CIRCADIAN-RHYTHM OF IN-VITRO BONE-RESORBING ACTIVITY IN HUMAN SERUM

Calciotropic hormones as well as biochemical parameters of bone format ion and resorption show circadian rhythms. In a previous study in the rat, me observed a circadian rhythm in serum bone-resorbing activity ( SBRA). In the present study, we investigated whether there was a circa dian rhythm of SBRA in human serum. For this purpose, we studied 10 he althy premenopausal women and 5 healthy men. Blood was collected every 2 h, and urine samples were collected during 4-h periods for 24 h. Fo r the determination of SBRA, media were prepared by reconstituting ser um samples with Dulbecco's Modified Eagle's Medium at a ratio of 20% s erum and 80% Dulbecco's Modified Eagle's Medium. Limb bones were disse cted from 19-day old fetal rats prelabelled with Ca-45 and were cultur ed for 72 h in the presence of the sera. Bone resorption was assessed from the Ca-45 released into the culture medium and from that retained in the bone and was expressed as percentage Ca-45 release. Serum calc ium, phosphorus, PTH, cortisol, and urinary pyridinium cross-links wer e also determined. SBRA in human serum followed a circadian rhythm wit h a peak at about 0300 h and a nadir at 0700 h. There was no significa nt difference between the rhythm of SBRA of women and men. At concurre nt time points, SBRA and serum PTH were positively correlated (r=0.629 ; P<0.01), and SBRA and serum cortisol were negatively correlated (r=0 .797; P<0.01). To further investigate the possible contribution of the se hormones to SBRA, either neutralizing anti-PTH antibody or RU-486 ( mifepristone), a glucocorticoid receptor antagonist, was added to the serum samples of 6 subjects. Neutralizing the effect of PTH did not ch ange the pattern of SBRA rhythm. The addition of RU-486 had a signific ant effect on the rhythm of SBRA, reducing the peak and nadir amplitud es. Thus we conclude that cortisol plays a major role in the rhythm of SBRA present in human serum; however, the influence of other factors cannot be excluded. Cortisol may be an important determinant of the ci rcadian rhythm of bone resorption in vivo.