Urinary 3H-Tetracycline and Pyridinium Crosslinks Differ in Their Response to Calcium Restriction in Mature and Aged Rats

TLDR: Data indicate that although both markers are elevated by CR, marker response differs with age, and variability exists for acute and chronic responses.

Abstract: The aim of this study was to evaluate bone resorption (BR) in rats by two methods: chronic 3H-tetracycline labeling (3HTC) and pyridinium crosslink excretion (PYDX), and compare the sensitivity of these markers in two age groups. Female Sprague-Dawley rats at 12–29 weeks of age (``mature'', n = 12) and at 40–57 weeks of age (``aged'', n = 22) were examined. Skeletal incorporation of 3HTC in aged rats was 43 ± 8% of that in mature animals (P < 0.01), indicating an age-related decrease in bone turnover. BR was modulated over 9 weeks by calcium restriction (CR), measured by urinary excretion of both 3HTC and PYDX, and compared with age-matched, calcium-adequate controls. At baseline, urinary excretion of 3HTC was not significantly different between age groups, whereas urinary PYDX was 14–20% higher in mature compared with aged rats (P < 0.01). CR produced a 32–39% peak increase in BR (P < 0.01) compared with controls that did not differ significantly between marker or age group. Urinary 3HTC was elevated at weeks 1–3 (P < 0.01) and reached maximal values at week 2 (32 ± 17%). Urinary PYDX, however, was not elevated until week 2, reached maximal levels at week 3 (39 ± 15%), and remained elevated until week 6 (P < 0.01). These data indicate that although both markers are elevated by CR, marker response differs with age, and variability exists for acute and chronic responses.