| objectives1 To investigate in vivo the effect of lactation on the BMD of the proximal tibia in adult rats.2 To investigate the changes of serum osteocalcin and alkaline phoshatase level during lactation.3 To investigate the relation between the number of pubs and the BMD of the proximal tibia in rats4 To investigate the relation between lactating time and the BMD of the proximal tibia in ratsMethodsA total of 40 female virgin Sprague-Dawley rats ,were randomized into four groups composed of ten rats each .1 Groups of experimental animals: Nonpregnant control group: virgin female rats, which would be killed on the forty-third days. Nonlactating group: all the pups of postpartum were removed immediately after delivery . pregnant rats would be killed on the twenty-first days after delivery Lactating A group: the number of lactating litters were adjusted to ten-fifty . pregnant rats would be killed on the twenty-first days after delivery Lactating B group: the number of lactating pups were Controlled to three-six. pregnant rats would be killed on the twenty-first days after delivery. Nonpregnant control group, Lactating groups , nonlactating group rats were fed standard reproductive forage, containing 1.8% calcium, 1.2% phosphorus, 20%proteinum ,and 1500IU of vitamin D /kg.2 The BMD of right proximal tibia of rats were observed in vivo by DEXA. The time of investigate were the first day , the end of the first week , the second week and the third week after parturition. The control group were investigated at the same time.3 The serum of rats were gathered at the same day of measuring pBMD respectively. Then the osteocalcin levels were measured by radioimmunoassay together.4 The serum of rats were gathered at the same day of measuring pBMD respectively, Total alkaline phosphatase activity was estimated together by Method of nitrobenzene phosphoric acidResults1 Changes of body weights of rats in different groups: The body weights of rats in three groups did not present difference at the beginning, which increased significantly at the end of pregnancy and approached to the peak level, While the body weights declined significantly after delivery in above three groups. the body weights of rats in lactation A group,lactation B group and nonlactation group after delivery were different with that of beginning experiment(P<0.05);the body weights of rats in lactation A group and lactation B group declined at the end of the second week, The body weights of rats in nonlactation group were decreased to the most lowest level at the end of the first week after delivery, Then started rising from the end of the second week .There was no difference among lactation A group,lactation B group and nonlactation group about the body weights of rats。2 Changes of pBMD in different groups:The pBMD of lactation groups after delivery were gradually significantly declined with time (P<0.05); The pBMD increased with time after delivery in nonlactation group, but the changes of that was nonsignificant。The pBMD of the lactation groups was significantly lower than that in the other groups(P<0.05); the differences in The lactation A group from the lactation B group were nonsignificant(P<0.05); The pBMD in the nonlactating group was similar to that in the control group。The decreasing mean levels of the pBMD in lactation groups were different from nonlactation group at the end of the third week after delivery (P<0.05),but the changes of mean between the lactation A group and the lactation B group were nonsignificant。3 Changes of serum osteocalcin levels in four groups : The serum osteocalcin levels of rats in nonlactation group rised with time after delivery,but the changes was not significant; The serum osteocalcin levels of rats in lactation groups gradually significantly increased after delivery (P<0.05)。the osteocalcin levels in lactation and nonlactation groups after delivery were higher than control group(P<0.05); The changes in nonlactation group after delivery were not different with lactation groups; minor differences in lactation A group and lactation B group。4 Changes of the serum Alkaline Phosphatase levels of rats in four groups:The serum Alkaline Phosphatase levels of rats in nonlactation group increased after delivery, but the changes did not present differences; The Alkaline Phosphatase levels in lactation groups significantly increased(P<0.05)。The Alkaline Phosphatase levels in the nonlactation group was similar to that in the control group; the Alkaline Phosphatase levels in lactation group were different with the control group (P<0.05); the Alkaline Phosphatase levels in lactation and nonlactation groups after delivery were different (P<0.05);that in the lactation A group was similar to that in the lactation B group。 5 The pBMD and the number of pups had strongly negative correlation at the end of lactation, according to the relative analysis(r= -0.931 r= -0.783 P =0.001 P =0.022)。Conclusions1 The pBMD significantly decreased during lactation.2 The elevating of serum Alkaline Phosphatase and osteocalcin levels are beneficial to recovery of bone mass.3 The number of pups and pBMD has negative correlation at the end of lactation, but were not relative with the decreasing deep of the pBMD.4 Reducing lactating time can diminish bone loss after delivery.
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