11Beta-hydroxysteroid dehydrogenase in the Dahl rat

Renal medullary 11 beta-hydroxysteroid dehydrogenase type 1 in Dahl salt-sensitive hypertension

The Dahl salt-sensitive rat is a widely used model of human salt-sensitive forms of hypertension. The kidney plays an important role in the pathogenesis of Dahl salt-sensitive hypertension, but the molecular mechanisms involved remain a subject of intensive investigation. Gene expression profiling studies suggested that 11 beta-hydroxysteroid dehydrogenase type 1 might be dysregulated in the renal medulla of Dahl salt-sensitive rats. Additional analysis confirmed that renal medullary expression of 11 beta-hydroxysteroid dehydrogenase type 1 was downregulated by a high-salt diet in SS-13BN rats, a consomic rat strain with reduced blood pressure salt sensitivity, but not in Dahl salt-sensitive rats. 11 beta-Hydroxysteroid dehydrogenase type 1 is known to convert inactive 11-dehydrocorticosterone to active corticosterone. The urinary corticosterone/11-dehydrocorticosterone ratio as well as urinary excretion of corticosterone was higher in Dahl salt-sensitive rats than in SS-13BN rats. Knockdown of renal medullary 11 beta-hydroxysteroid dehydrogenase type 1 with small-interfering RNA attenuated the early phase of salt-induced hypertension in Dahl salt-sensitive rats and reduced urinary excretion of corticosterone. Knockdown of 11 beta-hydroxysteroid dehydrogenase type 1 did not affect blood pressure in SS-13BN rats. Long-term attenuation of salt-induced hypertension was achieved with small hairpin RNA targeting renal medullary 11 beta-hydroxysteroid dehydrogenase type 1. In summary, we have demonstrated that suppression of 11 beta-hydroxysteroid dehydrogenase type 1 expression in the renal medulla attenuates salt-induced hypertension in Dahl salt-sensitive rats.

Increases in CSF [Na+] precede the increases in blood pressure in Dahl S rats and SHR on a high-salt diet

In Dahl salt-sensitive (S) and salt-resistant (R) rats, and spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats, at 5-6 wk of age, a cannula was placed in the cisterna magna, and cerebrospinal fluid (CSF) was withdrawn continuously at 75 microl/12 h. CSF was collected as day- and nighttime samples from rats on a regular salt intake (0.6% Na+; R-Na) and then on a high salt intake (8% Na+; H-Na). In separate groups of rats, the abdominal aorta was cannulated and blood pressure (BP) and heart rate (HR) measured at 10 AM and 10 PM, with rats first on R-Na and then on H-Na. On H-Na, CSF [Na+] started to increase in the daytime of day 2 in Dahl S rats and of day 3 in SHR. BP and HR did not rise until day 3 in Dahl S rats and day 4 in SHR. In Dahl R and WKY rats, high salt did not change CSF [Na+], BP, or HR. In a third set of Dahl S rats, sampling of both CSF and BP was performed in each individual rat. Again, significant increases in CSF [Na+] were observed 1-2 days earlier than the increases in BP and HR. In a fourth set of Dahl S rats, BP and HR were recorded continuously by means of radiotelemetry for 5 days on R-Na and 8 days on H-Na. On H-Na, BP (but not HR) increased first in the nighttime of day 2. In another set of Dahl S rats, intracerebroventricular infusion of antibody Fab fragments binding ouabain-like compounds (OLC) with high affinity prevented the increase in BP and HR by H-Na but further increased CSF [Na+]. Finally, in Wistar rats on H-Na, intracerebroventricular infusion of ouabain increased BP and HR but decreased CSF [Na+]. Thus, in both Dahl S and SHR on H-Na, increases in CSF [Na+] preceded the increases in BP and HR, consistent with a primary role of increased CSF [Na+] in the salt-induced hypertension. An increase in brain OLC in response to the initial increase in CSF [Na+] appears to attenuate further increases in CSF [Na+] but at the "expense" of sympathoexcitation and hypertension.

Sexual dimorphism of 11beta-hydroxysteroid dehydrogenase in hypertensive and normotensive rats

To evaluate the role of sexually dimorphic tissue expression of 11beta-oxidase activity of 11beta-hydroxysteroid dehydrogenase (11betaHSD) in gender-associated blood pressure differences, we have studied female and male hypertensive rats of two different strains and their normotensive controls: spontaneously hypertensive rats (SHR), Wistar-Kyoto rats (WKY) and Dahl salt-sensitive (SS/Jr) and salt-resistant rats (SR/Jr). In hypertensive SHR and SS/Jr, but not in normotensive strains WKY and SR/Jr, blood pressure reached a higher level in males than in females. The activity of 11betaHSD was higher in the renal cortex, medulla, colon and aorta of males than of females in all investigated strains with the exception of aortic 11betaHSD in SHR and WKY rats, both of which had very low 11beta-oxidase activity. In contrast to gender-dependent differences, strain differences of 11betaHSD were observed in a limited number of tissues only. Renal medullary 11betaHSD showed significantly lower activity in WKY than in SHR, whereas no difference was observed in the renal cortex. Similarly, colonic 11betaHSD activity was lower in WKY than in SHR. In Dahl rats the strain differences were observed in aortic 11betaHSD that had higher activity in SR/Jr than in SS/Jr rats; no difference was observed in the kidney or colon. These data demonstrate the following. 1) Sexual dimorphism of 11betaHSD activity exists in the kidney, colon, and aorta. 2) The sexual dimorphism of 11betaHSD does not play a role in gender-associated differences in blood pressure. 3) The reduced 11betaHSD activity in the aorta of hypertensive SS/Jr compared to SR/Jr rats suggests that this enzyme might play a role in the pathogenesis of salt-sensitive hypertension in Dahl rats.

Enhanced sympathoexcitatory and pressor responses to central Na+ in Dahl salt-sensitive vs. -resistant rats

An enhanced responsiveness to increases in cerebrospinal fluid (CSF) Na+ by high salt intake may contribute to salt-sensitive hypertension in Dahl salt-sensitive (S) rats. To test this hypothesis, sympathetic and pressor responses to acute and chronic increases in CSF Na+ were evaluated. In conscious young (5-6 wk old) and adult (10-11 wk old) Dahl S and salt-resistant (R) rats as well as weight-matched Wistar rats, hemodynamic [blood pressure (BP) and heart rate (HR)] and sympathetic [renal sympathetic nerve activity (RSNA)] responses to 10-min intracerebroventricular infusions of artificial CSF (aCSF) and Na+-rich aCSF (containing 0.2-0.45 M Na+) were evaluated. Intracerebroventricular Na+-rich aCSF increased BP, RSNA, and HR in a dose-related manner. The extent of these increases was significantly larger in Dahl S versus Dahl R or Wistar rats and young versus adult Dahl S rats. In a second set of experiments, young Dahl S and R rats received a chronic intracerebroventricular infusion of aCSF or Na+-rich (0.8 M) aCSF (5 microl/h) for 14 days, with the use of osmotic minipumps. On day 14 in conscious rats, CSF was sampled and BP, HR, and RSNA were recorded at rest and in response to air stress, intracerebroventricular alpha2-adrenoceptor agonist guanabenz, intracerebroventricular ouabain, and intravenous phenylephrine and nitroprusside to estimate baroreflex function. The infusion of Na+-rich aCSF versus aCSF increased CSF Na+ concentration to the same extent but caused severe versus mild hypertension in Dahl S and Dahl R rats, respectively. After central Na+ loading, hypothalamus "ouabain" significantly increased in Dahl S and only tended to increase in Dahl R rats. Moreover, sympathoexcitatory and pressor responses to intracerebroventricular exogenous ouabain were attenuated by Na+-rich aCSF to a greater extent in Dahl S versus Dahl R rats. Responses to air-jet stress or intracerebroventricular guanabenz were enhanced by Na+-rich aCSF in both strains, but the extent of enhancement was significantly larger in Dahl S versus Dahl R. Na+-rich aCSF impaired arterial baroreflex control of RSNA more markedly in Dahl S versus R rats. These findings indicate that genetic control of mechanisms linking CSF Na+ with brain "ouabain" is altered in Dahl S rats toward sympathetic hyperactivity and hypertension.

Effect of angiotensin II on rat renal cortical 11beta-hydroxysteroid dehydrogenase

Renal 11beta-hydroxysteroid dehydrogenases (11beta-HSDs) are subject to modulation by various endogenous factors. 11beta-HSDs convert glucocorticoids into inactive 11-ketones and thereby determine tissue levels of active glucocorticoids and thus the extent of glucocorticoid receptor (GR) and mineralocorticoid receptor (MR) activation. As such, modulation of the activity of renal 11beta-HSDs may contribute to the cascade of regulatory events involved in renal electrolyte water handling. We investigated whether renal 11beta-HSDs are modulated by elevated circulating angiotensin II. In rats infused for 2 wk with angiotensin II (250 ng/[kg x min] subcutaneously), plasma angiotensin II, aldosterone, and corticosterone were raised 5.1-, 10.7-, and 2.3-fold, respectively, compared with control rats. Angiotensin II infusion raised corticosterone 11beta-oxidation 1.46- and 1.35-fold in renal cortical proximal and distal tubules (enriched by Percoll centrifugation), respectively, but had no effect on 11beta-HSD1 and 11beta-HSD2 mRNA levels (semiquantitative reverse transcriptase polymerase chain reaction), except for distal tubular 11beta-HSD1 mRNA, which was decreased to 50%. In vitro treatment of freshly isolated tubules with angiotensin II for 45 min prior to assessment of 11beta-HSD activity showed no direct acute effects of angiotensin II on tubular corticosterone 11beta-oxidation. The enhanced renal tubular corticosterone 11beta-oxidation in vivo may partly protect renal GR and MR from elevated plasma corticosterone on angiotensin II infusion.

11Beta-hydroxysteroid dehydrogenase activity in spontaneously hypertensive and Dahl rats

The role of the enzyme 11beta-hydroxysteroid dehydrogenase (11betaHSD) in hypertension remains unknown even if it appears that the inappropriately decreased 11betaHSD activity might be involved in a process that leads to high blood pressure. The possible changes of 11betaHSD were therefore investigated in rats with spontaneous or salt-induced hypertension. The adult male rats of the following genotypes were used: spontaneously hypertensive rats (SHR), normotensive Wistar-Kyoto rats (WKY), Dahl salt-sensitive rats fed either a high-salt diet containing 8% NaCl (DS-HS) or low-salt diet containing 0.2% NaCl (DS-LS), and Dahl salt-resistant rats fed the same diets (DR-HS, DR-LS). 11betaHSD was investigated in colon, aorta, renal cortex, and renal medulla and was assessed as percentage conversion of [3H]corticosterone to [3H]11-dehydrocorticosterone in the presence of NAD or NADP. The results demonstrated that genotype exerts a significant effect on 11betaHSD. 11betaHSD activity was significantly increased in colon and renal medulla of SHR compared with WKY rats. No significant differences were observed in renal cortex and aorta. In Dahl rats kept on a low-salt diet, 11betaHSD activity was significantly higher in colon, renal medulla, and cortex of DS-LS than in DR-LS rats but no difference was observed in aorta. The differences disappeared in age-matched DS and DR rats fed the high-salt diet. Increased dietary sodium intake stimulated the activity of 11betaHSD in renal cortex and medulla of DR rats and decreased the activity in colon of DS rats. We conclude that the development of spontaneous and salt-induced hypertension is not associated with decreased activity of 11betaHSD. However, the results showed that salt intake is able to modulate the activity of 11betaHSD and that 11betaHSD in DS and DR rats responds to high dietary salt intake in a different manner.

Regulation of the genes for 11beta-hydroxysteroid dehydrogenase type 1 and type 2 in the kidney of the Dahl rat

BACKGROUND

An isoenzyme of 11beta-hydroxysteroid dehydrogenase (11beta-HSD), 11beta-HSD-2 confers aldosterone specificity on the mineralocorticoid receptor (MR) and is found collocated in renal cortical collecting duct cells with the MR. To investigate whether the salt sensitivity of the Dahl salt-sensitive (S) rat is due to 11beta-HSD deficiency, we measured 11beta-HSD-1 and 11beta-HSD-2 mRNA levels in the kidneys of Dahl-S and Dahl salt-resistant (R) rats. In addition, we studied the effects of gender, age and dietary sodium on expression of mRNA for the two isoforms. S and R rats were placed on low- or high-sodium (HNa) diets and sacrificed after 33 and 115 days. Rat kidney RNA was isolated and 11beta-HSD-1 and 11beta-HSD-2 mRNA levels were measured on Northern filter hybridization using isoform-specific probes.

RESULTS

No strain differences were observed in the mRNA expression of the two isoforms of 11beta-HSD under any of the experimental conditions. No gender or age differences were observed in 11beta-HSD-2 mRNA but HNa diet almost doubled 11beta-HSD-2 mRNA (P<0.0009). 11beta-HSD-1 mRNA levels were consistently higher, more than double, in male rats versus females rats (P<0.0001), and in the 115-day-old rats versus the 33-day-old rats (P<0.0001). Dietary sodium intake did not affect 11beta-HSD-1 mRNA levels.

CONCLUSIONS

There is no difference in the expression of the two isoforms of 11beta-HSD in the kidneys of the S and R rats, which might explain the salt sensitivity and higher blood pressure of the S rat. Renal 11beta-HSD-1 mRNA levels are higher in male than in female rats, and in the older rats of both strains. In the kidney, the 11beta-HSD-2 gene is regulated by sodium status but is not affected by gender or age.

Renal 11beta-hydroxysteroid dehydrogenase in genetically salt-sensitive hypertensive rats

-Renal 11beta-hydroxysteroid dehydrogenase II (11beta-HSDII) converts glucocorticoids into inactive metabolites and plays an important role in controlling blood pressure and sodium retention. To examine whether this enzyme may be involved in the pathophysiology of salt-sensitive hypertension, we determined 11beta-HSDII activity and mRNA levels in the blood vessel and kidney of Dahl Iwai salt-sensitive (DS) rats and Dahl Iwai salt-resistant (DR) rats. Urinary free corticosterone:free 11-dehydrocorticosterone ratio was measured to estimate renal 11beta-HSD activity. Vascular 11beta-HSDII activity was expressed as percent conversion of [3H]corticosterone to [3H]11-dehydrocorticosterone in homogenized mesenteric arteries. 11beta-HSDII mRNA was estimated with the use of competitive polymerase chain reaction (PCR). Renal 11beta-HSDII activity and mRNA levels were significantly decreased in 8- and 12-week-old high salt DS rats compared with DR, Sprague-Dawley (SD), or low salt DS rats of the same age. Decreased 11beta-HSDII activity and mRNA levels in mesenteric arteries were observed in 8- and 12-week-old high salt DS rats. Urinary excretion of 11beta-HSDII inhibitory factors was measured by inhibition of enzyme activity in microsomes from human kidney. The urinary inhibitors were significantly increased in 8- and 12-week-old high salt DS rats compared with DR, SD, or low salt DS rats of the same age. There were no significant differences in 11beta-HSDII activity and mRNA levels in mesenteric arteries and kidney or in urinary inhibitors between 4-week-old DS, DR, and SD rats. These results indicate that 11beta-HSDII may play a role in salt sensitivity and development of hypertension in the DS rat.