Restricted diffusion of bicarbonate and hypophosphite ions modulated by transport in suspensions of red blood cells

The central requirement for the determination of transport rates by NMR is to be able to distinguish between the intra- and extracellular species. We present two methods for characterizing fast transmembrane transport that are based on the pulsed field gradient NMR technique. The first method relies on the transported solute having different diffusion coefficients on either side of the membrane and the subsequent modulation of the observed diffusion coefficients in the presence of transmembrane transport. The second method, the absorbing wall technique, relies on attenuation of the spin-echo signal from the observed species diffusing into the "absorbing wall." The absorbing wall condition was established by addition of Mn²⁺ to the extracellular fluid. For these experiments the transport of both bicarbonate and hypophosphite ions across the membrane of the human red blood cell was studied.