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

Salt and nephrolithiasis

Dietary sodium chloride intake is nowadays globally known as one of the major threats for cardiovascular health. However, there is also important evidence that it may influence idiopathic calcium nephrolithiasis onset and recurrence. Higher salt intake has been associated with hypercalciuria and hypocitraturia, which are major risk factors for calcium stone formation. Dietary salt restriction can be an effective means for secondary prevention of nephrolithiasis as well. Thus in this paper, we review the complex relationship between salt and nephrolithiasis, pointing out the difference between dietary sodium and salt intake and the best methods to assess them, highlighting the main findings of epidemiologic, laboratory and intervention studies and focusing on open issues such as the role of dietary salt in secondary causes of nephrolithiasis.

High dietary salt does not significantly affect plasma 25-hydroxyvitamin D concentrations of Sprague Dawley rats

BACKGROUND

The Dahl salt-sensitive rat, but not the Dahl salt-resistant rat, develops hypertension and hypovitaminosis D when fed a high salt diet. Since the salt-sensitive rat and salt-resistant rat were bred from the Sprague Dawley rat, the aim of this research was to test the hypothesis that salt-resistant and Sprague Dawley rats would be similar in their vitamin D endocrine system response to high salt intake.

FINDINGS

Sprague Dawley, salt-sensitive, and salt-resistant rats were fed high (80 g/kg, 8%) or low (3 g/kg, 3%) salt diets for three weeks. The blood pressure of Sprague Dawley rats increased from baseline to week 3 during both high and low salt intake and the mean blood pressure at week 3 of high salt intake was higher than that at week 3 of low salt intake (P < 0.05). Mean plasma 25-hydroxyvitamin D concentrations (marker of vitamin D status) of Sprague Dawley, salt-sensitive, and salt-resistant rats were similar at week 3 of low salt intake. Mean plasma 25-hydroxyvitamin D concentrations of Sprague Dawley and salt-resistant rats were unaffected by high salt intake, whereas the mean plasma 25-hydroxyvitamin D concentration of salt-sensitive rats at week 3 of high salt intake was only 20% of that at week 3 of low salt intake.

CONCLUSIONS

These data indicate that the effect of high salt intake on the vitamin D endocrine system of Sprague Dawley rats at week 3 was similar to that of salt-resistant rats. The salt-sensitive rat, thus, appears to be a more appropriate model than the Sprague Dawley rat for assessing possible effects of salt-sensitivity on vitamin D status of humans.

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.

Black and white female adolescents lose vitamin D metabolites into urine

BACKGROUND

The black American population has a higher prevalence of salt sensitivity compared with the white American population. Dahl salt-sensitive rats, models of salt-induced hypertension, excrete protein-bound vitamin D metabolites into urine, a process that is accelerated during high salt intake. We tested the hypothesis that urinary vitamin D metabolite content and 25-hydroxyvitamin D (25-OHD) binding activity of black female adolescents would be greater than that of white female adolescents.

METHODS

Female adolescents (11-15 years old, 11 black and 10 white) were fed low (1.3 g, 56 mmol/24 hours sodium) and high salt (3.86 g, 168 mmol/24 hours sodium) diets for 3 weeks in a randomized order cross-over study design.

RESULTS

White and black adolescents had similar mean urinary vitamin D metabolite content (low salt, black versus white: 50 +/- 10 versus 58 +/- 17 pmol/24 hours; high salt, black versus white: 47 +/- 7 versus 79 +/- 16 pmol/24 hours). Mean urinary 25-OHD binding activities of the black and white adolescents did not significantly differ. Urinary 25-OHD binding activity of 10/11 black adolescents and 7/10 white adolescents was greater at week 3 of high salt intake than at week 3 of low salt intake (r = 0.50, P = 0.002, n = 17). Plasma 24,25-dihydroxyvitamin D concentrations of the white female adolescents were significantly higher than that of the black female adolescents (P < 0.001).

CONCLUSION

Urinary loss of vitamin D metabolites may be one cause of low vitamin D status, in addition to low dietary intake and reduced skin synthesis.

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.