Effects of exogenous insulin on placental transfer of maternal glucose to the rat fetus.

There is controversy concerning the possible modulation of glucose transfer to the fetus by insulin acting on the maternal side of the placenta. To study this question, 20.5 day pregnant rats were infused simultaneously with (U-14C)-D-glucose (via the jugular vein) and with different doses of insulin (via the left uterine artery) so that placentas from the left uterine horn were exposed to a higher insulin concentration than those from the right uterine horn. Placentas and fetuses from each uterine side were processed separately. No differences were detected in total blood radioactivity, plasma-14C-glucose, -14-C-lactate, -glucose, or -radioimmunoassayable insulin in fetuses from the left versus the right uterine horn. Total placenta radioactivity and 14C-glycogen were also similar in the left and right uterine sides at all insulin doses studied. Infusion of insulin (66 mU/min) to the pregnant rat caused hyperinsulinaemia and hypoglycaemia, decreased blood total radioactivity and plasma 14C-glucose, and increased plasma 14C-lactate in the mother. The level of fetal plasma 14C-glucose paralleled that of the mother. It is concluded that in the rat, placental glucose uptake, its transfer to the fetus, and fetal glucose utilization are not directly affected by maternal circulating insulin. Metabolic changes occurring in fetuses of hyperinsulinaemic mothers are secondary to the decreased availability of glucose.