Effect of placental lactogen on the number of intrinsic factor receptors in the pregnant mouse.

Experiments were designed to identify the factor in serum which has been shown to result in an increased number of intrinsic factor (IF) receptors in the small intestine of the mouse in late pregnancy. IF receptors for cobalamin (Cbl) were measured by determining the quantity of IF-57CoCbl bound in ileal homogenates. A Scatchard plot of the binding data confirmed that there was an increase in the number of IF receptors and revealed that there was a reduction in their affinity constant (K alpha). The K alpha for pregnant mice was 0.7 x 10(12) M-1 compared with 2.8 x 10(12) M-1 in nonpregnant mice. The number of receptors decreased between 2 and 4 hr after hysterectomy (Hx) in day 19 pregnant mice. Hourly injections of human placental lactogen (PL) maintained IF-Cbl binding at pregnant levels in the Hx mice. Growth hormone but not prolactin produced a similar response. The smallest quantity of PL which was effective was 1 microgram. Placental lactogen did not have an effect on IF-Cbl binding in nonpregnant female mice. Neither hypophysectomy nor ovariectomy affected IF-Cbl. It is proposed that placental lactogen is the serum factor which is responsible for the increased number of IF receptors in the pregnant mouse.