A Ca(2+)- and pH-dependent K+ channel of rat C6 glioma cells and its possible role in acidosis-induced cell swelling

Pancreatic KCa3.1 channels in health and disease

Ion channels play an important role for regulation of the exocrine and the endocrine pancreas. This review focuses on the Ca²⁺-regulated K⁺ channel K_Ca3.1, encoded by the KCNN4 gene, which is present in both parts of the pancreas. In the islets of Langerhans, K_Ca3.1 channels are involved in the regulation of membrane potential oscillations characterizing nutrient-stimulated islet activity. Channel upregulation is induced by gluco- or lipotoxic conditions and might contribute to micro-inflammation and impaired insulin release in type 2 diabetes mellitus as well as to diabetes-associated renal and vascular complications. In the exocrine pancreas K_Ca3.1 channels are expressed in acinar and ductal cells. They are thought to play a role for anion secretion during digestion but their physiological role has not been fully elucidated yet. Pancreatic carcinoma, especially pancreatic ductal adenocarcinoma (PDAC), is associated with drastic overexpression of K_Ca3.1. For pharmacological targeting of K_Ca3.1 channels, we are discussing the possible benefits K_Ca3.1 channel inhibitors might provide in the context of diabetes mellitus and pancreatic cancer, respectively. We are also giving a perspective for the use of a fluorescently labeled derivative of the K_Ca3.1 blocker senicapoc as a tool to monitor channel distribution in pancreatic tissue. In summary, modulating K_Ca3.1 channel activity is a useful strategy for exo-and endocrine pancreatic disease but further studies are needed to evaluate its clinical suitability.

pH-Channeling in Cancer: How pH-Dependence of Cation Channels Shapes Cancer Pathophysiology

Tissue acidosis plays a pivotal role in tumor progression: in particular, interstitial acidosis promotes tumor cell invasion, and is a major contributor to the dysregulation of tumor immunity and tumor stromal cells. The cell membrane and integral membrane proteins commonly act as important sensors and transducers of altered pH. Cell adhesion molecules and cation channels are prominent membrane proteins, the majority of which is regulated by protons. The pathophysiological consequences of proton-sensitive ion channel function in cancer, however, are scarcely considered in the literature. Thus, the main focus of this review is to highlight possible events in tumor progression and tumor immunity where the pH sensitivity of cation channels could be of great importance.

Lactacidosis-induced glial cell swelling depends on extracellular Ca2+

Cerebral tissue acidosis following ischemia or traumatic brain injury contributes to cytotoxic brain edema formation. In vitro lactacidosis induces swelling of glial cells by intracellular Na+- and Cl--accumulation by the Na+/H+-antiporter, Cl-/HCO3--antiporters and the Na+-K+-2Cl--cotransport. The present study aimed to elucidate whether mechanisms of lactacidosis-induced glial swelling are dependent on intra- or extracellular Ca2+-ions. Therefore, C6 glioma cells were exposed to a lactacidosis of pH 6.2 in standard or calcium-free medium and following intracellular calcium chelation. Cell volume and intracellular pH were assessed by flow cytometry. Lactacidosis of pH 6.2 induced a prompt and sustained swelling of suspended C6 glioma cells reaching a maximum of 128% within 60 min. Omission of Ca2+ from the suspension medium strongly attenuated cell swelling while chelation of intracellular Ca2+ had no effects. Intracellular acidosis was not affected by either treatment. The present data show a strong dependency of lactacidosis-induced glial swelling upon extracellular Ca2+ while intracellular acidosis is not affected by omission of [Ca2+]e. Therefore, our data suggest that the Na+-K+-2Cl--cotransporter, the only so far known transporter involved in cell volume regulation but not in pHi regulation during lactacidosis, is activated in a [Ca2+]e-dependent manner.

Astrocytic swelling in cerebral ischemia as a possible cause of injury and target for therapy

In this viewpoint article, I summarize data showing that the astrocytic swelling that occurs early after the acute CNS pathologies ischemia and traumatic brain injury is damaging. We have proposed that one reason may be the release of excitatory amino acids (EAA) via volume-activated anion channels (VRACs) that are activated by such swelling. This release could be a target for therapy, which could involve blocking the astrocytic swelling or the release mechanisms. The transport mechanisms likely causing the early astrocytic swelling are therefore summarized. In terms of targeting the release mechanisms, we have found a potent inhibitor of VRACs, tamoxifen, to be strongly neuroprotective in focal ischemia with a therapeutic window of 3 h after initiation of the ischemia. The question, however, of whether neuroprotection by tamoxifen can be solely attributed to VRAC inhibition in astrocytes has yet to be resolved.

Proton-induced calcitonin gene-related peptide release from rat sciatic nerve axons, in vitro, involving TRPV1

We have shown previously that rat sciatic nerve axons in vitro express sensitivity to capsaicin and heat and responded to these stimuli with a Ca2+-dependent and graded immunoreactive calcitonin gene-related peptide release. Morphological evidence for stimulated vesicular exocytosis and for the vanilloid receptor TRPV1 in the axolemma of the unmyelinated nerve fibres has also been presented. Here we used solutions of low pH, high K+ or 47 degrees C to stimulate isolated desheathed sciatic nerves measuring immunoreactive calcitonin gene-related peptide release. pH 6.1 increased immunoreactive calcitonin gene-related peptide release by 31% over baseline and pH 5.2 and 4.3 caused a log-linear concentration-dependent increase of 137 and 265%, respectively. The effect of pH 3.4 was out of the linear range and not reversible. Stimulation in Ca2+-free solutions and under increased intracellular Ca2+ buffering capacity strongly reduced the proton responses. The TRPV1 antagonists capsazepine and ruthenium red substantially reduced the effects of pH 5.2 but not pH 6.1. Combining a stimulus of 60 mm K+ with the subliminal pH 6.3 reduced the axonal immunoreactive calcitonin gene-related peptide response by 88%. The noxious heat response at pH 6.3, however, was only reduced by 39%, suggesting a hidden sensitization to heat by low pH. This was supported by an effect of capsazepine to reduce the combined response to half, indicative of an involvement of TRPV1 in the sensitization but not in the axonal heat response itself that was found to be resistant to capsazepine. Axonal calcitonin gene-related peptide release is thought to play a physiological role in activity-dependent autoregulation of endoneurial blood flow. Axonal sensitivity to and sensitization by protons may be a pathophysiological mechanism involved in certain peripheral neuropathies.

The role of nutritional habits during gestation and child life in pediatric brain tumor etiology

Our aim was to evaluate the role of maternal nutritional habits during the period of gestation and of children subsequent diet in the etiology of pediatric brain tumors. All cases of incident nervous system tumors under age 18, diagnosed between 1984 and 1993 (n = 300) in Israel were identified. Two matched population controls per case were selected (n = 574). Personal interviews, using a semi-quantified three-step food frequency questionnaire, were performed. Univariate analysis showed that increased child consumption of vegetable fat [p trend 0.01; 95% confidence interval (CI) 1.1-3.2], carbohydrates (p trend 0.05; CI 1.0-5.9), and vitamin E (p trend 0.05; CI 1.0-3.3), were significantly associated with brain tumor risk. No associations were found with nitrate, nitrite or vitamin C. A significant positive association with potassium consumption (p trend 0.01; CI 1.1-3.7) was noted during gestation. Results of multivariate analysis showed that the only persisting associations were with vegetable fat (OR = 1.36; CI 1.06-1.73) in the child diet and potassium intake during gestation (OR = 1.44; CI 1.04-1.99). In conclusion, nutritional associations with pediatric brain tumor etiology, remain unsubstantiated.

Enzymatic modulation of cell volume in C6 glioma cells

We monitored the volume of C6 glioma cells in suspension using a Coulter Counter and exposed the cells to micromolar or nanomolar levels of collagenase or clostripain. In 13 experiments, type IV collagenase (310 units ml-1; approximately 3 microM L-1) decreased the volume by 8-12%, 8 min after addition. In 13 of 21 experiments, the volume decrease was followed by a volume regulatory increase (VRI) back to control levels in the continued presence of collagenase. The shrinkage evoked by type IV collagenase was eliminated by heat-inactivation of the enzyme preparation. A highly purified collagenase (type VII) at the same concentration evoked a relatively minor decrease in volume. A well-known contaminating protease present in type IV collagenase, clostripain, which specifically cleaves arginyl peptide bonds, evoked a 7 +/- 2% shrinkage (100 nM L-1, 7 experiments). Clostripain did not evoke a volume regulatory increase. The initial velocity of shrinkage evoked by clostripain (0.0012 pL min-1, 0.0034 pL min-1, 0.0132 pL min-1; 1 pL = 10(-12) liters) scaled with its concentration (1 nM L-1, 10 nM L-1, 100 nM L-1). The effect of clostripain was inhibited by heat-inactivation of the enzyme. Leupeptin, an inhibitor of clostripain, prevented the decrease in volume evoked by clostripain. The activity of stretch-activated ion channels was unaffected by type IV collagenase. Barium, cesium, amiloride, DIDS, or bumetanide failed to block the shrinkage evoked by type IV collagenase. These results demonstrate that clostripain, present in crude collagenase enzyme preparations, causes the shrinkage, and that C6 glioma cells can undergo a volume regulatory increase at virtually constant osmotic pressure. In addition, cleavage of a cell surface moiety, which contains arginine, and possibly proline, causes shrinkage. This moiety may be part of the extracellular or intracellular matrix providing mechanical support to the cells. VRI reflect actions of another substance in the type IV crude collagenase preparations, on a receptor independent of the arg-pro moiety. The enzymatic modulation of glioma cell volume by these two receptors may reflect a new mechanism by which such cells, and possibly other glia, regulate their contact area and interactions with other cells in the central nervous system.

A proton-translocating H+-ATPase is involved in C6 glial pH regulation

Glial cells extrude acid equivalents to maintain pHi. Although four mechanisms have been described so far, pHi-control under physiological conditions is still not sufficiently explained. We therefore investigated whether a H+-translocating ATPase is involved in glial pHi homeostasis using an established glial cell line (C6 glioma). In the absence of bicarbonate, the inhibition of H+-ATPases by NEM led to a pHi decrease. The application of a more specific inhibitor (NBD-Cl) showed that the H+-ATPase involved is of the vacuolar type. Inhibition went along with delayed cell swelling. Together with the fact that glial acidification was far more pronounced in Na+-free media, this may serve as evidence for a secondary activation of Na+/H+-exchange once an activation setpoint is reached, which in turn causes secondary swelling from Na+-uptake. Stimulation of Na+/H+-exchange by PMA can increase the setpoint. pHi-recovery after an acid load was blocked by the inhibition of v-type H+-ATPase, if pHi did not reach 6.6 during the acid load. The inhibition of Na+/H+-exchange by amiloride inhibited recovery only if acidification was below the threshold. Finally, in bicarbonate-free media a v-type H+-ATPase contributes to pH-regulation in glial cells, especially during pH-homeostasis at physiological conditions, while Na+/H+-exchange gains significance during severe acid loads.

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.

Mechanotransducing ion channels in C6 glioma cells

Mechanotransducing (MS) ion channels and images of the patch membrane were studied in cell-attached patches in C6 glioma cells. MS channel density was approximately 0.08 to 0.5 channels/microns2, channel conductance was approximately 40 pS (at -40 mV), and the reversal potential was +15 mV. Replacement of NaCl with KCl, CsCl, or Na gluconate in the pipette solution was without substantial effect on the current-voltage relationship. Replacement of NaCl with NMDG (N-Methyl-D-Glucamine) Cl or reducing NaCl decreased the amplitude of inward currents at negative membrane potentials and caused the reversal potential to shift in the negative direction. Rapid application of suction to the back of the pipette usually elicited a fast (< 0.1 s) appearance of channel activity. The peak (phasic) in channel activity was followed by a decrease to a constant (tonic) level of activity. The reduction in channel activity--called adaptation--was reduced at depolarizing membrane potentials and disappeared if too much pressure was applied. Positive pressure caused the patch membrane to curve toward the pipette tip, move in the direction of the tip, and evoke MS channel activity. Removal of the positive pressure caused the patch to move back to the original position. Conversely, negative pressure caused the patch membrane to curve away from the pipette tip, move away from the tip, and elicit MS channel activity. Gigohm seal resistances were always maintained during translational movement of the patch membrane. Tonic MS channel activity was not associated with translational movements of the patch membrane. Phasic and tonic channel activity were independent of the sign of curvature of the patch membrane. C6 glioma cells have rapidly adapting voltage-dependent MS ion channels, which are non-selective for monovalent cations, and belong to the stretch-activating class of mechanosensory ion channels. Adaptation in MS channels may allow the cell to limit the influx of cations in response to mechanical input. The selective loss of adaptation suggests that the MS channel's gate receives input from two sources. A minimal viscoelastic mechanical model of adaptation and two alternative models for translational movement of the patch are presented.

[Ca2+]i oscillations induced by bradykinin in rat glioma cells associated with Ca2+ store-dependent Ca2+ influx are controlled by cell volume and by membrane potential

Long-term superfusion with bradykinin causes oscillations of cytosolic Ca2+ activity ([Ca2+]i) in Fura-2 loaded rat glioma cells. The [Ca2+]i rise is associated with synchronous plasma membrane hyperpolarization oscillating with a frequency of 0.8-1.8 per min. The initial large transient [Ca2+]i rise, induced immediately with bradykinin admission results from InsP3-mediated Ca2+ release, whereas the subsequent oscillations depend mainly on Ca2+ influx, as demonstrated: (i) by blockade of [Ca2+]i oscillations by reduction of [Ca2+]ex' or addition of Ca(2+)-channel blockers; and (ii) evidence from Mn2+ quench experiments. Suppression of [Ca2+]i oscillations with high K+ depolarization and with block of Ca(2+)-dependent K+ channels proves that membrane hyperpolarization is required for Ca2+ influx during the oscillation. Ca2+ release from intracellular stores by inhibitors of endoplasmic reticulum Ca(2+)-ATPase attenuates or blocks the [Ca2+]i oscillations. This suggests that bradykinin-induced Ca2+ influx is controlled by the filling state of the stores. The [Ca2+]i oscillations are suppressed by hypertonic medium and enhanced by hypotonic medium. Cell swelling enhances Ca2+ influx. We propose the following model for generation of the oscillations in the glial cell line: InsP3-induced Ca2+ release from internal stores periodically evokes Ca2+ influx through Ca(2+)-permeable cation channels. Hyperpolarization of the plasma membrane due to the activation of Ca(2+)-dependent K+ channels enhances the Ca2+ influx. The concomitant K+ efflux could lead to cell shrinkage which suppresses Ca2+ influx. Cell volume and membrane potential probably serve as feedback regulators during the [Ca2+]i oscillations.

Physiology of transformed glial cells

Much of our present knowledge of glial cell function stems from studies of glioma cell lines, both rodent (C6, C6 polyploid, and TR33B) and human (1321N1, 138MG, D384, R-111, T67, Tp-276MG, Tp-301MG, Tp-483MG, Tp-387MG, U-118MG, U-251MG, U-373MG, U-787MG, U-1242MG, and UC-11MG). New methods such as patch clamp and Ca2+ imaging have lead to rapid progress the last few years in our knowledge about glial cells, where an unexpected presence and diversity of receptors and ion channels have emerged. Basic mechanisms related to membrane potential and K+ transport and the presence of voltage gated ion channels (Na+, inwardly rectifying K+, Ca(2+)-activated K+, Ca2+, and Cl- channels) have been identified. Receptor function and intracellular signaling for glutamate, acetylcholine, histamine, serotonin, cathecolamines, and a large number of neuropeptides (bradykinin, cholecystokinin, endothelin, opioids, and tachykinins) have been characterized. Such studies are facilitated in cell lines which offer a more homogenous material than primary cultures. Although the expression of ion channels and receptors vary considerably between different cell lines and comparative studies are rare, a few differences (compared to astrocytes in primary culture) have been identified which may turn out to be characteristic for glioma cells. Future identification of specific markers for receptors on glial and glioma cells related to cell type and growth properties may have great potential in clinical diagnosis and therapy.