Magnitude of calcium influx required to induce dehydration of normal human red cells

Activation by [Ca²⁺]_i of Ca²⁺-sensitive K⁺ channels has long been known to cause dehydration of red cells suspended in low-K, plasma-like media. However, the fundamental question of the extent to which Ca influx must be increased to trigger dense cell formation in conditions likely to arise in the circulation has not been established. We report here that in ionophore permeabilized red cells, increasing Ca influx above 0.7 mmol/litre cells per h induces the formation of subpopulations of dehydrated cells within 1-2 hours. The presence or absence of glycolytic substrates had little effect suggesting that ATP depletion was not large enough to significantly inhibit the pump within that period. Below maximal dehydrating Ca influxes of about 1.2 mmol/litre cells per h, the trend was for the fraction of dense cells formed to remain steady in time. As Ca influx was increased, both the rate of dense cell formation and the fraction of dense cells formed increased. These results are analyzed in relation to mechanisms and to possible states of increased Ca²⁺ permeability in physiological and physiopathological conditions.