DEVELOPMENT AND VALIDATION OF A RADIOIMMUNOASSAY FOR MOUSE OSTEOCALCIN - PARADOXICAL RESPONSE IN THE HYP MOUSE

The hypophosphatemic (Hyp) mouse is a model for human familial hypopho sphatemic rickets. To test the hypothesis that there is an osteoblasti c defect in these animals, serum osteocalcin levels were measured in H yp mice and their normal littermates. Furthermore, the effects of phos phorus deprivation, phosphorus loading, and 1,25-dihydroxyvitamin D3 a dministration on serum osteocalcin levels were examined. Osteocalcin w as purified from mouse hindlimbs, and a polyclonal antibody to this ma terial was produced in a goat. The antibody recognized native and deca rboxylated mouse osteocalcin, but could not recognize osteocalcin from several other species. A RIA was developed which had a minimal detect ion limit of 0.4 nmol/liter (2.2-mu-g/liter) and half-maximal displace ment at 2.7-3.3 nmol/liter (14.8-18.2-mu-g/liter). The intraassay coef ficient of variation was 6.4%, while the interassay coefficient of var iation was 12%. Dilutions of mouse serum samples varied by less than 1 5%. Analytical recovery was typically greater than 90%. Serum osteocal cin concentrations in Hyp and normal mice were shown to decrease with age. However, circulating osteocalcin levels in Hyp mice were higher t han those in their normal littermates regardless of the age of the ani mal (P < 0.001). One week of a high phosphorus diet resulted in an inc rease in serum phosphate in normal and Hyp mice, but serum osteocalcin concentrations were unaffected. On the other hand, dietary phosphorus deprivation for 4 weeks resulted in comparable hypophosphatemia in bo th Hyp and normal mice, and serum osteocalcin increased in both groups of animals. Intraperitoneal injection of 30 ng/day 1,25-dihydroxyvita min D3 for 7 days resulted in a 215 +/- 33% increase in serum osteocal cin in normal animals, while the same regimen produced a 250 +/- 29% d ecrease in the Hyp mouse. Our results are consistent with the hypothes is that abnormal osteoblastic activity is present in Hyp mice. Further more, hypophosphatemia may be a general regulator of osteocalcin synth esis or secretion in the mouse.