Changes in the number of insulin receptors of isolated rat hepatocytes during pregnancy and lactation.

The weight of the rat liver increased during pregnancy and lactation due solely to an increase in hepatocyte size. Insulin receptors were identified using 125I-labelled insulin and isolated hepatocytes in vitro. Scatchard analysis was interpreted in terms of high affinity (KD 4 nM) and low affinity (KD 80 nM) receptors for insulin. No change in the affinity of either receptor site was detected during pregnancy or lactation; There was, however, a significant increase in the number of both types of receptor on the hepatocyte by day 12 of pregnancy which was maintained until at least day 20 of pregnancy. During lactation, the number of receptors declined to values similar to those of virgin rats. Serum insulin concentrations, determined by radioimmunoassay, were elevated during pregnancy, returned to values similar to virgin rats during early lactation and were significantly reduced compared with virgin rats by day 15 of lactation. These results illustrate that physiological conditions exist whereby the number of insulin receptors may increase, despite elevated serum insulin concentrations, in apparent conflict with the 'down-receptor' hypothesis.