Swelling-activated K-Cl cotransport: metabolic dependence and inhibition by vanadate and fluoride

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.

Calmodulin-dependent regulation of hypotonicity-induced translocation of ENaC in renal epithelial A6 cells

Hypotonicity stimulates translocation of epithelial Na(+) channel (ENaC) to the apical membrane from the intracellular store site of ENaC by activating protein tyrosine kinase (PTK) in renal epithelial A6 cells. Based upon the fact that calmodulin shows its action on other enzymes through PTK caused phosphorylation of tyrosine residues of calmodulin itself, we studied whether a calmodulin-dependent pathway is involved in the action of hypotonicity on ENaC. W7, an antagonist of calmodulin, diminished the stimulatory action of hypotonicity on ENaC, irrespective of W7 treatment before or after application of hypotonicity. Calmodulin is known to regulate three pathways: (1) protein phosphatase 2B (PP2B), (2) Ca(2+)/calmodulin-dependent protein kinase II (CaMK II), and (3) myosin light chain kinase (MLCK). Pretreatment with cyclosporin A, an inhibitor of PP2B, did not influence the hypotonicity action on ENaC. The hypotonicity action on ENaC was partially inhibited by pretreatment with KN93, an inhibitor of CaMK II, but not by addition of KN93 after hypotonic stimulation had been applied. ML-7, an inhibitor of MLCK, showed the action similar to KN93. These observations indicate that: (1) the hypotonicity-induced translocation of ENaC depends on CaMK II and MLCK and (2) ENaC translocated to the apical membrane by hypotonicity is maintained in its activity and/or stability at the apical membrane through a calmodulin-dependent pathway.

Molecular cloning and functional expression of the K-Cl cotransporter from rabbit, rat, and human. A new member of the cation-chloride cotransporter family

We report the cloning, sequence analysis, tissue distribution, and functional expression of the K-Cl cotransport protein, KCC1. KCC1 was identified by searching the human expressed sequence tag data base, based on the expectation that it would be distantly related to the Na-K-Cl cotransporter. Rabbit KCC1 (rbKCC1) and rat KCC1 (rtKCC1) were cloned by screening rabbit kidney and rat brain cDNA libraries using homologous cDNA probes. Human KCC1 (hKCC1) was obtained from I.M.A.G.E. clones and in part by reverse transcription-polymerase chain reaction; it exhibits 97% identity with rbKCC1. KCC1 encodes a 1085-residue polypeptide with substantial sequence homology (24-25% identity) to the bumetanide-sensitive Na-K-Cl cotransporter (NKCC or BSC) and the thiazide-sensitive Na-Cl cotransporter (NCC or TSC). Hydropathy analysis of KCC1 indicates structural homology to NKCC, including 12 transmembrane domains, a large extracellular loop with potential N-linked glycosylation sites, and cytoplasmic N- and C-terminal regions. Northern blot analysis revealed a ubiquitously expressed 3. 8-kilobase transcript. Much of the genomic sequence of hKCC1 is in the data base, and the gene has been previously localized to 16q22.1 (Larsen, F., Solhein, J., Kristensen, T., Kolsto, A. B., and Prydz, H.(1993) Hum. Mol. Genet. 2, 1589-1595). Epitope-tagged rbKCC1 was stably expressed in human embryonic kidney (HEK 293) cells, resulting in production of a approximately150-kDa glycoprotein. The initial rate of 86Rb efflux from cells expressing rbKCC1 was more than 7 times greater than efflux from control cells and was inhibited by 2 mM furosemide; 86Rb efflux was stimulated by cell swelling. Uptake of 86Rb into rbKCC1 cells after a 15-min pretreatment with 1 mM N-ethylmaleimide was dependent on external chloride but not on external sodium, and was inhibited by furosemide with a Ki of approximately 40 microM and by bumetanide with a Ki of approximately 60 microM. These data demonstrate that the KCC1 cDNAs encode a widely expressed K-Cl cotransporter with the characteristics of the K-Cl transporter that has been characterized in red cells.

KCl cotransport activation in human erythrocytes by high hydrostatic pressure

1. Pressure induced a 4- to 5-fold stimulation of the residual (i.e. oubain-bumetanide insensitive) 86Rb+ influx across the human red cell membrane. This enhancement showed a broad pHo dependence with a maximum stimulation around pHo 7. 2. At atmospheric pressure, the protein kinase inhibitors staurosporine and chelerythrine stimulated a normally silent component of 86Rb+ influx in a dose-dependent manner with a half-maximum stimulatory concentration at about 550 nM and 140 microM, respectively. The component stimulated by staurosporine was entirely Cl- dependent, but part of the chelerythrine effect was Cl- independent. 3. Staurosporine (3 microM), chelerythrine (200 microM) and N-ethylmaleimide (1 mM) stimulated further the increased residual 86Rb+ influx in cells at high pressure. 4. The serine/threonine protein phosphatase inhibitors okadaic acid, cantharidin and calyculin A inhibited the stimulatory pressure effect in a dose-dependent manner with half-maximum inhibitory concentrations of 70 nM, 2.5 microM and 3.3 nM, respectively. In contrast, deltamethrin, a specific protein phosphatase type 2B inhibitor, did not affect the stimulation by pressure, up to a concentration of 10 microM. 5. Decreasing the internal ionized magnesium concentration ([Mg2+]i) with A23187 and EDTA stimulated the increased residual 86Rb+ influx in cells at high pressure. On the other hand, increasing the [Mg2+]i nearly abolished the stimulatory pressure effect. 6. Decreasing the [Mg2+]i produced a marked change in the pHo dependence curve, with a linear increase of the 86Rb+ influx at higher pHo values. 7. We demonstrate that high pressure stimulates the normally silent component of 86Rb+ influx by modifying the phosphorylation/dephosphorylation ratio of the KCl cotransporter.

Chemical sensors and biosensors for fluoride: current status and developing technologies

Portable sensors for bioanalytes are becoming of increasing interest, because their development will permit analyses of biological samples at the point of sampling, i.e., at the time when an individual is being examined by a health professional. In this paper, the design requirements for a fluoride sensor are outlined. Currently, commercially available fluoride sensors, ion-selective electrodes, do provide the capability to measure fluoride content in samples directly. However, the sensitivity of these devices is lower than the normal content of fluoride in plasma, and thus, samples must be concentrated before measurement. Recent progress in improving the sensitivity of chemical sensors for fluoride shows sensitivities up to 0.1 mumol/L, which is within the range of interest. In addition, the use of biological systems for developing sensors (biosensors) is expanding rapidly, because of the potential of achieving high selectivity and sensitivity through the use of the amplification power of enzymes. However, relatively few enzyme systems have been discovered that are affected by fluoride--thus the need for expanded research to find appropriate enzymes for this purpose. The characteristics of some enzymes, such as adenylate cyclase, that could be used in the development of a fluoride biosensor are discussed.

Membrane transport of Na and K and cell dehydration in sickle erythrocytes

The cellular concentration of Hb S plays a central role in the kinetic of Hb S polymerization and cell sickling. Blood of patients with homozygous sickle cell (SS) anemia contains a variable fraction of cells which are markedly dehydrated and have increased Hb S concentration. Since a decrease in cellular Hb S concentration reduces Hb S polymerization and sickling, the study of the processes leading to sickle cell dehydration has important pathophysiological and therapeutic implications. Sickle cell dehydration is due to cellular loss of K and Cl. K loss in sickle cells can take place via either the Ca(2+)-activated K+ channel, or the K-Cl cotransport, or the combined effect of oxidative damage and deformation of the red cell membrane. Inhibitors of K transport through these pathways could be used to prevent dehydration of sickle cells in vivo, provided that they can be administered safely.