Chloride transport by self-exchange and by KCl salt diffusion in gramicidin-treated red blood cells

The permeability of gramicidin-treated human red blood cell membranes to K+ and Cl- has been measured at normal ionic strength (1) by tracer exchange at steady-state distribution of salt, and (2) by net transport of salt in the presence of a salt concentration gradient. Under both conditions KCl was the only inorganic salt in cells and medium. In the studies of self-exchanges the electrical driving force on the ions was zero. Calculaton of permeability coefficients from net salt transport was simplified because the experiment was designed as a special case of the Nerst-Planck diffusion regime, i.e. the single salt case. Gramicidin altered the cell membranes from being anion to become cation selective. Gramicidin increased the potassium exchange without affecting the chloride exchange measurably. The chloride exchange showed saturation kinetics as does chloride exchange in normal cells. The net transport of KCl in the presence of a constant concentration gradient increased to a constant value with increasing gramicidin concentration. At high gramicidin concentrations (0 degree C, pH 7.2) the "chloride permeability coefficient" calculated from tracer exchange (1.9 x 10(-6) cm/s) was 290 times the chloride permeability coefficient calculated from net salt transport (0.65 x 10(-8) cm/s). The latter value corresponds to a chloride conductance of 4.2 x 10(-6) ohm-1 cm-2. The chloride permeability coefficient was 2.1 x 10(-6) cm/s at 25 degrees C (pH 6.8) indicating a value of 3 for the Q25. It appears that normal red cells are anion selective in the sense that anion permeability exceeds cation permeability with a factor of more than a hundred between 0 degrees C and body temperature. The anion exchange, i.e. the Hamburger shift, is a tightly coupled transport process which is several orders of magnitude faster than anion transport by salt diffusion.