Microradiographic evaluation of bone from cows with experimental hypervitaminosis D, diet-induced hypocalcemia, and naturally occurring parturient paresis

Microradiographic and histologic evaluation of cortical and trabecular bone revealed a low turnover in cows which developed a syndrome of profound hypocalcemia and paresis near parturition. Trabecular and Haversian surfaces appeared inactive and were similar to sections from nonlactating-nonpregnant cows. The inactive surfaces were smooth and bordered by a zone of increased radiodensity. The failure of osteoclastic response was not due to the accumulation of osteoid along trabecular and Haversian surfaces. Lactation was associated with a threefold increase in resorption of trabecular bone at 7 to 10 days postpartum in control cows. Feeding a calcium-deficient diet to cows for 30 days resulted in a significant hypocalcemia and a twofold increase in bone resorption. The addition of pharmacologic doses of vitamin D (30 million units daily) from the 20th to 30th day of feeding the calcium-deficient diet further increased bone resorption (approximately fivefold), primarily along trabecular surfaces. The added vitamin D prevented hypocalcemia but did not produce a detectable hypercalcemia. Similar levels of vitamin D administered for 3 to 10 days to cows with a normal calcium and phosphorus intake resulted in hypercalcemia and numerous resorption spaces in cortical bone. Vitamin D appeared to be more effective than calcium-deficient diets in influencing calcium homeostasis by altering skeletal metabolism in adult cows. © 1972 Springer-Verlag