Skeletal mass, chemistry, and growth during and after multiple reproductive cycles in the rat

There are dramatic changes in skeletal physiology and metabolism to accommodate the mineral requirements of the developing fetus during pregnancy and milk production during lactation. The purpose of this study was to document changes in skeletal growth, chemistry, and mass during and after multiple reproductive cycles in the rat, with emphasis on the putative reconstitution of the skeleton after lactation. To determine skeletal changes, experimental groups included rats at the end of the first and second lactation, at the end of the second pregnancy, and at various times after the first and second lactation. These groups were also compared with aged-matched, nulliparous animals. There were decreases in femoral ash weights and bone mineral densities (BMDs) during the first and second lactations, but accelerated rates of gain after lactation, compared with the nulliparous controls. The changes in bone ash were even more pronounced when normalized to the maternal body weight changes during and after the reproductive cycles. The rates and amount of bone mineral (ash) gain after the first and second reproductive cycles were similar; however, neither bone mineral nor BMD returned to levels found in nulliparous animals after the first and subsequent reproductive cycle. There was also a decrease in ash/dry weight ratio of the femur during the second lactation, suggesting a preferential loss of more mineralized bone. This decrease in ash/dry weight ratio reversed after lactation, indicating a relative accumulation of bone mineral during the postlactational period. As expected, endochondral growth was substantially suppressed during lactation, but rebounded during the postlactational period. These data collectively support the notion that the female rat has excess skeletal mass to accommodate losses associated with the first reproductive cycle. After the first reproductive cycle, a new optimal skeletal mass is achieved. These data also demonstrate that the postlactational period is "anabolic" with accelerated rates of bone growth and accumulation of bone mineral and bone mineral density with increases in the ratios of the inorganic to organic composition of the bone. This postlactational recovery phase may serve to at least partially reconstitute skeletal mineral depleted during lactation and perhaps to prepare the skeleton for the next reproductive cycle.