Volume-regulatory responses of Amphiuma red cells in anisotonic media. The effect of amiloride

Water Homeostasis and Cell Volume Maintenance and Regulation

From early unicellular organisms that formed in salty water environments to complex organisms that live on land away from water, cells have had to protect a homeostatic internal environment favorable to the biochemical reactions necessary for life. In this chapter, we will outline what steps were necessary to conserve the water within our cells and how mechanisms have evolved to maintain and regulate our cellular and organismal volume. We will first examine whole body water homeostasis and the relationship between kidney function, regulation of blood pressure, and blood filtration in the process of producing urine. We will then discuss how the composition of the lipid-rich bilayer affects its permeability to water and salts, and how the cell uses this differential to drive physiological and biochemical cellular functions. The capacity to maintain cell volume is vital to epithelial transport, neurotransmission, cell cycle, apoptosis, and cell migration. Finally, we will wrap up the chapter by discussing in some detail specific channels, cotransporters, and exchangers that have evolved to facilitate the movement of cations and anions otherwise unable to cross the lipid-rich bilayer and that are involved in maintaining or regulating cell volume.

The permeability of red blood cells to chloride, urea and water

This study extends permeability (P) data on chloride, urea and water in red blood cells (RBC), and concludes that the urea transporter (UT-B) does not transport water. P of chick, duck, Amphiuma means, dog and human RBC to (36)Cl(-), (14)C-urea and (3)H2O was determined under self-exchange conditions. At 25°C and pH 7.2-7.5, PCl is 0.94 × 10(-4)-2.15 × 10(-4) cm s(-1) for all RBC species at [Cl]=127-150 mmol l(-1). In chick and duck RBC, P(urea) is 0.84 × 10(-6) and 1.65 × 10(-6) cm s(-1), respectively, at [urea]=1-500 mmol l(-1). In Amphiuma, dog and human RBC, P(urea) is concentration dependent (1-1000 mmol l(-1), Michaelis-Menten-like kinetics; K1/2;=127, 173 and 345 mmol l(-1)), and values at [urea]=1 mmol l(-1) are 29.5 × 10(-6), 467 × 10(-6) and 260 × 10(-6) cm s(-1), respectively. Diffusional water permeability, Pd, was 0.84 × 10(-3) (chick), 5.95 × 10(-3) (duck), 0.39 × 10(-3) (Amphiuma), 3.13 × 10(-3) (dog) and 2.35 × 10(-3) cm s(-1) (human). DIDS, DNDS and phloretin inhibit PCl by >99% in all RBC species. PCMBS, PCMB and phloretin inhibit P(urea) by >99% in Amphiuma, dog and human RBC, but not in chick and duck RBC. PCMBS and PCMB inhibit Pd in duck, dog and human RBC, but not in chick and Amphiuma RBC. Temperature dependence, as measured by apparent activation energy, EA, of PCl is 117.8 (duck), 74.9 (Amphiuma) and 89.6 kJ mol(-1) (dog). The EA of P(urea) is 69.6 (duck) and 53.3 kJ mol(-1) (Amphiuma), and that of Pd is 34.9 (duck) and 32.1 kJ mol(-1) (Amphiuma). The present and previous RBC studies indicate that anion (AE1), urea (UT-B) and water (AQP1) transporters only transport chloride (all species), water (duck, dog, human) and urea (Amphiuma, dog, human), respectively. Water does not share UT-B with urea, and the solute transport is not coupled under physiological conditions.

Coordinated control of volume regulatory Na+/H+ and K+/H+ exchange pathways in Amphiuma red blood cells

The Na(+)/H(+) and K(+)/H(+) exchange pathways of Amphiuma tridactylum red blood cells (RBCs) are quiescent at normal resting cell volume yet are selectively activated in response to cell shrinkage and swelling, respectively. These alkali metal/H(+) exchangers are activated by net kinase activity and deactivated by net phosphatase activity. We employed relaxation kinetic analyses to gain insight into the basis for coordinated control of these volume regulatory ion flux pathways. This approach enabled us to develop a model explaining how phosphorylation/dephosphorylation-dependent events control and coordinate the activity of the Na(+)/H(+) and K(+)/H(+) exchangers around the cell volume set point. We found that the transition between initial and final steady state for both activation and deactivation of the volume-induced Na(+)/H(+) and K(+)/H(+) exchange pathways in Amphiuma RBCs proceed as a single exponential function of time. The rate of Na(+)/H(+) exchange activation increases with cell shrinkage, whereas the rate of Na(+)/H(+) exchange deactivation increases as preshrunken cells are progressively swollen. Similarly, the rate of K(+)/H(+) exchange activation increases with cell swelling, whereas the rate of K(+)/H(+) exchange deactivation increases as preswollen cells are progressively shrunken. We propose a model in which the activities of the controlling kinases and phosphatases are volume sensitive and reciprocally regulated. Briefly, the activity of each kinase-phosphatase pair is reciprocally related, as a function of volume, and the volume sensitivities of kinases and phosphatases controlling K(+)/H(+) exchange are reciprocally related to those controlling Na(+)/H(+) exchange.

Oxygen-sensitive regulatory volume increase and Na transport in red blood cells from the cane toad,Bufo marinus

The red blood cells (RBCs) of cane toad, Bufo marinus, are only partially saturated with oxygen in most of the circulation due to cardiac shunts that cause desaturation of arterial blood. The present study examines the oxygen dependency of RBC ouabain-insensitive unidirectional Na transport,using 22Na, in control cells and in cells exposed to hyperosmotic shrinkage or the β-adrenergic agonist isoproterenol. Deoxygenation per se induced a slow, but significant Na influx, which was paralleled by a slow increase in RBC volume. Hyperosmotic shrinkage by a calculated 25% activated a robust Na influx that in the first 30 min had a strong PO2 dependency with maximal activation at low PO2 values and a P50of ∼5.5 kPa. This activation was completely abolished by the Na/H exchanger (NHE) inhibitor EIPA (10–4 mol l-1). Hyperosmotic shrinkage is particularly interesting in B. marinus as it withstands considerable elevation in extracellular osmolarity following dehydration. Parallel studies showed that deoxygenated B. marinusRBCs had a much faster regulatory volume increase (RVI) response than air-equilibrated RBCs, reflecting the difference in magnitude of Na influxes at the two PO2 values. The extent of RVI(∼60%) after 90 min, however, was similar under the two conditions,reflecting a more prolonged elevation of the shrinkage-induced Na influx in air-equilibrated RBCs. There were no significant differences in the ability to perform RVI between whole blood cells at a PCO2of 1 and 3 kPa or washed RBCs, and 10–4 mol l-1amiloride reduced the RVI under all conditions, whereas 10–5mol l-1 bumetanide had no effect. Isoproterenol(10–5 mol l-1) induced a significant and prolonged increase in an EIPA-sensitive and bumetanide-insensitive Na influx at low PO2 under iso-osmotic conditions, whilst there was no stimulation by isoproterenol for up to 45 min in air-equilibrated RBCs. The prolonged β-adrenergic activation of the Na influx at low PO2 is distinctly different from the rapid and transient stimulation in teleost RBCs, suggesting significant differences in the signal transduction pathways leading to transporter activation between vertebrate groups.

Potassium transport in red blood cells of frog Rana temporaria: demonstration of a K-Cl cotransport

Pathways of K+ movement across the erythrocyte membrane of frog Rana temporaria were studied using 86Rb as a tracer. The K+ influx was significantly blocked by 0.1 mmol.l-1 ouabain (by 30%) and 1 mmol.l-1 furosemide (by 56%) in the red cells incubated in saline at physiological K+ concentration (2.7 mmol.l-1). Ouabain and furosemide had an additive effect on K+ transport in frog red cells. The ouabain-sensitive and furosemide-sensitive components of K+ influx saturated as f(K+)e with apparent Km values for external Ke+ concentration of 0.96 +/- 0.11 and 4.6 +/- 0.5 mmol.l-1 and Vmax of 0.89 +/- 0.04 and 2.8 +/- 0.4 mmol.l cells-1.h-1, respectively. The residual ouabain-furosemide-resistant component was also a saturable function of Ke+ medium concentration. Total K+ influx was significantly reduced when frog erythrocytes were incubated in NO3- medium. Furosemide did not affect K+ transport in frog red cells in NO3- media. At the same Ke+ concentration the ouabain-furosemide-insensitive K+ influx in Cl- medium was significantly greater than that in NO3- medium. We found no inhibitory effect of 1 mmol.l-1 furosemide on Na+ influx in frog red cells in Cl- medium. K+ loss from the frog erythrocytes in a K(+)-free medium was significantly reduced (mean 58%) after replacement of Cl- with NO3-. Furosemide (0.5 mmol.l-1) did not produce any significant reduction in the K+ loss in both media. The Cl(-)-dependent component of K+ loss from frog red cells was 5.7 +/- 1.2 mmol.l-1.h-1.(ABSTRACT TRUNCATED AT 250 WORDS)

Labeling of the Amphiuma erythrocyte K+/H+ exchanger with H2DIDS

Subsequent to swelling, the Amphiuma red blood cells lose K+, Cl-, and water until normal cell volume is restored. Net solute loss is the result of K+/H+ and Cl-/HCO3- exchangers functionally coupled through changes in pH and therefore HCO3-. Whereas the Cl-/HCO3- exchanger is constitutively active, K+/H+ actively is induced by cell swelling. The constitutive Cl-/HCO3- exchanger is inhibited by low concentrations (< 1 microM) of 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) or H2DIDS, yet the concentration of H2DIDS > 25 microM irreversibly modifies the K+/H+ exchanger in swollen cells. We exploited the volume-dependent irreversible low-affinity reaction between H2DIDS and the K+/H+ to identify the protein(s) associated with K+/H+ exchange activity. Labeling of the membrane proteins of intact cells with 3H2DIDS results in high-affinity labeling of a broad 100-kDa band, thought to be the anion exchanger. Additional swelling-dependent low-affinity labeling at 110 kDa suggests the possibility of a volume-induced population of anion exchangers. Finally, the correlation between volume-sensitive K+/H+ modification and low-affinity labeling suggests that transport activity is associated with a protein of approximately 85 kDa. Although a 55-kDa protein is also labeled, it is a less likely candidate, since label incorporation and transport modification are less well correlated than that of the 85- and 110-kDa proteins.

pH regulatory Na/H exchange by Amphiuma red blood cells

In Amphiuma red blood cells, the Na/H exchanger has been shown to play a central role in the regulation of cell volume following cell shrinkage (Cala, P. M. 1980. Journal of General Physiology. 76:683-708.) The present study was designed to evaluate the existence of pH regulatory Na/H exchange in the Amphiuma red blood cell. The data illustrate that when the intracellular pHi was decreased below the normal value of 7.00, Na/H exchange was activated in proportion to the degree of acidification. Once activated, net Na/H exchange flux persisted until normal intracellular pH (6.9-7.0) was restored, with a half time of approximately 5 min. These observations established a pHi set point of 7.00 for the pH-activated Na/H exchange of Amphiuma red blood cell. This is in contrast to the behavior of osmotically shrunken Amphiuma red blood cells in which no pHi set point could be demonstrated. That is, when activated by cell shrinkage the Na/H exchange mediated net Na flux persisted until normal volume was restored regardless of pHi. In contrast, when activated by cell acidification, the Na/H exchanger functioned until pHi was restored to normal and cell volume appeared to have no effect on pH-activated Na/H exchange. Studies evaluating the kinetic and inferentially, the molecular equivalence of the volume and pHi-induced Amphiuma erythrocyte Na/H exchanger(s), indicated that the apparent Na affinity of the pH activated cells is four times greater than that of shrunken cells. The apparent Vmax is also higher (two times) in the pH activated cells, suggesting the involvement of two distinct populations of the transporter in pH and volume regulation. However, when analyzed in terms of a bisubstrate model, the same data are consistent with the conclusion that both pH and volume regulatory functions are mediated by the same transport protein. Taken together, these data support the conclusion that volume and pH are regulated by the same effector (Na/H exchanger) under the control of as yet unidentified, distinct and cross inhibitory volume and pH sensing mechanisms.

Swelling activation of K-Cl cotransport in LK sheep erythrocytes: a three-state process

K-Cl cotransport in LK sheep erythrocytes is activated by osmotic swelling and inhibited by shrinkage. The mechanism by which changes in cell volume are transduced into changes in transport was investigated by measuring time courses of changes in transport after osmotic challenges in cells with normal and reduced Mg concentrations. When cells of normal volume and normal Mg are swollen, there is a delay of 10 min or more before the final steady-state flux is achieved, as there is for swelling activation of K-Cl cotransport in erythrocytes of other species. The delay was shown to be independent of the extent of swelling. There was also a delay after shrinkage inactivation of cotransport. Reducing cellular Mg concentration activates cotransport. Swelling of low-Mg cells activates cotransport further, but with no measurable delay. In contrast, there is a delay in shrinkage inactivation of cotransport in low-Mg cells. The results are interpreted in terms of a three-state model: [formula see text] in which A state, B state, and C state transporters have relatively slow, intermediate, and fast transport rates, respectively. Most transporters in shrunken cells with normal Mg are in the A state. Swelling converts transporters to the B state in the rate-limiting process, followed by rapid conversion to the C state. Reducing cell Mg also promotes the A-->B conversion. Swelling of low-Mg cells activates transport rapidly because of the initial predominance of B state transporters. The results support the following conclusions about the rate constants of the three-state model: k21 is the rate constant for a Mg-promoted process that is inhibited by swelling; k12 is not volume sensitive. Both k23 and k32 are increased by swelling and reduced by shrinkage; they are rate constants for a single process, whereas k12 and k21 are rate constants for separate processes. Finally, the A-->B conversion entails an increase in Jmax of the transporters, and the B-->C conversion entails an increase in the affinity of the transporters for K.

Cell volume and ph regulation by the Amphiuma red blood cell: A model for hypoxia-induced cell injury

The Amphiuma red blood cell is one of the model systems employed early in the study of vertebrate cell volume regulation. Following both cell swelling and shrinkage the Amphiuma red blood cell demonstrates volume regulation to virtual completion in 90-120 min. When swollen the Amphiuma red blood cell loses K, Cl and osmotically obliged water, while following shrinkage volume regulation is the result of Na, Cl and therefore water uptake. The main contribution of the Amphiuma red cell as a model is that it was the first cell in which volume regulation was demonstrated to be electroneutral and more specifically that K/H and Na/H exchangers were responsible for regulation following cell swelling and shrinkage, respectively. Additionally, the Amphiuma red blood cell K/H and Na/H exchangers have been demonstrated to function in a pH regulatory capacity. The latter observation in turn led to the demonstration of the mutually exclusive and contradictory nature of volume and pH regulation predicted upon Na/H exchanger activity. These observations prompted our recent investigations of the Na/H exchanger as a contributor to hypoxia-induced cell damage, using the rabbit heart as a model. These studies illustrated that Na, and Ca imbalances characteristic of hypoxia-induced cell damage are ultimately referable to the Na/H exchanger's function in a pH regulatory capacity, which contributes fundamentally to cell volume and Ca derangement and ultimately cell injury.

Activation of ion transport pathways by changes in cell volume

Swelling-activated K+ and Cl- channels, which mediate RVD, are found in most cell types. Prominent exceptions to this rule include red cells, which together with some types of epithelia, utilize electroneutral [K(+)-Cl-] cotransport for down-regulation of volume. Shrinkage-activated Na+/H+ exchange and [Na(+)-K(+)-2 Cl-] cotransport mediate RVI in many cell types, although the activation of these systems may require special conditions, such as previous RVD. Swelling-activated K+/H+ exchange and Ca2+/Na+ exchange seem to be restricted to certain species of red cells. Swelling-activated calcium channels, although not carrying sufficient ion flux to contribute to volume changes may play an important role in the activation of transport pathways. In this review of volume-activated ion transport pathways we have concentrated on regulatory phenomena. We have listed known secondary messenger pathways that modulate volume-activated transporters, although the evidence that volume signals are transduced via these systems is preliminary. We have focused on several mechanisms that might function as volume sensors. In our view, the most important candidates for this role are the structures which detect deformation or stretching of the membrane and the skeletal filaments attached to it, and the extraordinary effects that small changes in concentration of cytoplasmic macromolecules may exert on the activities of cytoplasmic and membrane enzymes (macromolecular crowding). It is noteworthy that volume-activated ion transporters are intercalated into the cellular signaling network as receptors, messengers and effectors. Stretch-activated ion channels may serve as receptors for cell volume itself. Cell swelling or shrinkage may serve a messenger function in the communication between opposing surfaces of epithelia, or in the regulation of metabolic pathways in the liver. Finally, these transporters may act as effector systems when they perform regulatory volume increase or decrease. This review discusses several examples in which relatively simple methods of examining volume regulation led to the discovery of transporters ultimately found to play key roles in the transmission of information within the cell. So, why volume? Because it's functionally important, it's relatively cheap (if you happened to have everything else, you only need some distilled water or concentrated salt solution), and since it involves many disciplines of experimental biology, it's fun to do.

Regulatory changes in the K+, Cl- and water contents of HeLa cells incubated in an isosmotic high K(+)-medium

HeLa cells had their normal medium replaced by an isosmotic medium containing 80 mM K+, 70 mM Na+ and 100 microM ouabain. The cellular contents of K+ first increased and then decreased to the original values, that is, the cells showed a regulatory decrease (RVD) in size. The initial increase was not inhibited by various agents except by substitution of medium Cl- with gluconate. In contrast, the regulatory decrease was inhibited strongly by addition of either 1 mM quinine, 10 microM BAPTA-AM without medium Ca2+, or 0.5 mM DIDS, and partly by either 1 mM EGTA without medium Ca2+, 10 microM trifluoperazine, or substitution of medium Cl- with NO3-. Addition of DIDS to the NO3(-)-substituted medium further suppressed the K+ loss but the effect was incomplete. Intracellular Ca2+ showed a transient increase after the medium replacement. These results suggest that the initial increase in cell K+ is a phenomenon related to osmotic water movement toward Donnan equilibrium, whereas the regulatory K+ decrease is caused by K+ efflux through Ca(2+)-dependent K+ channels. The K+ decrease induced a decrease in cellular water, i.e., RVD. The K+ efflux may be more selectively associated with Cl- efflux through DIDS-sensitive channels than the efflux of other anions.

Volume regulation in rat pheochromocytoma cultured cells submitted to hypoosmotic conditions

The mechanisms at work in cell volume regulation have been studied in PC12 cultured cells. Results show, for the first time to our knowledge, that the volume readjustment process occurring after application of a hypoosmotic saline is sensitive to amiloride, IBMX and forskoline. The process is also inhibited by quinine hydrochloride and trifluoperazine. Volume readjustment is concomtant with a decrease in K+ and Cl- intracellular levels. The decrease in K+ level can be related to an assymetrical change in the fluxes in and out of the ion as shown by flux kinetics studies using Rb86. These results are interpreted considering that the control of the activity of the ion channel pathways associated with volume readjustment in PC12 cells may implicate the Ca(2+)-calmodulin - cAMP system.

Cell volume regulation by trout erythrocytes: characteristics of the transport systems activated by hypotonic swelling

1. An osmolality reduction of the suspending medium leads to osmotic swelling of trout erythrocytes, which is followed by a volume readjustment towards the original level. The regulatory volume decrease (RVD) was not complete after 1 h. 2. During RVD the cells lost K+ and Cl- but gained Na+. This entry of Na+, which is about half the K+ loss, explains the incomplete volume recovery (it was complete when Na+ was replaced by impermeant N-methyl-D-glucamine). The cells also lose large quantities of taurine, which accounts for about 53% of the volume recovery. In addition RVD is accompanied by the activation of a pathway allowing some large organic cations which are normally impermeant, such as choline or tetramethyl-ammonium, to rapidly penetrate the cells. 3. The swelling-activated K+ loss is not significantly affected by replacement of Cl- by NO3-, indicating that K+ moves through a Cl(-)-independent K+ pathway. Furosemide, DIDS (4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid) and niflumic acid inhibit the K+ loss. From experiments performed in high-K(+)-containing media, it appears that these compounds block the K+ flux, not by inhibiting Cl- movements but by interfering with the K+ pathway. 4. All the volume-activated pathways (K+, Na+, taurine, choline) are fully inhibited by furosemide and by inhibitors of the anion exchanger such as DIDS and niflumic acid. The concentration required for 50% inhibition (IC50) of both inorganic cations and taurine appears to be similar. It is proposed that DIDS interacts with a unique target which controls all the volume-sensitive transport systems.

Effects of hyperthermia on the membrane potential and Na+ transport of V79 fibroblasts

The effects of hyperthermia (41-43 degrees C) on the membrane potential (calculated from the transmembrane distribution of [3H]tetraphenylphosphonium) and Na+ transport of Chinese hamster V79 fibroblasts were studied. At 41 degrees C, hyperthermia induced a membrane hyperpolarization of log phase cells (5 to 26 mV) that was reversible upon returning to 37 degrees C. The hyperpolarization was inhibited 50% by 1 mM ouabain or 0.25 mM amiloride, an inhibitor of Na+:H+ exchange. Shifting temperature to 41 degrees C increased ouabain-sensitive Rb+ uptake indicating activation of the electrogenic Na+ pump. At 43 degrees C for 60 min, the membrane potential of log phase cells depolarized (20-35 mV). Parallel studies demonstrated enhanced Na+ uptake at 41 degrees C only in the presence of ouabain. At 43 degrees C, Na+ uptake was increased relative to controls with or without ouabain present. At both 41 and 43 degrees C, 0.25 mM amiloride inhibited heat-stimulated Na+ uptake. Na+ efflux was enhanced at 41 degrees C in a process inhibited by ouabain. Thus, one consequence of heat treatment at 41 degrees C is activation of Na+:H+ exchange with the resultant increase in cytosolic [Na+] activating the electrogenic Na+ pump. At temperatures greater than or equal to 43 degrees C, the Na+ pump is inhibited.

Effect of potassium on cell volume regulation in renal straight proximal tubules

The present study was designed to assess for the influence of extracellular potassium and of inhibitors of potassium transport on cell volume regulatory decrease in isolated perfused straight proximal tubules of the mouse kidney. Volume regulatory decrease is virtually unaffected when bath potassium concentration is elevated from 5 to 20 mmol/liter, and still persists, albeit significantly retarded, in the presence of the potassium channel blocker barium on both sides of the epithelium and during virtually complete dissipation of the transmembrane potassium gradient by increasing extracellular potassium concentration to 40 mmol/liter. As evident from electrophysiologic observations, barium blocks the potassium conductance of the basolateral cell membrane. Reduction of bicarbonate concentration and increase of H+ concentration in the bath solution cannot compensate for enhanced potassium concentration and cell volume regulatory decrease is not affected in the presence of the K/H exchange inhibitor omeprazole. Similarly cell volume regulatory decrease is not affected by ouabain. In conclusion, potassium movements through potassium channels in the basolateral cell membrane are important determinants of cell volume and may participate in cell volume regulatory decrease. However, a powerful component of cell volume regulatory decrease in straight proximal tubules of the mouse kidney is apparently independent of potassium conductive pathways, K/H exchange and Na+/K(+)-ATPase.

Volume regulation in liver: further characterization by inhibitors and ionic substitutions

The present study has been performed to elucidate the mechanisms of volume regulation in isolated perfused liver. Reduction of extracellular osmolarity by 80 mOsm/L leads to a release of potassium and a sustained alkalinization of effluent. Reexposure to isotonic perfusate leads to reuptake of potassium by the liver and acidification of effluent. Part of the alkalinization could be due to release of bicarbonate parallel to potassium release. Carboanhydrase inhibition and replacement of bicarbonate/CO2 by HEPES buffer, however, do not significantly modify volume regulatory potassium release or reuptake. Reduction of perfusate chloride to 37 mmol/L by replacement of NaCl with raffinose leads to a decrease of liver weight indicative of shrinkage of liver cells. Subsequent omission of 180 mmol/L raffinose leads to potassium and chloride release and to alkalinization of effluent. Volume regulatory release of potassium is impaired in 1 mmol/L quinidine, 1 mmol/L SITS and 5 mmol/L barium. Volume regulatory reuptake of potassium is impaired by 1 mmol/L amiloride. Volume regulatory release of potassium is not appreciably affected by either; 1 mmol/L furosemide, 1 mumol/L verapamil, 1 mmol/L amiloride or 1 mmol/L barium and volume regulatory potassium reuptake proved insensitive to 1 mmol/L furosemide or 1 mmol/L barium. The data suggest that the cells release potassium and chloride during regulatory volume decrease by quinidine, SITS and weakly barium-sensitive transport systems and that regulatory volume increase is accomplished by activation of Na/H exchange.

Na+/H+ exchange in glial cells of Necturus optic nerve

Single and double-barreled pH-sensitive electrodes were used to study intracellular pH (pHi) regulation in glial cells of Necturus optic nerve in the nominal absence of HCO3-/CO2. After the cells were acidified by the addition and withdrawal of NH4+, the pHi recovered toward the original steady-state pHi. The recovery from acidification was Na+-dependent and inhibited by 1 mM amiloride. These results suggest the existence in intact vertebrate glial cells of a Na+/H+ exchanger which functions in acid extrusion.

Characteristics of the volume- and chloride-dependent K transport in human erythrocytes homozygous for hemoglobin C

In human red cells homozygous for hemoglobin C (CC), cell swelling and acid pH increase K efflux and net K loss in the presence of ouabain (0.1 mM) and bumetanide. We report herein, that K influx is also dependent on cell volume in CC cells: cell swelling induces a marked increase in the maximal rate (from 6 to 18 mmol/liter cell X hr) and in the affinity for external K (from 77 +/- 16 mM to 28 +/- 3 mM) of K influx. When the external K concentration is varied from 0 to 140 mM. K efflux from CC and normal control cells is unaffected. Thus, K/K exchange is not a major component of this K movement. K transport through the pathway of CC cells is dependent on the presence of chloride or bromide; substitution with nitrate, acetate or thiocyanate inhibits the volume- and pH-dependent K efflux. When CC cells are separated according to density, a sizable volume-dependent component of K efflux can be identified in all the fractions and is the most active in the least dense fraction. N-ethylmaleimide (NEM) markedly stimulates K efflux from CC cells in chloride but not in nitrate media, and this effect is present in all the fractions of CC cells separated according to density. The persistence of this transport system in denser CC cells suggests that not only cell age, but also the presence of the positively charged C hemoglobin is an important determinant of the activity of this system. These data also indicate that the K transport pathway of CC cells is not an electrodiffusional process and is coupled to chloride.

The effect of hypoosmolarity on the electrical properties of Madin Darby canine kidney cells

The present study has been performed to test for the effect of hypotonic extracellular fluid on the electrical properties of Madin Darby canine kidney (MDCK)-cells. The volume of suspended MDCK-cells is 1,892 +/- 16 fl (n = 8) in isotonic (298.7 mosmol/l) extracellular fluid. Exposure of the cells to hypotonic (230.7 mosmol/l) extracellular fluid is followed by cellular swelling to 2,269 +/- 18 fl (n = 4) and subsequent volume regulatory decrease to 2,052 +/- 22 fl (n = 4) within 512 s. Volume regulatory decrease is abolished by quinidine (1 mmol/l) and by lipoxygenase inhibitor nordihydroguaiaretic acid (50 mumol/l). The potential difference across the cell membrane averages -53.6 +/- 0.9 mV (n = 49) in isotonic extracellular perfusates. Reduction of extracellular osmolarity depolarizes the cell membrane by +25.7 +/- 0.8 mV (n = 67), reduces the apparent potassium selectivity of the cell membrane, from 0.55 +/- 0.07 (n = 9) to 0.09 +/- 0.01 (n = 26), and increases the apparent chloride selectivity from close to zero to 0.34 +/- 0.02 (n = 21). Potassium channel blocker barium (1 mmol/l) depolarizes the cell membrane by +15.2 +/- 1.1 mV (n = 13). In the presence of barium, reduction of extracellular osmolarity leads to a further depolarization by +14.0 +/- 1.4 mV (n = 12). Addition of chloride channel blocker anthracene-9-COOH (1 mmol/l) leads to a hyperpolarization of the cell membrane by -6.7 +/- 2.2 mV (n = 11). In the presence of anthracene-9-COOH, reduction of the extracellular osmolarity leads to a depolarization by +22.4 +/- 1.7 mV (n = 11).(ABSTRACT TRUNCATED AT 250 WORDS)

Water, K+, H+, lactate and glucose fluxes during cell volume regulation in perfused rat liver

The present study has been performed to test for ion release from isolated perfused rat liver exposed to hypotonic perfusates. Replacement of 40 mmol/l NaCl in perfusate by 80 mmol/l raffinose leads to slight alkalinization and slight decrease of liver weight. Subsequent decrease of perfusate osmolarity by omission of raffinose results in an increase of liver weight and a parallel increase of effluent sodium, chloride and potassium activity pointing to net uptake of solute free water. While effluent chloride and sodium activities approach perfusate activities within less than 2 min, a second, 6 min lasting increase of effluent potassium activity is observed, pointing to potassium release by the liver. This transient increase of effluent potassium activity is paralleled by a decrease of liver weight. Throughout exposure to hypotonic perfusates, lactate, pyruvate and glucose release by the liver is significantly decreased and effluent pH is rendered alkaline. Readdition of 80 mmol/l raffinose leads to rapid decrease of liver weight and a parallel decrease of effluent sodium, chloride and potassium activities followed by a 10-20 min lasting decrease of effluent potassium activity, pointing to net uptake of potassium, which almost matches the net release observed before. The transient decrease of potassium activity is paralleled by an increase of liver weight, an increase of effluent glucose, lactate and pyruvate concentration and an acidification of the effluent. Similar decrease of effluent potassium activity, acidification of effluent and increase of effluent glucose, lactate and pyruvate concentration are observed, if perfusates are made hypertonic by addition of raffinose.(ABSTRACT TRUNCATED AT 250 WORDS)

Separate control of regulatory volume increase and Na+-H+ exchange by cultured renal cells

Suspensions of OK cells (a continuous epithelioid cell line from opossum kidney) are examined by electronic cell sizing, measurements of intracellular pH, and measurements of cellular Na+ and K+. The response of the cells to hypertonic solutions is evaluated in most detail. When shrunken by exposure to hyperosmotic medium (430 mosmol/kg), the cells do not demonstrate a regulatory volume increase (RVI) independent of the solute that is used to increase osmolality [NaCl, N-methyl-D-glucamine-HCl (NMGCl), or sucrose]. In contrast, when cells are preexposed to 190 mosmol/kg medium and then shrunken by exposure to 310 mosmol/kg medium, a volume increase is observed after the addition of 120 mosmol/kg NaCl or NMGCl, but not sucrose. This RVI is sensitive to 1 mM furosemide and removal of Na+ or K+ from the medium, but it is not inhibited by 1 mM amiloride. In the presence of a propionate-induced cellular acidification, a Na+-H+ exchanger in the cells is shown to have a large capacity for net solute uptake and to be inhibited by 1 mM amiloride. Net solute uptake by the Na+-H+ exchanger is sensitive to addition of parathyroid hormone or 8-bromoadenosine 3',5'-cyclic monophosphate but is not stimulated in response to cell shrinkage.

Ionic requirement for regulatory cell volume decrease in renal straight proximal tubules

The present study has been performed to test for the ionic requirement of regulatory cell volume decrease in isolated perfused straight proximal tubules of the mouse kidney. Reduction of peritubular osmolarity from 308 mosmol/l to 228 mosmol/l leads within 0.5 min to cell swelling by 16 +/- 1% (n = 26) of original cell volume (Vo). Within 2 min cell volume (V2) approaches 105 +/- 1% of Vo (n = 26) despite continued exposure to hypotonic bath perfusate. Reexposure of the tubules to isotonic bath perfusate shrinks the cells to 94 +/- 1% of Vo (n = 25). Within 2 min from omission of extracellular bicarbonate and CO2 regulatory cell volume decrease is impaired (V2 = 114 +/- 1% of Vo, n = 14). Similarly, regulatory volume decrease is blunted upon prior removal of extracellular sodium (V2 = 115 +/- 2% of Vo, n = 12). In contrast, regulatory volume decrease is not affected by prior removal of extracellular chloride (V2 = 104 +/- 2% of Vo, n = 9). Regulatory volume decrease is impaired in the presence of 1 mmol/l potassium channel blocker barium (V2 = 120 +/- 4% of Vo, n = 7) and of 1 mmol/l carbonic anhydrase inhibitor acetazolamide (V2 = 111 +/- 2% of Vo, n = 16) but is preserved in the presence of 1 mumol/l chloride channel blocker NPPB (V2 = 105 +/- 2% of Vo, n = 11). In conclusion, regulatory cell volume decrease apparently depends on potassium and bicarbonate, but does not depend on chloride.

Cytomegalovirus: sodium entry and development of cytomegaly in human fibroblasts

A possible relationship between net Na+ entry and the development of CMV-induced cytomegaly (cell enlargement) was investigated in human fibroblasts derived from skin-muscle and thyroid tissue. We found that inhibiting cellular Na+ uptake, either by pharmacological means (amiloride, an inhibitor of Na+/H+ exchange) or by replacement of extracellular Na+ (by N-methyl-D-glucamine or choline), inhibited the development of cytomegaly. Furthermore, we noted a temporal parallelism between the development of cytomegaly and enhancement of ouabain-sensitive (O-S) 86Rb+ uptake. O-S 86Rb+ uptake is a monitor for the activity of the sodium pump resident in the plasmalemma of the fibroblasts. The enhanced O-S 86Rb+ uptake reflects either an increased intracellular Na+ concentration or an increased number of sodium pump complexes per fibroblast. Amiloride inhibited the enhancement of O-S 86Rb+ uptake, as well as cytomegaly development. Addition of amiloride at selected times after infection suggested that the same phase of virus replication was sensitive to the inhibitory effect of this drug on the enhancement of O-S 86Rb+ uptake and on the development of cytomegaly. There was also a similar pattern of inhibition of O-S 86Rb+ uptake and cytomegaly with increasing concentrations of amiloride. Thus, there may be a relationship between CMV-induced Na+ entry through activation of the Na+/H+ exchanger and development of cytomegaly.

Electrophysiology of cell volume regulation in proximal tubules of the mouse kidney

The present study has been designed to test for the influence of cell swelling on the potential difference and conductive properties of the basolateral cell membrane in isolated perfused proximal tubules. During control conditions the potential difference across the basolateral cell membrane (PDbl) is -65 +/- 1 mV (n = 74). Decrease of peritubular osmolarity by 80 mosmol/l depolarizes the basolateral cell membrane by +7.8 +/- 0.5 mV (n = 42). An increase of bath potassium concentration from 5 to 20 mmol/l depolarizes the basolateral cell membrane by +25 +/- 1 mV (n = 11), an increase of bath bicarbonate concentration from 20 to 60 mmol/l hyperpolarizes the basolateral cell membrane by -3.2 +/- 0.5 mV (n = 13). A decrease of bath chloride concentration from 79.6 to 27 mmol/l hyperpolarizes the basolateral cell membrane by -1.8 +/- 0.7 mV (n = 6). During reduced bath osmolarity, the influence of altered bath potassium concentration on PDbl is decreased (delta PDbl = +16 +/- 2 mV, n = 11), the influence of altered bicarbonate concentration on PDbl is increased (delta PDbl = -6.0 +/- 0.8 mV, n = 13), and the influence of altered bath chloride concentration on PDbl is unaffected (delta PDbl = -1.8 +/- 0.6 mV, n = 6). Barium depolarizes the basolateral cell membrane to -28 +/- 2 mV (n = 16). In the presence of 1 mmol/l barium, decrease of peritubular osmolarity by 80 mosmol/l leads to a transient hyperpolarization of the basolateral cell membrane by -5.9 +/- 0.5 mV (n = 16).(ABSTRACT TRUNCATED AT 250 WORDS)

Volume-sensitive Cl-dependent K transport in human erythrocytes

Passive K fluxes, measured with 86Rb, were investigated in osmotically swollen human erythrocytes. K influx and efflux increased progressively with increased hypotonicity up to 167 mosmol/kg. No increase in K flux was seen when NO3 or methylSO4 were substituted for Cl. Substitution of choline or N-methylglucamine for external Na reduced the K flux in swollen cells by only 22%, compared with a 60% reduction in euvolumic cells. However, the magnitude of this Na-dependent component was slightly, but significantly, higher in swollen cells. The presence of Na-dependent K influx in swollen cells was confirmed by measurements of Na influx demonstrating a K-dependent Na influx of similar magnitude in isovolumic and swollen cells. The volume-sensitive K flux was inhibited by bumetanide, but significantly less so than was Cl-dependent flux in isovolumic cells (half-maximal inhibition at 1.0 X 10(-4) vs. 5.8 X 10(-7) M). Kinetic analysis revealed that Cl-dependent K influx had a lower affinity for external K in swollen cells than in euvolumic cells (Km was 29.8 vs. 6.1 mM). The increased K flux in swollen cells was found to be transient, decreasing substantially and reverting back to a predominantly Na-dependent and more bumetanide-sensitive form after 2 h. The results indicate that swelling of human erythrocytes activates a transient Cl-dependent K flux that differs significantly from that in isovolumic cells in that it is less Na dependent, less sensitive to bumetanide, and has a lower affinity for K. Na-K cotransport is either unaffected or slightly increased in swollen cells. The altered flux in swollen cells would thermodynamically favor a volume-regulatory KCl efflux.

Interactions of sodium-proton exchange mechanism in dog red blood cells with N-phenylmaleimide

Dog red blood cells (RBC) have a Na-H exchanger that is reversibly activated by cell shrinkage. The Na-H exchanger can be fixed in the on or off mode by treating the cells with N-phenylmaleimide. This action depends on the volume of the cells at the time of exposure to N-phenylmaleimide and also on the concentration of the reagent per number of cells. If the cells are swollen in hypotonic media during N-phenylmaleimide exposure, the Na-H exchanger becomes irreversibly inactivated, so that on subsequent shrinkage of the cells, no amiloride-sensitive Na flux is seen. This effect is maximal at N-phenylmaleimide concentrations of greater than 20 mumol/g hemoglobin. If the cells are shrunken in hypertonic media during N-phenylmaleimide exposure, the response of the Na-H exchanger depends critically on the concentration of the reagent. At N-phenylmaleimide concentrations of less than 20 mumol/g hemoglobin, the Na-H exchanger is fixed in the activated state, so that even when the volume stimulus is removed by subsequent cell swelling, an amiloride-sensitive flux is seen. Higher concentrations of N-phenylmaleimide applied to shrunken cells inhibit the Na-H exchanger. The results are accounted for in a model that envisions a volume-responsive switching mechanism for Na-H exchange that has two functional groups capable of reacting with N-phenylmaleimide. The accessibility of these groups is determined by cell volume.

Volume regulation in the early proximal tubule of the Necturus kidney

The ability of early proximal tubule cells of the Necturus kidney to regulate volume was evaluated using light microscopy, video analysis and conventional microelectrodes. Necturus proximal tubule cells regulate volume in both hyper- and hyposmotic solutions. Volume regulation in hyperosmotic fluids is HCO3- dependent and is associated with a decrease in the relative K+ conductance of the basolateral cell membrane and a decrease in the resistance ratio, Ra/Rbl. Volume regulation in hyposmotic solutions is also dependent upon the presence of HCO3- but is also inhibited by 2 mM Ba2+ in the basolateral solution. Hyposmotic regulation is accompanied by an increase in the relative K+ conductance of the basolateral cell membrane and an increase in Ra/Rbl. Neither hypo- nor hyposmotic regulation have any affect on the depolarization of the basolateral cell membrane potential induced by HCO3- removal. We conclude that volume regulation in the early proximal tubule of the kidney involves both HCO3(-)-dependent transport systems and the baso-lateral K+ conductance.

Control of cell volume and ion transport by beta-adrenergic catecholamines in erythrocytes of rainbow trout, Salmo gairdneri

1. Trout red cells suspended in an isotonic medium containing beta-adrenergic catecholamines or adenosine 3',5'-phosphate (cyclic AMP) enlarge rapidly to reach a new steady-state volume which is maintained as long as hormone is present. The volume response is not changed by inhibition of the Na+-K+ pump with ouabain. The new steady-state volume was shown to result from a dynamic equilibrium involving the simultaneous functioning of two regulatory processes induced by hormone: a volume increase response that causes cells to enlarge by gaining Na+ and a volume decrease response that causes cells to shrink by losing K+. 2. As previously described, the volume increase response due to NaCl entry, is mediated by the activation by cyclic AMP of a Na+-H+ antiport operating in parallel to Cl(-)-OH- exchanges. In addition, it is shown in this paper that the Na+ uptake is a discontinuous, oscillatory process and that NaCl entry continues for several hours, i.e. as long as hormone is present. 3. The volume decrease response involves a passive, Cl(-)-dependent K+ loss. Na+ cannot use this pathway. The response is blocked by replacement of Cl- by NO3-, by loop diuretics (furosemide, bumetanide) but also by inhibitors of the anion exchanger (4,4'-diisothiocyanostilbene-2,2'-disulphonic acid (DIDS), niflumic acid). The activation of this ouabain-insensitive, Cl(-)-dependent K+ transport system is not directly triggered by cyclic AMP. It involves an all-or-none type of switching phenomenon which occurs when the cells swell to a certain volume. Thus it is a regulatory response to the increase in cell volume induced by stimulation of the Na+-H+ exchange by cyclic AMP. Inactivation is also volume dependent: when the cell size approaches the initial size the pathway shuts off. Thus the controlling mechanism of the K+ pathway acts like a reversible on-off switch that operates around a given volume. Ca2+ was not found to be involved in this control. Cyclic AMP is not necessary to keep the activated K+ pathway open but it could be one of the factors involved in the activating process. 4. There are several lines of evidence indicating that in trout red cells the volume decrease and the volume increase responses may not be brought about by the same transport mechanism operating in different modes. The movements of Na+, K+ and Cl- account for the water movements during volume increase and decrease. Thus movements of other solutes such as amino acids need not be considered.

Isovolumetric regulation of isolated S2 proximal tubules in anisotonic media

Sudden alteration in medium osmolality causes an osmometric change in proximal tubule cell size followed by restoration of cell volume toward normal in hypotonic but not in hypertonic medium. We determined the capability of isolated nonperfused proximal tubules to prevent a change in cell volume in anisotonic media. The external osmolality was gradually changed over a range from 110 to 480 mosM. At 1.5 mosM/min, cell volume remained constant between 167 +/- 9 and 361 +/- 7 mosM, a phenomenon termed isovolumetric regulation (IVR). Cells lost intracellular solutes in hypotonic and gained intracellular solutes in hypertonic media. Raffinose or choline chloride substitution showed that osmolality, rather than NaCl, signalled cell volume maintenance in hyperosmotic media. Cooling (7-10 degrees C) blocked IVR. IVR was maintained when osmolality was lowered at a rate of 27, but not at 42 mosM/min. IVR was not observed when the rate of osmolality increase exceeded 3 mosM/min. We conclude that proximal tubule cells sensitively regulate intracellular volume in an osmolality range of pathophysiologic interest by mechanisms dependent on the rate of net water movement across basolateral membranes and the absolute intracellular content of critical solutes.

Anion transport systems in the plasma membrane of vertebrate cells

In the case of the red blood cell, anion transport is a highly specific one-for-one exchange catalyzed by a major membrane protein known as band 3 or as capnophorin. This red cell anion-exchange system mediates the Cl-(-)HCO3- exchange responsible for most of the bicarbonate transport capacity of the blood. The rapidly expanding knowledge of the molecular biology and the transport kinetics of this specialized transport system is very briefly reviewed in Section III. Exchange diffusion mechanisms for anions are found in many cells other than erythrocytes. The exchange diffusion system in Ehrlich cells has several similarities to that in red cells. In several cell types (subsection IV-B), there is evidence that intracellular pH regulation depends on Cl-(-)HCO3- exchange processes. Anion exchange in other single cells is described in Section IV, and its role in pH regulation is described in Section VII. Anion exchange mechanism operating in parallel with, and only functionally linked to Na+-H+ or K+-H+ exchange mechanisms can also play a role in cell volume regulation as described in Section VII. In the Ehrlich ascites cell and other vertebrate cells, electroneutral anion transfer has been found to occur also by a cotransport system for cations and chloride operating in parallel with the exchange diffusion system. The cotransport system is capable of mediating secondary active chloride influx. In avian red cells, the cotransport system has been shown to be activated by adrenergic agonists and by cyclic AMP, suggesting that the cotransport is involved in regulatory processes (see subsection V-A.). In several cell types, cotransport systems are activated and play a role during volume regulation, as described in Section V and in Section VII. It is also likely that this secondary active cotransport of chloride plays a significant role for the apparently active extrusion of acid equivalents from certain cells. If a continuous influx of chloride against an electrochemical gradient is maintained by a cotransport system, the chloride disequilibrium can drive an influx of bicarbonate through the anion exchange mechanism, as described in Section VII. Finally, even the electrodiffusion of anions is shown to be regulated, and in Ehrlich cells and human lymphocytes an activation of the anion diffusion pathway plays a major role in cell volume regulation as described in Section VI and subsection VII-B.(ABSTRACT TRUNCATED AT 250 WORDS)