In vitro studies on methyl mercury distribution in human blood

Studies comparing the methyl mercury (mHg) content of maternal and newborn blood have shown increased levels in the newborn. This has been attributed to facilitated transplacental diffusion because of high fetal hematocrit (Hct). This study shows the converse, that the diffusion of mHg diminishes progressively with increasing Hct. The diffusion of m203Hg across a Millipore membrane (0.45 microns) separating compartments A and B of a diffusion cell was studied. When both compartments contained saline or plasma alone, equilibration from A to B occurred in 5 h. Introduction of human red blood cells (RBC) in saline (Hct 20%) into B resulted in a twofold increase in diffusion of mHg when compared to saline alone. Increasing Hct in saline in compartment B resulted in a progressive decrease in diffusion (r = -0.95, P less than 0.001). The presence of RBC in plasma (Hct 20%) in B resulted in a 70% decrease in diffusion; with increasing Hct, diffusion was further reduced (r = -0.95, P less than 0.001). Direct addition of mHg to RBC in saline resulted in 98% RBC uptake. Increasing concentrations of plasma (at a constant Hct) resulted in a progressive decrease in RBC uptake. In undiluted plasma at Hct 14%, RBC uptake of mHg was 35%. Plasma electrophoresis showed that much of the mHg was associated with a high-molecular-weight lipoprotein fraction. Plasma components appear to be important in the distribution of mHg in blood, and may be a factor in the relatively higher blood levels in the fetus.