Insulin Binding to Erythrocytes from Pregnant, Postpartum, Follicular and Luteal States

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Abstract
Specific binding of [125I]insulin to isolated erythrocytes was investigated in 4 groups of women: (A) pregnant subjects between wk 38-40 of pregnancy (n = 18); (B) postpartum subjects within 6 days after delivery (n = 30); (C) normal women during the follicular phase of the menstrual cycle (n = 12); and (D) normal women during the luteal phase of the menstrual cycle (n = 11). The respective specific [125I]insulin binding (fraction), fasting plasma glucose concentrations (mmol/l) and the corresponding insulin concentrations (mU/l) were as follows: 0.074 .+-. 0.012/4.00 .+-. 0.58/29.4 .+-. 21.4 for group A, 0.065 .+-. 0.016/4.40 .+-. 0.75/41.5 .+-. 26.2 for group B; 0.052 .+-. 0.008/4.58 .+-. 0.62/6.7 .+-. 4.0 for group C; and 0.054 .+-. 0.011/4.49 .+-. 0.63/8.3 .+-. 5.9 for group D. By using a modified Scatchard analysis, statistically significant differences were observed between the receptor affinities of the groups A and D, B and D, and A and C. The receptor affinities and concentrations were not significantly different between the follicular and luteal phases. No inverse correlation between the plasma insulin concentration and receptor binding was seen; i.e., the phenomenon of downregulation of insulin receptor concentration with hyperinsulinemia seemed not to apply to erythrocytes. Insulin binding to erythrocytes could be modulated preferably, or even exclusively, by an alteration of receptor affinity. Short-term changes in insulin binding to erythrocytes apparently are not caused by an alteration of receptor concentration.

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