Effect of bicarbonate feeding on immobilization osteoporosis in the rat

A time course study of immobilization osteoporosis was carried out in an experimental model in which the hind half of the rat was paralyzed by spinal section between the 3rd and 4th lumbar vertebrae. The earliest changes in the femure were a reduction in water content and in bone volume (3 days after operation). A significant reduction in both hydroxyproline and phosphate content was observed 8 days after immobilization. Calcium, magnesium, sodium, citrate, and carbonate were found significantly decreased at 15 days. Bone potassium remained unchanged. The rate of calcium accretion and the size of the exchangeable pools, when expressed per unit of bone mass, were not significantly affected by treatment. Sham-operated rats fed a bicarbonate-enriched diet, showed increased rates of calcium accretion and bone calcium content. The latter was not accompanied by increases in phosphate and carbonate contents. Operated animals fed the same diet showed an increased rate of bone resorption attributed to an assumed higher rate of parathyroid hormone secretion induced by bicarbonate feeding.