Relation between plasma glucose levels of mother and fetus during maternal hyperglycemia, hypoglycemia, and fasting in the rat.

Abstract

Extract: The relation between the concentration of glucose in maternal and fetal plasma was examined in pregnant rats during the last three days of gestation. Rats were studied after being fasted for 16 hours following two hours of infusion of physiologic saline or insulin (1,000 μU/minute) in either 30% glucose or saline. After a constant concentration of glucose in maternal plasma had been achieved, the rats were sacrificed rapidly and fetal blood was obtained. In fasting animals, before day 20, the concentration of glucose in fetal plasma was low, and a gradient of 2.0 or greater prevailed between maternal and fetal plasma glucose. This gradient fell rapidly during days 20 and 21 to average 1.0 for the last 24–48 hours before term. In the fetus, the rise in the level of glucose paralleled that of hepatic glycogen. When maternal hyperglycemia was induced by infusion of glucose, the gradient in the younger fetuses was poorly maintained and fetal hyperglycemia occurred at all ages. When maternal hypoglycemia was induced by infusion of insulin, the younger fetuses displayed proportional hypoglycemia; however, after day 20, the level of plasma glucose in the fetus exceeded the maternal level, and the placental gradient for glucose was reversed. It is concluded that during maternal hypoglycemia, the near-term fetus is capable of independently regulating the level of glucose in blood for at least two hours. During maternal hyperglycemia, the normal placental gradient for glucose characteristic of the younger fetus fails to prevent completely fetal hyperglycemia, and near-term, maternal hyperglycemia is transmitted to the fetus without evident fetal counter regulation. Speculation: This study provides additional evidence for the thesis that the near-term fetus develops independent mechanisms for production and regulation of glucose during periods of glucose deprivation. The development and use of such mechanisms in utero during maternal fasting may represent an important preparation for extrauterine existence. It remains to be determined whether the prevailing hyperglycemia of diabetes during gestation might delay development of the pathways involved.