NMR study of water exchange across the hepatocyte membrane

An understanding of the cellular permeability for water is needed to evaluate MR images of complex tissues, such as liver, and to interpret the effects of contrast agents. To obtain data essential for such an understanding we measured water exchange across the isolated rodent hepatocyte membrane by proton NMR relaxation with dextranmagnetite as a relaxation agent. The results are treated as water exchange in a two-compartment system, and possible reasons for deviations from that behavior are analyzed. The mean residence time of intracellular water was approximately 40 ms at 37°C. We found the lower limit for the diffusional permeability of the hepatocyte membrane to be 8 × 10^-3 cm s^-1. These results, combined with consideration of hepatic anatomy indicate that the failure to observe effects on the T₁ of liver from particulate contrast agents such as magnetite, Gd-starch, and liposome encapsulated Mn²⁺ is due to the localization of these agents in the Kupffer cells. Also, the nonexponential T₁ decay observed in normal liver is unlikely to be due to slow exchange of water between compartments.