Vasopressin response in collecting ducts of rats resistant to mineralocorticoid 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.

Increased expression and shuttling of aquaporin-2 water channels in the kidney in DOCA-salt hypertensive rats

An altered role of AQP2 water channels in DOCA-salt hypertension was investigated. DOCA-salt hypertension was induced in rats. Control groups were either treated with DOCA alone or subjected to a high-salt intake without DOCA. Four weeks after inducing the hypertension, AQP2 expression and shuttling were determined in the kidney. Adenylate cyclase activity was also determined to examine the upstream affecting the AQP2 system. The AVP-evoked cAMP generation in the cortex and outer medulla was augmented following the treatment with DOCA either alone or combined with high-salt intake. Accordingly, the expression and shuttling of AQP2 proteins were increased in the cortex and outer medulla. These findings suggest that DOCA enhances cAMP generation and expression/shuttling of AQP2 water channels in the kidney, which may be causally related with the development of hypertension.

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.

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.

5-Carboxamidotryptamine attenuates the development of deoxycorticosterone acetate-salt hypertension in rats

The effect of chronic i.v. infusion of the 5-HT1 receptor agonist, 5-carboxamidotryptamine (5-CT), was evaluated during the development of deoxycorticosterone acetate-salt (DOCA-salt) hypertension in rats over 4 weeks. Vehicle-treated (n = 10) Sprague-Dawley rats given DOCA (100 mg/kg, s.c.) and 1% saline as drinking fluid developed hypertension with systolic blood pressure reaching 194.6 +/- 8.99 mm Hg at 27 days. In DOCA-salt rats treated with 5-CT infusions (15.0 micrograms/kg per day, n = 10) for 4 weeks via osmotic minimpumps, systolic blood pressure was significantly lower by 41.7 mm Hg at day 27 when compared to vehicle-treated DOCA-salt rats. Systolic blood pressure values on day 27 in 5-CT-treated DOCA-salt rats were however greater than those in vehicle-treated control rats which were not given DOCA. Systolic blood pressure in 5-CT-treated DOCA-salt rats was significantly lower by day 7 compared to vehicle-treated DOCA-salt rats and remained lowered for the rest of the observation period. Heart rate was significantly greater in 5-CT-treated DOCA-salt rats on day 7 when compared to vehicle-treated DOCA-salt rats. Baroreflex sensitivity on day 28 was significantly greater in 5-CT-treated DOCA-salt rats as compared to vehicle-treated DOCA-salt rats. On day 28, hypotensive responses to hexamethonium (20 mg/kg) in 5-CT-treated DOCA-salt rats were markedly reduced compared to those in vehicle-treated DOCA-salt rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of vasopressin receptors in deoxycorticosterone acetate-salt hypertension

Since arginine vasopressin may play a role in mineralocorticoid hypertension, we examined the effects of deoxycorticosterone acetate (DOCA)-salt on vasopressin V1 and V2 receptor binding and their second messengers, inositol phosphate and adenylate cyclase, respectively, in liver and kidney to determine whether altered vasopressin receptor binding is pathogenetic in mineralocorticoid hypertension. The mean arterial blood pressure of mineralocorticoid (DOCA-salt)-treated rats (163 +/- 1 mm Hg) was increased compared with control salt-treated rats (salt) (122 +/- 1 mm Hg) and water-treated rats (120 +/- 1 mm Hg; p less than 0.001). Mineralocorticoid treatment also increased plasma sodium, osmolality, and vasopressin concentration (p less than 0.001). In the hypertensive animals, there was a reduction in hepatic V1 (DOCA-salt, 91 +/- 12; salt, 132 +/- 13; and water, 145 +/- 13 fmol/mg protein; p less than 0.05) and renal V2 receptor binding density (DOCA-salt, 53 +/- 5; salt, 93 +/- 9; and water, 95 +/- 9 fmol/mg protein; p less than 0.01), although receptor affinities remained unaltered. In contrast, the density of renal V1 receptors was increased by mineralocorticoid treatment (DOCA-salt, 24 +/- 2; salt, 16 +/- 2; water, 18 +/- 1 fmol/mg protein; p less than 0.05), although the affinity was unchanged. Downregulation of V2 receptors was associated with a decrease in maximum cyclic adenosine monophosphate levels (DOCA-salt, 19 +/- 4; salt, 49 +/- 6; water, 53 +/- 9 pmol.mg protein-1.10 min-1; p less than 0.05), whereas changes in V1 receptor levels were not associated with changes in maximum inositol phosphate levels.(ABSTRACT TRUNCATED AT 250 WORDS)

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.