Mechanism of placental glycogen deposition in diabetes in the rat

Abstract

Summary. The metabolic basis for glycogen accumulation in the placenta of rats with diabetes induced by streptozotocin on day 12 of pregnancy was studied on days 15 and 20. On day 15 glycogen content of the placenta was 1.5-fold higher in the diabetic than in the control rats and this difference increased to > fivefold on day 20 of gestation whether calculated per g tissue or per total placenta. Accumulation of glycogen was associated with increased specific activities of both glycogen synthase and phosphorylase. The activities of these enzymes regulating synthase and phosphorylase activities and the activity of acid a-glucosidase were not significantly affected by diabetes. Glucose-6-phosphate concentration of the placenta was 67 and 23 nmol/g in diabetic and control rats, respectively. Incubation of placental homogenates with glucose increased the rate of inactivation of phosphorylase and activation of glycogen synthase. These results indicate that the enhanced glucogenesis in diabetes is not due to changes in the activities of these enzymes, as measured in vitro under standard conditions. The factors promoting glycogen accumulation in vivo are related to the abundance of glucose and glucose-6-phosphate as substrates for glycogen synthesis, which may also cause an increase in the activity ratio glycogen synthase a / phosphorylase a. In addition, the high intracellular glucose-6-phosphate concentration is likely to enable glycogen synthase b to contribute to glycogen synthesis.

Evaluation of placental glycogen storage in mild diabetic rats

“…Glycogen accumulation in the placenta, in face of maternal insulin deficiency and the fact thatbhyperinsulinemia on the fetal side is not more accentuated in diabetic than in control rats, indicates that insulin has no direct influence on glycogen metabolism in this tissue 10 . Glucose flow across the placental is substantially reduced in diabetic hyperglycemia, and there is no convincing demonstration, as yet, of an insulin-dependent, rate-limiting locus in the placenta 20 .…”

“…The placenta is glucose-dependent related with of energy. Maternal glucose transfer by maternal placenta is determined by the glucose concentration and glucose amount stored as glycogen 10 .…”

Effect of diabetes on the sugar nucleotides in several tissues of the rat

“…In addition, glycogen levels appear to change in a parallel manner. A recent study of placental glycogen synthesis has suggested that the increased formation of this polymer noted in vivo is more clearly reflected by the increased level of substrate present in the tissue (glucose 6-phosphate) than by measurement of the enzymes phosphorylase and glycogen synthese in vitro [30].…”

Placental Function in Maternal-Fetal Fat Transport in Diabetes

“…Too rapid FFA transfer may produce untoward effects in tissues with fat oxidation capacity not yet fully developed. An analogy may be seen in glucose transport -the pla centa accumulates glycogen proportionately to the level of diabetic hyperglycemia [2,3,5], perhaps as an attempt to moderate the excessive glucose transfer to the fetus. In deed, fetal glucose levels arc at most times no more than two thirds of the maternal ones.…”