(Hypertension. 1995;26:471-479.)
© 1995 American Heart Association, Inc.
Articles |
Effect of Captopril and Angiotensin II Receptor Blockade on Pressure Natriuresis in Transgenic TGR(mRen-2)27 Rats
From the Max Delbrück Center for Molecular Medicine, Franz Volhard Clinic, Virchow Klinikum, Humboldt University of Berlin (Germany).
Correspondence to Volkmar Gross, MD, Franz Gross Haus, Wiltberg Strasse 50, 13122 Berlin, FRG.
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Abstract |
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Abstract The pressure-natriuresis curve of transgenic rats harboring an extra mouse renin gene [TGR(mRen-2)27] is shifted rightward compared with controls; however, whether intrarenal angiotensin II effects are responsible for the rightward shift is unknown. To clarify this issue we infused the converting enzyme inhibitor captopril or the angiotensin II receptor blocker CV 11974 into transgenic and normotensive Sprague-Dawley Hannover control rats. We eliminated any other neural or endocrine regulatory differences between transgenic and control rats by renal denervation and infusion of vasopressin, aldosterone, corticosterone, and norepinephrine in sufficient quantities to occupy all receptors. Sodium excretion increased from 3.4±1.2 to 10.1±0.5 µmol/min per gram kidney weight in transgenic rats when renal perfusion pressure was increased from 158 to 201 mm Hg. Captopril (4 mg/kg) and CV 11974 (0.1 mg/kg) shifted the pressure-natriuresis curve of transgenic rats leftward, so that sodium excretion was threefold higher at similar renal perfusion pressures (150 to 160 mm Hg). Similarly, fractional sodium and water excretion curves were shifted leftward, so that values for transgenic and control rats were no longer different. Over the pressure range, renal blood flow in transgenic rats ranged from 3.1±0.7 to 4.4±0.5 mL/min per gram kidney weight and increased (P<.05) with both captopril and CV 11974 to ranges from 4.8±0.9 to 6.8±0.6 or from 4.5±0.7 to 6.9±1.0 mL/min per gram kidney weight, respectively. Glomerular filtration rate in transgenic rats, on the other hand, was not increased. Transgenic kidneys showed severe hypertension-induced nephrosclerosis. These results suggest that the hypertensive, rightward shift in the transgenic pressure-natriuresis curve is largely due to angiotensin II–dependent renal effects. Altered renal structure may also play a role in shifting the renal function curve rightward.
Key Words: captopril • angiotensin II • receptors, angiotensin • natriuresis • rats, transgenic
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Introduction |
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The transgenic rat TGR(mRen-2)27 is a new model in which transfection of the mouse Ren-2 gene into Sprague-Dawley rats causes hypertension.1 The hypertension is clearly related to the mouse Ren-2 gene; however, the mechanisms of blood pressure elevation are poorly understood. Captopril and angiotensin receptor blockers effectively lower blood pressure in TGR2 3 ; however, since plasma renin activity is low in TGR and Ang II values are also not elevated,1 3 4 5 a simple, plasma-mediated Ang II effect appears unlikely. The adrenal gland has been implicated, and the notion has been advanced that local renin expression and Ang II generation at this location may play a role.3 4 Attention has not focused on the kidneys because an earlier study suggested that transgene expression in the kidney is minimal or absent.5 On the other hand, we recently showed that the relationship between arterial pressure and sodium and water excretions is shifted rightward in TGR toward higher perfusion pressures compared with controls.6 Since these experiments were carried out under conditions in which neural and other humoral influences were controlled by renal denervation and intravenous infusion of aldosterone, corticosterone, norepinephrine, and vasopressin, the results suggested that the reduced excretory renal function in TGR is intrinsic to the kidneys themselves. Locally acting Ang II could be responsible for the abnormal pressure-natriuresis and -diuresis responses in TGR by enhancing tubular sodium reabsorption,7 8 influencing renal hemodynamics,9 10 11 and modulating tubuloglomerular feedback.12 13 We investigated the effects of renin-angiotensin system blockade on the pressure-natriuresis and -diuresis relationships in kidneys of TGR and control SDH rats to test the notion that intrarenal, Ang II–related effects are responsible for the rightward shift in the pressure-natriuresis curve.
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Methods |
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Experiments were conducted in 21 male TGR weighing 319±9 g (aged 71±2 days) and 20 male SDH weighing 318±6 g (aged 67±2 days) purchased from Moellegaard Breeding Center GmbH, Schoenewalde, Germany. The rats were allowed free access to standard rat chow (0.3% sodium, SSNIFF Spezialitäten GmbH) and drinking water ad libitum. Food but not drinking water was withdrawn the day before the experiment. The experimental protocol was approved by the local Council on Animal Care, whose standards correspond to those of the American Physiological Society. For pressure-natriuresis and -diuresis experiments, the rats were randomly assigned to either control, captopril, or CV 11974 groups.
Rats were prepared as previously described.14 Briefly, rats were anesthetized with ketamine (35 mg/kg IM, Parke-Davis GmbH) and thiobutabarbital (65 mg/kg IP, Res Biochemical Inc) and placed on a heated table for maintenance of body temperature at 37°C. Cannulas were placed into the trachea for facilitation of breathing, in the carotid and femoral arteries for measurement of systemic blood pressure and RPP, in the jugular vein for compound infusion, and in the left ureter for urine collection. The right kidney was removed through a midline incision, and the left kidney was denervated by stripping the visible nerves and swabbing the renal artery and vein with 10% phenol in ethanol. Adjustable clamps were placed around the abdominal aorta above and below the kidney, and ligatures were loosely placed around the celiac and mesenteric arteries for later occlusion so that RPP could be varied.
During preparation the rats received an intravenous infusion of a 0.9% sodium chloride solution containing 1% bovine albumin (Sigma Chemical Co) at a rate of 50 µL/min per 100 g body weight. Thereafter, the infusion rate was reduced to 33 µL/min per 100 g body weight. Inulin (20 mg/mL, Sigma) and PAH (1.2 mg/mL, Merck Sharp & Dohme) were included in the infusion solution for measurement of GFR and renal plasma flow. Circulating levels of sodium- and water-retaining hormones were fixed by an infusion of norepinephrine (333 ng/kg per minute, Sigma), aldosterone (66 ng/kg per minute, Sigma), cortisol (33 µg/kg per minute, Sigma), and vasopressin (0.17 ng/kg per minute, Sigma).
Tissue Preparation for Morphology
For conventional morphology the contralateral, right kidney from
TGR not receiving drugs was removed, cut sagitally, and fixed in 4%
buffered paraformaldehyde at room temperature. The
tissue was dehydrated in graded alcohols and embedded in paraffin.
Sections 2 to 3 µm thick were cut with a microtome (Leitz 1512). The
sections were deparaffinized and rehydrated before being stained as
outlined below.
Van Gieson's elastica was used for combined staining of elastic substrates and connective tissue. The sections were deparaffinized, rehydrated, and stained for 15 minutes in 0.5% resorcine-fuchsin solution in 70% ethanol. Thereafter, the sections were rinsed in distilled water, differentiated in 96% ethanol, and transferred to Weigert's iron–hematoxylin for 10 minutes. The sections were rinsed in distilled water, differentiated in HCl/ethanol, and rinsed in tap water for 30 minutes. In a third step the sections were stained in a picric acid–thiazine red mixture (10:0.2) for 10 minutes, rinsed in distilled water containing picric acid, and dehydrated and coverslipped by an automated coverslipper (Sakura).
Masson's trichrome was used to demonstrate infiltration of hyaline or fibrinoid material (red) in the vessel wall and to stain connective tissue (green). The stain outlines infiltrating blood cells and proteinaceous casts in the renal tubules. The tissue was stained with Weigert's iron–hematoxylin (nuclear staining). After rinsing in distilled water, the tissue was stained with Ponceau acid–fuchsin, rinsed in 1% acetic acid, differentiated with orange G–tungstic acid, rinsed again in 1% acetic acid, and counterstained with light green. The tissue was then again rinsed in 1% acetic acid, dehydrated, and coverslipped as above.
Periodic acid–silver methenamine was used to demonstrate the basement membranes and basement membrane–like material. The tissue was placed in 0.5% periodic acid, rinsed in distilled water, and incubated with silver nitrate. After rinsing in distilled water again, the tissue was placed into a 0.2% gold chloride solution, rinsed in distilled water and in sodium thiosulfate, and then counterstained with hematoxylin and eosin. The sections were rinsed in distilled water, dehydrated, and coverslipped.
Protocol 1: Captopril Effect on Pressure Natriuresis
Experiments were performed in eight TGR (335±6 g, aged 75±6
days) and seven SDH (319±8 g, aged 68±5 days). After surgery and a
1-hour equilibration period baseline MAP was recorded, and
thereafter, bolus captopril (4 mg/kg IV, Sigma) was given (0.1 mL/100 g
body wt). Fifteen to 20 minutes later RPP was lowered to approximately
100 mm Hg in SDH and to 115 mm Hg in TGR. After another 20- to
30-minute equilibration period urine flow, sodium excretion, GFR, and
RBF were determined in two 15- to 30-minute collection periods,
depending on the magnitude of the urine flow. The supra-aortic occluder
was than released to increase RPP approximately 35 to 40 mm Hg in both
groups. Urine and plasma samples were again collected during two
15-minute periods. RPP was then increased to 155 mm Hg in SDH and 185
mm Hg in TGR by ligating the mesenteric and celiac arteries and
occluding the aorta below the kidney. After a 10-minute equilibration
period urine and plasma samples were again collected during two
10-minute collection periods.
Protocol 2: AT1 Receptor Blockade With CV
11974
Experiments were performed in eight TGR (319±14 g, aged 72±2
days) and six SDH (302±11 g, aged 64±3 days). The rats were
surgically prepared and infused with the hormone cocktail as described
above. After an approximately 1-hour equilibration period and baseline
MAP recording the AT1 receptor blocker CV 11974
(0.1 mg/kg IV, 100 µL/100 g body wt; Takeda Chemical Industries, Ltd)
was injected as a bolus.15 Thirty minutes later the
pressure-natriuresis-diuresis relationship was determined as
described above. In the experiments with AT1 receptor
blockade the blood pressure levels in SDH ranged from 89 to 149 mm Hg
and in TGR from 109 to 196 mm Hg.
To define the effects of captopril and CV 11974 on the pres- sure-natriuresis-diuresis response we determined the pressure-natriuresis-diuresis relationship without injection of these drugs in five TGR (294±8 g, aged 63±2 days) and seven SDH (331±8 g, aged 69±2 days). The experimental protocol was identical to that used in the captopril and CV 11974 studies as well as to the protocol we used to define TGR and SDH renal pressure-natriuresis-diuresis curves in an earlier study.6
Continuous measurements of MAP were obtained throughout the experiment and recorded on a computer system (TSE GmbH). Representative MAP values were calculated for each period by averaging all recorded values during that time period. Urine flow was determined gravimetrically. Inulin and PAH concentrations of urine and plasma samples were determined according to methods outlined elsewhere.16 17 Urinary (FLM3, Radiometer) and plasma (Cobas Mira Plus) sodium concentrations were determined by flame photometry. GFR was calculated as the ratio of concentrations of urine to plasma inulin times urine flow. RBF was calculated as PAH clearance divided by (1-hematocrit). Urine flow, sodium excretion, GFR, and RBF were normalized per gram kwt.
Statistical Analysis
Data are presented as mean±SEM. Statistically
significant differences in mean values were tested by two-way ANOVA for
repeated measures and the Duncan multiple range test. A value of
P<.05 was considered statistically significant.
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Results |
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Protocol 1: Captopril Effect on Pressure Natriuresis
In anesthetized TGR and SDH, MAP before application of captopril averaged 169±8 and 134±4 mm Hg, respectively (P<.05). Fifteen to 20 minutes after captopril MAP decreased in TGR to 157±11 mm Hg (P<.05) but remained unchanged in SDH at 139±2 mm Hg. In control TGR and SDH without drugs the corresponding MAP averaged 180±78 and 129±8 mm Hg, respectively (P<.05).
Fig 1 shows the pressure-diuresis and -natriuresis responses of SDH and TGR with or without captopril. In SDH, urine flow (top) and sodium excretion (bottom) increased from 19.5±4.2 to 74.0±15.9 µL/min per gram kwt and from 4.7±1.0 to 15.8±2.9 µmol/min per gram kwt, respectively, as RPP was increased from 104 to 157 mm Hg. In SDH given captopril, increasing RPP from 100 to 138 mm Hg led to a similar increase in urine flow and sodium excretion compared with SDH with no drug. Thereafter, a further increase in RPP to 158 mm Hg significantly increased water and sodium excretions in SDH given captopril compared with SDH without captopril.
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TGR given captopril had pressure-diuresis (Fig 1, top) and pressure-natriuresis (Fig 1, bottom) relationships that were markedly shifted toward the left (P<.05) compared with TGR that received no drug. At an RPP of 158 mm Hg TGR without drug had a urine flow rate of 18.3±4.9 µL/min per gram kwt and sodium excretion of 3.4±1.2 µmol/min per gram kwt; in captopril-treated TGR, urine flow rate and sodium excretion averaged 48.4±5.1 µL/min per gram kwt and 8.8±1.1 µmol/min per gram kwt, respectively, at the same level of RPP and rose when RPP was further increased (196 mm Hg) to 86.6±8.6 µL/min per gram kwt and 14.7±1.7 µmol/min per gram kwt, respectively.
Fig 2 presents the effects of changing RPP on RBF and GFR in the pressure-natriuresis-diuresis experiments in control and captopril-treated SDH and TGR. RBF averaged between 5.1±0.4 and 7.7±0.7 mL/min per gram kwt in SDH without drug and between 7.3±0.9 and 9.2±1.6 mL/min per gram kwt in SDH given captopril, respectively. In both groups RBF reactions were similar over the range of pressures studied. Increasing RPP from approximately 100 to 135 mm Hg led to a significant increase in RBF; thereafter, RBF was autoregulated in these rats. In control TGR on the other hand, RBF was clearly lower than in all other groups (P<.05). RBF increased from 3.1±0.7 to 4.4±0.5 mL/min per gram kwt when RPP was increased from 158 to 201 mm Hg. In TGR given captopril RBF was greater compared with TGR without captopril (P<.05). The values averaged between 4.8±0.9 and 6.8±0.6 mL/min per gram kwt in these rats. Moreover, similar to the response in SDH an increase in RPP led to a significant increase in RBF.
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Over the range of RPP values studied, GFR was not significantly different between control SDH and SDH given captopril. GFR increased when RPP was increased from 100 to 135 mm Hg in both groups of SDH. Thereafter, GFR was well autoregulated. In both SDH groups GFR averaged between 1.2±0.1 and 1.7±0.2 mL/min per gram kwt. In TGR on the other hand, GFR was lower than in SDH (P<.05) and ranged between 0.69±0.09 and 1.04±0.04 mL/min per gram kwt. As in SDH rats, TGR given captopril displayed a significant increase in GFR as RPP was increased for the first pressure step from 115 to 150 mm Hg. TGR given no captopril showed a flat relationship across a range of pressures from 155 to 200 mm Hg between 0.8±0.1 and 0.9±0.1 mL/min per gram kwt. Captopril did not change GFR significantly. In SDH captopril had no influence on filtration fraction, which ranged between 20% and 22% in these rats. In contrast, in TGR captopril significantly reduced filtration fraction from 24%-29% to 15%-17%.
Fig 3 shows FEH2O and FENa. The most striking result was that FENa and FEH2O in TGR moved to values not significantly different from those in SDH rats with or without captopril across the range of perfusion pressures. Thus, captopril served to normalize the FENa and FEH2O relationship in TGR compared with TGR without captopril.
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Protocol 2: AT1 Receptor Blockade With CV
11974
Before injection of CV 11974, baseline MAP averaged 162±7 mm Hg
in TGR and 131±4 mm Hg in SDH. CV 11974 reduced MAP, so that 20 to 30
minutes after injection MAP averaged 152±7 mm Hg in TGR and 125±3
mm Hg in SDH (both P<.05).
Rats treated with CV 11974 showed changes in the pressure-natriuretic response that were similar to those observed with captopril. As shown in Fig 4 blockade of AT1 receptors did not significantly affect the influence of RPP on sodium and water excretions in SDH. At a perfusion pressure of 89 mm Hg water and sodium excretions in SDH averaged 19.0±3.9 µL/min per gram kwt and 5.5±0.6 µmol/min per gram kwt, respectively, with CV 11974. Increasing RPP from this level by 60 mm Hg led to a fourfold increase in water excretion and a threefold increase in sodium excretion. When CV 11974 was given, the pressure-natriuresis and -diuresis curves in TGR were significantly shifted to the left. At a control RPP of 109 mm Hg, urine flow and sodium excretion averaged 9.5±1.3 µL/min per gram kwt and 1.3±0.3 µmol/min per gram kwt, respectively, in TGR given CV 11974. As shown in Fig 4 these values increased as RPP was elevated to 196 mm Hg to 86.6±8.6 µL/min per gram kwt and 14.7±1.7 µmol/min per gram kwt, respectively.
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Fig 5 shows the effects of changes in RBF and GFR in the pressure-diuresis and -natriuresis experiments in CV 11974–treated SDH and TGR. CV 11974 increased RBF in SDH and TGR (P<.05). At an RPP of 89 mm Hg in CV 11974–treated SDH and at an RPP of 109 mm Hg in CV 11974–treated TGR, RBF averaged 7.2±1.1 and 4.5±0.7 mL/min per gram kwt, respectively. When RPP was elevated, these values increased significantly to 9.2±1.0 mL/min per gram kwt for SDH and 6.9±0.8 mL/min per gram kwt for TGR. RBF remained stable at this level when RPP was increased further to 140 or 196 mm Hg.
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GFR of CV 11974–treated SDH was higher than that of TGR treated with CV 11974. As in untreated SDH, GFR in SDH with CV 11974 increased significantly when RPP was increased from 89 to 125 mm Hg and remained stable thereafter. Filtration fraction ranged between 17% and 20% in CV 11974–treated SDH and between 12% and 16% in CV 11974–treated TGR.
Similar to the captopril effects, the improved pressure-diuresis and -natriuresis responses observed in CV 11974–treated TGR were also evident in the FEH2O and FENa data shown in Fig 6. Increasing RPP from 109 mm Hg by 87 mm Hg produced an 11-fold increase in FEH2O and an 8-fold increase in FENa.
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The differences in urine flow and sodium excretion in all groups of SDH and TGR were not likely related to differences in the animals' state of hydration, because the hematocrits of all experimental groups ranged between 0.39±0.02 and 0.43±0.01.
Renal Histology
The small arteries of TGR showed medial thickening with concentric
hyperplasia and subendothelial deposition of
hyaline material, which in some instances led to vascular occlusion.
Larger vessels such as the arcuate arteries were unremarkable.
Infiltration of leukocytes into the media was observed with disruption
(Fig 7, top). Masson's trichrome (Fig 7, middle) and periodic acid–silver methenamine
(Fig 7, bottom) documented the presence of fibrinoid
necrosis at the junction between the media and adventitia as well as an
invasion of cells. Proteinaceous casts were commonly observed within
the tubular lumen in TGR (Fig 8, top and middle) and
within some glomeruli (Fig 8, middle). The distal
tubules showed basophilia, signs of atrophy, and vacuolization (Fig 8, bottom). The kidneys of SDH, on the other hand, were
normal (Fig 9).
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Discussion |
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The important findings in this study were that the renal pressure-natriuresis and -diuresis relationships are shifted to the right in TGR compared with SDH and that blockade of Ang II production or blockade of its AT1 receptor moves these relationships leftward. These latter observations argue strongly for Ang II being important in the rightward shift of the pressure-natriuresis and -diuresis relationships in TGR.
All genetic and acquired models of hypertension exhibit a rightward shift in the pressure-natriuresis and -diuresis relationships.18 19 20 21 The configuration of the relationships and the influence of antihypertensive treatment permit some inference into the nature and pathogenesis of the hypertension. In an earlier study we defined the pressure-natriuresis and -diuresis relationships in TGR compared with control SDH rats6 and found that the relationships were shifted to a higher operating level of arterial pressure. This result was confirmed in the present study.
In our earlier study we were unable to answer the question of whether the altered pressure-natriuresis-diuresis relationship was related to structural alterations in renal components or whether it was a sequel of local renin expression by the transgene. In those experiments we also effectively clamped other humoral and neural factors that could influence the renal handling of sodium and water. In the present experiments we followed this experimental approach, which is generally used to study pressure natriuresis.6 14 18 19 20 21 22 23 24 25 To clarify the importance of the renin-angiotensin system for pressure-dependent sodium excretion under these conditions, we blocked Ang II production with captopril or blocked its renal receptor, the AT1 receptor. The latter precaution also allowed us to distinguish between Ang II–related effects and possible confounding effects of increased bradykinin production. The fact that both captopril and CV 11974 effectively shifted the renal pressure-natriuresis and -diuresis curves leftward toward control demonstrates the important role of the renin-angiotensin system for the rightward shift in the curves in TGR.
An earlier investigation showed that renin gene expression in the kidneys of adult TGR was suppressed compared with control rats.5 In addition, renal renin synthesis and content were reported to be extremely low in that study. Furthermore, plasma renin activity was found to be suppressed in TGR compared to SDH, and Ang II levels in TGR were not elevated.1 3 5 26 The fact that captopril decreases blood pressure in TGR very effectively2 nevertheless is a strong argument that Ang II production is related to the increase of blood pressure. Both captopril and CV 11974 lowered blood pressure in TGR in our experiments, supporting the notion that the renin-angiotensin system is important in causing hypertension and shifting the TGR pressure-natriuresis-diuresis relationship toward higher RPP values.
The adrenal gland and vascular wall, where renin is produced locally, have been raised as possible sites of increased renin expression.3 27 28 Glucocorticoids given to TGR decreased their blood pressure but not to normotensive values.29 We therefore directed our attention to the kidneys to elucidate the hypertension of TGR. Precedence for the role of the kidneys in renin-angiotensin–induced hypertension is well established.7 30 Our data suggest that the kidneys are probably intimately involved in the pathogenesis of hypertension in TGR. In kidneys "clamped" from other influences both captopril and AT1 blockade reverted the pressure- natriuresis-diuresis relationship to nearly normal. Long-term Ang II infusion is a well-established model of hypertension that features sodium retention and altered pressure-natriuresis relationships.24 Ang II is capable of influencing the distribution of RBF9 10 11 and the regulation of tubuloglomerular feedback12 13 and is directly able to control sodium reabsorption in the renal tubule.7 8 The agents we used work by inhibiting the generation or action of Ang II. In TGR the decreased Ang II content of the plasma may still be inappropriately high for the given perfusion pressure. Furthermore, the plasma and intrarenal levels of Ang II may be different, as was shown in two-kidney, one clip hypertensive rats as well as after converting enzyme inhibition.31 32 In those studies Ang II levels were elevated in certain regions of the kidney despite normal plasma Ang II levels. We suggest that the role of the kidneys in the increased blood pressure of TGR should be reassessed in light of these findings.
Our aim was to concentrate on Ang II–related effects on renal function because the hypertension in TGR is caused by genetically transmitted changes in the renin-angiotensin system. Thus, we tested no agents other than captopril and an AT1 receptor antagonist. Ample evidence has accumulated indicating that any change in renal function that improves sodium excretion is able to shift the pressure-natriuresis-diuresis curve leftward. Thus, other interventions would also be expected to shift the renal pressure-natriuresis-diuresis curve to the left in TGR. For instance, calcium antagonists reset the pres- sure-natriuresis-diuresis curve by Ang II–independent mechanisms.25 33 34 35 Furthermore, hydralazine shifted the pressure-natriuresis-diuresis relationship in spontaneously hypertensive rats leftward compared with control Wistar-Kyoto rats,23 indicating that natriuresis and diuresis were perhaps induced by modified renal perfusion or physical factors.
In contrast to our earlier report6 we found that RBF in TGR was lower than in SDH. In both TGR and SDH blockade of the renin-angiotensin system increased RBF, supporting the general conclusion that Ang II is important in regulating renal vascular tone. Both substances probably had little additional effect on renal autoregulation,36 as the relationships between RPP and RBF were similar with or without captopril or CV 11974. We found that GFR in TGR was also significantly lower than in SDH. This result confirms our previous finding6 ; however, it conflicts with other reports in which differences in GFR between TGR and SDH could not be found.33 37 Our experimental design probably accounts for the differences. We measured GFR in maximally hydrated, unconscious preparations with denervated kidneys and "clamped" plasma vasopressin, corticosterone, aldosterone, and norepinephrine levels. Since GFR was not affected by captopril and AT1 receptor blockade, we attributed the lower GFR in TGR to fixed hypertension-induced structural changes.
The fact that Ang II blockade can shift the pressure-natriuresis curve without influencing GFR supports the notion that the resetting of the pressure-natriuresis-diuresis relationship may be related to an Ang II–mediated proximal tubular sodium reabsorption. This interpretation is consistent with the view that intrarenal infusion of Ang II can induce sodium and water retention without altering GFR38 by a marked increase in proximal tubular fractional sodium and water reabsorption.22 24 The increase in RBF and decrease in filtration fraction after Ang II blockade in TGR may have indirectly contributed to changes in peritubular hydrostatic and oncotic pressures and thereby decreased sodium reabsorption in these rats.
The pressure-natriuresis-diuresis curves in TGR were not completely normalized by blockade of the renin-angiotensin system. In addition to incomplete blockade because of inaccessible intrarenal renin-angiotensin compartments,31 39 hypertension-related structural changes also may have been responsible. We identified fairly severe nephrosclerosis in TGR, including substantial vascular, tubular, and interstitial damage. The vascular changes included fibrinoid necrosis and were particularly prominent in small arteries and arterioles. In contrast to Bachmann et al,5 we did not observe medial thickening in larger arteries, such as the arcuate and lobular vessels.
In summary, our experiments indicate that blockade of Ang II generation by captopril or blockade of the AT1 receptor with CV 11974 in TGR is accompanied by a shift of the pressure-natriuresis and -diuresis relationships to a lower operating level of arterial pressure. This shift was observed under conditions in which differences in the neural and circulating humoral factors to the kidney were "clamped." Since neither converting enzyme inhibition nor AT1 receptor blockade influenced GFR in TGR but instead induced parallel changes in FENa and FEH2O as RPP was changed, the improved water and sodium excretory ability in TGR may have been primarily related to decreased tubular reabsorption. The results underscore the importance of the renin-angiotensin system for sodium and water balance in TGR at the level of the kidney. The role of the kidney in the pathogenesis of TGR hypertension warrants reassessment. We suggest that the kidneys are pivotal in this process.
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Selected Abbreviations and Acronyms |
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Acknowledgments |
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This study was supported by a grant-in-aid to Volkmar Gross by the Deutsche Forschungsgemeinschaft and a grant-in-aid to Friedrich Luft from the Bundesministerium für Bildung und Forschung. We are grateful to Regina Uhlmann for technical assistance. We thank Allen W. Cowley, Jr, PhD, Chairman, Department of Physiology, Medical College of Wisconsin, Milwaukee, for helpful suggestions. CV 11974 was a gift from Takeda Chemical Industries, Ltd, Osaka, Japan.
Received January 18, 1995; first decision March 3, 1995; accepted May 23, 1995.
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