Endogenous angiotensin concentrations in specific intrarenal fluid compartments of the rat

Modulation by locally produced luminal angiotensin II of proximal tubular sodium reabsorption via an AT1 receptor

The concentration of angiotensin II reported in proximal tubular fluid in anaesthetized rats is considerably higher than in plasma, indicating secretion of this peptide into the tubular lumen. Shrinking split-drop micropuncture was used to examine the effect of endogenous angiotensin on sodium and water absorption in the proximal convoluted tubule. Addition of losartan, a nonpeptide AT1 receptor blocker, to intratubular fluid increased fluid uptake by 15.7 +/- 3.9% (10(-5) M) and 24.7 +/- 9.4% (10(-4) M) whereas the AT2 inhibitor, PD123319 had no effect. We conclude that angiotensin II is secreted into proximal tubular fluid and, in the anaesthetized rat, is maintained at a concentration that inhibits sodium and water transport via AT1 receptors.

Angiotensin II type 2 receptor subtype mediates phospholipase A2-dependent signaling in rabbit proximal tubular epithelial cells

We investigated the ability of angiotensin II (Ang II) or the stable analogue [Sar1]-Ang II to increase intracellular and extracellular free arachidonic acid in primary cultures of rabbit proximal tubular epithelial cells to better characterize the receptor subtype and orientation of phospholipase A2 (PLA2)-mediated signaling. Proximal tubular cells were labeled with [3H]arachidonic acid for 4 hours and then treated with Ang II or [Sar1]-Ang II. Lipids were extracted from labeled cells, separated by thin-layer chromatography, and quantified by liquid scintillation counting. Ang II (10 mumol/L, 1 minute) stimulated an increase in intracellular free [3H]arachidonic acid from 21.0 +/- 2.0 to 32.2 +/- 2.8 disintegrations per minute/microgram protein, an effect that was potentiated by EGTA. [Sar1]-Ang II stimulated a time- and concentration-dependent increase in [3H]arachidonic acid release from labeled cells. Release of [3H]arachidonic acid was maximal at 10 mumol/L [Sar1]-Ang II, with an EC50 of approximately 3 mumol/L. Ang II receptor antagonists caused concentration-dependent inhibition of [Sar1]-Ang II-stimulated [3H]arachidonic acid release with the following order of potency: CGP 42112 = PD 123319 > losartan. Furthermore, in proximal tubular epithelial cells grown on polyester membrane filters, the Ang II receptor that mediated arachidonic acid release was predominantly apical rather than basolateral. These observations are consistent with activation of a Ca(2+)-independent, apical PLA2 isoform in epithelial cells through an Ang II type 2 receptor subtype.

beta-Adrenoceptors and dexamethasone synergistically stimulate the expression of the angiotensinogen gene in opossum kidney cells

We transiently co-transfected opossom kidney (OK) cells with the plasmid containing the cDNA for beta 1-adrenoceptor (pBC-beta 1 AR) or beta 2-adrenoceptor (pBC-beta 2 AR) and a fusion gene with the 5'-flanking region of the angiotensinogen (ANG) gene linked to a bacterial chloramphenicol acetyl transferase (CAT) coding sequence as a reporter, pOCAT (ANG N-1498/ +18). Co-transfection of plasmid pBC-beta 1 AR or pBC-beta 2 AR alone enhanced the expression of pOCAT (ANG N-1498/+18). The addition of isoproterenol further stimulated the expression of pOCAT (ANG N-1498/ +18) when co-transfected with pBC-beta 1AR, but not with pBC-beta 2AR. Moreover, the addition of a combination of dexamethasone and isoproterenol synergistically stimulated the expression of pOCAT (ANG N-1498/+18) when co-transfected with pBC-beta 1AR, but not when cotransfected with pBC-beta 2AR. The synergistic effect of dexamethasone and isoproterenol was inhibited by the presence of RU 486 (an antagonist of glucocorticoid) or Rp-cAMP (an inhibitor of cAMP-dependent protein kinase A I and II). To localize the putative cAMP-responsive element (CRE) and glucocorticoid responsive element (GRE) in the ANG gene, we constructed the fusion gene by inserting the DNA fragment, ANG N-806 to N-465 upstream of the thymidine kinase (TK) promoter fused to a CAT gene and introduced them with pBC-beta 1AR into OK cells. The addition of dexamethasone or isoproterenol alone stimulated the expression of pTKCAT (ANG N-806/-465). The addition of isoproterenol and dexamethasone synergistically stimulated the transcriptional activity of pTKCAT (N-806/-465). These studies demonstrate that the beta 1-adrenoceptor and dexamethasone act synergistically to stimulate the expression of the ANG gene in OK cells via the putative CRE and GREs in the 5'-flanking region of the rat ANG gene. These data should aid in the understanding of the molecular mechanism(s) of the stimulatory effect of catecholamines/glucocorticoid induced expression of the ANG gene in the kidney.

Dopamine decreases expression of type-1 angiotensin II receptors in renal proximal tubule

Systemic and/or locally produced angiotensin II stimulates salt and water reabsorption in the renal proximal tubule. In vivo, dopamine (DA) may serve as a counterregulatory hormone to angiotensin II's acute actions on the proximal tubule. We examined whether dopamine modulates AT1 receptor expression in cultured proximal tubule cells (RPTC) expressing DA1 receptors. Dopamine decreased basal RPTC AT1 receptor mRNA levels by 67 +/- 7% (n = 10; P < 0.005) and decreased 125I-angiotensin II binding by 41 +/- 7% (n = 4; P < 0.05). The DA1-specific agonist, SKF38393 decreased basal AT1 receptor mRNA levels (65 +/- 5% inhibition; n = 5; P < 0.025), and the DA1-specific antagonist, SCH23390 reversed dopamine's inhibition of AT1 receptor mRNA expression (24 +/- 10% inhibition; n = 8; NS) and angiotensin II binding (5 +/- 15%; n = 4; NS). DA2-specific antagonists were ineffective. In rats given L-DOPA in the drinking water for 5 d, there were decreases in both proximal tubule AT1 receptor mRNA expression (80 +/- 5%; n = 6; P < 0.005) and specific [125I] Ang II binding (control: 0.74 +/- 0.13 fmol/mg pro vs. 0.40 +/- 0.63 fmol/mg pro; n = 5; P < 0.05). In summary, dopamine, acting through DA1 receptors, decreased AT1 receptor expression in proximal tubule, an effect likely mediated by increased intracellular cAMP levels. Local dopamine production also led to decreased AT1 receptor expression, suggesting dopamine may reset sensitivity of the proximal tubule to angiotensin II.

Endogenous production of angiotensin II modulates rat proximal tubule transport

There is evidence that angiotensin II is synthesized by the proximal tubule and secreted into the tubular lumen. This study examined the functional significance of endogenously produced angiotensin II on proximal tubule transport in male Sprague-Dawley rats. Addition of 10(-11), 10(-8), and 10(-6) M angiotensin II to the lumen of proximal convoluted tubules perfused in vivo had no effect on the rate of fluid reabsorption. The absence of an effect of exogenous luminal angiotensin II could be due to its endogenous production and luminal secretion. Luminal 10(-8) M Dup 753 (an angiotensin II receptor antagonist) resulted in a 35% decrease in proximal tubule fluid reabsorption when compared to control (Jv = 1.64 +/- 0.12 nl/mm.min vs. 2.55 +/- 0.32 nl/mm.min, P < 0.05). Similarly, luminal 10(-4) M enalaprilat, an angiotensin converting enzyme inhibitor, decreased fluid reabsorption by 40% (Jv = 1.53 +/- 0.23 nl/mm.min vs. 2.55 +/- 0.32 nl/mm.min, P < 0.05). When 10(-11) or 10(-8) M exogenous angiotensin II was added to enalaprilat (10(-4) M) in the luminal perfusate, fluid reabsorption returned to its baseline rate (Jv = 2.78 +/- 0.35 nl/mm.min). Thus, addition of exogenous angiotensin II stimulates proximal tubule transport when endogenous production is inhibited. These experiments show that endogenously produced angiotensin II modulates fluid transport in the proximal tubule independent of systemic angiotensin II.

Renin regulation in cultured proximal tubular cells

Recent studies have documented the presence of a complete renin-angiotensin system in the proximal tubule of the kidney: however, little is known about the regulation of renin in this proximal tubular system. Therefore, we performed the present studies to learn whether the behavior of the renin system in cultured proximal tubule is similar to that of the juxtaglomerular renin system. Basal renin secretion from rabbit proximal tubular cells in primary culture was low and not affected by isoproterenol (10(-5) mol/L), diltiazem (10(-5) mol/L), or a zero-calcium bath (O nmol/L). Only the calcium ionophore A23187 (10(-4) mol/L) significantly reduced renin secretion in these cells (from 2.44 +/- 0.37 to 1.14 +/- O.08 ng angiotensin I/mg protein per hour, P<.05). When the proximal tubular cells were lysed so the effects of the test agents on intracellular renin content could be assessed, isoproterenol caused a significant twofold (107 percent) increase (from 2.02 +/- 0.56 to 4.18 +/- 0.81 ng angiotensin I/mg protein per hour, P<.05), whereas diltiazem, A23187, and zero- and high-calcium baths did not produce a significant change. The effects of these agents on renin mRNA were examined in rabbit and rat proximal tubular cells in primary culture with the use of an S1 nuclease protection assay. Densitometry analysis of renin mRNA and either GAPDH mRNA (rat) or alpha-actin (rabbit) showed no significant alterations in renin mRNA abundance. In summary, these results confirm the presence of renin mRNA in cultured proximal tubular cells and suggest that a low-level, constitutive secretion of renin occurs in this system that is decreased by A23187. Moreover, the results also suggest that proximal tubular renin is regulated, albeit differently from the juxtaglomerular renin system. Finally, short-term increments in proximal tubular renin occur without a change in renin mRNA.

Angiotensin II is mitogenic for cultured rat glomerular endothelial cells

Angiotensin II (Ang II) has growth-stimulatory properties on different renal cell types. However, possible growth effects of this vasoactive peptide on endothelial cells isolated from the glomerular microvasculature have not been formally investigated. Therefore, we isolated and characterized primary cultures of rat glomerular endothelial cells. We used a simple technique in which collagenase-treated glomeruli were sparsely plated in several 96-well culture plates and microscopically screened for cobblestone-like outgrowth. After two limiting dilutions, homogeneous cultures were obtained. Cells were characterized by positive staining for the endothelial markers factor VIII, CD 31, endothelial leukocyte adhesion molecule-1, and the lectin Bandeiraea simplificifolia. Ang II stimulated the synthesis and release of endothelin-1 in culture supernatants. Moreover, in contrast to syngeneic mesangial cells, glomerular endothelial cells expressed angiotensin-converting enzyme. Ang II stimulated a mild but significant proliferation of quiescent cells, as measured by [3H]thymidine incorporation and direct cell counting. This mitogenesis was transduced by losartan-blockade angiotensin type 1 receptors. Moreover, Ang II mediated phosphorylation of mitogen-activated protein kinase 2 and induction of transcripts for the immediate early gene Egr-1. Our results indicate that Ang II is a moderate mitogen for primary cultures of rat glomerular endothelial cells and activation of these metabolically active cells may play a role in the pathophysiology of several types of glomerulonephritis. Moreover, remodeling of glomerular endothelial cells by Ang II may be important in the progression of structural renal damage during the course of hypertensive injury.

Role of angiotensin II in dietary modulation of rat late distal tubule bicarbonate flux in vivo

We have reported that overnight fasting stimulates bicarbonate reabsorption (JtCo2) in rat distal tubules. The present in vivo microperfusion studies evaluated the hypothesis that endogenous angiotensin II (AII) mediates this response. Rat late distal (LD) tubules were perfused at 8 nl/min in vivo with a hypotonic solution containing 28 mM bicarbonate. In overnight-fasted rats, LD JtCO2 was significantly higher than in normally fed rats (50 +/- 4 vs. 16 +/- 6 pmol/min.mm, P < 0.05). When overnight-fasted rats were salt-loaded, JtCO2 fell significantly (38 +/- 3 pmol/min.mm, P < 0.05). Conversely, in fed rats ingesting a zero-salt diet, JtCO2 increased three-fold (45 +/- 5 pmol/min.mm, P < 0.05). Enalaprilat infusion (0.25 micrograms/kg body wt, intravenously), in these zero-salt and overnight-fasted rats, reduced LD JtCO2 values to normal. Further, infusion of losartan (5 mg/kg body wt, intravenously), the specific AII AT1 receptor blocker, reduced JtCO2 in overnight-fasted rats by two-thirds (16 +/- 4 pmol/min.mm, P < 0.05). Finally, we perfused 10(-11) M AII intraluminally with and without 10(-6) M losartan: AII increased JtCO2 to 45 +/- 6 pmol/min.mm, equal to the zero-salt flux. This was completely abrogated by simultaneous losartan perfusion. Therefore, these results suggest that AII is an in vivo stimulator of late distal tubule bicarbonate reabsorption.

A re-evaluation of the determinants of glomerular filtration rate

Several factors are potentially able to change the glomerular filtration rate (GFR) and thereby participate in its regulation, but only a few factors seem to be physiologically important. The variable nature of proximal tubular pressure should be recognized as important in the regulation of GFR. It is argued that a distinction should be made between the terms 'autoregulation of GFR' and 'regulation of GFR'. The tubuloglomerular feedback mechanism (TGF) is an important factor for autoregulatory control of GFR. When perturbations result in major increases in tubular flow, the TGF saturates. Proximal tubular pressure then increases and becomes the major factor responsible for the stabilization of GFR. Changes in the proximal reabsorption rate (APR) are important for long-term variations in GFR (regulation of GFR). Small changes in the APR cause near parallel changes in the GFR mainly through the TGF mechanism, while larger changes in the APR cause near parallel changes in the GFR mainly because of the effect on tubular pressure. The hydraulic resistance in the distal nephron segments is an additional factor in regulating GFR, through its effect on proximal tubular pressure. The stimulus to the TGF mechanism also depresses renin release. The resulting local angiotensin II concentration has effects both on the arteriolar resistances and on the APR. The renin-angiotensin system and TGF are therefore considered to be integrated parts of a common control system regulating GFR. According to the hypothesis advocated here, TGF-mediated changes in afferent arteriolar resistance and angiotensin-mediated changes in efferent arteriolar resistance and APR cooperate in counteracting perturbations in proximal tubular pressure and Henle loop flow. However, because of the biphasic proximal effect of angiotensin II, a major unresolved question is whether physiological increases in endogenous local angiotensin II concentrations stimulate or inhibit proximal reabsorption.

Nitric oxide antagonizes the actions of angiotensin II to enhance tubuloglomerular feedback responsiveness

The present study was designed to investigate whether nitric oxide (NO) antagonizes angiotensin II (Ang II) in modulating the tubuloglomerular feedback (TGF) system. Maximum TGF responses were assessed by evaluating stop-flow pressure (SFP) responses to late proximal perfusion with artificial tubular fluid (40 nl/min). Peritubular capillary (PTC) infusion of 10(-3) M NG-L-arginine (NLA) at a rate of 20 nl/min, and infusion of 10(-7) and 10(-6) M Ang II at rates that did not decrease SFP under conditions of zero flow to the macula densa (resting SFP), augmented maximum SFP feedback responses to 12.0 +/- 1.7, 12.1 +/- 2.4 and 16.9 +/- 3.0 mm Hg, respectively (all P < 0.01 vs. control response). Combined PTC infusion of NLA and 10(-7) M Ang II at a rate of 20 nl/min resulted in decreases in resting SFP in 7 of the 12 nephrons studied. When the infusion rate was decreased to 15 +/- 3 nl/min, concomitant PTC infusion of NLA and 10(-7) M Ang II was associated with a tremendous increase in maximum TGF responses (23.8 +/- 3.9 mm Hg; P < 0.01 vs. responses during PTC NLA or Ang II) in the absence of a decrease in resting SFP. During AT1 receptor blockade using losartan, SFP feedback responses were attenuated to 1.6 +/- 0.6 mm Hg and PTC infusion of NLA only augmented TGF responses to 3.8 +/- 1.0 mm Hg. These results strongly suggest that local NO antagonizes Ang II with respect to the regulation of TGF responsiveness. Disruption of this balance by NO synthesis inhibition strongly potentiates TGF-independent and TGF-dependent actions of Ang II on the preglomerular vasculature.

Renal circulation and blockade of the renin-angiotensin system. Is angiotensin-converting enzyme inhibition the last word?

The mechanism by which angiotensin-converting enzyme (ACE) inhibition influences renal perfusion and function has assumed growing importance as alternatives for blocking the system have emerged. Neither renin inhibitors nor angiotensin II (Ang II) antagonists are likely to trigger responses similar to ACE inhibitor-induced involvement of kinins, prostaglandins, or nitric oxide. Several observations suggest species variation in the contribution of these pathways to the renal response to ACE inhibition. In humans, recent investigation suggests that virtually all of the renal response is due to a fall in Ang II formation. Perhaps most persuasive is the surprising observation that the renal hemodynamic response to renin inhibitors exceeds by more than 50% the response to ACE inhibition in healthy humans. To the extent that kinins or prostaglandins contribute to the renal response to ACE inhibition, one would anticipate a smaller response to renin inhibition. Possible explanations include an unanticipated additional action of renin inhibitors, better tissue penetration of these highly lipophilic agents, or more effective blockade of Ang II formation through an action at the rate-limiting step or non-ACE-dependent Ang II generation. Substantial evidence favors the latter two possibilities. Whatever the explanation, these observations raise the intriguing possibility that the undoubted therapeutic efficacy of ACE inhibition in renal injury, documented most rigorously for type I diabetes mellitus, might be exceeded with the newer classes of agent.

Developmental consequences of the renin-angiotensin system

Molecular, cellular, and physiological studies indicate that the renin-angiotensin system (RAS) is highly expressed during early kidney development. We propose that a major function of the RAS during early embryonic development is the modulation of growth processes that lead the primitive kidney into a properly differentiated and architecturally organized organ suited for independent extrauterine life. As development progresses, the RAS acquires new and overlapping functions such as the endocrine and paracrine regulation of blood pressure and renal hemodynamics. Disease states in adult mammals often result in expression of RAS genes and phenotypic changes resembling the embryonic pattern, emphasizing the importance of undertaking developmental studies. Because of their importance in health and disease, the immediate challenge is to identify the mechanisms that regulate the unique development of the RAS and its role(s) in normal and abnormal growth processes.

In situ hybridization and immunohistochemistry of renal angiotensinogen in neonatal and adult rat kidneys

Recent evidence suggests that a local renin-angiotensin system is operational in the kidney and that it mediates some of the actions of angiotensin II on renal tubules. In this study the ontogeny and renal distribution of the unique precursor to angiotensin II formation, angiotensinogen, was investigated in rats by use of immunohistochemistry, immuno-electron microscopy and non-isotopic hybridization histochemistry. At the light-microscopic level, intense staining for angiotensinogen was found in the proximal convoluted tubules of the cortex, with lighter staining in the straight proximal tubules of the outer stripe. The strongest immunostaining was found in the kidneys of neonatal rats, where glomerular mesangial cells and medullary vascular bundles were also immunopositive. The angiotensinogen content of the kidneys in late gestation embryos and neonates showed the presence of angiotensinogen by day E18 and a peak content in the neonate. Non-isotopic hybridization histochemistry with biotinylated oligodeoxynucleotide probes confirmed the presence of angiotensinogen mRNA expression in the proximal convoluted tubules of the renal cortex. Electron-microscopic immunohistochemistry showed staining of relatively few electron-dense structures close to the apical membrane of proximal convoluted tubule cells in the adult kidney. In the neonatal rat kidney, angiotensinogen immunostaining at the electron-microscopic level was found throughout the proximal tubule cells and was markedly stronger than that seen in adult kidney. The presence of angiotensinogen, from embryonic day 18, in the proximal tubules, mesangial cells and vasculature of the kidney suggests multiple potential sites of intrarenal angiotensin II generation with an ontogeny in late gestation.

Renal pharmacology of GR138950, a novel non-peptide angiotensin AT1 receptor antagonist

This paper describes the renal pharmacology of the novel, specific, non-peptide angiotensin AT1 receptor antagonist, GR138950 (1-[[3-bromo-2-[2-[[(trifluoromethyl) sulphonyl] amino] phenyl]-5-benzofuranyl] methyl]-4-cyclopropyl-2-ethyl-1H-imidazole-5- carboxamide). When administered to anaesthetised salt-replete dogs, GR138950 caused renal vasodilatation and significant increases in sodium and urine excretion. No change in glomerular filtration rate was observed indicating that the natriuresis was a consequence of inhibition of tubular sodium reabsorption. Qualitatively similar but less marked changes in renal function were observed in response to the angiotensin converting enzyme inhibitor, captopril, although in contrast to GR138950, captopril also caused a small but significant fall in mean blood pressure. Intra-renal artery infusion of exogenous angiotensin II resulted in dose-related renal vasoconstriction and decreases in urine excretion, sodium excretion, fractional excretion of sodium and glomerular filtration rate. These renal effects of angiotensin II were all markedly antagonised by GR138950. We conclude that GR138950 is an effective antagonist of the renal haemodynamic and excretory actions of endogenous and exogenous angiotensin II.

Alpha-adrenoceptors and angiotensinogen gene expression in opossum kidney cells

To investigate whether alpha (alpha)-adrenoceptor agonists have a stimulatory effect on the expression of the angiotensinogen (Ang) gene in opossum kidney (OK) cells, we used OK 27 cells with a fusion gene containing the 5'-flanking regulatory sequence of the rat angiotensinogen gene fused with a human growth hormone (hGH) gene as a reporter, pOGH (Ang N-1498/+18), permanently integrated into their genomes. The level of expression of the pOGH (Ang N-1498/+18) was quantitated by the amount of immunoreactive-human growth hormone (IR-hGH) secreted into the medium. The addition of iodoclonidine (alpha 2-adrenoceptor agonist, 10(-13) to 10(-9) M) and phorbol 12-myristate 13-acetate (PMA, 10(-13) to 10(-5) M) stimulated the expression of pOGH (Ang N-1498/+18) in a dose-dependent manner, whereas the addition of phenylephrine (alpha 1-adrenoceptor agonist, 10(-13) to 10(-5) M) had no effect. The stimulatory effect of iodoclonidine was blocked by the presence of yohimbine (alpha 2-adrenoceptor antagonist) and staurosporine (an inhibitor of protein kinase C) but not blocked by the presence of prazosin (alpha 1-adrenoceptor antagonist) or Rp-cAMP (an inhibitor of cAMP-dependent protein kinase A). The addition of iodoclonidine, phenylephrine or PMA had no effect on the expression of pTKGH in OK 13 cells, an OK cell line, into which had been stably integrated a fusion gene, pTKGH containing the promoter/enhancer DNA sequence of the viral thymidine-kinase (TK) gene fused with a human growth hormone gene as a reporter.(ABSTRACT TRUNCATED AT 250 WORDS)

Parathyroidectomy does not prevent the renal PTH/PTHrP receptor down-regulation in uremic rats

In a recent study we demonstrated that the PTH/PTHrP receptor (PTH-R) mRNA was markedly down-regulated in the remnant kidney of uremic rats with severe secondary hyperparathyroidism. Among the factors potentially implicated in this down-regulation, to date only PTH has been demonstrated to modulate PTH-R expression. Here, we examined the effect of thyroparathyroidectomy (TPTX) on the renal expression of PTH-R in rats with normal renal function or with chronic renal failure (CRF) induced by 5/6 nephrectomy. Four groups of rats were studied: control, TPTX, CRF, and CRF + TPTX. Moderate-degree renal failure was documented by mean (+/- SD) creatinine clearances (microliter/min/100 g body wt) of 259 +/- 40 and 212 +/- 45 in CRF and CRF + TPTX rats, compared with 646 +/- 123 and 511 +/- 156 in control and TPTX rats, respectively. Plasma phosphorus, calcitriol, and ionized calcium were significantly lower in CRF and CRF + TPTX than in control animals. Plasma ionized calcium and calcitriol were also lower in TPTX than in control rats. Plasma PTH levels (pg/ml) were increased in CRF rats (41.8 +/- 29.4), and markedly decreased in TPTX (10.1 +/- 7.8) and CRF + TPTX (8.0 +/- 3.8) rats compared with control rats (21.7 +/- 7.5). Northern blot analysis showed that the level of the steady-state PTH-R mRNA in the kidney of CRF and CRF + TPTX rats was markedly decreased compared with that of control rats, the ratios of PTH-R mRNA/beta-actin mRNA being 0.28 +/- 0.04 and 0.27 +/- 0.03 versus 0.54 +/- 0.05, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Intrarenal angiotensin II formation in humans. Evidence from renin inhibition

The intrarenal production of angiotensin II (Ang II) as a local hormone, suggested by multiple lines of investigation, has been difficult to buttress with evidence of functional significance in humans. During studies designed to assess the renal vascular responses to the renin inhibitor enalkiren, an agent (like others in its class) with great substrate specificity, we noted in some subjects that the time course of the effect of enalkiren on renal plasma flow was not congruent with the time course of its influence on the renin-angiotensin system in the plasma compartment. We pursued this discrepancy in the current study of 18 healthy men and 9 men with essential hypertension, who each received one or more doses of enalkiren while on a fixed sodium diet. Plasma enalkiren and Ang II concentration and renal plasma flow were measured in each subject at intervals during and after discontinuation of the enalkiren infusion. Plasma enalkiren concentration fell progressively in each subject after administration was discontinued, the fall becoming evident 10 minutes after discontinuation without exception. In plasma samples obtained 90 minutes after the end of the infusion, drug levels were generally less than half of their peak value. Plasma Ang II concentration, at nadir levels by the end of the enalkiren administration, rose consistently during recovery. Renal plasma flow, in contrast, rose during infusion but did not begin to fall when enalkiren was discontinued.(ABSTRACT TRUNCATED AT 250 WORDS)

Angiotensin II upregulates type-1 angiotensin II receptors in renal proximal tubule

Angiotensin II (Ang II) is an important regulator of proximal tubule salt and water reabsorption. Recent studies indicate that rabbit proximal tubule angiotensin II receptors are the type-1 (AT1R) subtype. We studied the effect of Ang II on proximal tubule receptor expression. Rabbits were treated with either angiotensin converting enzyme inhibitors or a low salt diet to modulate endogenous Ang II levels. In captopril-treated rabbits, liver and glomerular AT1R mRNA levels increased 242 +/- 125 and 141 +/- 60%, respectively (n = 6-7; P < 0.05), as determined by quantitative PCR. In contrast, proximal tubule AT1R mRNA levels decreased 40 +/- 11% (n = 6; P < 0.05). Binding of 125I Ang II to renal cortical basolateral membranes of captopril-treated rabbits decreased from 2.9 +/- 0.55 to 1.4 +/- 0.17 fmol/mg protein (n = 6; P < 0.025). In rabbits fed a sodium chloride-deficient diet for 4 wk, AT1R mRNA levels decreased 52 +/- 11% in liver and 43 +/- 7% in glomeruli (n = 4-5; P < 0.05), whereas they increased 141 +/- 85% (n = 5; P < 0.05) in proximal tubule. In basolateral membranes from rabbits on the sodium chloride-deficient diet, specific binding of 125I Ang II increased from 2.1 +/- 0.2 to 4.3 +/- 1.1 fmol/mg protein (n = 7; P < 0.05). To determine whether Ang II directly regulates expression of proximal tubule AT1 receptors, further studies were performed in cultured proximal tubule cells grown from microdissected S1 segments of rabbit proximal tubules and immortalized by transfection with a replication-defective SV40 vector. Incubation of these cells with Ang II (10(-11) to 10(-7) M) led to concentration-dependent increases in both AT1R mRNA levels and specific 125I Ang II binding. Pretreatment with pertussis toxin inhibited Ang II stimulation of AT1R mRNA. AT1R mRNA expression was decreased by either forskolin or a nonhydrolyzable cAMP analogue (dibutryl cAMP). Simultaneous Ang II administration overcame the inhibitory effect of forskolin but not dibutryl cAMP. These results indicate that proximal tubule AT1R expression is regulated by ambient Ang II levels, and Ang II increases AT1R mRNA at least in part by decreasing proximal tubule cAMP generation through a pertussis toxin-sensitive mechanism. Upregulation of proximal tubule AT1R by Ang II may be important in mediating enhanced proximal tubule sodium reabsorption in states of elevated systemic or intrarenal Ang II.

Expression of angiotensin-converting enzyme in renovascular hypertensive rat kidney

We hypothesized that the gene expression of angiotensinogen, angiotensin-converting enzyme, and angiotensin II type 1 receptor, in addition to renin, is increased in kidneys after renal artery stenosis. Two-kidney, one clip renovascular hypertension was initiated in Sprague-Dawley rats by clipping of the left renal artery; control rats were sham operated. Blood pressure was not changed for the first 2 days after clipping but was elevated on day 4 (mean arterial pressure, 104 +/- 4 versus 87 +/- 2 mm Hg in sham-operated control rats, P < .002) and increased further during the next 24 days. Rats were killed 2, 4, 7, 14, and 28 days after clipping or sham operation, and poly(A)(+)-purified renal cortical RNA was analyzed by Northern blotting. Autoradiographs were quantitated by densitometry and normalized for the expression of a housekeeping gene. Renin expression was increased in the clipped kidney (by 149% on day 2) and decreased in the nonclipped kidney (by 82% on day 2), compared with kidneys of control rats. Expression of the angiotensin-converting enzyme was increased in clipped kidneys from the first day after clipping (158%) and throughout the experiment (66% on day 28), but was unchanged or slightly decreased in nonclipped kidneys. Angiotensinogen mRNA showed little change. Angiotensin II type 1 receptor expression was decreased in nonclipped kidneys but unchanged during the first 7 days in clipped kidneys. Our results show that components of the renin-angiotensin system other than renin are also differentially expressed in clipped kidneys.(ABSTRACT TRUNCATED AT 250 WORDS)

Internephron heterogeneity of growth factors and sclerosis--modulation of platelet-derived growth factor by angiotensin II

We studied the early phase after 5/6 nephrectomy in Munich-Wistar rats to determine whether treatment with angiotensin II receptor antagonist (AIIRA) modulates the expression of platelet-derived growth factor (PDGF) mRNA and its protein among the glomeruli which are undergoing progressive hypertrophy and sclerosis. Average PDGF-B immunohistochemistry staining score (IHS, 0 to 3 scale) in glomeruli and PDGF-B chain mRNA from kidneys were both increased in 5/6 nephrectomy rats (N = 6) versus age-matched normal (N = 5) at week 4, when glomeruli were at early stages of sclerosis (IHS, 0.81 +/- 0.12 vs. 0.19 +/- 0.05; sclerosis index, S.I., 0 to 4 scale: 0.41 +/- 0.04 vs. 0.05 +/- 0.01, both P < 0.05). AIIRA (80 mg/liter drinking water, N = 6) started at time of 5/6 nephrectomy prevented the development of sclerosis (S.I. 0.08 +/- 0.03) and decreased PDGF-B protein (IHS 0.22 +/- 0.08, both P = NS vs. normal), and PDGF-B chain mRNA. In contrast, triple therapy (TRX; hydralazine, reserpine and hydrochlorothiazide, N = 5) in doses which controlled systemic blood pressure resulted in intermediate level of glomerulosclerosis at this early time point of progressive injury. Concurrently, TRX failed to affect the expression of PDGF-B protein (IHS 0.86 +/- 0.19) or its mRNA expression. The PDGF-B distribution was not uniform amongst the glomeruli with varying stages of sclerosis. There was a strong correlation in individual glomeruli of increased PDGF-B staining with early sclerotic changes (P < 0.01), with the disappearance of this correlation in glomeruli with advanced sclerosis.(ABSTRACT TRUNCATED AT 250 WORDS)

The renin angiotensin aldosterone system and frusemide response in congestive heart failure

1. To test the hypothesis that basal renin angiotensin aldosterone system (RAAS) activity impairs the acute natriuretic response to frusemide in patients with mild or moderate congestive heart failure (CHF), we studied eight adult volunteers with preserved renal function, stable New York Heart Association Class II or III CHF, and echocardiographic evidence of left ventricular dysfunction due to myocardial infarction, hypertension, or both causes. 2. All patients received three dosing regimens administered in random order: (a) intravenous frusemide: 40 mg bolus then 40 mg h-1 for 3 h, (b) captopril: two 12.5 mg oral doses separated by 2 h, (c) combined dosing: the first captopril dose preceded the frusemide bolus by 30 min. Sodium balance on an 80 mmol day-1 sodium diet was documented prior to each dosing regimen. Sodium excretion was quantitated in urine collected at intervals until 3.5 h after initiating drug administration. During this time, urine output was replaced intravenously with an equivalent volume of 0.45% saline. 3. Captopril significantly lowered plasma angiotensin converting enzyme (ACE) activity and plasma aldosterone concentration, and raised inulin clearance. The drug had essentially no effect on the time course of magnitude of frusemide's natriuretic effect. Maximal fractional sodium excretion during frusemide infused by itself and in combination with captopril was 24.7 +/- 1.9% vs 28.2 +/- 3.8%, respectively (difference 3.5%; 95% CI, -4.0 to 11.0%; P > 0.05). Cumulative sodium excretion ending at 3.5 h was 429 +/- 53 mmol when frusemide was given alone and 455 +/- 69 mmol when captopril was added (difference, 26 mmol; CI, -121 to 174 mmol; P > 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Application of the fast-evaporation sample preparation method for improving quantification of angiotensin II by matrix-assisted laser desorption/ionization

The fast-evaporation method of sample preparation has been applied for quantitative analysis using matrix-assisted laser desorption/ionization (MALDI) mass spectrometry. An instrumental protocol focusing on improvement of shot-to-shot repeatability and compensation for signal degradation has been developed for quantification of angiotensin II using the fast-evaporation technique and an internal standard. The fast-evaporation method was compared to the standard method of sample preparation (using a multicomponent matrix) in the quantitative analysis of angiotensin II, and found to be superior in several respects. Improvement in sample homogeneity using the fast-evaporation method enhanced both point-to-point repeatibility and sample-to-sample reproducibility. The relative standard deviations of the analyte/internal standard ratios (point RSD) were decreased by a factor of three compared to those obtained using the multicomponent matrix method. The average point RSD was found to be ca. 5% for the fast-evaporation technique. Two internal standards were evaluated for quantification of angiotensin II. The better one, 1-SAR-8-Ile angiotensin II, yielded a relative standard deviation of the standard curve slope of ca. 2.2% over two orders of magnitude of concentration (45 nM to 3000 nM), an improvement by a factor of two over the standard preparation method. Renal microdialysate samples, spiked with angiotensin II and the internal standard 1-SAR-8-Ile angiotensin II, were also analyzed using the fast-evaporation technique. The detection limit was calculated to be in the high attomole range (675 amol). Furthermore, the accuracy for a single determination of angiotensin II concentration in these samples was found to be 13.9% with a relative error of 8.19%.

Efficacy of intrarenal ACE-inhibition estimated from the renal response to angiotensin I and II in humans

Recent studies on the nature of the renin-angiotensin system (RAS) in animals have led to the concept that systemic and intrarenal RAS can be influenced to different degrees by angiotensin converting enzyme (ACE) inhibitors. Assessment of efficacy of intrarenal ACE inhibition by ACE inhibitors in humans is necessarily indirect and has not been reported. We therefore monitored the renal response to acute angiotensin (Ang) I infusion in volunteers taking 20 mg enalapril twice daily, and related the responses to the obtained increments in plasma Ang II levels. Ang I infusion rates of 4, 8, 16, and 32 pmol/kg/min caused gradual increments in plasma Ang I (maximal change from 26 +/- 18 to 578 +/- 120 pmol/liter, P < 0.05) and, despite treatment with enalapril, also of Ang II (from 3 +/- 1 to 29 +/- 5 pmol/liter, P < 0.05). This was associated with large reductions in renal plasma flow (paraaminohippurate clearance), filtration fraction, maximal urine flow, sodium excretion, lithium and uric acid clearance, and increments in mean arterial pressure and plasma aldosterone (P < 0.05 for each variable). Strong correlations existed between the changes in either variable and the increment in plasma Ang II. Infusions of Ang II at 1 and 4 pmol/kg/min in the same subjects caused comparable increments in plasma Ang II and had similar physiological effects as found during the Ang I infusion. Analysis of covariance of the changes in plasma Ang II and each of the measured variables revealed no differences between Ang I and Ang II infusions.(ABSTRACT TRUNCATED AT 250 WORDS)

Angiotensin as a renal growth promoting factor

The kidney has been traditionally considered to be one the pivotal organs involved in the systemic actions of the renin-angiotensin system (RAS) with renin produced in the juxtaglomerular apparatus and angiotensin II (ANG II) as a key player in the regulation of glomerular hemodynamics. However, many studies in the last decade, facilitated by a throughout molecular characterization of all elements of the RAS, have provided convincing evidence that the kidney exhibits a local RAS which may independently function from the systemic actions of the endocrine RAS. Moreover, even local distinct cell populations along the nephron possess all components of a functioning RAS. For example, proximal tubular cells express mRNA and protein for angiotensinogen, renin, and angiotensin converting enzyme (ACE). They bear different types of ANG II receptors with the appropriate signal transduction systems, and these cells also exhibit surface proteases like angiotensinase A which are required for the inactivation of ANG II. Moreover, recent studies in the isolated perfused kidney have clearly shown that proximal tubular cells produce considerable amounts of ANG II and these concentrations exceed approximately hundred times the systemic concentration of the peptide. Besides the well-known regulation of glomerular hemodynamics by contraction of the efferent glomerular arteriole and mesangium cells, ANG II influences transport and acidification processes in proximal and distal tubules. In addition, the octapeptide stimulates metabolic pathways like tubular gluconeogenesis and ammoniagenesis. Accumulating data over the last years derived from in vivo and in vitro studies have demonstrated that ANG II is also a growth factor for renal cells. For example, cell culture experiments have shown that the octapeptide stimulates proliferation or hypertrophy of mesangial cells. In contrast, proliferation of cultured proximal tubular cells is inhibited by ANG II and cellular hypertrophy of these cells is induced. Many studies have provided evidence that early mesangial proliferation/hypertrophy and tubular hypertrophy is a predecessor of the subsequent development of glomerulosclerosis and interstitial fibrosis, situations with irreversible morphological changes of the kidney's architecture leading finally to end-stage renal disease. Therefore, the identification of ANG II as a renal growth factor and a better understanding of its local intrarenal synthesis and growth stimulating effects on different cell types along the nephron may help to develop rational therapeutic interventions to prevent the progression of renal disease.

Cellular and ultrastructural location of angiotensinogen in rat and sheep kidney

Recent evidence suggests the involvement of a local renin-angiotensin system in some renal actions of angiotensin II (Ang II). In this study the renal distribution of the precursor to angiotensin formation, angiotensinogen, was investigated in rats and sheep using immunohistochemistry, immunoelectron microscopy and non-isotopic hybridization histochemistry. Immunostaining for angiotensinogen was seen in proximal tubules (PCT) of both rat and sheep kidneys. In the rat the strongest immunostaining was found in the kidneys of neonatal (1 day old) rats. Staining declined after birth. Non-isotopic hybridization histochemistry using oligodeoxynucleotide probes labeled with biotin confirmed the presence of angiotensinogen mRNA expression in PCT of the rat renal cortex. Electron microscopic immunohistochemistry using antibodies raised against rat angiotensinogen showed weak staining in the adult of granule-like structures close to the apical membrane of PCT cells. In the neonatal rat kidney, angiotensinogen immunostaining was found throughout the PCT cells and was markedly stronger than that seen in adult rat kidney. In sheep, angiotensinogen immunostaining with an antibody raised against purified ovine angiotensinogen showed staining of PCT in fetal, newborn and adult sheep kidney. The strongest immunostaining seen was in fetal sheep kidney with a decline seen after birth. Reverse transcription polymerase chain reaction (RT-PCR) showed that angiotensinogen mRNA was expressed in the sheep kidney at all ages studied. Angiotensinogen expression was higher in fetal sheep kidneys (77 day and 141 day gestation) than in adult sheep kidney. In conclusion, angiotensinogen mRNA expression was detected in both rat and sheep kidneys.(ABSTRACT TRUNCATED AT 250 WORDS)

Angiotensin II-dependent proximal tubule sodium transport is mediated by cAMP modulation of phospholipase C

Angiotensin II (ANG II) stimulates proximal tubule sodium transport by decreasing adenylyl cyclase activity. The role of ANG II-dependent phospholipase C is less certain. To determine the contribution of phospholipase C and adenylyl cyclase to apical (AP) ANG II-dependent sodium transport, unidirectional (AP to basolateral) 22Na flux was measured in rat proximal tubule cells cultured on permeable supports. AP ANG II (100 nM)-dependent sodium flux was prevented by preincubation with concentrations of the phospholipase C inhibitor U-73122 (1 microM) that blocked ANG II-dependent inositol phosphate formation. AP ANG II-dependent sodium flux was also abolished by preincubation with the intracellular calcium mobilization inhibitor 3,4,5-trimethoxybenzoic acid 8-(diethylamino)octyl ester (TMB-8), further suggesting that ANG II-dependent sodium transport was mediated by inositol phosphates. Neither U-73122 nor TMB-8 prevented ANG II-dependent adenosine 3',5'-cyclic monophosphate (cAMP) decreases. Incubation with dibutyryl cAMP (10 microM) or forskolin (10 microM) prevented ANG II-dependent sodium flux as well as ANG II-dependent inositol phosphate formation. In conclusion, ANG II-dependent proximal tubule sodium transport in cultured cells was transduced by phospholipase C and adenylyl cyclase. The adenylyl cyclase effect on ANG II-dependent sodium transport was mediated by phospholipase C.

Converting enzyme inhibition and renal tissue angiotensin II in the rat

Multiple observations suggest local control of renal function via an intrarenal renin-angiotensin system, including evidence for local angiotensin (Ang) II production. Our first goal was to examine renal tissue Ang I:Ang II relations to ascertain whether Ang II formation differs in the circulation and in renal tissue. We have recently shown an authentic Ang II/Ang I ratio of 1.5:1 in renal lymph, the opposite of the Ang II:Ang I relation in plasma. Our second goal was to examine the influence of maximal angiotensin converting enzyme inhibition on these relations in plasma and in renal tissue. We used two converting enzyme inhibitors with differing lipid solubility, on the premise that tissue penetration and action might differ on that basis. We measured Ang I and Ang II in plasma and renal tissue of rats given an intravenous dose of either vehicle, enalapril, or ramipril, over a wide dose range, from 0.1 to 10.0 mg/kg i.v. Renal and plasma angiotensin concentrations were measured by high-performance liquid chromatography and radioimmunoassay. Whereas the Ang I concentration in normal rat plasma (273 +/- 84 fmol/mL) was over threefold the plasma Ang II concentration (83 +/- 12 fmol/mL), the ratio was reversed in the kidney (Ang II, 178 +/- 12 versus Ang I, 91 +/- 3 fmol/g; P < .001). Although ramipril and enalapril induced an indistinguishable dose-related acute fall in blood pressure and plasma Ang II concentration, lower enalapril doses were less effective in reducing renal tissue Ang I:Ang II conversion and Ang II concentration (P < .025).(ABSTRACT TRUNCATED AT 250 WORDS)

Angiotensin II metabolic clearance rate and pressor responses in nonpregnant and pregnant women

OBJECTIVES

Normal pregnancy is associated with reduced pressor dose-responses to infused angiotensin II. We tested the hypotheses that alterations in the metabolic clearance rate and the half-life of angiotensin II account for reduced pressor dose-responses during gestation and that angiotensin II increases circulating levels of vasodilatory prostaglandins I2 and E2 relative to thromboxane A2.

STUDY DESIGN

Eleven nonpregnant and 37 pregnant (30 +/- 0.3 weeks' gestation, mean +/- SE) women were infused with angiotensin II (3.11 to 22.36 ng/min.kg) for 15 minutes, and blood was obtained to evaluate steady-state immunoreactive plasma angiotensin II and eicosanoid concentrations.

RESULTS

Angiotensin II pressor responses were dose dependent in all groups and reduced in pregnant women (p < 0.001). Basal immunoreactive plasma angiotensin II concentrations were 2.7-fold greater (p < 0.001) in pregnant versus nonpregnant women. Plasma levels reached steady state by 5 minutes of infusion, and at similar angiotensin II concentrations the increase in blood pressure was greater in nonpregnant versus pregnant women (p < 0.001). The angiotensin II metabolic clearance rate and half-life were similar in nonpregnant and pregnant women: metabolic clearance rate = 85 +/- 10 versus 68 +/- 3 ml/min.kg, respectively (p = 0.130), and half-life = 48 and 49 seconds, respectively. Plasma prostaglandin I2 (6-keto-prostaglandin F1 alpha) prostaglandin E2, and thromboxane B2 levels in pregnant women were unaffected by angiotensin II infusions.

CONCLUSION

Neither changes in angiotensin II metabolism nor angiotensin II-induced increases in plasma levels of prostaglandin I2, prostaglandin E2, or the prostaglandin I2/thromboxane A2 ratio appear responsible for the decreased pressor response sensitivity to infused angiotensin II observed during normal human pregnancy.

Isoproterenol and 8-bromo-cyclic adenosine monophosphate stimulate the expression of the angiotensinogen gene in opossum kidney cells

To investigate whether the expression of the renal angiotensinogen (ANG) gene is regulated by beta-adrenoceptors and the cAMP-dependent protein kinase A pathway, we introduced stably the fusion gene containing the 5'-flanking regulatory sequence of the ANG gene with a human growth hormone (hGH) gene as a reporter, pOGH (ANG N-1498/+18), into opossum kidney (OK) cells. We successfully obtained several stable transformants with a high expression of the pOGH (ANG N-1498/+18) fusion gene. One stable transformant (OK 27) that is able to maintain the expression of pOGH (ANG N-1498/+18) in culture for more than a year was used in the present study. The level of expression of the pOGH (ANG N-1498/+18) in OK 27 was evaluated by the amount of immunoreactive-hGH (IR-hGH) secreted into the culture medium. The addition of isoproterenol (10(-11) M to 10(-9) M) stimulated the expression of pOGH (ANG N-1498/+18) and increased the accumulation of intracellular cAMP. Higher concentrations of isoproterenol (that is, greater than 10(-9) M) had low or minimal effect. In contrast, the addition of 8-bromo-cAMP (8-Br-cAMP) and forskolin stimulated the expression of pOGH (ANG N-1498/+18) in a dose-dependent manner. The stimulatory effect of isoproterenol was blocked by the presence of propranolol, atenolol and ICI 118,551. The addition of ICI 118,551, however, was less effective than atenolol. Furthermore, the stimulatory effect of isoproterenol and 8-Br-cAMP on the expression of the pOGH (ANG N-1498/+18) was inhibited by the presence of Rp-cAMP (an inhibitor of cAMP-dependent protein kinase A I and II).(ABSTRACT TRUNCATED AT 250 WORDS)

Role of angiotensin II in renal injury of deoxycorticosterone acetate-salt hypertensive rats

To investigate the role of angiotensin II (Ang II) in hypertension-induced tissue injury, we gave TCV-116 (1 mg/kg per day PO), a nonpeptide Ang II type I receptor antagonist, or enalapril (10 mg/kg per day PO) to deoxycorticosterone acetate (DOCA)-salt hypertensive rats for 3 weeks and examined the effects on tissue mRNA levels for transforming growth factor-beta 1 (TGF-beta 1) and extracellular matrix components. Tissue mRNA levels were measured by Northern blot analysis. Renal mRNA levels for TGF-beta 1; types I, III, and IV collagen; and fibronectin in DOCA-salt hypertensive rats were increased by severalfold (P < .01) compared with sham-operated rats. In the aorta of DOCA-salt hypertensive rats, TGF-beta 1 and fibronectin mRNA levels were increased, but types I, III, and IV collagen mRNAs did not increase. In the heart, increased mRNA was found only for fibronectin. Thus, these gene expressions are regulated in a tissue-specific manner. TCV-116 or enalapril did not lower blood pressure in DOCA-salt hypertensive rats. However, the increase in renal mRNAs for TGF-beta 1 and extracellular matrix components in DOCA-salt hypertensive rats was significantly inhibited by treatment with TCV-116 or enalapril, which was associated with a significant decrease in urinary protein and albumin excretions and histological improvement of renal lesions. In contrast, in the aorta and heart these gene expressions were not affected by TCV-116 or enalapril. Thus, local Ang II may contribute to renal injury of DOCA-salt hypertension by stimulating the gene expression of TGF-beta 1 and extracellular matrix components.

Nitric oxide in the kidney: synthesis, localization, and function

The characterization and cloning of constitutive and inducible nitric oxide (NO)-synthesizing enzymes and the development of specific inhibitors of the L-arginine NO pathway have provided powerful tools to define the role of NO in renal physiology and pathophysiology. There is increasing evidence that endothelium-derived NO is tonically synthesized within the kidney and that NO plays a crucial role in the regulation of renal hemodynamics and excretory function. Bradykinin and acetylcholine induce renal vasodilation by increasing NO synthesis, which in turn leads to enhancement of diuresis and natriuresis. The blockade of basal NO synthesis has been shown to result in decreases of renal blood flow and sodium excretion. These effects are partly mediated by an interaction between NO and the renin angiotensin system. Intrarenal inhibition of NO synthesis leads to reduction of sodium excretory responses to changes in renal arterial pressure without an effect on renal autoregulation, suggesting that NO exerts a permissive or a mediatory role in pressure natriuresis. Nitric oxide released from the macula densa may modulate tubuloglomerular feedback response by affecting afferent arteriolar constriction. Nitric oxide produced in the proximal tubule possibly mediates the effects of angiotensin on tubular reabsorption. In the collecting duct, an NO-dependent inhibition of solute transport is suggested. The L-arginine NO pathway is also active in the glomerulus. Under pathologic conditions such as glomerulonephritis, NO generation is markedly enhanced due to the induction of NO synthase, which is mainly derived from infiltrating macrophages. An implication of NO in the mechanism of proteinuria, thrombosis mesangial proliferation, and leukocyte infiltration is considered. In summary, the data presented on NO and renal function have an obvious clinical implication. A role for NO in glomerular pathology has been established. Nitric oxide is the only vasodilator that closely corresponds to the characteristics of essential hypertension. Using chronic NO blockade, models of systemic hypertension will provide new insights into mechanisms of the development of high blood pressure.

Renin and renin mRNA in proximal tubules of the rat kidney

The present study was undertaken to assess the presence of renin enzymatic activity and renin mRNA in proximal tubules of rat kidneys, and to determine the effect of converting enzyme inhibition (CEI) on proximal tubule renin gene expression. Proximal convoluted tubules (PCT), proximal straight tubules (PST), outer medullary collecting ducts (OMCD), and glomeruli (Gloms) were isolated by microdissection. Renin activity was measured in sonicated segments by radioimmunoassay. Renin mRNA levels were assessed using a quantitative PCR. Renin activity in PCT averaged 51 +/- 15 microGU/mm compared to 405 +/- 120 microGU/glomerulus. No measurable renin activity was found in PST and OMCD. Renin activity in both glomeruli and tubules had the same pH optimum, between 7.0 and 7.5. Renin mRNA was consistently detectable in cDNA prepared from PCT and PST, although its abundance per mm tubule was about 1/500th that found in one glomerulus. Renin mRNA was not detectable in OMCD. Tubular renin PCR product identity was confirmed by restriction digestion. CEI administration increased glomerular renin activity and renin mRNA, but not proximal tubular renin. The absence of a stimulatory effect of CEI on proximal tubule renin gene expression suggests the operation of different intracellular signals in control of renin synthesis in the proximal tubule than in the vascular compartment.

Effects of dietary protein and uninephrectomy on renal angiotensin converting enzyme activity in the rat

This study examines the effects of dietary protein and of uninephrectomy on angiotensin converting enzyme (ACE) in the normotensive rat, with particular regard to the kidney. Male Wistar Kyoto rats were fed isocaloric diets containing 5, 16 or 50% protein for three weeks. Other groups of rats were subjected to either left unilateral nephrectomy or sham operations, and the rats were killed eight days after surgery. ACE activity was measured in the kidney medulla, cortex, proximal tubule brush border membrane and in the plasma, heart and lung. Renal cortex and brush border ACE activity increased in parallel with protein intake, whereas plasma and lung ACE activity decreased; heart and kidney medulla ACE activity did not vary significantly. Uninephrectomy also led to a high increase in brush border ACE activity in the contralateral kidney, with no effect in the renal medulla or in the other tissues. The increase in ACE activity in the brush border membrane corresponded to a similar increase in the maximum number of binding sites of 3H-ramiprilat. This suggested that the increase in ACE activity corresponded to an increase in ACE concentration. The increase in renal tubular ACE activity could result in higher angiotensin II levels, and could consequently play a role in the modification of sodium reabsorption and cellular growth which occurs in the proximal tubule in these experimental models.

Angiotensin II stimulates extracellular matrix protein synthesis through induction of transforming growth factor-beta expression in rat glomerular mesangial cells

Angiotensin II (Ang II) has been implicated in the development of progressive glomerulosclerosis, but the precise mechanism of this effect remains unclear. In an experimental model, we have shown previously that TGF-beta plays a key role in glomerulosclerosis by stimulating extracellular matrix protein synthesis, increasing matrix protein receptors, and altering protease/protease-inhibitor balance, thereby inhibiting matrix degradation. We hypothesized that Ang II contributes to glomerulosclerosis through induction of TGF-beta. Ang II treatment of rat mesangial cells in culture increased TGF-beta and matrix components biglycan, fibronectin, and collagen type I at both the mRNA and protein levels in a time- and dose-dependent manner. Saralasin, a competitive inhibitor of Ang II, prevented the stimulation. Ang II also promoted conversion of latent TGF-beta to the biologically active form. Coincubation of mesangial cells with Ang II and neutralizing antibody to TGF-beta blocked the Ang II-induced increases in matrix protein expression. Continuous in vivo administration of Ang II to normal rats for 7 d resulted in 70% increases in glomerular mRNA for both TGF-beta and collagen type I. These results indicate that Ang II induces mesangial cell synthesis of matrix proteins and show that these effects are mediated by Ang II induction of TGF-beta expression. This mechanism may well contribute to glomerulosclerosis in vivo.

In vivo occupancy of angiotensin II subtype 1 receptors in rat renal medullary interstitial cells

Angiotensin II receptor binding sites in type 1 interstitial cells in the inner stripe of the outer medulla are readily labeled in vitro by the radioligand but not in vivo after systemic radioligand administration. In anesthetized rats, we investigated if reduced vascular delivery due to angiotensin II-induced renal vasoconstriction or, alternatively, prior occupancy of these sites by endogenous angiotensins modulates angiotensin II subtype 1 receptor binding to renal medullary interstitial cells in vivo using electron microscopic autoradiography. Using 125I-angiotensin II, administered systemically, as a radioligand, binding in control rats occurred predominantly in the glomeruli and proximal tubules, while only low binding was observed in the inner stripe of the outer medulla. Pretreatment of rats with unlabeled [Sar1,Ile8]angiotensin II or with the angiotensin II subtype 1 receptor antagonist losartan before receiving the radioligand completely abolished binding to all sites. Renal vasodilatation induced by sodium nitroprusside or use of the radiolabeled antagonist analogue 125I-[Sar1,Ile8]angiotensin II did not alter binding to the inner stripe. In contrast, chronic salt loading or inhibition of angiotensin-converting enzyme by perindopril significantly increased binding not only to the cortical sites but also to the sites in the inner stripe of the outer medulla. Electron microscopic autoradiographs of the inner stripe detected binding in the interstitial cells only in rats treated with chronic salt loading or perindopril. These results suggest that endogenous angiotensins may modulate binding of circulating angiotensin II to the interstitial cells in vivo, and these angiotensin II receptor-bearing cells are more likely to be more responsive to interstitial angiotensin II than to the circulating hormone.

Angiotensin II-dependent proximal tubule sodium transport requires receptor-mediated endocytosis

Angiotensin II (ANG II) receptors are present on apical and basolateral surfaces of proximal tubule cells. To determine the cellular mechanisms of proximal tubule ANG II receptor-mediated Na transport, apical-to-basolateral 22Na flux was measured in cultured proximal tubule cells. Apical ANG II caused increases in 22Na flux (maximum response: 100 nM, 30 min). Basolateral ANG II resulted in 22Na flux that was 23-56% greater than 22Na flux observed with equimolar apical ANG II. Apical ANG II-induced 22Na flux was prevented by preincubation with amiloride, ouabain, and the AT1 receptor antagonist losartan. Because apical ANG II signaling was previously shown to be endocytosis dependent, we questioned whether endocytosis was required for ANG II-stimulated proximal tubule Na transport as well. Apical (but not basolateral) ANG II-dependent 22Na flux was inhibited by phenylarsine oxide, an agent which prevents ANG II receptor internalization. In conclusion, apical and basolateral ANG II caused proximal tubule Na transport. Apical ANG II-dependent Na flux was mediated by AT1 receptors, transcellular transport pathways, and receptor-mediated endocytosis.

The paradox of the renin-angiotensin system in chronic renal disease

Despite normal to suppressed levels of renin activity in chronic renal disease, multiple lines of evidence suggest a role for the RAS, especially its intrarenal expression, in several critical aspects of this condition. Alterations in the distribution and control of components of the renal RAS could account for localized areas of activation of this system. Renal scarring may be particularly important as a major stimulus to renin synthesis in the diseased kidney. While both intrarenal and systemic hypertension may depend in part upon actions of the RAS, other non-hemodynamic actions of the RAS may also contribute to the adaptation of residual nephrons as well as their progressive injury.

Angiotensin converting enzyme in renal ontogeny: hypothesis for multiple roles

Angiotensin converting enzyme (ACE) has multiple effects both as the enzyme which cleaves angiotensin II from angiotensin I and as that which breaks down bradykinin. The present study examines ACE mRNA and protein expression in the rat kidney during development. Changes in distribution and expression during development are consistent with suggestions that the renin angiotensin system is important in growth modulation, vascular development and regulation, and protein reabsorption.

Renal responses to acute angiotensin II inhibition and administered angiotensin II in the aging, conscious, chronically catheterized rat

Studies of conscious, chronically catheterized, young (3 to 5 months of age) and old (19 to 22 months of age) male Sprague Dawley rats in the baseline state showed that glomerular filtration rate (factored for body weight) was lower and urine flow higher in old compared with young rats. Acute blockade of endogenous angiotensin II (AII) with either converting enzyme inhibitor (CEI) or Losartan (Dupont Merck, Wilmington, DE) caused small variable decreases in blood pressure in both age groups and produced a significant renal vasodilation with increases in renal plasma flow in the older rats. In separate studies using low-dose AII infusion (5 ng/kg body weight/min), an increase in filtration fraction was the only effect seen in both young and old rats with an intact renin/AII system; no effect was seen with CEI. During high-dose AII (20 ng/kg body weight/min), significant and similar increases were seen in blood pressure and renal vascular resistance in old and young rats, and the patterns of blood pressure and renal hemodynamic responses were similar with and without acute CEI. A natriuretic and diuretic response to high-dose AII was seen in young rats with intact endogenous renin/AII, whereas old rats were completely refractory to this action of AII. Against a background of acute CEI, no natriuretic/diuretic response to high-dose AII was seen in either age group. Thus, in the baseline state, renal hemodynamics in the old kidney are controlled by endogenous AII since CEI and Losartan produce renal vasodilation in old but not young rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Renin expression in renal proximal tubule

Angiotensinogen, angiotensin-converting enzyme, and renin constitute the components of the renin-angiotensin system. The mammalian renal proximal tubule contains angiotensinogen, angiotensin-converting enzyme, and angiotensin receptors. Previous immunohistochemical studies describing the presence of renin in the proximal tubule could not distinguish synthesized renin from renin trapped from the glomerular filtrate. In the present study, we examined the presence of renin activity and mRNA in rabbit proximal tubule cells in primary culture and renin mRNA in microdissected proximal tubules. Renin activity was present in lysates of proximal tubule cells in primary culture. Cellular renin content in cultured proximal tubule cells was increased by incubation with 10(-5) M isoproterenol and 10(-5) M forskolin by 150 and 110%, respectively. In addition, renin transcripts were detected in poly(A)+ RNA from cultured proximal tubule cells by RNA blots under high stringency conditions. In microdissected tubules from normal rats, renin mRNA was not detectable with reverse transcription and polymerase chain reaction. However, in tubules from rats administered the angiotensinogen-converting-enzyme inhibitor, enalapril, renin was easily detected in the S2 segment of the proximal tubule. We postulate the existence of a local renin-angiotensin system that enables the proximal tubule to generate angiotensin II, thereby providing an autocrine system that could locally modulate NaHCO3 and NaCl absorption.

Cytoskeleton-dependent endocytosis is required for apical type 1 angiotensin II receptor-mediated phospholipase C activation in cultured rat proximal tubule cells

Renal proximal tubule sodium reabsorption is enhanced by apical or basolateral angiotensin II (AII). Although AII activates phospholipase C (PLC) in other tissues, AII coupling to PLC on either apical or basolateral surfaces of proximal tubule cells is unclear. To determine if AII causes PLC activation, and the differences between apical and basolateral AII receptor function, receptors were unilaterally activated in rat proximal tubule cells cultured on permeable, collagen-coated supports. Apical AII incubation resulted in concentration- and time-dependent inositol trisphosphate (IP3) formation. Basolateral AII caused greater IP3 responses. Apical AII-induced IP3 generation was inhibited by DuP 753, suggesting that the type 1 AII receptor subtype mediated proximal tubule PLC activation. Apical AII signaling did not result from paracellular ligand leak to basolateral receptors since AII-induced PLC activation occurred when basolateral AII receptors were occupied by Sar-Leu AII or DuP 753. Inhibition of endocytosis with phenylarsine oxide prevented apical (but not basolateral) AII-induced IP3 formation. Cytoskeletal disruption with colchicine or cytochalasin D also prevented apical AII-induced IP3 generation. These results demonstrate that in cultured rat proximal tubule cells, AII is coupled to PLC via type 1 AII receptors and cytoskeleton-dependent endocytosis is required for apical (but not basolateral) AII receptor-mediated PLC activation.

High resolution localization of angiotensin II receptors in rat renal medulla

The cellular localization of angiotensin II (Ang II) receptors in the inner stripe of the outer medulla of the rat kidney was investigated by using high resolution light and electron microscopic autoradiography. Fresh tissue blocks from the inner stripe of the outer medulla were incubated with 125I-[Sar1, Ile8] Ang II and prepared for microscopic autoradiography. At the light microscopic level, 125I-[Sar1, Ile8] Ang II was found to penetrate into the tissue and to bind specifically to sites outlining renal tubules and vasa recta bundles. Electron microscopic autoradiography revealed that silver grains were detected over interstitial cells located between the tubules and components of the vasa recta bundles, but no silver grains were detected overlying the cells of the thin descending or thick ascending limbs of the loop of Henle, the collecting ducts, the vasa recta, or other blood vessels. These interstitial cells contained abundant endoplasmic reticulum, microfilaments, occasional lipid droplets and extensive cytoplasmic processes which closely related to the basement membranes of the vasa recta and loops of Henle. The cells therefore closely resemble type 1 interstitial cells. Since Ang II binding sites are absent in the inner medulla, the cells labelled by this technique must be a subset of type 1 interstitial cells, distinct from the typical lipid-laden interstitial cells most abundant in the inner medulla. These findings demonstrate that type 1 interstitial cells are the primary sites for a high density of Ang II receptors located in the inner stripe of the outer medulla.

Modulation of Na-H exchange activity by angiotensin II in opossum kidney cells

Angiotensin II (ANG II) was shown to modulate transport in the renal proximal tubule through both inhibition of adenylate cyclase and protein kinase C (PKC) activation. We evaluated the effects of ANG II on adenosine 3',5'-cyclic monophosphate (cAMP) content and Na-H exchange activity (amiloride-sensitive Na influx) in two strains of opossum kidney (OK) cells originating from different sources, OK-VD and OK-RR cells. In OK-VD cells, ANG II inhibited basal and parathyroid hormone (PTH)-induced cAMP generation in a pertussis toxin-sensitive manner and reversed PTH inhibition of Na-H exchange. These effects of ANG II were prevented by PD 123319, a selective nonpeptide antagonist of AT2 receptors. In contrast, DuP 753, which antagonizes selectively AT1 receptors, had no effect. In OK-RR cells, ANG II had no effect on cAMP content and decreased Na-H exchange activity. The effect of ANG II persisted in the presence of PTH but was abolished by PKC downregulation and by DuP 753, but not by PD 123319. In conclusion, two types of ANG II receptors, coupled to distinct signaling pathways, were expressed independently in OK cells originating from two different sources and mediated opposite effects of ANG II on Na-H exchange activity. Those models provide a powerful tool for studying the intracellular steps involved in the tubular effects of ANG II and to evaluate the effect of pharmacological inhibitors of ANG II binding to its receptors.

Evidence for tissue-specific activation of renal angiotensinogen mRNA expression in chronic stable experimental heart failure

The intrarenal renin-angiotensin system (RAS) may contribute to the pathophysiology of heart failure by the generation of angiotensin II at local sites within the kidneys. Angiotensin II may directly influence renal hemodynamics, glomerular contractility, and tubular sodium reabsorption, thereby promoting sodium and fluid retention in this syndrome. In the present study, we examined components of the circulating RAS as well as the intrarenal expressions of renin and angiotensinogen mRNA in rats with stable compensated heart failure (HF) 12 wk after experimental myocardial infarction. Renal angiotensinogen mRNA level in vehicle-treated HF rats increased 47%, as compared with sham control rats (P = 0.001). The increase in angiotensinogen mRNA levels was more pronounced in animals with medium (46%, P < 0.05) and large (66%, P < 0.05) infarcts than in those with small infarcts (31%, P = NS). There were no differences in liver angiotensinogen mRNA, circulating angiotensinogen, angiotensin II, plasma renin concentration (PRC), kidney renin content (KRC), and renal renin mRNA level between sham and HFv. In addition, in a separate group of rats with heart failure, we demonstrated that renal angiotensin II concentration increased twofold (P < 0.05) as compared with that of age-matched sham operated controls. A parallel group of heart failure rats (HFe, n = 11) was treated with enalapril (25 mg/kg per d) in drinking water for 6 wk before these measurements. Blood pressure decreased significantly during treatment (91 vs. 103 mm Hg, P < 0.05). Enalapril treatment in HF rats increased renin mRNA level (2.5-fold, P < 0.005), KRC (5.6-fold, P = 0.005), and PRC (15.5-fold, P < 0.005). The increase in renal angiotensinogen mRNA level observed in HFv rats was markedly attenuated in enalapril treated HF rats (P < 0.001), suggesting a positive feedback of angiotensin II on renal angiotensinogen synthesis. These findings demonstrate an activation of intrarenal RAS, but no changes in the circulating counterpart in this model of experimental heart failure, and they support the concept that the intrinsic renal RAS may contribute to the pathophysiology in this syndrome.

Lewis K. Dahl Memorial Lecture. The renin system and four lines fo hypertension research. Nephron heterogeneity, the calcium connection, the prorenin vasodilator limb, and plasma renin and heart attack

As the major regulator of arterial blood pressure and sodium balance, the renin axis supports normotension or hypertension via angiotensin-mediated vasoconstriction and angiotensin plus aldosterone-induced renal sodium retention. In this endocrine servo control, renal renin is released by hypotension or salt depletion; conversely, with hypertension or volume excess, plasma renin activity falls to zero. Accordingly, any renal renin secretion is abnormal in the face of arterial hypertension. Human hypertensive disorders comprise a spectrum of abnormal vasoconstriction-volume products (renin-sodium profiles). Excess plasma renin activity for the sodium balance is created by nephron heterogeneity in which a subpopulation of ischemic nephrons hypersecretes renin and retains sodium. This excess renin impairs adaptive natriuresis of neighboring normal nephrons. Research defining the pivotal role of vascular cytosolic calcium for transducing sodium or renin-mediated vasoconstriction explains the selective value of calcium antagonists for correcting the sodium-volume-mediated, and beta-blockers or angiotensin converting enzyme inhibitors for correcting renin-mediated, arteriolar vasoconstriction. The renin precursor prorenin appears to be physiologically active, causing selective vasodilation that offsets renin-mediated vasoconstriction. Overactivity of prorenin may be involved in the hyperperfusion vascular injuries of diabetes mellitus and toxemias. Prorenin underactivity may facilitate renin-mediated ischemic vascular injury. In essential hypertension, undue plasma renin activity is powerfully and independently associated with heart attack risk. Conversely, patients with low renin activity are protected from heart attack despite higher blood pressures and greater age. Also, renin or angiotensin administration consistently causes vascular injury in the heart, brain, and kidneys of animals. These data suggest new potentials for the prevention of cardiovascular sequelae (heart attack and stroke) by using explicit strategies to curtail plasma renin activity.

Blood pressure-independent effect of angiotensin inhibition on vascular lesions of chronic renal failure

Previous studies in experimental models of progressive renal failure have shown that the capacity of antihypertensive drugs to protect glomeruli from sclerosis is often unpredictable from their effect on systemic blood pressure. The present study was undertaken to ascertain whether this systemic blood pressure-independent structure-preserving effect of antihypertensives, particularly angiotensin II converting enzyme inhibitors (ACEI), is confined to the glomerulus or not, as well as whether this effect is mediated via angiotensin II (Ang II). The following experimental drug regimens were used in the rat model of subtotal nephrectomy (sNPX): so-called triple therapy [TRX; a combination of reserpine 5 mg/liter drinking water (DW), hydralazine 80 mg/liter DW and hydrochlorothiazide 25 mg/liter DW], or ACEI (either captopril, CPL, 600 mg/liter DW, enalapril, ENL, 400 mg/liter DW or lisinopril, LSL, 200 mg/liter DW), or a novel Ang II receptor antagonist (Ang IIR, L-158,809, 20 mg/liter DW). These dosages were identified in pilot studies to be the minimum required to control systemic blood pressure in the early phase up to 12 weeks. In addition, a separate group was treated with a higher dose of L-158,809 (80 mg/liter DW) with equipotent systemic pressor effect. Treatment was initiated eight weeks after subtotal nephrectomy following renal biopsy, and animals were sacrificed at 16 weeks. In ACEI treated rats, carotid arterial wall thickening (WT), defined as ratio of media thickening to radius of outer vessel wall, was similar to normal age-matched control (0.073 in all ACEI treated rats, vs. 0.074 in normal control) and significantly less than with TRX (ratio 0.118) or untreated sNPX (0.130). Even more remarkably, coronary arteriole WT in ACEI-treated rats averaged 0.139, a value less than one half and one third of TRX (0.298) and untreated sNPX control (0.388), respectively. Similar results were obtained for mesenteric artery WT. These findings were closely paralleled by changes of glomerular sclerosis. In untreated sNPX control rats, glomerular sclerosis increased from biopsy to autopsy specimens by an average of 458%. Although TRX dampened the degree of increase in sclerosis to on average 212%, this protective effect was far less than that achieved by ACEI. In the latter, sclerosis increased on average only 65% from biopsy to autopsy. Although all ACEIs were more effective than TRX, captopril and lisinopril groups showed greatest benefit at these doses. Ang IIR also protected renal and extrarenal structures with 34% increase of sclerosis index in low dose and WT 0.088, 0.117 and 0.112, respectively in carotid, mesenteric and coronary arteries.(ABSTRACT TRUNCATED AT 400 WORDS)