Actions of thiocyanate and N-phenylmaleimide on volume-responsive Na and K transport in dog red cells

Muscarinic receptor antagonists: effects on pulmonary function

In healthy lungs, muscarinic receptors control smooth muscle tone, mucus secretion, vasodilation, and inflammation. In chronic obstructive pulmonary disease (COPD) and asthma, cholinergic mechanisms contribute to increased bronchoconstriction and mucus secretion that limit airflow. This chapter reviews neuronal and nonneuronal sources of acetylcholine in the lung and the expression and role of M₁, M₂, and M₃ muscarinic receptor subtypes in lung physiology. It also discusses the evidence for and against the role of parasympathetic nerves in asthma, and the current use and therapeutic potential of muscarinic receptor antagonists in COPD and asthma.

Regulation of K-Cl cotransport: from function to genes

This review intends to summarize the vast literature on K-Cl cotransport (COT) regulation from a functional and genetic viewpoint. Special attention has been given to the signaling pathways involved in the transporter's regulation found in several tissues and cell types, and more specifically, in vascular smooth muscle cells (VSMCs). The number of publications on K-Cl COT has been steadily increasing since its discovery at the beginning of the 1980s, with red blood cells (RBCs) from different species (human, sheep, dog, rabbit, guinea pig, turkey, duck, frog, rat, mouse, fish, and lamprey) being the most studied model. Other tissues/cell types under study are brain, kidney, epithelia, muscle/smooth muscle, tumor cells, heart, liver, insect cells, endothelial cells, bone, platelets, thymocytes and Leishmania donovani. One of the salient properties of K-Cl-COT is its activation by cell swelling and its participation in the recovery of cell volume, a process known as regulatory volume decrease (RVD). Activation by thiol modification with N-ethylmaleimide (NEM) has spawned investigations on the redox dependence of K-Cl COT, and is used as a positive control for the operation of the system in many tissues and cells. The most accepted model of K-Cl COT regulation proposes protein kinases and phosphatases linked in a chain of phosphorylation/dephosphorylation events. More recent studies include regulatory pathways involving the phosphatidyl inositol/protein kinase C (PKC)-mediated pathway for regulation by lithium (Li) in low-K sheep red blood cells (LK SRBCs), and the nitric oxide (NO)/cGMP/protein kinase G (PKG) pathway as well as the platelet-derived growth factor (PDGF)-mediated mechanism in VSMCs. Studies on VSM transfected cells containing the PKG catalytic domain demonstrated the participation of this enzyme in K-Cl COT regulation. Commonly used vasodilators activate K-Cl COT in a dose-dependent manner through the NO/cGMP/PKG pathway. Interaction between the cotransporter and the cytoskeleton appears to depend on the cellular origin and experimental conditions. Pathophysiologically, K-Cl COT is altered in sickle cell anemia and neuropathies, and it has also been proposed to play a role in blood pressure control. Four closely related human genes code for KCCs (KCC1-4). Although considerable information is accumulating on tissue distribution, function and pathologies associated with the different isoforms, little is known about the genetic regulation of the KCC genes in terms of transcriptional and post-transcriptional regulation. A few reports indicate that the NO/cGMP/PKG signaling pathway regulates KCC1 and KCC3 mRNA expression in VSMCs at the post-transcriptional level. However, the detailed mechanisms of post-transcriptional regulation of KCC genes and of regulation of KCC2 and KCC4 mRNA expression are unknown. The K-Cl COT field is expected to expand further over the next decades, as new isoforms and/or regulatory pathways are discovered and its implication in health and disease is revealed.

Coordinate modulation of Na-K-2Cl cotransport and K-Cl cotransport by cell volume and chloride

Na-K-2Cl cotransporter (NKCC) and K-Cl cotransporter (KCC) play key roles in cell volume regulation and epithelial Cl(-) transport. Reductions in either cell volume or cytosolic Cl(-) concentration ([Cl(-)](i)) stimulate a corrective uptake of KCl and water via NKCC, whereas cell swelling triggers KCl loss via KCC. The dependence of these transporters on volume and [Cl(-)](i) was evaluated in model duck red blood cells. Replacement of [Cl(-)](i) with methanesulfonate elevated the volume set point at which NKCC activates and KCC inactivates. The set point was insensitive to cytosolic ionic strength. Reducing [Cl(-)](i) at a constant driving force for inward NKCC and outward KCC caused the cells to adopt the new set point volume. Phosphopeptide maps of NKCC indicated that activation by cell shrinkage or low [Cl(-)](i) is associated with phosphorylation of a similar constellation of Ser/Thr sites. Like shrinkage, reduction of [Cl(-)](i) accelerated NKCC phosphorylation after abrupt inhibition of the deactivating phosphatase with calyculin A in vivo, whereas [Cl(-)] had no specific effect on dephosphorylation in vitro. Our results indicate that NKCC and KCC are reciprocally regulated by a negative feedback system dually modulated by cell volume and [Cl(-)]. The major effect of Cl(-) on NKCC is exerted through the volume-sensitive kinase that phosphorylates the transport protein.

Macromolecular crowding and its role as intracellular signalling of cell volume regulation

Macromolecular crowding has been proposed as a mechanism by means of which a cell can sense relatively small changes in volume or, more accurately, the concentration of intracellular solutes. According to the macromolecular theory, the kinetics and equilibria of enzymes can be greatly influenced by small changes in the concentration of ambient, inert macromolecules. A 10% change in the concentration of intracellular proteins can lead to changes of up to a factor of ten in the thermodynamic activity of putative molecular regulatory species, and consequently, the extent to which such regulator(s) may bind to and activate membrane-associated ion transporters. The aim of this review is to examine the concept of macromolecular crowding and how it profoundly affects macromolecular association in an intact cell with particular emphasis on its implication as a sensor and a mechanism through which cell volume is regulated.

Copper effects on ion transport across lamprey erythrocyte membrane: Cl(-)/OH(-) exchange induced by cuprous ions

We studied the effects of prelytic copper concentrations on cell volume, intracellular pH, and ion transport in lamprey erythrocytes. Ion fluxes and pH were measured by radioactive tracer technique, patch clamp, and flame photometry. Prelytic CuSO(4) concentration of 100 microM caused anion-dependent intracellular acidification and increase in Cl(-) influx after 2 min lag-phase. In the presence of ascorbate copper effect was amplified and lag-phase was skipped. Pretreatment of the cells with N-phenyl maleimide abolished copper-induced changes completely. Copper treatment caused an increase in Na(+) fluxes in both directions and a net Na(+) uptake. Copper-induced Na(+) transport was partially amiloride(MIA)-sensitive representing Na(+)/H(+) exchange. The nature of the amiloride-insensitive fraction of copper-activated Na(+) influx remains unknown. Cell swelling after 15 min of copper exposure induced regulatory volume decrease response involving KCl extrusion via K(+) and Cl(-) volume-sensitive channels. We suggest that the effects of copper on ion transport fit the following sequence of events: (i) cupric ions are reduced to cuprous state on the membrane surface, (ii) electroneutral pairs CuCl and CuOH mediate chloride/hydroxyl exchange, as shown before for trialkyltin, dissipating transmembrane pH gradient, and (iii) changes in intracellular pH result in the activation of the Na(+)/H(+) exchange and consecutive volume changes cause the RVD response.

Regulation of cation transport pathways and glycolytic enzyme activity by alterations in red cell volume

In the presence of NH4Cl and hypotonic solutions, Rana balcanica red cells respond by increasing their volume. The stimulation of cellular volume by hypotonicity is more rapid than that of NH4Cl, while the maximum value is less than that observed in the presence of NH4Cl. Depending on the cause of swelling, (nct uptake of NH4Cl or decrease in external osmolality) cells show specific responses. The NH4Cl treatment causes a significant increase in intracellular Na+, from 5.14 +/- 0.78 to 29.84 +/- 0.47 mmoles l-1 cell, while hypotonicity leads to a significant decrease of this cation, to 3.85 +/- 0.25 mmoles l-1 cell in relation to the control, after 30 min of incubation of Rana balcanica erythrocytes. In addition, amiloiride significantly reverses the NH4Cl effect with respect to intracellular Na+. Both treatments cause a significant K+ loss in comparison with controls. Two glycolytic enzymes glyceraldehyde phosphate dehydrogenase (GAPDH) and pyruvate kinase (PK) of Rana balcanica haemolysate were found to respond to the NH4Cl effect by significantly decreasing their activity.

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.

Effects of ionic strength on the regulation of Na/H exchange and K-Cl cotransport in dog red blood cells

Dog red cell membranes contain two distinct volume-sensitive transporters: swelling-activated K-Cl cotransport and shrinkage-activated Na/H exchange. Cells were prepared with intracellular salt concentration and weight percentage of cell water (%cw) varied independently by transient permeabilization of the cell membrane to cations. The dependence of transporter-mediated Na and K influxes upon %cw and upon extracellular salt concentration (c(ext)) was measured in cells so prepared. It was found that the critical value of %cw at which transporters are activated, called the set point, is similar for the two transporters, and that the set points for the two transporters decrease similarly with increasing extracellular salt concentration. These findings suggest a common mechanism of regulation of these two transporters. Cellular Na, K, and Cl concentrations were measured as functions of %cw and c(ext). Using these data together with data from the literature for other solute concentrations, empirical expressions were developed to describe the dependence of the intracellular concentrations of all significant small molecule electrolytes, and therefore the intracellular ionic strength, upon %cw and c(ext). A mechanistic model for the dependence of the set point of an individual transporter upon intracellular ionic strength is proposed. According to this model, the set point represents a critical extent of association between the transporter and a postulated soluble regulatory protein, called regulator. Model functions are presented for the calculation of the thermodynamic activity of regulator, and hence extent of regulator-transporter association, as a function of total intracellular protein concentration (or %cw) and ionic strength. The experimentally observed dependence of set point %cw on c(ext) are simulated using these functions and the empirical expressions described above, together with reasonable but not uniquely determined values of model parameters.

Inhibition by thiocyanate of muscarinic-induced cytosolic acidification and Ca2+ entry in rat sublingual acini

Thiocyanate (SCN-) plays a critical part in an oral antimicrobial system by acting as a substrate for peroxidases. Salivary glands concentrate SCN- from blood up to 5 mM in saliva; however, the influence of SCN- on salivary acinar-cell function is unknown. The present study examined the effects of SCN- on the regulation of cytosolic pH (pHi) and free Ca2+ concentration ([Ca2+]i) in rat sublingual mucous acini using the pH- and Ca(2+)-sensitive fluorescent indicators, 2',7'-bis-(2-carboxyethyl)-5,6-carboxyfluorescein and fura-2, respectively. SCN- induced a concentration-dependent inhibition of the carbachol-stimulated cytosolic acidification (K1/2, approx. 1.4 mM SCN-). Cytosolic pH recovery from an acid load was not changed by substitution of Cl- by SCN-, suggesting that Na+/H+ exchange activity was not affected by SCN-. SCN- did not alter the initial carbachol-stimulated increase in [Ca2+]i; however, the sustained [Ca2+]i increase was inhibited by > 65% (K1/2, approx. 1.0 mM SCN-). Furthermore, SCN- prevented the carbachol-stimulated Mn2+ influx, indicating that it inhibits the divalent-cation entry pathway. Consistent with decreased Ca2+ mobilization being involved in the blockade of the agonist-induced acidification by SCN-, only total replacement of Cl- with SCN- significantly inhibited the acidification induced by the Ca2+ ionophore ionomycin. The permeability to SCN- through the Ca(2+)-dependent Cl- channels was 5.2-fold higher than the permeability to Cl-. These results suggest that inhibition of the agonist-induced cytosolic acidification by high-concentration SCN- may be mediated by both competitive inhibition of HCO3- efflux and by blockade of Ca2+ influx.

Volume-activated cation transport in dog red cells: detection and transduction of the volume stimulus

1. Carnivore red cells lose their Na/K pump capability as they develop from erythroblasts to reticulocytes to mature cells. They defend their fluid volume by utilizing the Ca/Na exchanger as a Na extrusion pump, the energy for which is ultimately derived from active Ca transport. 2. Swelling-induced [K-Cl] cotransport and shrinkage-induced Na/H exchange are regulated in a coordinated fashion in dog red cells. Circumstantial evidence points to a regulatory protein kinase-phosphatase system. 3. Dog red cells detect changes in their fluid volume, not by virtue of membrane distortion, but by alterations in the concentrations of cytoplasmic macromolecules induced by swelling or shrinkage.