Chloride conductance of the amphiuma red cell membrane

Like most other red cells, the giant erythrocytes of Amphiuma means possess a system for rapid exchange of chloride across the membrane. Also, there are indications that the net transport of chloride in these cells is slow. The size of Amphiuma erythrocytes allows direct measurements of membrane potential with microelectrodes. The present work exploits the possibility that such measurements can be used to give a quantitative estimate of the chloride conductance (GCl) of the Amphiuma red cell membrane. The membrane potential was measured as a function of extracellular chloride concentration (5-120mM), using an impermeant anion (Para-amino-hippurate) as a substitute. Furthermore, the effect of different pH values (6.0-7.2) was studied. For each extracellular chloride concentration the membrane potential was determined at a pH at which hydroxyl, hydrogen, and bicarbonate ions were in electrochemical equilibrium. From these membrane potentials and the corresponding chloride concentrations in the medium (at constant intracellular ion concentrations), the GCl of the membrane was calculated to be 3.9 x 10-7 omega-1 cm-2. This value is some six orders of magnitude smaller than that calculated from the rate of tracer exchange under equilibrium conditions. The experimental strategy used gives the values for a "partial transference number" which takes into account only ions which are not in electrochemical equilibrium. Whereas this approach gives a value for GCl, it does not permit calculation of the overall membrane conductance. From the calculated value of GCl it is possible to estimate that the maximal value of the combined conductances of hydroxyl (or proton) and bicarbonate ions is 0.6 x 10-7 omega-1 cm-2. The large discrepancy between the rate of exchange of chloride and its conductance is in agreement with measurements on human and sheep red cells employing the ionophore valinomycin to increase the potassium conductance of the membrane. The results in the present study were, however, obtained without valinomycin and an accompaning assumption of a constant field in the membrane. Therefore, the present measurements give independent support to the above mentioned conclusions.