Decreasing rates of bone resorption in growing rats in vivo: comparison of different types of bones.

This study quantifies rates of bone resorption by measuring the kinetic loss of 3H-tetracycline in vivo from the five different types of whole bones in four ages of normal male rats: appendicular bone (femur), cephalic bone (calvarium), axial bones (sixth lumbar vertebrae and sternum), and pelvis. The skeletons of newborn (0-2 weeks), weanling (4-7 weeks), adolescence (10-14 weeks), and mature (15-23 weeks) of Sprague Dawley rats were multiply labeled with 3H-tetracycline. The radioactivity in whole calvaria, the 6th lumbar vertebra, sternum, pelvis and femur was quantified to calculate the time course of tetracycline loss. The isotopic half lives (t1/2) were different for each age group and each bone, but the rates of resorption were usually the highest in the newborn rats. In the youngest animals, the t1/2 for the femur, vertebrae, and sternum was 1.5 week, for the pelvis it was 2.0 week, and for the calvarium it was 2.5 week. In the oldest age group, t1/2 for the femur was 28 week, for the calvarium was 19 week, for the sternum and pelvis was 16.5 week, and for the vertebrae was 13 week. For cephalic bones and femur, the greatest decrement in the rate of resorption occurred in the first 7 weeks of life (-52% and -72%, respectively). The axial bones and pelvis maintained a uniformly high resorptive rate until 7 weeks of age, which waned sharply at adolescence (-70% to -78%). At maturity the resorptive rate of these bones was less than 16% of the rate in the newborn.(ABSTRACT TRUNCATED AT 250 WORDS)