In vitro bone metabolism in hypophosphatemic mice: 32P and 45Ca release from cultured calvaria

Abstract

Calvaria were explanted from 6-day-old normal and hypophosphatemic mice and cultured for 3–4 days. The release of 32P and 45Ca into a chemically defined physiological medium was studied. During a 72-h incubation period, normal calvaria released 39.9% 32P and 25.5% 45Ca. Calvaria explanted from mutant mice released 26.0% 32P and 18.1% 45Ca. At the same time, efflux from dead tissue amounted to 16.4% 32P and 11.9% 45Ca in normal and 13.6% 32P and 10.0% 45 Ca in hypophosphatemic animals. Differences between living- and dead-tissue releases increased with time. Such differences representing cell-mediated releases of 32P and 45Ca are more conspicuous in normal than in hypophosphatemic mice. During the same period, normal calvaria responded to a physiological concentration of 1,25-dihydroxycholecalciferol with increases (treated-control) of 11.9% in 32P release and 9.5% in 45Ca release. Stimulations by parathyroid extract were respectively, 26.0 and 15.7% in normal, and 15.4 and 9.3% in hypophosphatemic mice. In the presence of both agents, stimulation of 32P mobilization was greater than the sum of their independent effects. Therefore, vitamin D and the parathyroid extract exerted a synergistic effect on 32P release from calvaria. In combination or independently, these hormones showed stimulations that increased progressively with time. Absolute hormonal effects (treated-control) on mineral release were greater in normal than in hypophosphatemic mice. Nonetheless, percentage effects 100(treated-control)Jcontrol increased at equal rates in both strains. Our data show that the hypophosphatemic bone releases its minerals at slower rates than does normal bone, presumably a metabolic defect related to cellular activity. It is suggested, therefore, that the differences in hormonal responses between normal and hypophosphatemic mice may simply be quantitative and related to bone size. In fact, the nature of these responses may be the same in both strains.