Evidence against the role of calcium deficiency in genetic hypertension

Epidemiological studies suggest an association between reduced calcium uptake and hypertension, while clinical trials and rat experiments indicate a small but significant hypotensive effect with oral calcium supplements. These data imply that calcium deficiency has a role in genetic hypertension. We reasoned that if the hypothesis is correct, the hypertension should be aggravated by further reducing calcium balance but attenuated by augmenting calcium balance. We tested this hypothesis by evaluating the blood pressure response in spontaneously hypertensive rats (SHR) as calcium balance was decreased by dietary restriction of calcium or increased by supplementation with magnesium or 1 alpha, 25-dihydroxycholecalciferol (calcitriol). A low calcium diet within the physiological range did not accentuate the hypertension in SHR during the 11 weeks of treatment, even though calcium balance was reduced by half. Similar results were obtained with dietary calcium restriction in parathyroidectomized SHR, which excludes any offsetting effects of changes in parathyroid hormone levels. Conversely, 7 weeks of a high magnesium diet, which increased calcium balance without reducing PO4 balance, did not correct the hypertension of SHR. Similarly, long-term administration of calcitriol failed to reduce the blood pressure of parathyroidectomized SHR and normotensive Wistar-Kyoto (WKY) controls, despite the presence of increased serum calcium levels comparable to those produced by oral calcium loading. Finally, external calcium balance was measured directly in 25-day-old, prehypertensive SHR. As a result of the increased calcium absorption and reduced calcium excretion, SHR retained more calcium than did the normotensive WKY, which directly refutes the existence of calcium deficiency at this normotensive stage. These data do not support the role of calcium deficiency in genetic hypertension.