Variations in Circulating Osteoprotegerin and Soluble RANKL during Diurnal and Menstrual Cycles in Young Women

Abstract

Background/Aims:Physiological bone turnover shows diurnal variations and changes within the menstrual cycle. The aim of this study was to assess the variability of osteoprotegerin (OPG) and soluble RANKL (sRANKL) serum levels during diurnal and menstrual cycles. Method: Blood was collected from 15 young women at 6-hour intervals between 08.00 and 20.00 h during the follicular phase. Moreover, to compare the follicular and luteal phases, blood was also collected at 14.00 h during the luteal phase. Serum bone-specific alkaline phosphatase (BAP), N-telopeptide of type I collagen (NTX), OPG and free sRANKL were measured. Results: No diurnal variations in BAP, OPG, sRANKL and sRANKL/OPG ratio were detected. NTX was significantly higher in the morning than in the afternoon and at night (p = 0.02). There were no menstrual variations in either. Conclusions: The consistent absence of diurnal variations in circulating OPG and sRANKL levels may reflect the absence of diurnal variation in their expression in the bone microenvironment. In this case, the nocturnal rise and the fall in bone resorption in the luteal phase should be accounted for by other factors than RANKL/OPG-mediated factors. Timing of sampling is unlikely to influence the results of circulating OPG and sRANKL measurement.