Insulin receptor binding to erythrocytes during normal pregnancy: an update of the method

Normal values of the parameters of insulin receptors in erythrocytes were provided to make a control group throughout gestation, 24 h postpartum and six weeks after delivery with the aim of comparing them with other parameters with insulin receptor-related pathologies. Thus, one of the purposes of this study was the update of a method to calculate the parameters of insulin receptors in erythrocytes, carrying out several modifications that improved the assay: during binding studies incubation was in continuous rotation shaking, and increasing maximal insulin binding. Other modifications were the increase in the concentration of insulin 125I to 1 ng/mL, and the maximal concentration of unlabeled insulin, 100 ng/mL. Erythrocyte age was considered by the intracellular creatine content providing control values and allowing the normalization of the parameters of insulin receptor during gestation. Data obtained in this study indicated that changes at receptor binding level may be also considered to explain insulin resistance: week 28 showed maximal insulin secretion (16.70+/-1.44 microU/mL) whereas plasma glucose concentrations remained almost constant (91.14+/-2.37 mg/100 mL) with respect to the 1st and 2nd trimester of pregnancy (89.73+/-1.38 and 91.71+/-2.10 mg/100 mL respectively); insulin reached the point of maximal resistance, which is explained by a decrease of maximum specific insulin binding, %Bo (6.32+/-0.51) at minimal values due to a decrease of high-affinity receptor number per erythrocyte, N-AA (16+/-2) at minimal values. Moreover, the negative correlation between progesterone (31.2+/-0.2 ng/mL) and Ka-AA (r=-0.71) could possibly be related to this maximal resistance.