Bone mineral density in spontaneous hypertension: differential effects of dietary calcium and sodium

Dietary calcium and sodium have been postulated to modify both bone mineral status and blood pressure regulation in humans and animals. The spontaneously hypertensive rat (SHR) manifests several defects in calcium metabolism that may contribute to its hypertension. Blood pressure and bone mineral status were measured in SHR and normotensive Wistar-Kyoto rats (WKY) as a marker of whole animal calcium metabolism. In addition, the effect of alterations in dietary calcium and sodium on bone status were examined. At 6 weeks of age, seven male SHR and seven male WKY were placed on a control diet. At the same age, 28 SHR and 28 WKY were randomized to four diets containing either 2.0% or 0.1% calcium and 1.0% or 0.25% sodium. Four markers of bone mineral status were analyzed: bone density measured by direct photon absorptiometry, and total bone calcium, phosphorus, and magnesium content measured by atomic absorption spectrophotometry. The SHR exhibited significantly lower levels (p less than 0.001) of bone density and bone magnesium content than the WKY, whereas bone phosphorus and calcium did not differ between the two strains. The 2.0% calcium diets resulted in increased bone density and bone calcium content, and lower bone magnesium in both strains. The 1.0% sodium diets were associated with decreased bone density in the SHR, but not in the WKY. These findings identify another indicator of disturbed calcium metabolism in the SHR that may be related to impaired renal calcium handling. They are consistent with previously reported reductions in renal calcium reabsorption and decreased intestinal calcium transport in older SHR.