Model for the role of macromolecular crowding in regulation of cellular volume.

A simple model is proposed to account for large increases in transporter-mediated ion flux across cell membranes that are elicited by small fractional changes of cell volume. The model is based upon the concept that, as a result of large excluded volume effects in cytoplasm (macromolecular crowding), the tendency of soluble macromolecules to associate with membrane proteins is much more sensitive to changes in cell water content than expected on the basis of simple considerations of mass action. The model postulates that an ion transporter may exist in either an active dephosphorylated state or an inactive phosphorylated state and that the steady-state activity of the transporter reflects a balance between the rates of phosphatase-catalyzed activation and kinase-catalyzed inactivation. Cell swelling results in the inhibition of kinase relative to phosphatase activity, thereby increasing the steady-state concentration of the active form of the transporter. Calculated volume-dependent stimulation of ion flux is comparable to that observed experimentally.