Differential Effect of Renal Cortical and Medullary Interstitial Fluid Calcium on Blood Pressure Regulation in Salt-Sensitive Hypertension.

BACKGROUND: Hypercalciuria is a frequent characteristic of hypertension. In this report we extend our earlier studies investigating the role of renal interstitial fluid calcium (ISF(Ca))(2+) as a link between urinary calcium excretion and blood pressure in the Dahl salt-sensitive (DS) hypertensive model. METHODS: Dahl salt-sensitive and salt-resistant (DR) rats were placed on control (0.45%) and high (8%) salt diets to determine if changes in renal cortical and medullary ISF(Ca)(2+)correlated with changes in urinary calcium excretion and blood pressure. RESULTS: We observed that renal ISFCa(2+) was predicted by urinary calcium excretion (P < 0.05) in DS rats but not DR rats. Renal cortical ISF(Ca)(2+) was negatively associated with blood pressure (P < 0.03) while renal medullary ISF(Ca)(2+) was positively associated with blood pressure in DS rats (P < 0.04). In contrast, neither urinary calcium excretion nor renal ISF(Ca)(2+) was associated with blood pressure in the DR rats under the conditions of this study. CONCLUSION: We interpret these findings to suggest that decreased renal cortical ISF(Ca)(2+) plays a role in the increase in blood pressure following a high salt diet in salt hypertension perhaps by mediating renal vasoconstriction; the role of medullary calcium remains to be fully understood. Further studies are needed to determine the mechanism of the altered renal ISF(Ca)(2+) and its role in blood pressure regulation.

Action of thiazide on renal interstitial calcium.

BACKGROUND: Although thiazides increase urinary sodium excretion, they also decrease urinary calcium excretion. Recent studies in our laboratory have shown that increased dietary salt significantly reduces interstitial fluid calcium in Dahl salt-sensitive (DS) rats, and this was associated with a rise in blood pressure and increased urinary calcium excretion. Owing to the vasorelaxant actions of increased extracellular fluid calcium, we reasoned that the antihypertensive action of hydrochlorothiazide (HCTZ), a commonly used thiazide, may be the result of increased interstitial fluid calcium as a consequence of decreased urinary calcium excretion. METHODS: To test this hypothesis, DS and Dahl salt-resistant (DR) rats were given high salt alone or in combination with HCTZ for 1 week. Renal cortical interstitial fluid calcium was determined by the zero net flux method. RESULTS: High salt decreased cortical interstitial fluid calcium (1.69 +/- 0.25 vs. 1.13 +/- 0.05 mmol/l; P < 0.05) in DS rats as previously reported; thiazide treatment had no effect on the high salt interstitial fluid calcium response in salt-sensitive animals. However, thiazide decreased interstitial fluid calcium in DS on a normal salt diet. Cortical interstitial fluid calcium was unchanged by dietary salt in DR rats, and thiazide did not alter this interstitial fluid calcium response. CONCLUSION: We interpret these data to mean that (i) short-term thiazide treatment does not reduce blood pressure by restoring renal cortical interstitial fluid calcium concentration and (ii) a decrease in renal cortical interstitial fluid calcium may not contribute to the increased renal vasoconstriction seen in salt-sensitivity.