The plasma membrane potential of human neutrophils. Role of ion channels and the sodium/potassium pump

ATP-Sensitive Potassium Channel Blockage Attenuates Cisplatin-Induced Renal Damage

BACKGROUND

Cisplatin-induced renal damage was associated with an inflammatory process. ATP-sensitive potassium channels can be involved in neutrophil migration. This study evaluated the effects of glibenclamide, an ATP-sensitive potassium channel blocker, on cisplatin-induced renal damage.

METHODS

A total of 48 Wistar rats received glibenclamide (20 mg/kg/day, s.c.) and 24 h later, these animals, and an additional group of 45 rats, were injected with cisplatin (5 mg/kg, i.p.). In addition, 38 control rats were injected with saline, i.p. Twenty-four hours and 5 days after saline or cisplatin injections blood and urine samples were collected to evaluate renal function and the kidneys were removed for analysis of neutrophil accumulation, tumor necrosis factor-alpha and interleukin-1beta and histological and immunohistochemical studies.

RESULTS

Cisplatin injection induced neutrophil recruitment and increased tumor necrosis factor-alpha and interleukin-1beta contents in renal cortices and outer medullae tissues. Cisplatin-treated rats also presented reduction in the glomerular filtration rate, as well as greater immunostaining for ED1 (macrophages/monocytes) and acute tubular necrosis. All of these alterations were reduced by treatment with glibenclamide. These effects seem to be related, at least in part, to the restriction of neutrophil recruitment and inflammatory process observed in the kidneys from glibenclamide+cisplatin-treated rats.

Ion channel clustering enhances weak electric field detection by neutrophils: apparent roles of SKF96365-sensitive cation channels and myeloperoxidase trafficking in cellular responses

We have tested Galvanovskis and Sandblom's prediction that ion channel clustering enhances weak electric field detection by cells as well as how the elicited signals couple to metabolic alterations. Electric field application was timed to coincide with certain known intracellular chemical oscillators (phase-matched conditions). Polarized, but not spherical, neutrophils labeled with anti-K(v)1.3, FL-DHP, and anti-TRP1, but not anti-T-type Ca(2+) channels, displayed clusters at the lamellipodium. Resonance energy transfer experiments showed that these channel pairs were in close proximity. Dose-field sensitivity studies of channel blockers suggested that K(+) and Ca(2+) channels participate in field detection, as judged by enhanced oscillatory NAD(P)H amplitudes. Further studies suggested that K(+) channel blockers act by reducing the neutrophil's membrane potential. Mibefradil and SKF93635, which block T-type Ca(2+) channels and SOCs, respectively, affected field detection at appropriate doses. Microfluorometry and high-speed imaging of indo-1-labeled neutrophils was used to examine Ca(2+) signaling. Electric fields enhanced Ca(2+) spike amplitude and triggered formation of a second traveling Ca(2+) wave. Mibefradil blocked Ca(2+) spikes and waves. Although 10 microM SKF96365 mimicked mibefradil, 7 microM SKF96365 specifically inhibited electric field-induced Ca(2+) signals, suggesting that one SKF96365-senstive site is influenced by electric fields. Although cells remained morphologically polarized, ion channel clusters at the lamellipodium and electric field sensitivity were inhibited by methyl-beta-cyclodextrin. As a result of phase-matched electric field application in the presence of ion channel clusters, myeloperoxidase (MPO) was found to traffic to the cell surface. As MPO participates in high amplitude metabolic oscillations, this suggests a link between the signaling apparatus and metabolic changes. Furthermore, electric field effects could be blocked by MPO inhibition or removal while certain electric field effects were mimicked by the addition of MPO to untreated cells. Therefore, channel clustering plays an important role in electric field detection and downstream responses of morphologically polarized neutrophils. In addition to providing new mechanistic insights concerning electric field interactions with cells, our work suggests novel methods to remotely manipulate physiological pathways.

The NADPH oxidase of professional phagocytes--prototype of the NOX electron transport chain systems

The NADPH oxidase is an electron transport chain in "professional" phagocytic cells that transfers electrons from NADPH in the cytoplasm, across the wall of the phagocytic vacuole, to form superoxide. The electron transporting flavocytochrome b is activated by the integrated function of four cytoplasmic proteins. The antimicrobial function of this system involves pumping K+ into the vacuole through BKCa channels, the effect of which is to elevate the vacuolar pH and activate neutral proteases. A number of homologous systems have been discovered in plants and lower animals as well as in man. Their function remains to be established.

Voltage-gated proton channels and other proton transfer pathways

Proton channels exist in a wide variety of membrane proteins where they transport protons rapidly and efficiently. Usually the proton pathway is formed mainly by water molecules present in the protein, but its function is regulated by titratable groups on critical amino acid residues in the pathway. All proton channels conduct protons by a hydrogen-bonded chain mechanism in which the proton hops from one water or titratable group to the next. Voltage-gated proton channels represent a specific subset of proton channels that have voltage- and time-dependent gating like other ion channels. However, they differ from most ion channels in their extraordinarily high selectivity, tiny conductance, strong temperature and deuterium isotope effects on conductance and gating kinetics, and insensitivity to block by steric occlusion. Gating of H(+) channels is regulated tightly by pH and voltage, ensuring that they open only when the electrochemical gradient is outward. Thus they function to extrude acid from cells. H(+) channels are expressed in many cells. During the respiratory burst in phagocytes, H(+) current compensates for electron extrusion by NADPH oxidase. Most evidence indicates that the H(+) channel is not part of the NADPH oxidase complex, but rather is a distinct and as yet unidentified molecule.

The role of ATP-sensitive potassium channels in neutrophil migration and plasma exudation

Neutrophil activation and migration during an inflammatory response is preceded or accompanied by plasma membrane electrical changes. Besides changes in calcium currents, neutrophils have a high permeability to potassium, mainly through potassium channels. However, the significance of potassium channels in neutrophil physiology is still unclear. Here, we show that the treatment of rats with the ATP-sensitive potassium channel blocker glibenclamide (4, 20, or 40 micromol/kg) dose dependently decreased carrageenan-, N-formyl-methionyl-leucyl-phenylalanine (fMLP)-, and lipopolysaccharide-induced neutrophil influx and fluid leakage into the interpleural space. On the other hand, minoxidil (an ATP-sensitive potassium channel opener; 25, 50, and 100 micromol/kg) increased both neutrophil influx and fluid leakage induced by a submaximal dose of carrageenan. In addition, in vitro human neutrophil chemotaxis induced by leukotriene B4 or fMLP (both 1 microM) was fully blocked by glibenclamide (10, 30, and 100 microM) or tetraethylammonium (a nonselective potassium channel blocker; 1, 3, and 10 mM). Thus, our results disclose the possibility that ATP-sensitive potassium channels may have a role in neutrophil migration and chemotaxis and plasma exudation in the inflammatory response.

The interaction of a new anti-tumour drug, KAR-2 with calmodulin

1. KAR-2 (3"-(beta-chloroethyl)-2",4"-dioxo-3,5" -spiro-oxazolidino-4-deacetoxy-vinblastine) is a semisynthetic bis-indol derivative, with high anti-microtubular and anti-tumour activities but with low toxicity. KAR-2, in contrast to other biologically active bis-indols (e.g. vinblastine) did not show anti-calmodulin activity in vitro (enzyme kinetic, fluorescence anisotropy and immunological tests). 2. Direct binding studies (fluorescence resonance energy transfer, circular dichroism) provided evidence for the binding of KAR-2 to calmodulin. The binding affinity of KAR-2 to calmodulin (dissociation constant was about 5 microM) in the presence of Ca2+ was comparable to that of vinblastine. 3. KAR-2 was able to interact with apo-calmodulin as well; in the absence of Ca2+ the binding was of cooperative nature. 4. The effect of drugs on Ca2+ homeostasis in human neutrophil cells was investigated by means of a specific fluorescent probe. Trifluoperazine extensively inhibited the elevation of intracellular Ca2+ level, vinblastine did not appreciably affect it, KAR-2 stimulated the Ca2+ influx and after a transient enhancement the Ca2+ concentration reached a new steady-state level. 5. Comparison of the data obtained with KAR-2 and bis-indols used in chemotherapy suggests that the lack of anti-calmodulin potency resides on the spiro-oxazolidino portion of KAR-2. This character of KAR-2 manifested itself in various systems and might result in its low in vivo toxicity, established in an anti-tumour test.

Depletion of calcium stores by thapsigargin induces membrane depolarization by cation entry in human neutrophils

The ability of various cations to change the electrical potential of the plasma membrane was examined in human neutrophils by the use of the fluorescent cationic dye 3,3'-dipropylthiadicarbocyanine. When the cells were suspended in 140 mM KCl, the fluorescence was high, indicating depolarized neutrophils. Suspension in 145 mM N-methyl-D-glucamine chloride (NMG), replacing sodium and potassium chloride, resulted in hyperpolarized neutrophils. After depletion of the intracellular calcium stores of the NMG-suspended cells with thapsigargin and EDTA or EGTA, the addition of cations depolarized the neutrophils, suggesting the existence of pathways for cation entry. Besides Na+ and K+, several divalent cations were effective in the sequence: Ca2+ > Mn2+ > Ba2+ > Cd2+ > Mg2+ > Co2+ > Zn2+ > Ni2+. Pretreatment of the neutrophils with 0.5 or 1 mM CaCl2, resulting in loading of calcium stores, reduced the ability of some of the cations to depolarize the NMG-suspended cells. From the depolarizing effects of the cations it is concluded that the entries of Ca2+, Mg2+, Mn2+, Ba2+, probably Co2+, to some extent Na+ and K+, but hardly Cd2+, Zn2+, or Ni2+, are regulated by the filling state of the intracellular calcium stores in human neutrophils. The store-regulated entry pathway may contribute to the control of the membrane potential and become active when the neutrophils are stimulated.

Phorbol 12-myristate 13-acetate activates an electrogenic H(+)-conducting pathway in the membrane of neutrophils

The mode of activation of an H(+)-conducting pathway present in the membrane of neutrophils was investigated. (1) Resting neutrophils released protons through an electrogenic Cd(2+)-inhibitable (K0.5 approximately 20 microM) route when a pH gradient and appropriate charge compensation was provided. (2) The rate of H+ efflux was stimulated over 2.5-fold by 4 beta-phorbol 12-myristate 13-acetate (PMA; K0.5 approximately 0.7 nM) or by 4 beta-phorbol 12,13-dibutyrate (K0.5 approximately 20 nM) even when the NADPH oxidase was blocked by p-chloromercuribenzoate. (3) Staurosporine inhibited the effect of PMA. (4) The H+ egress was not enhanced by 4 alpha-phorbol 12,13-didecanoate. (5) Low concentrations of Cd2+ (less than 40 microM) inhibited the H+ flux without influencing the oxidase. The results raise the possibility that protein kinase C could be involved in the activation of an electrogenic H(+)-conducting pathway in the membrane of neutrophils. The activation of this route by phorbol esters seems to be independent of the stimulation of NADPH oxidase.