Effect of insulin on glucose uptake by the maternal hindlimb and uterus, and by the fetus in conscious pregnant sheep

Previous studies have demonstrated the presence of insulin receptors on the maternal surface of the placenta in several species and the specific binding of insulin to the placenta in sheep. However, both in-vitro and in-vivo studies have produced conflicting evidence concerning the effect of insulin on placental glucose uptake. To clarify this problem, we measured maternal hindlimb, uterine and fetal glucose and oxygen extractions and glucose/oxygen quotients in chronically catheterized, non-stressed, late-gestation pregnant sheep over 1 h at a constant concentration of arterial plasma glucose, and again during the next 2 h at the same glucose level but at a higher insulin concentration using glucose 'clamp' methodology. Insulin produced a 4.9-fold increase in glucose extraction and a 3.5-fold increase in glucose/oxygen quotient across the hindlimb; in contrast, insulin did not significantly affect uterine or fetal glucose extraction or glucose/oxygen quotient. We conclude that in contrast to other tissues of the pregnant ewe, placental glucose uptake and transfer are insensitive to variations in maternal insulin concentration.