AJP - Regulatory, Integrative and Comparative Physiology, Vol 250, Issue 3 459-R464, Copyright © 1986 by American Physiological Society
Diffusion permeability of cyanocobalamin in human placenta
D. M. Willis, J. P. O'Grady, J. J. Faber and K. L. Thornburg
The molecular weight of cyanocobalamin is well suited to distinguish the
two patterns of size discrimination found in the hemochorial and
epitheliochorial placentas. The concentrations of cyanocobalamin that were
used ensured that nondiffusional transport was negligible, and experiments
on guinea pigs confirmed that the diffusion permeabilities measured with
cyanocobalamin were the same as those expected for inert hydrophilic
substances of similar molecular weight. All human studies were performed on
volunteers who were scheduled for elective cesarean section under spinal or
epidural anesthesia. Endogenous plasma concentrations of cobalamin were
measured in 10 pregnant volunteers and their newborns. Ten additional
volunteers were given 1 mg of cyanocobalamin by intramuscular injection 44
min before delivery. Placental permeability, calculated as the ratio of
fetal uptake and the concentration-time integral between maternal and fetal
plasmas, was 14.3 microliter X min-1 X g placental wt-1. The permeability
of the human placenta parallels that of the histologically similar guinea
pig placenta, at a slightly higher level, up to molecular weights of 1,355.
