Macromolecular crowding and volume perception in dog red cells

To differentiate whether the primary volume signal in dog red cells arises from a change in cell configuration or the concentration and dilution of cell contents, we prepared resealed ghosts that had the same surface area and hemoglobin concentration as intact cells but less than 1/3 their volume. Shrinkage of both intact cells and resealed ghosts triggered Na/H exchange. Activation of this transporter in the two preparations correlated closely with cytosolic protein concentration but not at all with volume. The Na/H exchanger was more sensitive to shrinkage in albumin-loaded resealed ghosts than in intact cells or ghosts containing only hemoglobin. Similar results were obtained for the swelling-induced [K-Cl] cotransporter. We believe perception of cell volume originates with changes in cytoplasmic protein concentration. We think the kinases and phosphatases that control the activation of membrane transporters in response to cell swelling or shrinkage are regulated by the mechanism of macromolecular crowding.