IgG binding and expression of its receptor in rat intestine during postnatal development.

The binding of 125I labelled IgG to the microvillus membranes (MVM) has been studied during postnatal development of rat intestine. The levels of mRNA encoding IgG receptor were also analyzed by liquid hybridization under these conditions. The IgG binding to MVM reached maximum levels by day 12 and showed a gradual decline upon weaning. The FcRn mRNA was markedly low in adult rats and was maximum during second week of postnatal development. Administration of cortisone or thyroxine to suckling rats, induced precocious decline of both IgG binding and the receptor expression. However, insulin administration did not affect the receptor expression. Scatchard analysis of IgG binding to MVM in cortisone injected pups revealed that the observed inhibition in IgG binding was a consequence of a decrease, both in the affinity constant (-Ka) as well as in the number of receptor sites (n) while thyroxine administration caused a reduction in the number of receptor sites from 2.29 in control to 1.14 nmoles/mg protein in thyroxine injected pups. These observations indicate that expression of IgG receptor during postnatal development is a hormone regulated process.