Effect of menopause and hormone replacement therapy on the urinary excretion of pyridinium cross-links

Abstract

Pyridinoline (Pyr) and deoxypyridinoline (D-Pyr) are two cross-links of collagen molecules that are present in the extracellular matrix and released during its degradation. In contrast to the wide distribution of collagen, Pyr is present in bone and cartilage, but not in significant amounts in other connective tissues, and D-Pyr appears to be specific for bone tissue. Therefore, the urinary excretion of Pyr and D-Pyr might be a sensitive marker of bone matrix degradation. Using a specific high pressure liquid chromatography assay we have measured Pyr and D-Pyr cross-links in a 24-h and a fasting urine sample in 60 early postmenopausal women and 19 premenopausal women matched for age. Menopause induced a 62% increase in Fu Pyr (49.8 ± 18.7 vs. 30.8 ± 8.0 pmol/μmol creatinine; P < 0.001) and an 82% increase in Fu D-Pyr (8.2 ± 3.4 vs. 4.5 ± 1.4 pmol/μmol creatinine; P < 0.001). In 20 postmenopausal women on hormone replacement therapy, urinary Pyr and D-Pyr returned to premenopausal levels within 6 months, contrasting with unchanged levels during placebo treatment. The 24-h excretion of Pyr and D-Pyr was significantly lower than the fasting excretion, but was similarly decreased after hormone replacement therapy. Pyr and D-Pyr excretion measured in the same urinary sample were highly correlated (r = 0.85 for fasting and 0.83 for 24-h sampling), but correlations between fasting and 24-h values were weak (D-Pyr, r = 0.30; Pyr, r= 0.29; P < 0.05 for both). Correlations between urinary cross-links and other markers of bone turnover (Fu hydroxyproline/creatinine and plasma osteocalcin) were significant but low (Pyr vs. osteocalcin, r = 0.29, P < 0.05; Pyr vs. hydroxyproline, r = 0/.34; P < 0.01; D-Pyr vs. osteocalcin, r = 0.39; P < 0.01), except for D-Pyr vs. hydroxyproline (r = 0.24; P = 0,07), suggesting that these markers reflect different events of bone metabolism. Finally, a single measurement of the fasting excretion, but not of the 24-h excretion, of cross-links was significantly correlated (Pyr, r = 0.34; P < 0.05; D-Pyr, r = -0.46; P < 0.01), with the subsequent spontaneous rate of bone loss assessed by repeated measurements of the radial bone mineral content in 37 postmenopausal women. The correlation with the rate of bone loss was improved when the simultaneous measurement of plasma osteocalcin and urinary hydroxyproline/creatinine was combined (Pyr, r = 0.75; P < 0.001; D-Pyr, r = 0.77; P < 0.001). We conclude that the fasting excretion of Pyr and D-Pyr reflects bone turnover changes at the menopause and the effects of hormone replacement therapy. The combination of cross-links, hydroxyproline, and osteocalcin measurement seems promising to evaluate the rate of bone loss in postmenopausal women.