We studied insulin receptor characteristics and glucose uptake in erythrocytes (RBCs) of sheep during maturation. RBC insulin receptors were characterized with respect to binding of monoiodinated 125I-insulin and by Scatchard analysis to determine binding sites, and their affinities. Glucose uptake by RBCs was assessed by incorporation of 14C-2-deoxy D-glucose in the presence of D-glucose. The percent specific 125I-insulin bound to RBCs from fetal sheep (4.81 ± 0.48 SE; N = 11) was significantly higher when compared with those from 1–7-day-old newborn lambs (3.41 ± 0.24, N = 7). Thereafter, insulin binding progressively decreased to 2.45 ± 0.46 at 8–14 days, 0.99 ± 0.08 at 15–21 days, and 0.41 ± 0.25 at 22–35 days. No specific insulin binding was observed in adult sheep RBCs. When individual percent specific 125I-insulin binding was plotted against age, there was a significant negative correlation (r = −0.79; P < 0.01). Both high (HA) and low (LA) affinity binding sites per erythrocyte were significantly higher in the fetus (HA, 2.2 ± 0.1; LA, 11.6 ± 0.4) compared with those in newborn lambs (HA, 1.3 ± 0.1; LA, 6.4 ± 0.2). Although the association constant (Ka) for HA sites was significantly lower (P < 0.01) for fetal RBCs (2.4 ± 0.1 × 109 M−1) compared with that for newborn RBCs (3.2 ± 0.1 × 109 M−1), no significant difference was observed for LA Ka in both groups. Because of low percent specific 125l-insulin binding after 1 wk of age, accurate competition curves could not be calculated. Glucose uptake by fetal sheep RBCs (53 ± 2 nmol/min/L × 109 cells, N = 6) was significantly higher than that from adult sheep (25 ± 7, N = 7). A negative correlation existed between RBC glucose uptake and the age of the animals. Although both glucose uptake and insulin receptors decrease postnatally, they appear to be independent phenomena, since insulin (120 μU/ml) did not augment glucose uptake in either fetal cells possessing insulin receptors or adult cells without suchreceptors. We conclude that disappearance of insulin receptors and decreased glucose uptake in sheep RBCs reflect ongoing and independent maturation of membrane function.

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