Chloride and non-selective cation channels in unstimulated trout red blood cells

Inhibition of cation channels in human erythrocytes by spermine

In erythrocytes, spermine concentration decreases gradually with age, which is paralleled by increases of cytosolic Ca²+ concentration, with subsequent cell shrinkage and cell membrane scrambling. Cytosolic Ca²+ was estimated from fluo-3 fluorescence, cell volume from forward scatter, cell membrane scrambling from annexin V binding and cation channel activity with whole-cell patch-clamp in human erythrocytes. Extracellular spermine exerted a dual effect on erythrocyte survival. At 200 μM spermine blunted the increase of intracellular Ca²+, cell shrinkage and annexin V binding following 48 h exposure of cells at +37 °C. In contrast, short exposure (10-30 min) of cells to 2 mM spermine was accompanied by increased cytosolic Ca²+ and annexin binding. Intracellular addition of spermine at subphysiological concentration (0.2 μM) significantly decreased the conductance of monovalent cations (Na+, K+, NMDG+) and of Ca²+. Moreover, spermine (0.2 μM) blunted the stimulation of voltage-independent cation channels by Cl⁻ removal. Spermine (0.2 and 200 μM) added to the extracellular bath solution similarly inhibited the cation conductance in Cl⁻-containing bath solution. The effect of 0.2 μM spermine, but not the effect of 200 μM, was rapidly reversible. Acute addition (250 μM) of a naphthyl acetyl derivative of spermine (200 μM) again significantly decreased basal cation conductance in NaCl bath solution and inhibited voltage-independent cation channels. Spermine is a powerful regulator of erythrocyte cation channel cytosolic Ca²+ activity and, thus, cell survival.

Further characterization of cation channels present in the chicken red blood cell membrane

In this paper, we provide an update on cation channels in nucleated chicken erythrocytes. Patch-clamp techniques were used to further characterize the two different types of cation channels present in the membrane of chicken red blood. In the whole-cell mode, with Ringer in the bath and internal K+ saline in the pipette solution, the membrane conductance was generated by cationic currents, since the reversal potential was shifted toward cations equilibrium when the impermeant cation NMDG was substituted to small cations. The membrane conductance could be increased by application of mechanical deformation or by the addition of agonists of the cAMP-dependent pathway. At the unitary level, two different types of cationic channels were revealed and could account for the cationic conductance observed in whole-cell configuration. One of them belongs to the family of stretch-activated cationic channel showing changes in activity under conditions of membrane deformation, whereas the second one belongs to the family of the cAMP activated cationic channels. These two channels could be distinguished according to their unitary conductances and drug sensitivities. The stretch-activated channel was sensitive to Gd(3+) and the cAMP-dependent channel was sensitive to flufenamic acid. Possible role of these channels in cell volume regulation process is discussed.

Chloride channels in normal and cystic fibrosis human erythrocyte membrane

Electrophysiological studies on human RBCs have been difficult due to fragility and small size of cells, and little is known of ionic conductive pathways present in the RBC membrane in health and disease. We report on anionic channels in cells of healthy donors (control) and cystic fibrosis (CF) patients. Anion channel activity (8-12 pS, linear) was induced in cell-attached configuration by forskolin (50 microM) and in excised inside-out configuration by PKA (100 nM) and ATP (1 mM) but control and CF RBCs differed by their respective kinetics and gating properties. These channels were permeable to ATP (100 mM, symmetrical Tris-ATP). These data suggest either the existence of two different anionic channel types or regulation of a single channel type either by the CFTR (cystic fibrosis transmembrane regulator) protein or by different cytosolic factors. Another anionic channel type displaying outward rectification (approximately 80 pS, outward conductance) was present in 30% of CF cell patches but was not observed in normal cell patches. The frequently recorded activity of this channel in CF patches suggests a down-regulation in normal RBCs.

Hemolysis of erythrocytes by granulysin-derived peptides but not by granulysin

Granulysin, a 9-kDa protein localized in human cytolytic T lymphocytes and natural killer cell granules, is cytolytic against tumors and microbes but not against red blood cells. Synthetic peptides corresponding to the central region of granulysin recapitulate the lytic activity of the intact molecule, and some peptides cause hemolysis of red blood cells. Peptides in which cysteine residues were replaced by serine maintain their activity against microbes but lose activity against human cells, suggesting their potential as antibiotics. Studies were undertaken to determine the mechanism of resistance of red blood cells to granulysin and sensitivity to a subset of granulysin-derived peptides. Granulysin lyses immature reticulocytes, which have mitochondria, but not red blood cells. Granulysin lyses U937 cells but not U937 cells lacking mitochondrial DNA and a functional respiratory chain (U937rho(o) degrees cells), further demonstrating the requirement of intact mitochondria for granulysin-mediated death. Peptide G8, which corresponds to helix 2/loop 2/helix 3, lyses red blood cells, while peptide G9, which is identical except that the cysteine residues were replaced by serine, does not lyse red blood cells. Granulysin peptide-induced hemolysis is markedly inhibited by an anion transporter inhibitor and by Na(+), K(+), and Ca(2+) channel blockers but not by Na(+)/K(+) pump, cotransport, or Cl(-) channel blockers. Although recombinant granulysin and G9 peptide do not induce hemolysis, they both competitively inhibit G8-induced hemolysis. The finding that some derivatives of granulysin are hemolytic may have important implications for the design of granulysin-based antimicrobial therapeutics.

Patch-clamp analysis of the "new permeability pathways" in malaria-infected erythrocytes

The intraerythrocytic amplification of the malaria parasite Plasmodium falciparum induces new pathways of solute permeability in the host cell's membrane. These pathways play a pivotal role in parasite development by supplying the parasite with nutrients, disposing of the parasite's metabolic waste and organic osmolytes, and adapting the host's electrolyte composition to the parasite's needs. The "new permeability pathways" allow the fast electrogenic diffusion of ions and thus can be analyzed by patch-clamp single-channel or whole-cell recording. By employing these techniques, several ion-channel types with different electrophysiological profiles have been identified in P. falciparum-infected erythrocytes; they have also been identified in noninfected cells. This review discusses a possible contribution of these channels to the new permeability pathways on the one hand and their supposed functions in noninfected erythrocytes on the other.

Mechanosensitive cation channels in human leukaemia cells: calcium permeation and blocking effect

Cell-attached and inside-out patch-clamp methods were employed to identify and characterize mechanosensitive (MS) ionic channels in the plasma membrane of human myeloid leukaemia K562 cells. A reversible activation of gadolinium-blockable mechanogated currents in response to negative pressure application was found in 58 % of stable patches (n = 317). I-V relationships measured with a sodium-containing pipette solution showed slight inward rectification. Data analysis revealed the presence of two different populations of channels that were distinguishable by their conductance properties (17.2 +/- 0.3 pS and 24.5 +/- 0.5 pS), but were indistinguishable with regard to their selective and pharmacological properties. Ion-substitution experiments indicated that MS channels in leukaemia cells were permeable to cations but not to anions and do not discriminate between Na(+) and K(+). The channels were fully impermeable to large organic cations such as Tris(+) and N-methyl-D-glucamine ions (NMDG(+)). Ca(2+) permeation and blockade of MS channels were examined using pipettes containing different concentrations of Ca(2+). In the presence of 2 mM CaCl(2), when other cations were impermeant, both outward and inward single-channel currents were observed; the I-V relationship showed a unitary conductance of 7.7 +/- 1.0 pS. The relative permeability value, P(Ca)/P(K), was equal to 0.75, as estimated at physiological Ca(2+) concentrations. Partial or full inhibition of inward Ca(2+) currents through MS channels was observed at higher concentrations of external Ca(2+) (10 or 20 mM). No MS channels were activated when using a pipette containing 90 mM CaCl(2). Monovalent mechanogated currents were not significantly affected by extracellular Ca(2+) at concentrations within the physiological range (0-2 mM), and at some higher Ca(2+) concentrations.

The role of anion and cation channels in volume regulatory responses in trout red blood cells

(1) An outwardly rectifying chloride channel (ORCC) of large conductance has been detected under isotonic conditions (320 mosM 1(-1)) in the plasma membrane of trout red blood cells (RBCs) using the excised inside-out configuration. The channel, with a permeability ratio P(Cl)/Pcation of 12, was inhibited by the Cl- channel blockers 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB) (50 microM), and 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) (100 microM) in the bathing solution. (2) In hypotonic conditions (215 mosM 1(-1)), 44% of cell-attached patches showed spontaneous single channel activity identified as nonselective cationic (NSC) channels. A second group, corresponding to 7% of cell-attached patches, showed spontaneous activity corresponding to a channel type presenting outward rectification and anionic selectivity. Finally, 49% of patches displayed a complex spontaneous signal corresponding to the superimposition of inward and outward currents probably due to activation of different channel types. (3) Giga-seals obtained without suction in intact cells under isotonic conditions possessed NSC channels that were quiescent but which could be activated either by mechanical deformation of cell membrane or by hypotonic cell swelling. (4) Hypotonically swollen RBCs exhibited regulatory volume decrease (RVD) over 3 h, which was linked to a fivefold to sixfold increase in unidirectional fluxes of K+, a net loss of intracellular K+ and net gain of extracellular Na+. RVD and the hypotonically activated, unidirectional K+ influx continued after replacement of Cl- by methylsulfonate (MeSF) albeit more slowly. (5) The NSC channel inhibitor, barium, and the Cl- channel inhibitor, NPPB, both inhibited the RVD response by approximately 50% in Cl- containing saline. When Cl- was replaced by MeSF, the inhibition was > 90% suggesting that NSC channels and ORCC play key roles in the chloride-independent component of RVD.

O(2)-dependent K(+) fluxes in trout red blood cells: the nature of O(2) sensing revealed by the O(2) affinity, cooperativity and pH dependence of transport

The effects of pH and O(2) tension on the isotonic ouabain-resistant K(+) (Rb+) flux pathway and on haemoglobin O2 binding were studied in trout red blood cells (RBCs) in order to test for a direct effect of haemoglobin O(2) saturation on K(+) transport across the RBC membrane. At pH values corresponding to in vivo control arterial plasma pH and higher, elevation of the O(2) partial pressure (PO(2)) from 7.8 to 157 mmHg increased unidirectional K(+) influx across the RBC membrane several-fold. At lower extracellular pH values, stimulation of K(+) influx by O(2) was depressed, exhibiting an apparent pK(a) (pK'(a)) for the process of 8.0. Under similar conditions the pK'(a) for acid-induced deoxygenation of haemoglobin (Hb) was 7.3. When trout RBCs were exposed to PO(2) values between 0 and 747 mmHg, O(2) equilibrium curves typical of Hb O(2) saturation were also obtained for K(+) influx and efflux. However, at pH 7.9, the PO(2) for half-maximal K(+) efflux and K(+) influx (P50) was about 8- to 12-fold higher than the P(50) for Hb-O(2) binding. While K(+) influx and efflux stimulation by O(2) was essentially non-cooperative, Hb-O(2) equilibrium curves were distinctly sigmoidal (Hill parameters close to 1 and 3, respectively). O(2)-stimulated K(+) influx and efflux were strongly pH dependent. When the definition of the Bohr factor for respiratory pigments (Phi = delta logP50 x delta pH(-1)) was extended to the effect of pH on O(2)-dependent K(+) influx and efflux, extracellular Bohr factors (Phi(o) of -2.00 and -2.06 were obtained, values much higher than that for Hb (Phi(o) = -0.49). The results of this study are consistent with an O(2) sensing mechanism differing markedly in affinity and cooperativity of O(2) binding, as well as in pH sensitivity, from bulk Hb.