Evolution of our understanding of cell volume regulation by the pump-leak mechanism

Abstract

All animal cells are surrounded by a flexible plasma membrane that is permeable to water and to small ions. Cells thus face a fundamental problem: the considerable tension that their membranes would experience if the osmotic influx of water, driven by the presence of impermeant intracellular ions, was left unopposed. The pivotal study that described the cell's remedy for this impending osmotic catastrophe-the "pump-leak mechanism" (PLM)-was published in the Journal of General Physiology by Tosteson and Hoffman in 1960. Their work revealed how the sodium pump stabilizes cell volume by eliminating the osmotic gradient. Here we describe the mechanistic basis of the PLM, trace the history of its discovery, and place it into the context of our current understanding.

Figure 1.

The essentials of the PLM. The asymmetry in ion distributions between the cytoplasm and extracellular space is established by an NKA (top). The flexible membrane is permeable to Na⁺, K⁺, Cl⁻, and water. Impermeant molecules (X), mostly anions, with an average charge valence z, are trapped in the cytoplasm.

Figure 2.

The action of the NKA stabilizes cell volume. The line labeled “NKA on” indicates when the pump is turned on and then off (p = 0.5 μC cm⁻² s⁻¹). The composition of the extracellular solution is changed during the period indicated by “ΔOsmo,” to 350 from 300 mOsm/liter, by adding an impermeant uncharged molecule. The intracellular ion concentrations ([Conc.], top panel) are plotted as a function of time, with the concentrations stacked on top of one another. The voltage (middle panel) and cell volume (lower panel) are plotted on the same timescale. The inset on the lower panel shows the change in volume, at a higher temporal resolution (bar, 0.5 s), when the osmolarity is increased. When the pump is turned off, the cell volume increases, slowly but without limit, showing that the system is unstable. The volume is expressed as a percentage of the steady-state volume (% SS vol.). The equations and parameters are given in the Appendix.

Figure 3.

Constraints on the PLM. The steady-state intracellular ion concentrations ([Conc.], top), voltage (middle), and volume (bottom) are plotted as a function of the NKA pump rate (p). The concentrations are stacked as in Fig. 2. The steady-state solutions shown are the analytical Keener–Sneyd solution given in the Appendix. The hyperpolarization induced by the action of the NKA drives Cl⁻ out of the cell. Water moves out of the cell, preserving osmotic and charge balance, leading to a decrease in volume and an increase in the concentration of $X^z.$ As p is increased further, K⁺ progressively substitutes for Na⁺. Osmotic balance is shown again by the fact that the sum of all intracellular ions at any p equals the extracellular osmotic strength. Electroneutrality is also preserved at all p; the sum of anions = sum of cations; $z=-1.$ The volume is normalized by that at p = 2 (% Norm. vol.). The figure has been modified from Kay (2017), with a temperature of 37°C rather than 25°C.

Figure 4.

Joseph Hoffman (left) and Daniel Tosteson (right). Photograph from 1955 or 1956 at the Museum of National History, Frederiksborg Castle, Hillerød, Denmark, taken when Tosteson was a postdoctoral fellow in Hans Ussing’s laboratory in Copenhagen before he moved to join Alan Hodgkin’s laboratory at the University of Cambridge, UK. Photo courtesy of Dr. Magdalena Tosteson.