Daily rhythm in bone resorption in humans: preliminary observations on the effect of hypoparathyroidism or blindness

Abstract

While 24-hour rhythmicity of bone formation is largely dependent on serum cortisol, the physiological basis for daily changes in bone resorption remains uncertain. The present study was carried out to assess the effect of a chronic lack of PTH or blindness on overall daily bone resorption in a small group of human subjects. Three groups of subjects were examined: (a) hypoparathyroid (n = 6, 4 female); (b) congenitally blind (n = 6, 2 female); (c) control (n = 6, 6 female). All individuals were admitted to the hospital at 0600 h and stayed there for 26 h with normal day/night activity. Urine samples were obtained every 4 h, starting at 0800 h. Urinary cross-linked N-telopeptide of type I collagen (NTX) was measured by ELISA and expressed as the ratio with urinary creatinine levels. Mean 24-h concentration of urinary NTX was lower in hypoparathyroid and blind subjects as compared to controls (p < 0.001). Significant 24-h profiles of urinary NTX/creatinine concentration (expressed as % of change of the mean 24 h level) were observed in the three groups. While control and hypoparathyroid individuals showed maximal levels at early morning and a minimum at late afternoon (2000 h), blind subjects exhibited a minimum at early afternoon (1600 h). The results suggest that neither the lack of PTH nor the lack of vision suppress the circadian rhythmicity of urinary NTX in humans. Further studies comprising larger amounts of subjects are needed to support this conclusion.

Keywords:

• Circadian rhythm
• bone resorption
• hypoparathyroism
• blindness

Acknowledgements

This work was supported by grants from Fundación Argentina de Osteología, the Agencia Nacional de Promoción Científica y Tecnológica, Argentina (PICT 2431) and the Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Argentina. BO, DPC and SNZ are established investigators from CONICET.