The use of transmembrane differences in saturation transfer for measuring fast membrane transport; application to H¹³C0₃ ^- exchange across the human erythrocyte

We consider the NMR signal intensity of a solute B, that is both in exchange across the membrane of cells in suspension and also in chemical exchange with another solute C, when the nuclei of C are saturated with selective radiofrequency radiation. This extension of the classical saturation transfer experiment potentially enables the measurement of fast unidirectional membrane transport rates of solutes that are in chemical equilibrium, when the chemical shifts of the intra- and extracellular solute are identical. The application of the theory and experimental methods to a biological system is illustrated by using ¹³C NMR with the H¹³CO₃ ^{- 13}CO₂ exchange system in human erythrocytes. In the buffer conditions employed the estimated value of the exchange rate of H¹³C0₃ ^- was comparable to values estimated with NMR and other non-NMR methods.