretic peptide did not differ between control and diabetic patients. In the control group atrial natriuretic peptide levels increased significantly (p<0.01) in response to NaCl whereas atrial natriuretic peptide levels did not change in the diabetic group. The results of this study show that patients with Type 1 diabetes have a blunted natriuresis in response to isotonic NaCl. This abnormality seems mainly to be due to impaired inhibition of proximal tubular sodium reabsorption, which may be the result of defective intrarenal dopamine mobilisation.     

Role of SRC family kinase in extracellular renal cyclic guanosine 3',5'-monophosphate- and pressure-induced natriuresis

cGMP functions as an extracellular (paracrine) messenger acting at the renal proximal tubule and is an important modulator of pressure-natriuresis (P-N). The signaling pathway activated by cGMP in the tubule cell basolateral membrane remains unknown. We hypothesized that renal interstitial microinfusion of cGMP (50 nmol/kg per minute) or P-N would be accompanied by increased renal protein levels of phospho-Src (Tyr 416) and that the natriuresis would be decreased by Src inhibition. Renal interstitial cGMP-induced natriuresis was blocked by Src inhibitor PP2 (2.0±0.4 versus 0.5±0.01 μEq/g per minute; P<0.001). The inactive analog of PP2, PP3, had no effect on cGMP-induced natriuresis. SU6656, another Src inhibitor, also inhibited cGMP-induced natriuresis (2.0±0.4 versus 1.02±0.01 μEq/g per minute; P<0.001). Renal interstitial cGMP infusion increased phospho-Src protein levels 5.6-fold at 15 minutes and 6.8-fold at 30 minutes compared with vehicle infusion but returned toward basal levels after 60 minutes. PP2 also blunted P-N (3.1±0.1 versus 1.1±0.3 μEq/g per minute; P<0.01) despite a similar increase in blood pressure. PP3 had no effect on P-N. Phospho-Src protein levels increased during P-N in vehicle- (1.8-fold) and PP3-treated (2.1-fold) groups compared with the sham-operated group. PP2 blocked the pressure-induced increase in renal phospho-Src protein levels. PP2 had no effect on renal hemodynamics but decreased both fractional excretion of Na(+) and lithium. Both extracellular cGMP and increased renal perfusion pressure increased renal phospho-Src protein levels and induced natriuresis in an Src-dependent manner, demonstrating that Src is an important downstream signaling molecule for extracellular cGMP-induced natriuresis.     

Enhanced antinatriuresis in response to angiotensin II in essential hypertension

Angiotensin II regulates sodium homeostasis by modulating aldosterone secretion, renal vascular response, and tubular sodium reabsorption. We hypothesized that the antinatriuretic response to angiotensin II is enhanced in human essential hypertension. We therefore studied 48 white men with essential hypertension (defined by ambulatory blood pressure measurement) and 72 normotensive white control persons, and measured mean arterial pressure, sodium excretion, renal plasma flow, glomerular filtration rate, and aldosterone secretion in response to angiotensin II infusion (0.5 and 3.0 ng/kg/min). Hypertensive subjects exhibited a greater increase of mean arterial pressure (16.7+/-8.2 mm Hg v 13.4+/-7.1 mm Hg in normotensives, P < .05) and a greater decrease of renal plasma flow (-151.5+/-73.9 mL/ min v -112.6+/-68.0 mL/min in controls, P < .01) when 3.0 ng/kg/min angiotensin II was infused. The increase of glomerular filtration rate and serum aldosterone concentration was similar in both groups. Sodium excretion in response to 3.0 ng/kg/min angiotensin II was diminished in both groups (P < .01). However, the decrease in sodium excretion was more pronounced in hypertensives than in normotensives (-0.18+/-0.2 mmol/min v -0.09+/-0.2 mmol/min, P < .05), even if baseline mean arterial pressure and body mass index were taken into account (P < .05). We conclude that increased sodium retention in response to angiotensin II exists in subjects with essential hypertension, which is unrelated to changes in glomerular filtration rate and aldosterone concentration. Our data suggest a hyperresponsiveness to angiotensin II in essential hypertension that could lead to increased sodium retention.     

Hypotensive effect of chronic intrarenal infusion of acetylcholine during angiotensin hypertension

We examined the role of the pressure natriuresis phenomenon in long-term arterial pressure control. Uninephrectomized dogs were housed in metabolic cages and made hypertensive with a continuous background intravenous infusion of angiotensin II (AngII, 12 ng/kg/min). To increase the ability of the kidney to excrete salt and water, we infused acetylcholine (ACH, 2.0 micrograms/kg/min), a potent natriuretic agen, directly into the renal artery. In four dogs, ACH decreased mean arterial pressure (MAP) from 144 +/- 5 mm Hg to 113 +/- 3 mm Hg. Sodium excretion increased by about 60% on the first day of infusion and then returned rapidly toward the control value. On cessation of the ACH infusion, there was a transient but marked sodium retention, and the hypertension returned. A control infusion of ACH intravenously rather than into the renal artery in the same four dogs did not affect MAP or sodium excretion during AngII hypertension.     

Exogenous L-arginine ameliorates angiotensin II-induced hypertension and renal damage in rats

Experiments were performed to determine whether exogenous L-arginine could ameliorate angiotensin II-induced hypertension and renal damage. Rats were instrumented with chronic indwelling femoral venous and arterial catheters for infusions of drugs and measurement of conscious arterial pressure. Arterial blood pressure significantly increased from 124+/-1 to 199+/-4 mm Hg, after 9 days of continuous infusion of angiotensin II (20 ng/kg per minute; IV; n=6 to 9). In contrast, the increase in arterial pressure after 9 days of angiotensin II infusion was significantly blunted by 45% (P=0.0003) in rats coadministered L-arginine (300 microg/kg per minute; IV; n=7 to 9). The glomerular injury index was significantly greater in rats administered angiotensin II in comparison with rats administered saline vehicle (P<0.001). Coinfusion of L-arginine significantly increased plasma nitrate/nitrite concentrations (P<0.001) and completely prevented angiotensin II-induced glomerular damage (P<0.001). Angiotensin II infusion alone and combined angiotensin II plus L-arginine infusion significantly increased urinary albumin excretion. Albuminuria in rats administered angiotensin II plus L-arginine is likely to be because of increased intraglomerular pressure. Our experiments demonstrate that L-arginine can blunt angiotensin II-induced hypertension and associated renal damage. This latter observation is most exciting because it indicates that increasing NO bioavailability, in addition to lowering arterial pressure, can greatly reduce hypertension-induced renal damage.