The triphasic volume response of human red cells in low ionic strength media: demonstration of a special bicarbonate transport.

When human red cells are first placed in isotonic media of low ionic strength there is a triphasic volume change. The initial shrinkage (phase A) starting immediately is followed by a rapid reflation (phase B). The final shrinkage (phase C) is slower and has more variable properties. The first two phases need Cl- and HCO3- and they have been shown to involve both the band 3 anion exchanger and the red cell carbonic anhydrase. Phase C is only seen when phase B is present. Both phase B and phase C were accelerated by the lower monohydric alcohols. With methanol the acceleration was maximal near 2M, thereafter inhibition developed. The hypothesis is advanced that phase C involves a permeability for bicarbonate which is independent of the band 3 anion exchanger, and which may be a dimer of carbonic anhydrase. The unique kinetics, the physiological significance, and implications of the special bicarbonate transport system are discussed.