Bone structural and metabolic changes at the end of pregnancy and lactation in rats

OBJECTIVE: The objective of this study was to elucidate the net change of bone structure and metabolism in the lumbar trabecular bone of rats at the end of the pregnancy and lactation. STUDY DESIGN: Female virgin Sprague-Dawley rats aged 200 days were mated, and bone mineral density by dual-energy x-ray absorptiometry, bone histomorphometry, and serum bone metabolic markers were measured at the end of pregnancy (day 22 of pregnancy), after delivery (day 5 post partum), and at the end of lactation (day 21 post partum). RESULTS: At the end of pregnancy bone mineral density, bone volume, trabecular thickness, and serum calcium decreased; serum parathyroid hormone increased; and the histomorphometric parameters indicated that bone resorption were higher than those variables in nonpregnant rats; but bone formation was suppressed, as demonstrated by the low histomorphometric parameters and by the low serum alkaline phosphatase levels. After delivery the bone mineral density of nonlactating rats recovered rapidly, as in nonpregnant rats, but nonlactating rats showed more bone formation by histomorphometry than nonpregnant rats did. At the end of lactation, bone mineral density and serum calcium levels decreased considerably, and lactating rats showed substantial bone formation, bone resorption, and high serum alkaline phosphatase levels. The correlation between the number of pups (x) of the lactating and nonlactating groups and the bone mineral density (y, in grams per square centimeter) showed simple linear regression (y = -0.0067 . x + 0.2517, r=0.949, p<0.0001). CONCLUSIONS: These results indicate that significant decreases occur in the trabecular bone of rats at the end of pregnancy and lactation and that lactational intensity is related to bone mineral density.