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

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.

The effect of calcium intake on bone composition and bone resorption in the young growing rat

“…Pyridinium crosslinks, pyridinoline (Pyr) and deoxypyridinoline (Dpyr), are products of the post-translational modification of collagen, and their urinary excretion has been used as a specific and sensitive index of bone resorption in man (Eyre, 1992;Robins & New, 1997) and rats (Black et al 1989;Egger et al 1994). Low Ca intakes by both rats (Egger et al 1994;Talbott et al 1999) and man (Shapses et al 1995;Ginty et al 1998) have been shown to increase the level of excretion of these pyridinium crosslinks in urine.…”

3H-tetracycline as a proxy for 41Ca for measuring dietary perturbations of bone resorption

“…It is unlikely that a lack of significant diet effect on urinary 3 H-TC beyond 1 month post dose was due to draining of 3 H-TC from the skeleton as a result of calcium deficient diet, as proposed by Talbott et al [12], because 3 H-TC could still be detected in the urine and skeleton up to 6 months post dose. It is also unlikely that a lack of diet effect on urinary 3 H-TC excretion beyond 1 month was due to the single 3 H-TC dosing regimen, because chronic pre-labeling [12] showed similar transient results. It has been reported that repeated exposure to the labeled material during early rapid skeletal growth period, such as in utero or shortly after birth, is required in order to study the normal behavior of calcium in adult rat skeleton [13,18].…”

“…Age and estrogen deficiency should combine to reduce the ability to adapt with time and would lessen differences between high and low calcium intake. However, Talbott et al [12] showed little differences in response to dietary calcium with age. The effects of low dietary calcium-induced bone resorption have been shown using urinary 3 H-TC to occur as early as 6 hours after the initiation of a low calcium diet [8].…”

“…Biochemical markers of bone turnover have a sustained response to other interventions such as estrogen and bisphosphonate therapies that perturb bone turnover that result in differences in BMD [16,17]. It is unlikely that a lack of significant diet effect on urinary 3 H-TC beyond 1 month post dose was due to draining of 3 H-TC from the skeleton as a result of calcium deficient diet, as proposed by Talbott et al [12], because 3 H-TC could still be detected in the urine and skeleton up to 6 months post dose. It is also unlikely that a lack of diet effect on urinary 3 H-TC excretion beyond 1 month was due to the single 3 H-TC dosing regimen, because chronic pre-labeling [12] showed similar transient results.…”

“…The effects of low dietary calcium-induced bone resorption have been shown using urinary 3 H-TC to occur as early as 6 hours after the initiation of a low calcium diet [8]. Interestingly, Talbott et al [12] demonstrated an increase in urinary 3 H-TC levels only during the first 3 weeks of a 9 week period of dietary calcium restriction (0.1% calcium) in 20 and 48 week-old rats. These rats started receiving chronic 3 H-TC labeling over a 6 week period when they were 12 and 40 weeks old.…”