Resistance to mineralocorticoid-induced hypertensive vascular disease

Antagonistic effects of bone morphogenetic protein-4 and -7 on renal mesangial cell proliferation induced by aldosterone through MAPK activation

Aldosterone and angiotensin II (ANG II) contribute to the development and progression of renal damage. Here we investigated the effects of bone morphogenetic proteins (BMPs) on renal cell proliferation evoked by aldosterone and ANG II with mouse mesangial cells, which express mineralocorticoid receptors (MR), ANG II type 1 receptors, and BMP signaling molecules. Aldosterone and ANG II stimulated mesangial cell mitosis and activated ERK1/2 and SAPK/JNK signaling. These aldosterone effects were neutralized by the MR antagonist eplerenone and inhibition of transcription or translation, suggesting the involvement of genomic activation via MR. BMP-4 and BMP-7 stimulated Smad1, -5, -8 signaling more potently than BMP-2 and BMP-6, leading to suppression of mesangial cell mitosis and MR expression. MAPK inhibitors including U-0126 and SP-600125, but not SB-203580, suppressed aldosterone-induced cellular DNA synthesis, implying that ERK1/2 and SAPK/JNK pathways play crucial roles in mesangial cell proliferation. BMP-4 and BMP-7 inhibited phosphorylation of ERK1/2 and SAPK/JNK induced by aldosterone while activating p38 pathway, resulting in inhibition of aldosterone-induced cell mitosis. In contrast, aldosterone modulated the mesangial BMP system by decreasing expression of ALK-3, BMP-4, and BMP-7 while increasing inhibitory Smad6 expression. Thus novel functional cross talk between the mesangial BMP system and aldosterone signaling was uncovered, in which inhibition of MAPK signaling and MR expression by BMP-4 and BMP-7 may be involved in ameliorating renal damage due to mesangial proliferation caused by aldosterone.

Arterial expression of 5-HT2B and 5-HT1B receptors during development of DOCA-salt hypertension

BACKGROUND

5-hydroxytryptamine (5-HT)2B and 5-HT1B receptors are upregulated in arteries from hypertensive DOCA-salt rats and directly by mineralocorticoids. We hypothesized that increased 5-HT2B and 5-HT1B receptor density and contractile function would precede increased blood pressure in DOCA-high salt rats. We performed DOCA-salt time course (days 1, 3, 5 and 7) studies using treatment groups of: DOCA-high salt, DOCA-low salt, Sham and Sham-high salt rats.

RESULTS

In isolated-tissue baths, DOCA-high salt aorta contracted to the 5-HT2B receptor agonist BW723C86 on day 1; Sham aorta did not contract. The 5-HT1B receptor agonist CP93129 had no effect in arteries from any group. On days 3, 5 and 7 CP93129 and BW723C86 contracted DOCA-high salt and Sham-high salt aorta; Sham and DOCA-low salt aorta did not respond. Western analysis of DOCA-high salt aortic homogenates revealed increased 5-HT2B receptor levels by day 3; 5-HT1B receptor density was unchanged. Aortic homogenates from the other groups showed unchanged 5-HT2B and 5-HT1B receptor levels.

CONCLUSION

These data suggest that functional changes of 5-HT2B but not 5-HT1B receptors may play a role in the development of DOCA-salt hypertension.

Potential importance of glomerular citrate synthase activity in remnant nephropathy

BACKGROUND

Aldosterone fosters progressive renal injury, but the mechanism is unknown. Both Wistar-Furth rats, which are resistant to aldosterone actions, and adrenalectomized Sprague-Dawley rats, which lack aldosterone, are characterized by resistance to remnant nephropathy and by reduced whole kidney citrate synthase activity. Increase in citrate synthase activity is a well-characterized, specific renal response to aldosterone. Therefore, we performed experiments to test the hypothesis that enhanced citrate synthase activity contributes to remnant nephropathy.

METHODS

Rat models included Wistar (control for Wistar-Furth), Wistar-Furth (resistant to aldosterone), Sprague-Dawley (normal), adrenalectomy (lacking aldosterone), and 5/6 nephrectomy (renal injury). Glomeruli were obtained by differential sieving. Citrate synthase activity was determined spectrophotometrically. Binding characteristics of cytosolic mineralocorticoid receptors were determined by equilibrium competition binding between tritiated and unlabeled aldosterone. Gene sequencing was performed with reverse transcription-polymerase chain reaction (RT-PCR) and fluorescent dye terminators.

RESULTS

In glomeruli isolated from adrenalectomized Wistar rats with intact renal mass, aldosterone stimulated a threefold increase in citrate synthase activity; this stimulation was not observed in glomeruli from Wistar-Furth rats. Similarly, citrate synthase activity in glomeruli isolated from adrenally intact Sprague-Dawley rats was 65% greater than that from adrenalectomized Sprague-Dawley rats. Compared to sham surgery, subtotal nephrectomy resulted in 100% greater glomerular citrate synthase activity in Sprague-Dawley rats. In Wistar-Furth rats, mineralocorticoid receptor binding was not reduced, and mutations in the mineralocorticoid receptor DNA binding segment were not found.

CONCLUSION

Citrate synthase activity is elevated in remnant glomeruli, and experimental models characterized by reduced glomerular citrate synthase activity (Wistar-Furth rats, adrenalectomized Sprague-Dawley rats) are protected from remnant nephropathy.

Upregulation of arterial serotonin 1B and 2B receptors in deoxycorticosterone acetate-salt hypertension

Previous studies have established a role for 5-hydroxytryptamine (5-HT)(2B) and 5-HT(1B) receptors in mediating enhanced contraction to serotonin (5-HT) in arteries from hypertensive deoxycorticosterone acetate (DOCA)-salt rats. To determine whether the observed increase in responsiveness was due to upregulation of 5-HT receptors, we used Western analysis to measure 5-HT(1B) and 5-HT(2B) receptor protein density. In endothelium-denuded aortas from hypertensive DOCA-salt rats (mean systolic blood pressure 192 +/- 6 mm Hg), 5-HT(1B) and 5-HT(2B) receptor proteins were upregulated approximately 2-fold compared with the response in the aortas of sham-operated control rats (mean systolic blood pressure 119 +/- 2 mm Hg). Contraction to 5-HT(2B) receptor agonists was also enhanced in arteries from Wistar-Furth rats given DOCA and salt. This strain is relatively resistant to the hypertensive effects of DOCA and salt treatment. A common factor between the model of DOCA-salt hypertension and the DOCA-salt--treated Wistar-Furth rats is the presence of mineralocorticoids. Therefore, we tested the hypothesis that mineralocorticoids can upregulate 5-HT(1B) and 5-HT(2B) receptors. Aortas from normal Sprague-Dawley rats were incubated with aldosterone (100 nmol/L) for 8, 12, 24, and 48 hours. The expression of 5-HT(2B) and 5-HT(1B) receptor proteins was significantly increased (approximately 2- fold over vehicle treatment) by 8 hours. 5-HT(2B) and 5-HT(1B) receptors were upregulated by aldosterone in a concentration-dependent manner, and incubation with spironolactone (10 micromol/L) blocked this upregulation. These data support the conclusion that the increased expression of 5-HT(1B) and 5-HT(2B) receptors observed in arteries from DOCA-salt rats may be partially due to mineralocorticoids acting via the mineralocorticoid receptor to modulate gene expression.

Mineralocorticoids upregulate arterial contraction to epidermal growth factor

The present studies test the hypothesis that contraction to EGF is dependent on mineralocorticoids and/or an elevation in systolic blood pressure (SBP). Endothelium-denuded thoracic aortas from sham normotensive, N(omega)-nitro-L-arginine (L-NNA) hypertensive, Wistar-Kyoto (WKY), and spontaneously hypertensive rats (SHR) were used in isolated tissue-bath experiments. Maximal contraction to epidermal growth factor [EGF; percentage of phenylephrine (PE; 10 umol/l)-induced contraction] was greater in strips from L-NNA (32 +/- 5%) and SHR (53 +/- 8%) rats compared with sham and WKY rats (17 +/- 1 and 12 +/- 4%, respectively). Wistar-Furth rats became only mildly hypertensive when given DOCA salt (134 +/- 6 mmHg) compared with Wistar rats (176 +/- 9 mmHg), but aortas from both strains had a similarly enhanced contraction to EGF (approximately 9 times the maximal contraction of sham aorta). Furthermore, in vitro incubation of aortas from Wistar and Wistar-Furth rats with aldosterone (10 nmol/l) increased EGF-receptor mRNA expression by >50%. These data indicate that arterial contraction to EGF may occur independent of hypertension and be stimulated by mineralocorticoids.

Role of citrate synthase in aldosterone-mediated sodium reabsorption

Aldosterone and other mineralocorticoids increase citrate synthase activity in the kidney and enhance renal sodium reabsorption, but it is unclear whether the increased citrate synthase activity is involved in renal sodium transport. We used the Wistar-Furth rat, an inbred strain found to be deficient in renal citrate synthase activity, as an experimental model to investigate this issue. We confirmed that renal citrate synthase activity from adrenalectomized Wistar-Furth rats was decreased compared with that from control Wistar rats (by 28%). Similarly, urinary citrate excretion was 23% lower in Wistar-Furth rats. Subnormal citrate formation in Wistar-Furth rats could not be accounted for by differences in systemic pH or circulating potassium levels. Because renal citrate synthase activity was reduced in Wistar-Furth rats, we hypothesized that renal sodium excretory responses to mineralocorticoids would be reduced as well. Four-hour sodium excretion after intraperitoneal injection of 5 microg of aldosterone was reduced by 56% in adrenalectomized Wistar rats and by 52% in adrenalectomized Wistar-Furth rats (both P<0.01 compared with vehicle injection). Similarly, the pattern of urinary sodium excretion in response to subcutaneous injections of deoxycorticosterone acetate over a 2-week period was similar in adrenalectomized Wistar and Wistar-Furth rats. In summary, acute and chronic antinatriuretic responses to mineralocorticoids are maintained in Wistar-Furth rats at the level of Wistar rats, despite the marked reduction in citrate synthase activity. These findings are not consistent with an important role for citrate synthase activity in mineralocorticoid-mediated renal sodium transport.

Is functional upregulation of the 5-HT2B receptor in deoxycorticosterone acetate salt-treated rats blood pressure dependent?

This study tests the hypothesis that the functional upregulation of the arterial 5-hydroxytryptamine (5-HT)2B receptor in arteries of deoxycorticosterone acetate (DOCA)-salt hypertensive rats depends on the development of high blood pressure. Wistar-Furth and Wistar rats were given sham or DOCA-salt treatment (200 mg/kg DOCA, SC; 1.0% NaCl and 0.2% KCI in drinking water). Systolic blood pressures (4 week; mm Hg) were: Wistar Sham (120+/-3), Wistar DOCA (176+/-6), Wistar-Furth Sham (112+/-3) and Wistar-Furth DOCA (136+/-4). Isolated mesenteric arteries from Wistar DOCA and Wistar-Furth DOCA rats displayed a three- to fivefold leftward shift in contraction to 5-HT that was insensitive to blockade by the 5-HT2A receptor antagonist ketanserin (10 nM) and a significantly increased maximal contraction to the 5-HT2B receptor agonist BW723C86 [Wistar DOCA = 90+/-17% phenylephrine contraction; Wistar Sham = 1+/-1%; Wistar-Furth DOCA = 33+/-8%; Wistar-Furth Sham = 0%]. Arteries from Sprague-Dawley rats receiving salt or DOCA alone displayed similar systolic blood pressures (151+/-11 mm Hg and 144+/-5 mm Hg, respectively), but only tissues from rats receiving DOCA displayed an increased contraction to BW723C86 (DOCA alone = 60.7+/-16% vs. sham = 13+/-5.3%). These data suggest that upregulation of the arterial 5-HT2B receptor is largely independent of an increase in blood pressure.

Resistance to remnant nephropathy in the Wistar-Furth rat

The Wistar-Furth rat, an inbred strain resistant to actions of mineralocorticoids, was used to study the concept that mineralocorticoids contribute to progressive renal injury. It was postulated that if chronic nephropathy depends on aldosterone and if Wistar-Furth rats are resistant to aldosterone, remnant nephropathy would be attenuated in Wistar-Furth rats. Wistar-Furth rats and control Wistar rats were subjected to 5/6 nephrectomy or a sham procedure and then followed for 4 wk. Renal ablation resulted in hypertension at 4 wk in both strains (164+/-5 [Wistar-Furth] versus 184+/-7 [Wistar] mm Hg mean arterial pressure), with sham animals remaining normotensive (134+/-6 mm Hg). Renal damage in response to 5/6 nephrectomy was greatly decreased in Wistar-Furth rats compared with Wistar rats. Albuminuria was markedly less in Wistar-Furth rats (12.7+/-4.2 [Wistar-Furth] versus 97.4+/-22.6 [Wistar] mg/d per 100 g body wt, P<0.01). Glomerular damage, consisting of mesangial proliferation, mesangial lysis, and segmental necrosis, was observed in 42% of glomeruli from Wistar rats but in 0% of glomeruli from Wistar-Furth rats (P<0.01). To address the possibility that higher BP in partially nephrectomized Wistar rats mediated the greater renal damage, the study was repeated, with Wistar rats (not Wistar-Furth rats) being treated with a hydralazine-reserpine-hydrochlorothiazide regimen. Although this antihypertensive regimen equalized BP (conscious systolic) (144+/-8 mm Hg [Wistar] versus 157+/-7 mm Hg [Wistar-Furth] at 4 wk), albuminuria remained more than 10-fold greater in Wistar rats. In summary, renal damage upon 5/6 nephrectomy was markedly reduced in Wistar-Furth rats, a finding not attributable to reduced systemic BP. Since Wistar-Furth rats have been shown previously to be resistant to the actions of mineralocorticoids, the data from the present study support the hypothesis that aldosterone mediates, at least in part, the renal injury attendant to renal mass reduction.

NaCl appetite in two strains of rat reported to be resistant to mineralocorticoid-induced hypertension

Both Long-Evans (LE) and Wistar-Furth (WF) strains of rat are known to be resistant to development of hypertension by mineralocorticoid (MC) treatment. MC-induced hypertension is, in part, mediated by the brain. We have examined another aspect of central MC action, the induction of NaCl appetite in these strains, by using the more common Sprague-Dawley (SD) and Wistar (WS) strains for comparison. In the first experiment, LE and SD rats were administered three treatments known to induce an appetite for NaCl solution in rats. Administration of deoxycorticosterone acetate (DOCA) increased the intake of 0.45 M NaCl in both strains, but the amounts consumed were about 2-fold greater in LE rats than in SD rats. Administration of captopril also increased NaCl intake, but there were no differences between LE and SD rats. NaCl depletion with furosemide induced NaCl appetite in both strains, but the amounts consumed were about 2-fold greater in LE rats than in SD rats. In the second experiment, adult male WF and WS rats were administered DOCA, enalapril, or furosemide and NaCl appetite was determined. Both strains showed comparable NaCl appetite during each of these treatments. However, during a 5-week regimen of DOCA with only NaCl-KCl solution to drink, uninephrectomized WF rats consumed less than WS rats. Thus, despite reported resistance to MC-induced hypertension, neither LE nor WF strains of rats showed correspondingly marked deficits in induced NaCl appetite.

Genetic susceptibility to hypertension-induced renal damage in the rat. Evidence based on kidney-specific genome transfer

To test the hypothesis that genetic factors can determine susceptibility to hypertension-induced renal damage, we derived an experimental animal model in which two genetically different yet histocompatible kidneys are chronically and simultaneously exposed to the same blood pressure profile and metabolic environment within the same host. Kidneys from normotensive Brown Norway rats were transplanted into unilaterally nephrectomized spontaneously hypertensive rats (SHR-RT1.N strain) that harbor the major histocompatibility complex of the Brown Norway strain. 25 d after the induction of severe hypertension with deoxycorticosterone acetate and salt, proteinuria, impaired glomerular filtration rate, and extensive vascular and glomerular injury were observed in the Brown Norway donor kidneys, but not in the SHR-RT1.N kidneys. Control experiments demonstrated that the strain differences in kidney damage could not be attributed to effects of transplantation-induced renal injury, immunologic rejection phenomena, or preexisting strain differences in blood pressure. These studies (a) demonstrate that the kidney of the normotensive Brown Norway rat is inherently much more susceptible to hypertension-induced damage than is the kidney of the spontaneously hypertensive rat, and (b) establish the feasibility of using organ-specific genome transplants to map genes expressed in the kidney that determine susceptibility to hypertension-induced renal injury in the rat.

The resistance of the Wistar/Furth rat strain to steroid hypertension

We have previously reported that the Wistar/Furth (W/Fu) rat strain is resistant to mineralocorticoid hypertension. In the current study, we have examined renal mRNA levels for mineralocorticoid receptor (MR), glucocorticoid receptor (GR), renin and Na+, K(+)-ATPase in response to treatment with mineralocorticoids. Uninephrectomized male Wistar (WI) and W/Fu rats were treated with aldosterone or deoxycorticosterone acetate (DOCA) and were given 1% NaCl to drink. Rats were sacrificed after 1, 3 or 7 days of treatment. Renal MR and ATPase mRNA levels were significantly reduced in aldosterone and DOCA-treated WI rats (e.g. MR was 30% on day 3 and ATPase was 50% of control on day 7 of aldosterone treatment). Unexpectedly, GR mRNA levels paralleled the changes in MR. In W/Fu rats the level of message was either unchanged or only moderately altered by this treatment. In vivo administration of the MR antagonist RU28318 or the GR antagonist RU38486 to WI rats for 4 days reduced renal mRNA levels for both subunits of ATPase. In the W/Fu rat, this treatment resulted in no change in the alpha subunit and an increase in the beta subunit of ATPase. In preliminary studies, we have determined that the W/Fu rat is also resistant to dexamethasone-induced hypertension. These studies suggest that altered MR- and GR-mediated mechanisms may contribute to the resistance of the W/Fu rat strain to steroid-induced hypertension.

Vascular responsiveness in rats resistant to aldosterone-salt hypertension

Wistar-Furth rats have been shown to be resistant to mineralocorticoid-salt hypertension, but the mechanism for this resistance is unknown. In the current experiments, adult male Wistar and Wistar-Furth rats were given a subcutaneous aldosterone infusion (0.15 microgram/hr) for 4 weeks, and changes in blood pressure and vascular reactivity were studied. Rats received a 1% NaCl, 0.2% KCl solution to drink. After 4 weeks of aldosterone infusion, systolic blood pressure measured using a tail-cuff technique had increased by 60 mm Hg in Wistar rats but was unchanged in Wistar-Furth rats. Hypokalemia occurred in both strains in response to the aldosterone infusion. Isolated, helically cut strips of common carotid artery and aorta were prepared for isometric force recording. Cumulative concentration-response curves to norepinephrine, serotonin, KCl, calcium, nitroprusside, and acetylcholine were performed in carotid artery strips, and concentration-response curves to ouabain were performed in aortic strips. Increased vascular contractile sensitivity to KCl, ouabain, norepinephrine, and serotonin was observed in vessels from Wistar rats treated with aldosterone and salt. The same treatment in Wistar-Furth rats produced only increased vascular sensitivity to ouabain and serotonin, and these changes were of smaller magnitude than those seen in Wistar rats. Aldosterone-salt treatment produced decreased vascular sensitivity to acetylcholine and nitroprusside in both Wistar and Wistar-Furth rats. These results support the hypothesis that resistance of Wistar-Furth rats to aldosterone-salt hypertension is due to resistance to the effects of aldosterone-salt treatment that normally result in increased vasoconstrictor sensitivity.

Vasopressin response in collecting ducts of rats resistant to mineralocorticoid hypertension

In previous studies we found that vasopressin stimulation of both cyclic AMP (cAMP) formation in cortical collecting tubules (CCT) and sodium reabsorption in isolated perfused kidneys was markedly exaggerated in rats with mineralocorticoid hypertension. In the present study, we tested the response (cAMP accumulation) of cortical and outer medullary collecting tubules (OMCT) to vasopressin in two rat models that are resistant to deoxycorticosterone acetate (DOCA)-induced hypertension, the Wistar-Furth strain and NaCl-deficient rats. The blood pressure of normal outbred Wistar rats rose to hypertensive levels (systolic pressure more than 165 mm Hg) during a 5-week treatment with DOCA (10 mg/week) and 1% saline to drink. Significant hypertrophy of the heart and kidneys was also observed. Vasopressin (10(-8) M)-induced cAMP formation was enhanced 3.4-fold in the CCT (OMCT unchanged) of hypertensive rats compared with normotensive controls. Significant hypertrophy (as indexed by tubule diameter) of the CCT but not the OMCT was also observed in DOCA-salt hypertensive rats. Restriction of dietary NaCl (0.13% in chow, tap water to drink) completely prevented DOCA-induced hypertension, organ and CCT hypertrophy, and enhancement of vasopressin-stimulated cAMP formation in the CCT. In Wistar-Furth rats, DOCA-salt treatment did not alter blood pressure or cause significant organ hypertrophy. However, DOCA-salt treatment enhanced vasopressin-stimulated cAMP formation by 4.1-fold in CCT of Wistar-Furth rats, with significant tubular hypertrophy in the CCT but not the OMCT. We conclude that DOCA-induced hypertension and changes in CCT function are dependent on excess dietary NaCl.(ABSTRACT TRUNCATED AT 250 WORDS)