Humoral regulation of vitamin B12 absorption by pregnant mouse small intestine.

In vivo experiments in which pregnant mice were fed an oral dose of 40 ng of [57Co]B12 demonstrated that there was a progressive increase in B12 absorption in the 3rd week of pregnancy. The absorptive mechanism resembled that in nonpregnant animals on the grounds that the mechanism was saturable, and presumably intrinsic factor (IF) dependent, and that it was localized to the ileum. In vitro studies of IF-57Co B12 binding to ileal homogenates confirmed the requirement for IF. Studies in late pregnancy showed that IF-[57Co]B12 binding to ileal homogenates, expressed as nanograms of IF-[57Co]B12 bound per milligram of protein, was twice that of nonpregnant animals. When ileal homogenates were prepared from mice on days 20 to 21 of gestation, IF-[57Co]B12 binding fell to nonpregnant levels within 2 to 4 hr of hysterectomy. Hourly injections of serum from mice in late pregnancy, or from a female at the 37th week of gestation, maintained IF-B12 binding at levels reached during pregnancy, whereas nonpregnant serum was ineffective.