Excretion of pyridinium cross-links of collagen in ovariectomized rats as urinary markers for increased bone resorption

Groups of 19-day-old rats were ovariectomized or were given sham operations. Measurements in urine of the pyridinium cross-links of collagen, pyridinoline and deoxypyridinoline, 7 weeks after surgery showed significantly higher amounts of cross-links relative to creatinine in the ovariectomized groups compared with the controls. Analyses before and after acid hydrolysis of the urine revealed that the increased excretion was only as free cross-link with no change in the concentrations of the bound forms. The loss of trabecular bone in the ovariectomized group was confirmed by immunocytochemical staining with antibodies to type I collagen. There were no differences between the ovariectomized and control groups in the concentrations of cross-links in the tibial bone or the articular cartilage. Measurements of free pyridinoline and deoxypyridinoline in urine therefore appear to provide a good index of the increased bone resorption induced by estrogen deficiency.