The influence of high salt intake on intestinal Na,K-ATPase in Wistar and Dahl rats

Biphasic time course of the changes in aldosterone biosynthesis under high-salt conditions in Dahl salt-sensitive rats

OBJECTIVE

The comorbidity of excess salt and elevated plasma aldosterone has deleterious effects in cardiovascular disease. We evaluated the mechanisms behind the paradoxical increase in aldosterone biosynthesis in relation to dietary intake of salt.

METHODS AND RESULTS

Dahl salt-sensitive (Dahl-S) and salt-resistant (Dahl-R) rats were fed a high-salt diet, and plasma and tissue levels of aldosterone in the adrenal gland and heart were quantified by liquid chromatography-electrospray ionization-tandem mass spectrometry. In Dahl-S rats, we found that the delayed and paradoxical increase in aldosterone biosynthesis after the initial and appropriate response to high salt. The late rise in aldosterone biosynthesis was accompanied by upregulation of CYP11B2 expression in the zona glomerulosa and increased adrenal angiotensin II levels and renin-angiotensin system components. It preceded the appearance of left ventricular systolic dysfunction and renal insufficiency. Blockade of angiotensin AT(1) receptors reversed the paradoxical increase in aldosterone biosynthesis. In contrast, Dahl-R rats maintained the initial suppression of aldosterone biosynthesis. Aldosterone levels in the heart closely paralleled those in the plasma and adrenal gland and disappeared after bilateral adrenalectomy.

CONCLUSIONS

Chronic salt overload in Dahl-S rats stimulates aberrant aldosterone production via activation of the local renin-angiotensin system in the adrenal gland, thereby creating the comorbidity of excess salt and elevated plasma aldosterone.

The mammalian mineralocorticoid receptor: tying down a promiscuous receptor

The mineralocorticoid receptor (MR) has been called a promiscuous receptor because its intrinsic affinity for aldosterone, cortisol and corticosterone is similar. Since glucocorticoids circulate in concentrations 100- to 1000-fold those of aldosterone, stoichiometry dictates that MR should be activated by glucocorticoids, not aldosterone, yet MRs are expressed in many tissues and regulate diverse functions, many of them under the regulation of the renin-angiotensin-aldosterone system. A relatively small number of brain MRs are aldosterone selective and modulate blood pressure. Evidence for possible mechanisms conferring ligand specificity in the context of mineralocorticoid-induced hypertension and the brain are discussed. These include factors (or mechanisms) intrinsic to the receptor, such as alternative splice variants and translation start sites, and extrinsic to the MR, including differential access through the blood-brain barrier, differential recruitment of co-regulators and scaffolding proteins, 11beta-steroid dehydrogenase activity, synthesis of potent acylated aldosterone derivatives and the synthesis of relevant amounts of aldosterone in areas of the brain that modulate blood pressure.

Aldosterone synthesis in the brain contributes to Dahl salt-sensitive rat hypertension

The enzymes required for aldosterone synthesis from cholesterol are expressed in rat and human brains. The hypertension of Dahl salt-sensitive (SS) rats is mitigated by the intracerebroventricular (i.c.v.) infusion of antagonists of the mineralocorticoid receptor (MR) and downstream effectors of mineralocorticoid action, as well as ablations of brain areas that also abrogate mineralocorticoid-salt excess hypertension in normotensive rats. We used real time RT-PCR to measure mRNA of aldosterone synthase and 11beta-hydroxylase, the requisite enzymes for the last step in the synthesis of aldosterone and corticosterone, respectively, MR and the determinants of MR ligand specificity, 11beta-hydroxysteroid dehydrogenase types 1 and 2 (11beta-HSD1&2) and hexose-6-phosphate dehydrogenase (H6PDH). A combination of extraction and ELISA was used to measure aldosterone concentrations in tissue and urine of SS and Sprague-Dawley (SD) rats. Aldosterone synthase mRNA expression was higher in the brains and lower in the adrenal glands of SS compared with SD rats. The amounts of mRNA for MR, 11beta-hydroxylase, 11beta-HSD1&2 and H6PD were similar. Aldosterone concentrations were greater in brains of SS than SD rats, yet, in keeping with the literature, the circulating and total aldosterone production of aldosterone in SS rats were not. The selective inhibitor of aldosterone synthase, FAD286, was infused i.c.v. or subcutaneously in a cross-over blood pressure study in hypertensive SS rats further challenged by a high-salt diet. The i.c.v. infusion of FAD286, at a dose that had no effect systemically, significantly and reversibly lowered blood pressure in SS rats. Aldosterone synthesis in brains of SS rats is greater than in SD rats and is important in the genesis of their salt-sensitive hypertension.

Alterations in aldosterone and angiotensin II levels in salt-induced hypertension

Several studies have demonstrated that plasma renin-angiotensin activity is reduced in rats administered a high salt diet. We evaluated changes in plasma and tissue levels of aldosterone (ALDO) and angiotensin II (A-II), as well as the reduced-to-oxidized glutathione ratio. Male Dahl salt-sensitive (SS) rats were placed on either a high-salt (8% NaCl; HS) or a normal-salt (0.3% NaCl; NS) diet for 3 weeks. Prior to and weekly on the diets, systolic blood pressure was measured by tail cuff plethysmography. Levels of A-II and ALDO in plasma, heart, and kidney were analyzed by enzyme immunoassay. Reduced and oxidized gluthatione were simultaneously measured by HPLC fluorescence detection. Heart and kidney tissues were prepared for histological analysis. Systolic blood pressure in animals on a HS diet was significantly elevated above that of those on a NS diet. High salt caused a reduction in both plasma A-II and ALDO levels; while their levels in the heart and kidney were increased. Exposure to a high-salt diet led to the enlargement of both heart and kidney. The reduced-to-oxidized glutathione ratio in plasma, heart and kidney was lowered by exposure to a HS diet. Kidneys from animals on a high-salt diet showed fibroid necrosis associated with wrinkling and thickening of the glomerular capillary wall, while hearts were hypertrophic. Taken together, high dietary salt induces inappropriate activation of the local renin-angiotensin-aldosterone systems. Tissue levels of angiotensin II and aldosterone may be more reflective of the severity of vascular maladaptations than are plasma levels, and may play a greater role in the maintenance of hypertension.

Marinobufagenin, an endogenous alpha-1 sodium pump ligand, in hypertensive Dahl salt-sensitive rats

Dahl salt-sensitive rats (DS), which have a mutation in the alpha-1 subunit of Na(+)/K(+)-ATPase, exhibit impaired pressure natriuresis and on a high-salt diet, retain Na(+) and exhibit increased blood pressure. Recently, we have shown that mammalian tissues contain a bufadienolide Na(+)/K(+)-ATPase inhibitory factor, marinobufagenin (MBG), that exhibits greater affinity for the alpha-1 than alpha-3 sodium pump isoform. The present study investigated the possible role of MBG in hypertension in DS on a high NaCl intake. Eight DS and 8 Dahl salt-resistant rats (DR) were placed on an 8% NaCl diet. Within 2 weeks, systolic blood pressure increased in DS (162+/-9 mm Hg at week 2 versus 110+/-2 mm Hg in baseline, P<0.01), and increased less in DR (124+/-3 mm Hg at week 2 versus 112+/-2 mm Hg in baseline). Renal excretion of MBG increased 4-fold (38.9+/-7.6 pmol versus 9.1+/-1.3 pmol in baseline, P<0.01) in DS, but by only 25% in DR (13.2+/-0.9 pmol versus 10.3+/-0.7 pmol in baseline). Excretion of endogenous ouabain did not change in either strain. MBG-immunoreactive material was purified from the urine of hypertensive DS by means of 2 steps of reverse-phase high performance liquid chromatography (HPLC) and compared with plant ouabain and amphibian MBG for its ability to inhibit the Na(+)/K(+)-ATPase from rat kidney (which expresses only alpha-1 Na(+)/K(+)-ATPase isoform). Unlike ouabain (IC(50)=248 micromol/L), serially diluted, HPLC-purified MBG immunoreactivity from DS and authentic MBG potently inhibited rat kidney Na(+)/K(+)-ATPase (IC(50)=70 and 78 nmol/L, respectively). Our results suggest that an alpha-1 Na(+)/K(+)-ATPase ligand, MBG, is elaborated to promote natriuresis in hypertensive DS. MBG acts as a selective inhibitor of the ouabain-resistant alpha-1 Na(+)/K(+)-ATPase subunit, ie, the major sodium pump isoform of the kidneys, as would be expected of a putative natriuretic hormone.

Sodium balance and jejunal ion and water absorption in Dahl salt-sensitive and salt-resistant rats

1. Apparent Na+ absorption and jejunal water, Na+, Cl- and K+ absorption in vivo was evaluated in young (prepubertal) and adult Dahl salt-sensitive (DS) and Dahl salt-resistant (DR) rats kept on a low-salt (low-salt rat chow + distilled water) or a high-salt diet (HS1 diet: NaCl-enriched rat chow + distilled water; HS2 diet: standard rat chow + 1% saline as drinking fluid). These two high-salt diets were chosen because the HS1 regimen has been shown to increase blood pressure (BP) in DS rats and the HS2 regimen decreases jejunal water and ion absorption in normotensive Wistar rats. 2. The HS1 or HS2 diet increased BP in young and adult DS rats but had no effect on the BP of young and adult DR rats. 3. Irrespective of dietary Na+ intake, no significant difference of apparent Na+ absorption (dietary Na+ intake minus faecal Na+ output) was observed between DS and DR rats both in prepuberty and in adulthood. Young DS rats kept on a low-salt diet had increased faecal Na+ output in comparison with young DR rats. This difference disappeared with increasing dietary Na+ intake. 4. There were no interstrain differences on the effect of a high-salt diet on jejunal Na+ and K+ absorption in young and adult DS and DR rats. However, high-salt diets stimulated jejunal water and Cl- absorption in young DS rats, but not in adult DS rats and young and adult DR rats. Interstrain differences of water and Cl- absorption were observed only in adulthood. Adult DR rats kept on an HS2 diet absorbed more water and Cl- than their DS counterparts. 5. Our results do not indicate any abnormalities of apparent Na+ absorption and jejunal water and electrolyte transport in DS and DR rats. We conclude that there is no relationship between intestinal Na+ absorption and sensitivity or resistance to induction of experimental salt hypertension.