Cell Volume Regulation in Lower Vertebrates

Functional significance of cell volume regulatory mechanisms

To survive, cells have to avoid excessive alterations of cell volume that jeopardize structural integrity and constancy of intracellular milieu. The function of cellular proteins seems specifically sensitive to dilution and concentration, determining the extent of macromolecular crowding. Even at constant extracellular osmolarity, volume constancy of any mammalian cell is permanently challenged by transport of osmotically active substances across the cell membrane and formation or disappearance of cellular osmolarity by metabolism. Thus cell volume constancy requires the continued operation of cell volume regulatory mechanisms, including ion transport across the cell membrane as well as accumulation or disposal of organic osmolytes and metabolites. The various cell volume regulatory mechanisms are triggered by a multitude of intracellular signaling events including alterations of cell membrane potential and of intracellular ion composition, various second messenger cascades, phosphorylation of diverse target proteins, and altered gene expression. Hormones and mediators have been shown to exploit the volume regulatory machinery to exert their effects. Thus cell volume may be considered a second message in the transmission of hormonal signals. Accordingly, alterations of cell volume and volume regulatory mechanisms participate in a wide variety of cellular functions including epithelial transport, metabolism, excitation, hormone release, migration, cell proliferation, and cell death.

Membrane mechanisms and intracellular signalling in cell volume regulation

Recent work on selected aspects of the cellular and molecular physiology of cell volume regulation is reviewed. First, the physiological significance of the regulation of cell volume is discussed. Membrane transporters involved in cell volume regulation are reviewed, including volume-sensitive K+ and Cl- channels, K+, Cl- and Na+, K+, 2Cl- cotransporters, and the Na+, H+, Cl-, HCO3-, and K+, H+ exchangers. The role of amino acids, particularly taurine, as cellular osmolytes is discussed. Possible mechanisms by which cells sense their volumes, along with the sensors of these signals, are discussed. The signals are mechanical changes in the membrane and changes in macromolecular crowding. Sensors of these signals include stretch-activated channels, the cytoskeleton, and specific membrane or cytoplasmic enzymes. Mechanisms for transduction of the signal from sensors to transporters are reviewed. These include the Ca(2+)-calmodulin system, phospholipases, polyphosphoinositide metabolism, eicosanoid metabolism, and protein kinases and phosphatases. A detailed model is presented for the swelling-initiated signal transduction pathway in Ehrlich ascites tumor cells. Finally, the coordinated control of volume-regulatory transport processes and changes in the expression of organic osmolyte transporters with long-term adaptation to osmotic stress are reviewed briefly.

Selected aspects of cell volume control in renal cortical and medullary tissue

Under normal physiological conditions, demands placed on mammalian renal cortical cells are quite different from those in the medulla. Cortical proximal tubule cells exist in an isotonic environment, but must resorb vast amounts of filtered fluid and solute, and also adjust to solute generated from cellular metabolism. In addition, cortical cells must also adjust to occasional pathological derangements in blood osmolality. By contrast, human medullary cells have a smaller solute resorptive load, but exist in a milieu where osmolality varies from 40 to more than 1200 mosmol/kg H2O, depending on water intake. Remarkably, the cells maintain a near normal size despite these stresses. Under isosmotic conditions, the primary regulator of cell volume is Na-K ATPase. In its absence, factors such as external protein, extracellular matrix and basement membrane, cytoskeleton, and perhaps formation of cytoplasmic vesicular-like structures help prevent cells from swelling massively. Under anisosmotic conditions, a variety of transport processes operating across basolateral and apical membranes either remove solute from or add solute (and water) to cells to minimize changes in their size. Medullary cells have the additional ability to accumulate organic, non-toxic, osmolytes that offset external hypertonicity and allow cells to maintain normal size without increasing cellular inorganic ion concentrations.

pCMBS-induced swelling of dogfish (Squalus acanthias) rectal gland cells: role of the Na+,K(+)-ATPase and the cytoskeleton

(1) 0.1-1.0 mM p-chloromercuribenzene sulfonate (pCMBS) and some other organic mercurials produce a swelling of slices of dogfish shark (Squalus acanthias) rectal glands, with an uptake of cell Na+ and a loss of K+. In contrast, 1 mM N-ethylmaleimide (NEM) does not swell rectal gland cells (RGC), while affecting cell cations. (2) The slow entry of [203Hg]pCMBS is linearly related to its external concentration (10 microM-1 mM) and a small accumulation of pCMBS (apparent gradient about 3) in the cells occurs in 2 h. Cell 203Hg rapidly washes out of the cells (fast rate constant 0.153.min-1; slow rate constant 0.0067.min-1), and this efflux is accelerated by 1mM dithiothreitol. Thus, a major portion of pCMBS inter-acts rather loosely with cell components. (3) pCMBS and NEM share: (a) a negligible effect on the efflux of 86Rb+ and of [14C]urea; (b) a gradual inhibition of the cell Na+,K(+)-ATPase activity. (4) NEM as well as agents lowering cell glutathione accelerate and increase the pCMBS-induced cell swelling. Conditions inhibiting the Na+,K(+)-ATPase (ouabain, absence of Na+) have the same effect. (5) pCMBS, but not NEM produce a disappearance of the F-actin-phalloidin fluorescence independent of cell volume changes, particularly at the basolateral RGC membrane. (6) The data are consistent with the following set of events: (a) pCMBS (but not NEM) affects the cell membrane by increasing the efflux of the cell osmolyte taurine (Ziyadeh et al. (1988) Biochim. Biophys. Acta 943, 43-52 and unpublished data); (b) on entry into the cells, pCMBS and NEM interact with cell -SH, including those of the Na+,K(+)-ATPase; this action produces the observed changes in cell cations. Also, pCMBS, but not NEM, decrease F-actin at the membrane; (c) the inhibition of the Na+,K(+)-ATPase activity together with the decreased resistance of the cell membrane to stretch (absence of F-actin) produces the observed pCMBS-induced cell swelling by osmotic forces (intracellular non-diffusible anions).

K+-induced swelling of the dogfish shark (Squalus acanthias) rectal gland cells is associated with changes of the cytoskeleton

Dogfish shark (Squalus acanthias) rectal gland cells swell massively when incubated in elasmobranch media in which Na+ was equivalently replaced by K+; this swelling was abolished when the impermeant gluconate replaced Cl-, while the cell depolarization was comparable in both media. The K+-effect was associated with (a) an increase of the steady-state 42K (and 86Rb) efflux (particularly of the rate constant of the fast cellular efflux component) and a rearrangement of the respective cellular pools of K+; (b) an alteration of cell morphology and the pattern of the F-actin staining along the basolateral cell membrane as revealed with fluorescent analogs of phallacidin. These changes were independent of cell volume, being identical in KCl and K-gluconate media. The observations were specific for K+ (and Rb+): replacement of media Na+ by Li+ (which is not actively extruded by the cells), or the presence of ouabain, produced only minor swelling without affecting cell morphology and F-acting distribution. The results are consistent with the following view: as opposed to Na+ or Li+ media, the K+-induced changes of the cortical F-actin component of the cytoskeleton permit the observed massive cell swelling due to the osmotic contribution of intracellular impermeant anion(s).

Taurine and cell volume maintenance in the shark rectal gland: cellular fluxes and kinetics

Tissue slices of shark rectal gland are studied to examine the kinetics of the cellular fluxes of taurine, a major intracellular osmolyte in this organ. Maintenance of high steady-state cell taurine (50 mM) is achieved by a ouabain-sensitive active Na+-dependent uptake process and a relatively slow efflux. Uptake kinetics are described by two saturable taurine transport components (high-affinity, Km 60 microM; and low-affinity, Km 9 mM). [14C]Taurine uptake is enhanced by external Cl-, inhibited by beta-alanine and unaffected by inhibitors of the Na+/K+/2Cl- co-transport system. Two cellular efflux components of taurine are documented. Incubation of slices in p-chloromercuribenzene sulfonate (1 mM) reduces taurine uptake, increases efflux of taurine and induces cell swelling. Studies of efflux in isotonic media with various cation and anion substitutions demonstrate that high-K+ markedly enhances taurine efflux irrespective of cell volume changes (i.e. membrane stretching is not involved). Moreover, iso-osmotic cell swelling induced in media containing propionate is not associated with enhanced efflux of taurine from the cells. It is suggested that external K+ exerts a specific effect on the cytoplasmic membrane to increase its permeability to taurine.