Diurnal rhythms in Ca transfer into bone, Ca release from bone, and bone resorbing activity in serum of rats

To characterize diurnal rhythms in calcium transfer in and out of bone, rats were adapted to a 12:12 h light-dark illumination program (with light from 0600 to 1800 h). For studies of deposition, rats were injected with ⁴⁵CaCl₂ at 0300, 0700, 1100, 1500, 1900 or 2300 h and killed with chloroform inhalation 60 min later, and radioactivity was determined in tibia and mandibular incisor. Peak deposition occurred at 2400 h, with a nadir at 1200 h. For studies of radionuclide release, rats were prelabeled with ⁴⁵CaCl₂ for 6 days and serum obtained from the tail at 4-h intervals. ⁴⁵Ca and specific activity were maximal around 1200-1600 h and lowest late in the dark period. To determine the role of humoral factor(s), fetal rat limb bones were cultured in media prepared with sera from light-dark-adapted rats. Activity was maximal in serum collected at 0800 h and minimal at 1600 h. Heat inactivation at 100°C for 5 min eliminated the difference between the peak and nadir. The results suggest that a heat-sensitive humoral factor(s) regulates diurnal rhythms in calcium metabolism.