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We evaluated in normal and hypophosphatemic (Hyp) mice whether changes in serum levels of osteocalcin in response to dietary phosphate supplementation, parathyroid hormone (PTH) and 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) administration were related to perturbations in calcium phosphate homeostasis. In normal mice, serum osteocalcin levels were not altered by phosphate supplementation. In contrast, phosphate supplementation in Hyp mice led to a 2-fold decrease in serum osteocalcin to normal levels after 3 days and to an increase in osteocalcin levels after 14 days. The decrease in osteocalcin was associated with normophosphatemia, severe hypocalcemia, and marked increases in circulating 1,25(OH)2D3 levels, whereas the increase in osteocalcin levels was associated with normophosphatemia and no change in serum calcium and 1,25(OH)2D3. Administration of PTH decreased serum osteocalcin in both genotypes. Infusion of 1,25(OH)2D3 for 3 days elicited increases in serum osteocalcin and calcium levels in normal mice, whereas in Hyp mice it produced significant decreases in osteocalcin levels and no change in serum calcium. However, with a more prolonged infusion of 1,25(OH)2D3, hypercalcemia and increases in serum osteocalcin were induced in mutant mice. Our results suggest that the abnormal osteocalcin response of Hyp mice is not directly attributable to an osteoblast dysfunction but is secondary, at least in part, to perturbations in factors that modulate the osteoblast activity, especially serum calcium and/or PTH.

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