Insulin binding is a specific marker of fetal erythrocytes in ruminants

The ability of erythrocytes (RBC) from sheep and cattle of various gestational and postnatal ages to bind insulin specifically was studied. Insulin binding to RBC decreased as gestational and postnatal age advanced and was absent in blood obtained from adult animals. Maximal percentage 125I-insulin bound to RBC (3.6 X 10(9)/ml) was highest in the fetuses of sheep and cattle (7.3 +/- .6 and 7.8 +/- .9, respectively) compared to postnatal animals (2.3 +/- .2 and 2.2 +/- .3, respectively), or adults (no binding) of the same species. The decrease in binding began antenatally, and binding was projected to be insignificant by the end of the second postnatal month. Most of the observed decrease was due to a progressive decrease in the number of receptors on the cell surface. The time course of this phenomenon, as well as the total absence of insulin receptors on the RBC of adult ruminants, provides independent evidence that two distinct populations of RBC in ruminants exist. The gradual appearance of the adult RBC with no insulin binding results in a decrease in observed binding to RBC in a given blood specimen as fetuses and postnatal animals age.