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

The binding of I-125 labelled IgG to the microvillus membranes (MVM) has be en studied during postnatal development of rat intestine. The levels of mRN A encoding IgG receptor were also analyzed by liquid hybridization under th ese 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 a dult rats and was maximum during second week of postnatal development. Admi nistration of cortisone or thyroxine to suckling rats, induced precocious d ecline of both Ige binding and the receptor expression. However, insulin ad ministration did not affect the receptor expression. Scatchard analysis of IgG binding to MVM in cortisone injected pups revealed that the observed in hibition in IgG binding was a consequence of a decrease, both in the affini ty constant (-Ka) as well as in the number of receptor sites (n) while thyr oxine administration caused a reduction in the number of receptor sites fro m 2.29 in control to 1.14 nmoles/mg protein in thyroxine injected pups. The se observations indicate that expression of IgG receptor during postnatal d evelopment is a hormone regulated process.