Urinary H-3-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 m ethods: chronic H-3-tetracycline labeling ((HTC)-H-3) and pyridinium crossl ink excretion (PYDX), and compare the sensitivity of these markers in two a ge 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 inco rporation 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 modulat ed over 9 weeks by calcium restriction (CR), measured by urinary excretion of both 3HTC and PYDX, and compared with age-matched, calcium-adequate cont rols. At baseline, urinary excretion of 3HTC was not significantly differen t between age groups, whereas urinary PYDX was 14-20% higher in mature comp ared 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 m arker or age group. Urinary 3HTC was elevated at weeks 1-3 (P < 0.01) and r eached maximal values at week 2 (32 +/- 17%). Urinary PYDX, however, was no t 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 althou gh both markers are elevated by CR, marker response differs with age, and v ariability exists for acute and chronic responses.