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

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.