CARBOHYDRATE METABOLISM IN PREGNANCY: THE DEGRADATION OF INSULIN BY EXTRACTS OF MATERNAL AND FETAL STRUCTURES IN THE PREGNANT RAT1

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Abstract
Cell-free extracts have been prepared from livers, placentas, and fetuses of pregnant rats on day 12, 15, 18 and 21 of gestation and from livers of non-pregnant control animals. With the use of I131 -insulin and nitrogen reference standards, the insulin-degrading capacities of these structures have been compared on the basis of activity per unit mass (specific activity) and activity per total tissue (total potential activity). It has been demonstrated that mechanisms for the proteolytic degradation of insulin are present within the conceptus throughout gestation. Placental specific activity remained constant and at all times equal to that of maternal and control liver. Thus, for pooled placentas, total potential activity paralleled the increase in placental mass to reach one-third the value of maternal liver at term. Fetal specific activity increased late in pregnancy to approximately twice the level found in the other tissues. In isolated fetal parts, activity was greatest in fetal skin. At term, the total potential activity of the pooled fetuses was almost three times as great as that of maternal liver. The demonstration of insulin-degrading systems in rat placenta and fetus at multiple intervals during pregnancy lends further support to the thesis that proteolysis of hormones within the conceptus may be of significance in the gestational changes in endocrine economy.