Increased intestinal uptake of cobalamin in pregnancy does not require synthesis of new receptors

Abstract

Increased intrinsic factor cobalamin binding to receptors present in ileal mucosa from mice in the late stages of pregnancy is regulated by placental lactogen. In mice at day 18–20 of pregnancy given an intraperitoneal injection of cycloheximide, 0.5 mg/kg, receptor binding was reduced from 0.42 ng/mg protein to 0.18 ng/mg protein 4 h later. Intestinal mucosal protein synthesis was less than 20% of control values after this dose of cycloheximide. Although this result could be interpreted to mean that the increase in receptors in pregnancy was due to new protein synthesis, cycloheximide-treated mice also had reduced concentrations of placental lactogen in serum. Supplementation with the hormone in cycloheximide-treated mice maintained receptor binding at pregnant levels. Analysis of binding data showed receptor number to be 3.1·10 11/mg protein and the binding constant ( K a) to be 0.5·10 12 M −1, which were similar to values found in untreated pregnant mice. It is concluded that, because the increase in receptors cannot be explained on the grounds of new protein synthesis, placental lactogen may recruit cryptic intrinsic factor cobalamin receptors.