Exogenous 25-hydroxycholecalciferol does not attenuate salt-induced hypertension

Vitamin D status and arterial hypertension: a systematic review

Vitamin D deficiency is common and is primarily caused by a lack of ultraviolet-B (UVB) radiation from reduced sun exposure, and the consequent limiting of vitamin D production in the skin. The vitamin D endocrine system regulates about 3% of the human genome. Observational data support the concept that vitamin D is involved in the pathogenesis of cardiovascular diseases and arterial hypertension. The antihypertensive properties of vitamin D include renoprotective effects, suppression of the renin-angiotensin-aldosterone system, direct effects on vascular cells, and effects on calcium metabolism, including prevention of secondary hyperparathyroidism. The results of clinical studies largely, but not consistently, favor the hypothesis that vitamin D sufficiency promotes lowering of arterial blood pressure. Randomized, placebo-controlled trials are greatly needed to clarify and definitively prove the effect of vitamin D on blood pressure. In general, the antihypertensive effects of vitamin D seem to be particularly prominent in vitamin-D-deficient patients with elevated blood pressure. Thus, in view of the relatively safe and inexpensive way in which vitamin D can be supplemented, we believe that vitamin D supplementation should be prescribed to patients with hypertension and 25-hydroxyvitamin D levels below target values.

High dietary cholecalciferol increases plasma 25-hydroxycholecalciferol concentration, but does not attenuate the hypertension of Dahl salt-sensitive rats fed a high salt diet

The Dahl salt-sensitive rat, a model for salt-induced hypertension, develops hypovitaminosis D during high salt intake, which is caused by loss of protein-bound vitamin D metabolites into urine. We tested the hypothesis that high dietary cholecalciferol (5- and 10-fold standard) would increase plasma 25-hydroxycholecalciferol (25-OHD(3)) concentration (indicator of vitamin D status) of salt-sensitive rats during high salt intake. Salt-sensitive rats were fed 0.3% salt (low salt, LS), 3% salt (HS), 3% salt and 7.5 microg cholecalciferol/d (HS-D5), or 3% salt and 15 microg cholecalciferol/d (HS-D10) and sacrificed at week 4. Plasma 25-OHD(3) concentrations of the two groups of HS-D rats were similar to that of LS rats and more than twice that of HS rats. Urinary cholecalciferol metabolite content of HS-D rats was more than seven times that of HS rats. Systolic blood pressures of the hypertensive HS and HS-D rats did not significantly differ, whereas LS rats were not hypertensive. We conclude that high dietary cholecalciferol increases plasma 25-OHD(3) concentration, but does not attenuate the hypertension of salt-sensitive rats during high salt intake. Low salt intake may be necessary to both maintain optimal vitamin D status and prevent hypertension in salt-sensitive individuals.

Dahl salt-sensitive rats develop hypovitaminosis D and hyperparathyroidism when fed a standard diet

The Dahl salt-sensitive rat (S), a model for salt-sensitive hypertension, excretes protein-bound 25-hydroxyvitamin D (25-OHD) into urine when fed a low salt diet. Urinary 25-OHD increases during high salt intake. We tested the hypothesis that continuous loss of 25-OHD into urine would result in low plasma 25-OHD concentration in mature S rats raised on a standard diet. Dahl S and salt-resistant (R) male rats were raised to maturity (12-month-old) on a commercial rat diet (1% salt) and switched to 0.3% (low) or 2% (high) salt diets 3 weeks before euthanasia. Urine (24 h) was collected at the end of the dietary treatments. Urinary 25-OHD and urinary 25-OHD binding activity of S rats were three times that of R rats, resulting in lower plasma 25-OHD and 24,25-dihydroxyvitamin D concentrations in S rats than in R rats (P < 0.001). Plasma parathyroid hormone concentrations of S rats were twice that of R rats. S rats fed 2% salt had higher plasma 1,25-dihydroxyvitamin D concentrations than those fed 0.3% salt (P = 0.002). S rats excreted more calcium into urine than R rats (P < 0.001) and did not exhibit the expected calciuric response to salt. Proteinuria of the S rats was three times that of the R rats, suggesting kidney damage in the S rats. Low plasma 25-OHD and 24,25-dihydroxyvitamin D and high plasma 1,25-dihydroxyvitamin D and PTH concentrations seen in the mature S rats have also been reported for elderly patients with low-renin (salt-induced) hypertension. An implication of this study is that low vitamin D status may occur with age in salt-sensitive individuals, even when salt intake is normal.

Dahl salt-sensitive rats excrete 25-hydroxyvitamin D into urine

The plasma 25-hydroxyvitamin D concentration of Dahl salt-sensitive rats (S) is markedly decreased in response to high sodium chloride (salt) intake. We tested the hypothesis that urinary excretion is a mechanism for the decrease. Female S rats excreted 0.26 +/- 0.04 nmol 25-hydroxyvitamin D/24 h at wk 2 of high salt (80 g/kg) intake, five times that of female salt-resistant (R) rats at wk 2 of high salt intake and nine times that of S rats at wk 2 of low salt (3 g/kg) intake. The 25-hydroxyvitamin D binding activity in 24-h urine of S rats was 79 +/- 11 pmol/h at wk 2 of high salt intake, two times that in urine of S rats at wk 2 of low salt intake and > 35 times that in urine of R rats at wk 2 of low or high salt intake. We conclude that markedly decreased plasma 25-hydroxyvitamin D concentrations of S rats during high salt intake result in part from excretion of protein-bound 25-hydroxyvitamin D. Low plasma 25-hydroxyvitamin D concentrations in humans may also result in part from salt sensitivity, which is prevalent in > 50% of the United States hypertensive population.

Plasma 24,25-dihydroxyvitamin D concentration of Dahl salt-sensitive rats decreases during high salt intake

Dahl salt-sensitive rats, but not salt-resistant rats, develop hypertension in response to high salt intake. We have previously shown an inverse relationship between plasma 25-hydroxyvitamin D (25-OHD) concentration and blood pressure of Dahl salt-sensitive rats during high salt intake. In this study, we report on the relationship between high salt intake and plasma 24,25-dihydroxyvitamin D (24,25-(OH)(2)D) concentration of Dahl salt-sensitive and salt-resistant rats. Rats were fed a high salt diet (8%) and sacrificed at day 2, 7, 14, 21, and 28. Plasma 24,25-(OH)(2)D concentrations of salt-sensitive rats were reduced to 50% of that at baseline at day 2-when blood pressure and plasma 25-OHD concentration were unchanged, but 25-OHD content in the kidney was 81% of that at baseline. Plasma 24,25-(OH)(2)D concentration was reduced further to 10% of that at baseline from day 7 to 14 of high salt intake, a reduction that was prevented in rats switched to a low salt (0.3%) diet at day 7. Exogenous 24,25-dihydroxycholecalciferol (24,25-(OH)(2)D(3)), administered at a level that increased plasma 24,25-(OH)(2)D concentration to five times normal, did not attenuate the salt-induced hypertension of salt-sensitive rats. Plasma 24,25-(OH)(2)D concentration of salt-resistant rats was gradually reduced to 50% of that at baseline at day 14 and returned to baseline value at day 28 of high salt intake. We conclude that the decrease in plasma 24,25-(OH)(2)D concentration in salt-sensitive rats during high salt intake is caused by decreased 25-OHD content in the kidney and also by another unidentified mechanism.

The calciuric response to dietary salt of Dahl salt-sensitive and salt-resistant female rats

BACKGROUND

We have shown previously that the calciuric response to salt does not differ in Dahl salt-sensitive (S) and salt-resistant (R) male rats. Clinical studies with women, however, suggest an effect of salt sensitivity on the calciuric response to salt. The objective of this study was to determine whether there is an effect of salt sensitivity on the calciuric response to salt of female S and R rats.

METHOD

Dahl S and R female rats were fed high- (8%) or low- (0.3%) salt diets for 3 weeks. The rats were placed in metabolic cages for 24-hour urine collection at baseline and weekly (for sodium and calcium determination).

RESULTS

Blood pressure of female S rats was 177+/-3.0 mm Hg at week 3 of high salt intake compared with 96+/-1 mm Hg for female R rats. Female S rats excreted significantly more calcium than female R rats at baseline (P < 0.001), when fed a nonpurified diet, and during high salt intake (P = 0.004). Salt sensitivity significantly increased calcium excretion, water intake, and urine output when rats were fed a high-salt diet. Calcium excretion, water intake, and urine output of female S rats were time-dependent during high salt intake. Plasma 25-hydroxyvitamin D and 24,25-dihydroxyvitamin D concentrations were markedly lower in female S rats fed a high-salt diet, but not in female R rats. Plasma parathyroid hormone and 1,25-dihydroxyvitamin D concentrations did not significantly differ between female S and R rats, but plasma concentrations of these two hormones at week 3 were significantly higher in S rats fed a high-salt diet compared with S rats fed a low-salt diet.

CONCLUSIONS

Our data indicate that the calciuric response to salt is greater in female S compared with female R rats, thus supporting findings on the effect of salt sensitivity reported in several clinical studies with women. The greater calciuric response to salt of female S rats compared with female R rats, which was not seen in a previous study when male S rats were compared to male R rats, suggest a gender difference in the calciuric response to salt.

The Calciuric Response to Dietary Salt of Dahl Salt-Sensitive and Salt-Resistant Male Rats

BACKGROUND

There are conflicting reports regarding the effect of salt sensitivity on the calciuric response to salt, perhaps because of gender differences and different modes of salt administration. We tested the hypothesis that the calciuric response to dietary salt would not differ for male Dahl salt-sensitive (S) and salt-resistant (R) rats.

METHOD

S and R rats were fed high- (80 g/kg) or low- (3 g/kg) salt diets for 3 weeks and urine (24 hour) was collected weekly.

RESULTS

Urinary calcium excretion was up to 20-fold greater for S and R rats fed a high-salt diet (P < 0.001) than for S and R rats fed a low-salt diet and did not differ significantly between S and R rats. S rats, however, excreted calcium in significantly higher urine volumes (P< 0.001) during high salt intake and developed hypertension. Plasma parathyroid hormone concentrations of S and R rats did not differ during low salt intake and increased significantly to the same concentration after 3 weeks of high salt intake.

CONCLUSIONS

We have previously reported that plasma 25-hydroxyvitamin D and 24,25-dihydroxyvitamin D concentrations of male S rats, but not male R rats, were drastically reduced by 3 weeks of high salt intake. These data suggest that salt-induced hypertension and salt-induced alterations in the vitamin D endocrine system of male S rats do not affect the calciuric response to dietary salt.

Regulation of sodium, calcium and vitamin D metabolism in Dahl rats on a high-salt/low-potassium diet: genetic and neural influences

1. A dietary combination of high salt and low potassium (HSLK) exacerbates hypertension in Dahl salt-sensitive (DS) rats and renders previously normotensive Dahl salt-resistant (DR) rats hypertensive. In both strains, the severity of hypertension correlates with urinary calcium loss. However, the magnitude of excretory calcium losses is significantly greater in DS rats and is potentiated by chemical sympathectomy in both strains. 2. We hypothesized that a defect in vitamin D metabolism may underlie the observed strain-dependent differences in calcium balance. 3. Arterial blood pressure (ABP), water and mineral balance and serum concentrations of 1,25-dihydroxyvitamin D3 (1,25(OH)2 D3) and 25-hydroxyvitamin D3 (25(OH)D3) were measured in intact and chemically sympathectomized (6-hydroxydopamine; 6-OHDA) DS and DR rats after 8 weeks on a HSLK diet. 4. Chronic ingestion of this diet resulted in marked and moderate levels of hypertension in DS and DR rats, respectively. The hypertension was abated and eliminated by 6-OHDA in the DS and DR strains, respectively. Independent of treatment, DS rats had significantly higher urinary excretion of calcium and reduced intestinal absorption of the ion compared with DR rats. The DS rats had significantly higher serum levels of 1,25(OH)2 D3 and markedly lower serum levels of 25(OH)D3 than DR rats. Chemical sympathectomy tended to increase 1,25(OH)2 D3 and to decrease 25(OH)D3 levels in both strains. 5. These data indicate a genetic difference in vitamin D metabolism between DS and DR rats. The abnormally elevated levels of 1,25(OH)2 D3 in DS rats may be an appropriate compensatory response to excessive excretory calcium loss and reduced target organ sensitivity to the hormone and may, maladaptively, directly contribute to hypertension, by stimulating vascular smooth muscle contractility.