Fig. 1

A simple model of correlation between changes in cytosolic Cl⁻ concentration ([Cl⁻]_c) and cell volume under isosmotic conditions. This simple model shows contents and concentrations of K⁺, Cl⁻ and membrane-impermeable fixed negative charges such as proteins, although other ions including Na⁺ and HCO₃⁻ significantly exist in cell. A Cell volume = 100%. Cell contains 150 mmoles/L K⁺ (150 mM), 45 mmoles/L Cl⁻ and 105 mEq/L fixed negative charges. B Cell volume = 80%. When the cell volume reduces to 80% under an isosmotic condition, 20% K⁺ (30 mmoles/L) is released from the cytosolic space to the extracellular space; K⁺ remaining in the cytosolic space is 120 mmoles/L and the cytosolic K⁺ concentration (150 mM) is same as before the cell volume change occurs. On the one hand, when cell volume decreases by 20%, the same amount of Cl⁻ (30 mmoles/L) as K⁺ must be released from the cytosolic space to the extracellular space to keep electroneutrality. This means that the cytosolic Cl⁻ is reduced to 15 mmoles/L from 45 mmoles/L after the occurrence of 20% cell volume decrease, and the cytosolic Cl⁻ concentration is reduced to 19 mM (15 mmoles/0.8 L) from 45 mM before the cell volume decrease occurs. This is because even though the cytoplasmic Cl⁻ content (45 mmoles/L) is much lower than the cytoplasmic K⁺ content (150 mmoles/L), the same amount of Cl⁻ as K⁺ must be released from the cytosolic space due to the presence of a large amount of membrane-impermeable fixed negative charge