Development of a delayed outward-rectifying K+ conductance in cultured mouse peritoneal macrophages

NCX1 represents an ionic Na+ sensing mechanism in macrophages

Inflammation and infection can trigger local tissue Na+ accumulation. This Na+-rich environment boosts proinflammatory activation of monocyte/macrophage-like cells (MΦs) and their antimicrobial activity. Enhanced Na+-driven MΦ function requires the osmoprotective transcription factor nuclear factor of activated T cells 5 (NFAT5), which augments nitric oxide (NO) production and contributes to increased autophagy. However, the mechanism of Na+ sensing in MΦs remained unclear. High extracellular Na+ levels (high salt [HS]) trigger a substantial Na+ influx and Ca2+ loss. Here, we show that the Na+/Ca2+ exchanger 1 (NCX1, also known as solute carrier family 8 member A1 [SLC8A1]) plays a critical role in HS-triggered Na+ influx, concomitant Ca2+ efflux, and subsequent augmented NFAT5 accumulation. Moreover, interfering with NCX1 activity impairs HS-boosted inflammatory signaling, infection-triggered autolysosome formation, and subsequent antibacterial activity. Taken together, this demonstrates that NCX1 is able to sense Na+ and is required for amplifying inflammatory and antimicrobial MΦ responses upon HS exposure. Manipulating NCX1 offers a new strategy to regulate MΦ function.

PARP1 promotes gene expression at the post-transcriptiona level by modulating the RNA-binding protein HuR

Poly(ADP-ribosyl)ation (PARylation) is mainly catalysed by poly-ADP-ribose polymerase 1 (PARP1), whose role in gene transcription modulation has been well established. Here we show that, in response to LPS exposure, PARP1 interacts with the adenylateuridylate-rich element-binding protein embryonic lethal abnormal vision-like 1 (Elavl1)/human antigen R (HuR), resulting in its PARylation, primarily at site D226. PARP inhibition and the D226 mutation impair HuR's PARylation, nucleocytoplasmic shuttling and mRNA binding. Increases in mRNA level or stability of pro-inflammatory cytokines/chemokines are abolished by PARP1 ablation or inhibition, or blocked in D226A HuR-expressing cells. The present study demonstrates a mechanism to regulate gene expression at the post-transcriptional level, and suggests that blocking the interaction of PARP1 with HuR could be a strategy to treat inflammation-related diseases that involve increased mRNA stability.

PARP1, in addition to its role in DNA repair, has a role in regulating gene transcription via PARylation of target proteins. Here the authors show that HuR is targeted after lipopolysaccharide exposure to regulate the inflammatory gene expression at post-transcriptional level.

Role of Kir2.1 in human monocyte-derived foam cell maturation

The role of K⁺ channels in macrophage immunomodulation has been well‐established. However, it remains unclear whether K⁺ channels are involved in the lipid uptake of macrophages. The expression and function of the inward rectifier potassium channel (Kir2.1, KCNJ2) in Human acute monocytic leukemia cell line (THP‐1) cells and human monocytes derived macrophages (HMDMs) were investigated using RT‐PCR and western blotting, and patch clamp technique. The expression of scavenger receptors in THP‐1–derived macrophages was detected using western blotting. Expressions of Kir2.1 mRNA and protein in HMDMs were significantly decreased by 60% (P < 0.05) and 90% (P < 0.001) on macrophage maturation, but overexpressed by approximately 1.3 (P > 0.05) and 3.8 times (P = 0.001) after foam cell formation respectively. Concurrently, the Kir2.1 peak current density in HMDMs, mature macrophages and foam cells, measured at −150 mV, were −22.61 ± 2.1 pA/pF, −7.88 ± 0.60 pA/pF and −13.39 ± 0.80 pA/pF respectively (P < 0.05). In association with an up‐regulation of Kir2.1 in foam cells, the SR‐A protein level was significantly increased by over 1.5 times compared with macrophages (P < 0.05). THP‐1 cells contained much less lipids upon Kir2.1 knockdown and cholesterol ester/total cholesterol ratio was 29.46 ± 2.01% (P < 0.05), and the SR‐BI protein level was increased by over 6.2 times, compared to that of macrophages (P < 0.001). Kir2.1 may participate in macrophage maturation and differentiation, and play a key role in lipid uptake and foam cell formation through modulating the expression of scavenger receptors.

Inactivation of TRPM7 kinase activity does not impair its channel function in mice

Transient receptor potential (TRP) family channels are involved in sensory pathways and respond to various environmental stimuli. Among the members of this family, TRPM7 is a unique fusion of an ion channel and a C-terminus kinase domain that is highly expressed in immune cells. TRPM7 serves as a key molecule governing cellular Mg²⁺ homeostasis in mammals since its channel pore is permeable to Mg²⁺ ions and can act as a Mg²⁺ influx pathway. However, mechanistic links between its kinase activity and channel function have remained uncertain. In this study, we generated kinase inactive knock-in mutant mice by mutagenesis of a key lysine residue involved in Mg²⁺-ATP binding. These mutant mice were normal in development and general locomotor activity. In peritoneal macrophages isolated from adult animals the basal activity of TRPM7 channels prior to cytoplasmic Mg²⁺ depletion was significantly potentiated, while maximal current densities measured after Mg²⁺ depletion were unchanged in the absence of detectable kinase function. Serum total Ca²⁺ and Mg²⁺ levels were not significantly altered in kinase-inactive mutant mice. Our findings suggest that abolishing TRPM7 kinase activity does not impair its channel activity and kinase activity is not essential for regulation of mammalian Mg²⁺ homeostasis.

Modulation of voltage-dependent and inward rectifier potassium channels by 15-epi-lipoxin-A4 in activated murine macrophages: implications in innate immunity

Potassium channels modulate macrophage physiology. Blockade of voltage-dependent potassium channels (Kv) by specific antagonists decreases macrophage cytokine production and inhibits proliferation. In the presence of aspirin, acetylated cyclooxygenase-2 loses the activity required to synthesize PGs but maintains the oxygenase activity to produce 15R-HETE from arachidonate. This intermediate product is transformed via 5-LOX into epimeric lipoxins, termed 15-epi-lipoxins (15-epi-lipoxin A4 [e-LXA4]). Kv have been proposed as anti-inflammatory targets. Therefore, we studied the effects of e-LXA4 on signaling and on Kv and inward rectifier potassium channels (Kir) in mice bone marrow-derived macrophages (BMDM). Electrophysiological recordings were performed in these cells by the whole-cell patch-clamp technique. Treatment of BMDM with e-LXA4 inhibited LPS-dependent activation of NF-κB and IκB kinase β activity, protected against LPS activation-dependent apoptosis, and enhanced the accumulation of the Nrf-2 transcription factor. Moreover, treatment of LPS-stimulated BMDM with e-LXA4 resulted in a rapid decrease of Kv currents, compatible with attenuation of the inflammatory response. Long-term treatment of LPS-stimulated BMDM with e-LXA4 significantly reverted LPS effects on Kv and Kir currents. Under these conditions, e-LXA4 decreased the calcium influx versus that observed in LPS-stimulated BMDM. These effects were partially mediated via the lipoxin receptor (ALX), because they were significantly reverted by a selective ALX receptor antagonist. We provide evidence for a new mechanism by which e-LXA4 contributes to inflammation resolution, consisting of the reversion of LPS effects on Kv and Kir currents in macrophages.

Kv1.5 in the immune system: the good, the bad, or the ugly?

For the last 20 years, knowledge of the physiological role of voltage-dependent potassium channels (Kv) in the immune system has grown exponentially. Leukocytes express a limited repertoire of Kv channels, which contribute to the membrane potential. These proteins are involved in the immune response and are therefore considered good pharmacological targets. Although there is a clear consensus about the physiological relevance of Kv1.3, the expression and the role of Kv1.5 are controversial. However, recent reports indicate that certain heteromeric Kv1.3/Kv1.5 associations may provide insight on Kv1.5. Here, we summarize what is known about this issue and highlight the role of Kv1.5 partnership interactions that could be responsible for this debate. The Kv1.3/Kv1.5 heterotetrameric composition of the channel and their possible differential associations with accessory regulatory proteins warrant further investigation.

Voltage-dependent calcium and chloride currents in S17 bone marrow stromal cell line

The bone marrow stromal cell line S17 has been used to study hematopoiesis in vitro. In this study, we demonstrate the presence of calcium and chloride currents in cultured S17 cells. Calcium currents were of low amplitude or barely detectable (50-100 pA). Hence to amplify the currents, we have used barium as a charge carrier. Barium currents were identified based on their distinct voltage-dependence, and sensitivity to dihydropyridines. S17 cells also exhibited a slowly activating outward current without inactivation, most commonly seen when the sodium of the extracellular solution was replaced either by TEA (TEA/Cs saline) or NMDG (NMDG saline), or by addition of amiloride to the extracellular solution. This current was abolished either by 500 microM SITS (4,4'-diisothiocyanatostilbene-2-2'-disulfonic acid) or 500 microM DPC (diphenylamine-2-carboxylic acid) a cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel blocker, identifying it as a Cl(-) current. RT-PCR identified the presence of ENaC and CFTR transcripts. CFTR blockade reduced cell proliferation, suggesting that this channel plays a physiological role in regulation of S17 cell proliferation.

Leishmania amazonensis infection may affect the ability of the host macrophage to be activated by altering their outward potassium currents

Understanding the impact of intracellular pathogens on the behaviour of their host cells is key to designing new interventions. We are interested in how Leishmania alters the electrical functioning of the plasma membrane of the macrophage it infects. The specific question addressed here is whether Leishmania amazonensis infection alters the macrophage's outward currents and what the consequences of such changes might be. Using the whole cell configuration of the patch clamp technique, we show that outward peak current density remains constant over the period studied but that time to peak and sensitivity to inhibitors vary during infection. Infected cells take 40% longer to activate and are more sensitive to the potassium channel inhibitor tetraethyl ammonium, compared to control cells, indicating increased potassium outward current activity. Activation of macrophages is associated with increases of nitric oxide production and membrane area, depolarization of the macrophage membrane, down regulation of inward potassium and up regulation of outward currents. After Leishmania infection, macrophage activation is characterised by a reduction of nitric oxide production and of outward current density. We therefore suggest that this reflects a weaker activation.

hKv1.5 channels play a pivotal role in the functions of human alveolar macrophages

We examined the pharmacological properties, the molecular identity, and the functional roles of hKv1.5 channel in human alveolar macrophage. Some of outward K(+) current was inhibited by 4-aminopyridine and antisense oligodeoxynucleotides against hKv1.5 mRNA. Consistently, the protein and mRNA expressions of hKv1.5 channel were detected. Furthermore, the phagocytosis and migration of human alveolar macrophages were significantly suppressed when the protein expression of hKv1.5 channel was lowered by the antisense hKv1.5 oligodeoxynucleotides. These results suggest that hKv1.5 channel is expressed in human alveolar macrophages and it plays a role in phagocytosis and migration of the human alveolar macrophage.

Potassium channels, memory T cells, and multiple sclerosis

Multiple sclerosis is a chronic inflammatory autoimmune disease of the central nervous system characterized by demyelination and axonal damage that result in disabling neurological deficits. Here the authors explain the rationale for the use of inhibitors of the Kv1.3 K+ channel in immune cells as a therapy for multiple sclerosis and other autoimmune disorders.

The induction of NOS2 expression by the hybrid cecropin A-melittin antibiotic peptide CA(1-8)M(1-18) in the monocytic line RAW 264.7 is triggered by a temporary and reversible plasma membrane permeation

There is an increasing awareness of immune cell modulation by antimicrobial peptides. While this process often requires specific receptors for the peptides involved, several reports point out to a receptor-independent process. The cecropin A-melittin hybrid peptide CA(1-8)M(1-18) (KWKLFKKIGIGAVLKVLTTGLPALIS-amide) modifies gene expression in the macrophage line RAW 264.7 in the absence of any previous macrophage priming, suggesting a membrane permeation process. To further analyze the initial steps of this mechanism, we have studied the interaction of the peptide with these cells. Below 2 microM, CA(1-8)M(1-18) causes a concentration-dependent membrane depolarization partially reversible with time. At 2 microM, the accumulation of the SYTOX green vital dye is one half of that achieved with 0.05% Triton X-100. The binding level, as assessed by fluorescein-labeled CA(1-8)M(1-18), varies from 7.7+/-1.2 to 37.4+/-3.9 x 10(6) molecules/cell over a 0.5-4.0 microM concentration range. Electrophysiological experiments with 0.5 microM CA(1-8)M(1-18), a concentration that triggers maximal NOS2 expression and minimal toxicity, show a reversible current induction in the RAW 264.7 plasma membrane that is maintained as far as peptide is present. This activation of the macrophage involves the production of nitric oxide, a metabolite lethal for many pathogens that results from unspecific membrane permeation by antimicrobial peptides, and represents a new mode of action that may open new therapeutic possibilities for these compounds against intracellular pathogens.

The voltage-gated potassium channel Kv1.3 is highly expressed on inflammatory infiltrates in multiple sclerosis brain

Multiple Sclerosis (MS) is characterized by central nervous system perivenular and parenchymal mononuclear cell infiltrates consisting of activated T cells and macrophages. We recently demonstrated that elevated expression of the voltage-gated potassium channel, Kv1.3, is a functional marker of activated effector memory T (T(EM)) cells in experimental allergic encephalomyelitis and in myelin-specific T cells derived from the peripheral blood of patients with MS. Herein, we show that Kv1.3 is highly expressed in postmortem MS brain inflammatory infiltrates. The expression pattern revealed not only Kv1.3(+) T cells in the perivenular infiltrate but also high expression in the parenchyma of demyelinated MS lesions and both normal appearing gray and white matter. These cells were uniformly chemokine receptor 7 negative (CCR7(-)), consistent with an effector memory phenotype. Using double-labeling immunohistochemistry and confocal microscopy, we demonstrated colocalization of Kv1.3 with CD3, CD4, CD8, and some CD68 cells. The expression patterns mirrored in vitro experiments showing polarization of Kv1.3 to the immunological synapse. Kv1.3 was expressed in low to moderate levels on CCR7(+) central memory T cells from cerebrospinal fluid, but, when these cells were stimulated in vitro, they rapidly became Kv1.3(high)/CCR7(-) T(EM), suggesting that a subset of cerebrospinal fluid cells existed in a primed state ready to become T(EM). These studies provide further rationale for the use of specific Kv1.3 antagonists in MS.

Silicon chip-based patch-clamp electrodes integrated with PDMS microfluidics

We report on a silicon wafer-based device that can be used for recording macroscopic ion channel protein activities across a diverse group of cell-types. Gigaohm seals were achieved for CHO-K1 and RIN m5F cells, and both cell-attached and whole-cell mode configurations were also demonstrated. Two distinct intrinsic potassium ion channels were recorded in whole-cell mode for HIT-T15 and RAW 264.7 cells. Polydimethylsiloxane (PDMS) microfluidics were also coupled with the micromachined silicon chips in order to demonstrate that a single cell could be selectively directed to a micropore, and membrane protein currents could subsequently be recorded. These silicon chip-based devices have significant advantages over traditional micropipette approaches, and may serve as combinatorial tools for investigating membrane biophysics, pharmaceutical screening, and other bio-sensing tasks.

Isolation and characterization of resident macrophages from the smooth muscle layers of murine small intestine

Macrophages within the murine tunica muscularis were isolated and cultured for physiological studies. Following dispersion, macrophages were identified by phagocytotic activity of fluorescein isothiocyanate (FITC)-dextran. Immediately following isolation, macrophages were rounded and possessed fluorescent granula but developed a ramified shape after 3-4 days in culture. Resident and cultured macrophages were immunopositive for F4/80 and I-Ad/I-Ed. Greater than 90% of F4/80 positive cultured cells were FITC-dextran positive. Macrophages had resting membrane potentials (RMP) of -33.3 +/- 1.5 mV after 1 day in culture, which increased to -53.9 +/- 4.4 mV after 3-4 days. The change in RMP was associated with the development of an inward rectifying K+ current, and a decrease in a voltage-dependent, inactivating outward current. After 3-4 days in culture the inflammatory mediated substances adenosine triphosphate (ATP), platelet-activating factor and bacterial lipopolysaccharide induced increases in cytoplasmic Ca2+ ([Ca2+]i). Forskolin suppressed the ATP-induced increase in [Ca2+]i. Macrophages exhibited oxidative bursts, measured by oxidation of dihydrorhodamine-123 to rhodamine-123. Oxidative bursts coincided with a reduction in intracellular pH. Macrophages expressed a proton conductance that may participate in pH maintenance during reactive oxygen production. These results suggest that resident macrophages in the intestine may play a role in the immunological protection of the gut.

Differential voltage-dependent K+ channel responses during proliferation and activation in macrophages

Voltage-dependent K+ channels (VDPC) are expressed in most mammalian cells and involved in the proliferation and activation of lymphocytes. However, the role of VDPC in macrophage responses is not well established. This study was undertaken to characterize VDPC in macrophages and determine their physiological role during proliferation and activation. Macrophages proliferate until an endotoxic shock halts cell growth and they become activated. By inducing a schedule that is similar to the physiological pattern, we have identified the VDPC in non-transformed bone marrow-derived macrophages and studied their regulation. Patch clamp studies demonstrated that cells expressed outward delayed and inwardly rectifying K+ currents. Pharmacological data, mRNA, and protein analysis suggest that these currents were mainly mediated by Kv1.3 and Kir2.1 channels. Macrophage colony-stimulating factor-dependent proliferation induced both channels. Lipopolysaccharide (LPS)-induced activation differentially regulated VDPC expression. While Kv1.3 was further induced, Kir2.1 was down-regulated. TNF-alpha mimicked LPS effects, and studies with TNF-alpha receptor I/II double knockout mice demonstrated that LPS regulation mediates such expression by TNF-alpha-dependent and -independent mechanisms. This modulation was dependent on mRNA and protein synthesis. In addition, bone marrow-derived macrophages expressed Kv1.5 mRNA with no apparent regulation. VDPC activities seem to play a critical role during proliferation and activation because not only cell growth, but also inducible nitric-oxide synthase expression were inhibited by blocking their activities. Taken together, our results demonstrate that the differential regulation of VDPC is crucial in intracellular signals determining the specific macrophage response.

A potassium ion channel is involved in cytokine production by activated human macrophages

Macrophages play an important role in immune and inflammatory responses, largely through secretion of bioactive molecule such as cytokines. While calcium is known to be an important regulator of this process, less is known about the role of other ions and the ion channels that regulate them. We have previously implicated an outwardly rectifying potassium channel (Kor) in this process and for this reason we have investigated the role of potassium (K+) and K+ channels in the regulation of tumour necrosis factor-alpha (TNF-alpha)and interleukin (IL)-8 production by activated human culture-derived macrophages. The effect of blockade of Kor is to inhibit phorbol myristate acetate (PMA)-induced cytokine production by translational or post-translational mechanisms, an effect that is duplicated by increasing extracellular K+. By contrast, the effects of K+ on LPS-stimulated cells are far more complex and are probably mediated through the change of osmolality and occur largely at the mRNA level. This data directly implicates K+, and its regulation through Kor, in early events following PMA stimulation of these cells.

Characterization of a delayed rectifier potassium channel in the slowly adapting stretch receptor neuron of crayfish

Single channel recordings were performed on enzyme-cleaned slowly adapting sensory neurons of crayfish, in cell-attached configuration, with a physiological K(+) gradient across the neuronal membrane. An outward rectifying, voltage-gated K(+) channel with a slope conductance of 13 pS and a K(+) ion permeability of P(K)=6.5 x 10(-14) cm(3)/s was characterized. This 13 pS K(+) channel started to be activated at around 20 mV depolarization. Its open probability increased upon depolarization with V(0.5)= -25.3 mV and P(max)=0.83. The averaged currents showed a delay following the onset of depolarization. The activation time constant was voltage-dependent. The maximal value was 17.0 ms at -25 mV and at +35 mV the time constant was 1.7 ms. Little inactivation was observed throughout the 80- or 1500-ms long depolarization pulses. A sum of two exponentials provided the optimal fit for open time and closed time distribution. At 80-mV depolarization, the open time constants were 0.4 and 10.4 ms; the close time constants were 0.4 and 2.3 ms. The first-latency distribution suggested that at least two closed states preceded two open states. This 13 pS delayed rectifier plays a minor role in the maintenance of the resting membrane potential but contributes to the action potential repolarization. It may also modify the stretch-induced receptor potential and affect the adaptation behaviours in this neuron.

Mechanical stimulation regulates voltage-gated potassium currents in cardiac microvascular endothelial cells

Vascular endothelial cells are constantly exposed to mechanical forces resulting from blood flow and transmural pressure. The goal of this study was to determine whether mechanical stimulation alters the properties of endothelial voltage-gated K+ channels. Cardiac microvascular endothelial cells (CMECs) were isolated from rat ventricular muscle and cultured on thin sheets of silastic membranes. Membrane currents were measured with the use of the whole-cell arrangement of the patch-clamp technique in endothelial cells subjected to static stretch for 24 hours and compared with measurements from control, nonstretched cells. Voltage steps positive to -30 mV resulted in the activation of a time-dependent, delayed rectifier K+current (IK) in the endothelial cells. Mechanically induced increases of 97%, 355%, and 106% at +30 mV were measured in the peak amplitude of IK in cells stretched for 24 hours by 5%, 10%, and 15%, respectively. In addition, the half-maximal voltage required for IK activation was shifted from +34 mV in the nonstretched cells to -5 mV in the stretched cells. Although IK in both groups of CMECs was blocked to a similar extent by tetraethylammonium, currents in the stretched endothelial cells displayed an enhanced sensitivity to inhibition by charybdotoxin. Preincubation of the CMECs with either pertussis toxin or phorbol 12-myristate 13-acetate during the 24 hours of cell stretch did not prevent the increase in IK. The application of phorbol 12-myristate 13-acetate and static stretch stimulated the proliferation of CMECs. Stretch-induced regulation of K+ channels may be important to control the resting potential of the endothelium and may contribute to capillary growth during periods of mechanical perturbation.

Ion channels in microglia (brain macrophages)

Microglia are immunocompetent cells in the brain that have many similarities with macrophages of peripheral tissues. In normal adult brain, microglial cells are in a resting state, but they become activated during inflammation of the central nervous system, after neuronal injury, and in several neurological diseases. Patch-clamp studies of microglial cells in cell culture and in tissue slices demonstrate that microglia express a wide variety of ion channels. Six different types of K+ channels have been identified in microglia, namely, inward rectifier, delayed rectifier, HERG-like, G protein-activated, as well as voltage-dependent and voltage-independent Ca2+-activated K+ channels. Moreover, microglia express H+ channels, Na+ channels, voltage-gated Ca2+ channels, Ca2+-release activated Ca2+ channels, and voltage-dependent and voltage-independent Cl- channels. With respect to their kinetic and pharmacological properties, most microglial ion channels closely resemble ion channels characterized in other macrophage preparations. Expression patterns of ion channels in microglia depend on the functional state of the cells. Microglial ion channels can be modulated by exposure to lipopolysaccharide or various cytokines, by activation of protein kinase C or G proteins, by factors released from astrocytes, by changes in the concentration of internal free Ca2+, and by variations of the internal or external pH. There is evidence suggesting that ion channels in microglia are involved in maintaining the membrane potential and are also involved in proliferation, ramification, and the respiratory burst. Further possible functional roles of microglial ion channels are discussed.

HERG-like K+ channels in microglia

A voltage-gated K+ conductance resembling that of the human ether-à-go-go-related gene product (HERG) was studied using whole-cell voltage-clamp recording, and found to be the predominant conductance at hyperpolarized potentials in a cell line (MLS-9) derived from primary cultures of rat microglia. Its behavior differed markedly from the classical inward rectifier K+ currents described previously in microglia, but closely resembled HERG currents in cardiac muscle and neuronal tissue. The HERG-like channels opened rapidly on hyperpolarization from 0 mV, and then decayed slowly into an absorbing closed state. The peak K+ conductance-voltage relation was half maximal at -59 mV with a slope factor of 18.6 mV. Availability, assessed by a hyperpolarizing test pulse from different holding potentials, was more steeply voltage dependent, and the midpoint was more positive (-14 vs. -39 mV) when determined by making the holding potential progressively more positive than more negative. The origin of this hysteresis is explored in a companion paper (Pennefather, P.S., W. Zhou, and T.E. DeCoursey. 1998. J. Gen. Physiol. 111:795-805). The pharmacological profile of the current differed from classical inward rectifier but closely resembled HERG. Block by Cs+ or Ba2+ occurred only at millimolar concentrations, La3+ blocked with Ki = approximately 40 microM, and the HERG-selective blocker, E-4031, blocked with Ki = 37 nM. Implications of the presence of HERG-like K+ channels for the ontogeny of microglia are discussed.

Role of cyclic ADP-ribose in ATP-activated potassium currents in alveolar macrophages

There is growing evidence that extracellular ATP causes a dramatic change in the membrane conductance of a variety of inflammatory cells. In the present study, using the nystatin perforated patch recording configuration, we found that ATP (0.3-30 microM) induced a transient outward current in a concentration-dependent manner and that the reversal potential of the ATP-induced outward current was close to the K+ equilibrium potential, indicating that the membrane behaves like a K+ electrode in the presence of ATP. The first application of ATP to alveolar macrophages perfused with Ca2+-free external solution could induce the outward current, but the response to ATP was diminished with successive applications. Intracellular perfusion with a Ca2+ chelator, 1,2-bis(2-aminophenoxy)ethane-N,N,N', N'-tetraacetic acid, also diminished the response. When cyclic ADP-ribose (cADPR) was applied to the macrophage cytoplasm, a transient outward current was elicited. Thereafter, the successive outward current was inhibited, suggesting the involvement of cADPR in the response. Intracellular perfusion with inositol 1,4, 5-trisphosphate also induced a transient outward current, but the successive current was not inhibited. The ATP-induced outward current was abolished when 8-amino-cADPR (as a blocker of cADPR, 10(-6)-10(-5) M) was introduced into the cytoplasm. Homogenates of alveolar macrophages showed both ADP-ribosyl cyclase and cADPR hydrolase activities, and CD38 (ADP-ribosyl cyclase/cADPR hydrolase) expression was confirmed by reverse transcriptase-polymerase chain reaction and Western blot analyses. These results indicate that ATP activates K+ currents by releasing Ca2+ from cADPR-sensitive internal Ca2+ stores.

Suppression of the voltage-gated K+ current of human megakaryocytes by thrombin and prostacyclin

We examined the effects of platelet activators and inhibitors of platelet function on the voltage-gated delayed rectifier K+ current of human megakaryocytes. We found that both the activators such as thrombin, the thrombin receptor peptide (TRP42-47) and ADP and the inhibitors such as prostacyclin suppressed the delayed rectifier current through two different mechanisms. The cAMP dependent protein kinase (A-kinase) inhibitor IP20 blocked the suppression of the delayed rectifier current by prostacyclin and failed to block the suppression by thrombin, TRP42-47 and ADP. The effects of IP20 suggest that the action of prostacyclin is mediated by A-kinase and the action of the three activators is not mediated by A-kinase. Pertussis toxin (PTX) an inhibitor of the inhibitory GTP-binding proteins (Gi) blocked the suppression of the delayed rectifier current by thrombin, TRP42-47 and ADP and failed to block the suppression by prostacyclin. The effects of PTX suggests that the action of the three activators is mediated by Gi or some other PTX-sensitive GTP-binding protein. We speculate that thrombin and other platelet activators that activate Gi may be suppressing the delayed rectifier current via a direct interaction of Gi or a subunit of it with the delayed rectifier potassium channel itself.

Properties of voltage-gated potassium currents of microglia differentiated with granulocyte/macrophage colony-stimulating factor

Voltage-gated whole-cell currents were recorded from cultured microglial cells which had been developed in the presence of the macrophage/microglial growth factor granulocyte/macrophage colony-stimulating factor. Outward K+ currents (IK) were most prominent in these cells. IK could be activated at potentials more positive than -40 mV. Half-maximal activation of IK was achieved at -13.8 mV and half-maximal inactivation of IK was determined at -33.8 mV. The recovery of IK from inactivation was described by a time constant of 7.9 sec. For a tenfold change in extracellular K+ concentration the reversal potential of IK shifted by 54 mV. Extracellularly applied 10 mM tetraethylammonium chloride reduced IK by about 50%, while 5 mM 4-aminopyridine almost completely abolished IK. Several divalent cations (Ba2+, Cd2+, Co2+, Zn2+) reduced current amplitudes and shifted the activation curve of IK to more positive values. Charybdotoxin (IC50 = 1.14 nM) and noxiustoxin (IC50 = 0.89 nM) blocked IK in a concentration-dependent manner, whereas dendrotoxin and mast cell degranulating peptide had no effect on the current amplitudes. The outward K+ currents showed a frequency dependence when depolarizing pulses were applied at a frequency of 1 Hz. A frequency-independent outward current (IK') characterized by the same activation behavior as IK was detected. IK' was blocked completely by 10 nM charybdotoxin or by 10 nM noxiustoxin. In contrast to its effect on IK, 10 mM tetraethylammonium chloride did not reduce IK'.

Potassium currents in cultured rabbit retinal pigment epithelial cells

Membrane potential and ionic currents were studied in cultured rabbit retinal pigment epithelial (RPE) cells using whole-cell patch clamp and perforated-patch recording techniques. RPE cells exhibited both outward and inward voltage-dependent currents and had a mean membrane capacitance of 26 +/- 12 pF (SD, n = 92). The resting membrane potential averaged -31 +/- 15 mV (n = 37), but it was as high as -60 mV in some cells. When K+ was the principal cation in the recording electrode, depolarization-activated outward currents were apparent in 91% of cells studied. Tail current analysis revealed that the outward currents were primarily K+ selective. The most frequently observed outward K+ current was a voltage- and time-dependent outward current (IK) which resembled the delayed rectifier K+ current described in other cells. IK was blocked by tetraethylammonium ions (TEA) and barium (Ba2+) and reduced by 4-aminopyridine (4-AP). In a few cells (3-4%), depolarization to -50 mV or more negative potentials evoked an outwardly rectifying K+ current (IKt) which showed more rapid inactivation at depolarized potentials. Inwardly rectifying K+ current (IKI) was also present in 41% of cells. IKI was blocked by extracellular Ba2+ or Cs+ and exhibited time-dependent decay, due to Na+ blockade, at negative potentials. We conclude that cultured rabbit RPE cells exhibit at least three voltage-dependent K+ currents. The K+ conductances reported here may provide conductive pathways important in maintaining ion and fluid homeostasis in the subretinal space.

Activation of a potassium outward current by zymosan and opsonized zymosan in mouse peritoneal macrophages

The effects of zymosan and human serum opsonized zymosan on membrane currents of adherent mouse peritoneal macrophages which had been cultured for 5 to 20 days were investigated with the whole-cell voltage-clamp technique. Both stimuli activated an outward current. The outward current activation was transient and lasted about 5 min. In solutions with 10 or 50 mmol/l extracellular potassium concentration the activation of an outwardly directed current occurred at test potentials positive to the respective potassium equilibrium potential. This particle-induced current resembled a calcium-activated potassium current which could be activated with the calcium ionophore A 23187 and with platelet activating factor. The order of maximal responses (test potential + 55 mV, amplitude given as percentage of the respective control) was: 0.1 mumol/l platelet activating factor (222 +/- 36%, n = 8, P < 0.01) > 1 mumol/l A 23187 (190 +/- 24%, n = 11, P < 0.01) > 900 micrograms/ml opsonized zymosan (134 +/- 7%, n = 22, P < 0.01) > 900 micrograms/ml zymosan (116 +/- 5%, n = 21, P < 0.01). The lower efficiency of zymosan as compared to opsonized zymosan is explained in part by a lower percentage of responding cells which was 48% for zymosan and 73% for opsonized zymosan. Macrophages which were pretreated with particles showed a greater reactivity to calcium as compared to untreated cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Voltage-dependent ion channels in glial cells

Glial cells, although non-excitable, express a wealth of voltage-activated ion channels that are typically characteristic of excitable cells. Since these channels are also observed in acutely isolated cells and in brain slices, they have to be considered functional in the intact brain. Numerous studies over the past 10 years have yielded detailed characterizations of glial channels permitting comparison of their properties to those of their neuronal counterparts. While for the most part such comparisons have demonstrated a high degree of similarity, they also provide evidence for the expression of some uniquely glial ion channels. An increasing number of studies indicate that the expression of "glial" channels is influenced by the cells' microenvironment. For example, the presence of neurons can induce or inhibit (depending on the preparation and type of channel studied) the expression of glial ion channels. Like ion channels in excitable cells, glial channels can be functionally regulated by activation of second-messenger pathways, allowing for short-term modulation of their membrane properties. Although the extent to which most of the characterized ion channels are involved in glial function is presently unclear, a growing body of data suggests that certain channels play an active role in glial function. Thus inwardly rectifying K+ channels in concert with delayed rectifying K+ channels are thought to be involved in the removal and redistribution of excess K+ in the brain, a process referred to as "spatial buffering". Glial K+ channels may also be crucial in modulating glial proliferation. Cl- channels and stretch-activated cation channels are believed to be involved in volume regulation. Na+ channels appear to be important in fueling the glial Na+/K(+)-pump, and Ca2+ channels are likely involved in numerous cellular events in which intracellular Ca2+ is a critical second messenger.

Differential depolarization-activated calcium responses in fetal and neonatal rat osteoblast-like cells

The present study evaluates differential occurrence of voltage-dependent calcium channels (VDCC) in the membranes of fetal (FROB) and neonatal (NROB) calvarian rat osteoblastic cells in primary culture. The intracellular calcium concentration ([Ca2+]i) was monitored upon depolarization of the cell membrane with the use of high K+ containing extracellular solutions. [Ca2+]i was measured in populations of cells as well as in individual cells using Fura-2, whereas the membrane potential (Em) was recorded in parallel experiments using patch-clamp techniques. Increasing the extracellular K+ concentration resulted in an instantaneous depolarization of Em of both FROB and NROB. This depolarization of Em did not significantly affect [Ca2+]i of populations of FROB and neonatal osteoblast precursors (NpROB). In contrast to FROB and NpROB, NROB populations responded to depolarization with significant transient [Ca2+]i increases that could be blocked by the calcium antagonist verapamil and were absent if extracellular Na+ was replaced for choline instead of K+. In individual cell measurements, response frequencies as well as the magnitude of [Ca2+]i responses upon depolarization of NROB were much higher than those of FROB, suggesting that more NROB than FROB possess VDCC. This phenomenon might point to a development-related expression of VDCC in the membranes of osteoblast-like cells.

Voltage-dependent potassium channels in activated rat microglia

1. Voltage-dependent currents of untreated (proliferating) and lipopolysaccharide (LPS)-treated rat microglial cells in culture were recorded using the whole-cell patch-clamp technique. 2. Membrane potentials showed prominent peaks at -35 mV and -70 mV. Membrane potentials of LPS-treated cells alternated between the two values. This may be due to a negative slope region of the I-V relation resulting in two zero current potentials. 3. From a holding potential of -70 mV, hyperpolarizing steps evoked an inwardly rectifying current both in proliferating and in LPS-treated cells, while depolarizing steps below -50 mV evoked an outwardly rectifying current only in LPS-treated microglia. The currents were K+ selective, as indicated by their reversal potential of approximately 0 mV in symmetric K+ concentrations (150 mM both intra- and extracellularly) and the reversal potential of the outward tail currents of approximately -90 mV at a normal extracellular K+ concentration (4.5 mM). 4. The activation of the outward current could be fitted by Hodgkin-Huxley-type n4 kinetics. The time constant of activation depended on voltage. 5. The inactivation of the inward and outward currents could be fitted by a single exponential. The time constant of the inward current inactivation was dependent on voltage, whereas the time constant of the outward current inactivation was virtually independent of voltage, except near the threshold of activation. Recovery of the outward from inactivation was slow and could be fitted by two exponentials. Responses to depolarizing steps were stable at 0.125 Hz, but greatly decreased from the first to the second pulse at 1 Hz. 6. The inactivation of the inward, but not of the outward, current disappeared in a low Na(+)-containing medium (5 mM). The inward current was selectively inhibited by extracellular Cs+ and Ba2+. The outward current was selectively inhibited by Cd2+, 4-aminopyridine and charybdotoxin. Replacement of intracellular K+ by an equimolar concentration of Cs+, and the extracellular application of tetraethylammonium and quinine inhibited both currents. 7. An increase of extracellular Ca2+ from 2 to 20 mM resulted in outwardly rectifying K+ channels activating at more positive potentials. Omission of Ca2+ from the extracellular medium had the opposite effect. When the intracellular free Ca2+ was increased from 0.01 to 1 microM, the outward current amplitudes were depressed. The Ca2+ ionophore A23187 had a similar effect. 8. LPS-treated microglial cells possess inwardly and outwardly rectifying K+ channels. The physiological and pharmacological characteristics of these two channel populations are markedly different.(ABSTRACT TRUNCATED AT 400 WORDS)

Activation of membrane outward currents by human low density lipoprotein in mouse peritoneal macrophages

The aim of the present study was to search for electrophysiological effects of human lipoproteins on membrane currents in mouse peritoneal macrophages which had been cultured for 5 to 20 days. Whole-cell currents were recorded by using a voltage-clamp technique. Low density lipoprotein (LDL, 100 micrograms/ml) increased a slowly activating nonspecific cation current (iso) in the positive potential range to 244 +/- 23% of the reference (test potential + 55 mV, n = 13, P < 0.005). Augmentation of current resulted out of a negative shift of the activation curve along the voltage axis (-22 mV) and an increase of maximally available current. Furthermore, LDL increased a rapidly activating outward current (ifo) at test potentials positive to the potassium equilibrium potential. At +55 mV ifo-amplitude increased to 165 +/- 14% of reference (n = 16, P < 0.005). LDL-induced effects on ifo-current could be mimicked by application of the calcium ionophore A 23,187 (1 mumol/l) which led to an increase of ifo-current to 161 +/- 25% of the reference (test potential +55 mV, n = 11, P < 0.005). Acetylated-LDL (100 micrograms/ml, 5-15 min) produced no significant effect on the membrane currents under investigation.

Calcium and potassium currents in porcine granulosa cells maintained in follicular or monolayer tissue culture

We studied membrane currents in granulosa cells (GC), immediately after collection or after variable culture time in the everted-follicle wall or in the monolayer. GC in both systems express an inward calcium current (ICa) with T-type kinetics and voltage dependence. GC in the everted-follicle culture express an outward potassium current (IK) kinetics, which remains unchanged during three days in culture. IK has delayed-rectifier kinetics, but is insensitive to TEA, 4-AP and apamine. GC in monolayer culture develop a new, inactivating delayed-rectifier potassium current (InK), which progressively dominates as cells advance from day one to day three in culture. A similar InK was recorded in large luteal cells. A possible link between luteinization and the appearance of InK is hypothesized.

Alterations in a voltage-gated K+ current during the differentiation of ML-1 human myeloblastic leukemia cells

A voltage-gated K+ current has been identified in ML-1 human myeloid leukemia cells, with the use of the whole-cell patch-clamp technique. ML-1 cells proliferate in tissue culture as immature myeloblasts and can be induced to differentiate to nonproliferative monocyte/macrophages. In the myeloblastic cells, activation of the K+ current occurs upon depolarization of the membrane potential to above -40 mV; inactivation of this current is also voltage dependent and follows a simple exponential time course with a time constant (Ti) of 900 msec at 0 mV. The current is inhibited by 4-aminopyridine (IC50 of 80 microM at 0 mV), but is much less sensitive to tetraethylammonium of Ba2+. In cells exposed to the differentiation-inducer 12-O-tetradecanoylphorbol-13-acetate (TPA), dramatic alterations in the K+ current occur: upon exposure to 10 nM TPA during whole-cell recording, the amplitude of the voltage-activated current initially increases (within 4 min) and later decreases (at approximately 30-50 min). Upon addition of 0.5 nM TPA to cells in tissue culture, the current shows suppressed activation and accelerated inactivation in the early stages of differentiation (10-fold decrease in Ti at approximately 7 hr) and is completely suppressed in the later stages (3 days). Thus, this voltage-gated K+ current is suppressed early in the induction of differentiation and associated loss of proliferation in myeloid ML-1 cells exposed to TPA; this parallels the fact that channels of a similar type are activated upon the stimulation of proliferation in lymphoid cells exposed to mitogens.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrophysiological behavior of microglia

The present knowledge of voltage- and ligand-activated ion channels of cultured microglial cells is described and its relevance is discussed. All microglial cells cultured from rat or mouse brain express an inward rectifying K+ channel but no outward currents. This expression is not changed by the length of the cultivation period, nor is it different in freshly isolated cells. It makes the microglial cells distinct from peritoneal macrophages, which possess an outward rectifying K+ channel. In bone marrow, 2 populations of cells could be distinguished electrophysiologically, one with the channel pattern of macrophages and one with that of microglial cells. This finding is interesting in light of the fact that it is presently hypothesized that the differentiation of monocytes into microglia takes place exclusively during embryonic development but not in the adult. The available data thus support the hypothesis that within the bone marrow a population of macrophage precursor cells exists with a possible lineage relationship to brain macrophages. The lack of outward currents in the microglial cells has the functional consequence that even a small inward current leads to a large membrane depolarization, since K+ outward currents are not activated with the depolarization. The microglial cell is thus very sensitive to depolarizing events. We found that ATP induced an inward current and an increase in the conductance, whereas ADP, AMP, and adenosine did not. These relative potencies indicate that microglia possess a P2 purinoceptor linked to an ion channel. The amplitude of the inward current elicited by ATP is about 80 pA and is sufficient to depolarize microglial cells close to 0 mV.(ABSTRACT TRUNCATED AT 250 WORDS)

Flow cytometric analysis of altered mononuclear cell transmembrane potential induced by cyclosporin

Immunosuppression by the fungal metabolite cyclosporin A (CsA) is characterized by functional inhibition, rather than destruction of cells. Because activation of immune cells involves intracellular signalling events associated with modulations of cell transmembrane potential (TMP), we tested the ability of cyclosporin A (CsA) to modulate immune mononuclear cell TMP in vitro using a TMP sensitive cationic dye, dihexyloxacarbocyanine (DIOC6(3)). All analyses were performed by flow cytometry. CsA increased TMP in monocytes and lymphocytes isolated from the blood of healthy human volunteers. CsA-induced hyperpolarization was time and concentration dependent in monocytes while the lymphocyte hyperpolarization, although time dependent, was evident over the entire range of CsA concentrations tested. CsA-induced hyperpolarization of lymphocytes was dependent on potassium ion (K+) efflux as indicated by the absence of hyperpolarization in 154 mM KCl or with pretreatment with 100 microM quinine (an inhibitor of K+ channels). Monocyte hyperpolarization by CsA was not inhibited in either system. Dihydrocyclosporin C (DH-CsC), an immunosuppressive analog of CsA, also hyperpolarized mononuclear cells. The anionic TMP sensitive dye bis oxonal (diBA-C4) indicated that CsA treatment depolarized mononuclear cell plasma membranes. The mitochondrial poison carbonyl cyanide p-trifluoromethoxy-phenyl-hydrazine (FCCP) eliminated CsA induced hyperpolarization and also indicated that CsA caused plasma membrane depolarization. We conclude that brief in vitro exposure to cyclosporin alters the transmembrane electrical potential of human lymphocytes and monocytes.

G-protein activators induce a potassium conductance in murine macrophages

The whole-cell patch clamp technique was used to test whether intracellular application of G-protein activators affect ionic currents in murine macrophages. Both the J774.1 macrophage-like cell line and primary bone marrow derived macrophages were used. Cells were bathed in Na Hanks' solution and intracellularly dialyzed (via the patch pipette) with K Hanks (145 mM KCl, < 100 nM Ca) plus or minus the G-protein activators GTP gamma S (10 microM), GppNHp (10 microM), or AIF4- (200 microM AlCl3 + 5 mM KF). In the absence of G-protein activators, only two K currents, an inwardly rectifying K current (Kir) and an outward, inactivating K current (Ko) were observed. In the presence of protein activators, two effects were observed: (i) the Kir conductance, which is stable for up to 30 min under control conditions, decayed twice as fast and (ii) an outwardly rectifying, noninactivating current appeared. The induced outward current appeared < 2 min after attaining the whole-cell patch clamp configuration. The current could be distinguished from the Kir and Ko currents on the basis of its direction of rectification (outward), barium sensitivity (> 1 mM), and kinetics (no time-dependent inactivation). Intracellular application of GTP (500 microM), GDP (500 microM), cAMP (100 microM + 0.5 mM ATP), or IP3 (20 microM) did not induce the current; 100 microM ATP gamma S activated a half-maximal amount of current. Induction of outward current by 10 microM GTP gamma S could be prevented by pre-exposing cells to pertussis toxin but not cholera toxin. This current is K selective since (i) its induction was accompanied by hyperpolarization of the cell toward EK, even after Kir had "washed out", (ii) it was present after > 90% of both intracellular and extracellular Cl were replaced by isethionate, and (iii) the induced outward conductance was absent when Ki was completely replaced by Cs, and was reduced by approximately 1/3 when [K]i was reduced by 1/3. Quinidine (1 mM) and 4-aminopyridine (10 mM) inhibited the current, but apamin (1 microM) and charybdotoxin (1 microM) did not.

Characterization of a delayed rectifier potassium current in chicken growth plate chondrocytes

With the use of the whole cell arrangement of the patch-clamp technique, an outward-directed time-dependent potassium current was identified in cultured chicken growth plate chondrocytes. This delayed rectifier potassium current (IK) activated with a sigmoidal time course during voltage steps to potentials positive to -40 mV. The half-maximal voltage required for current activation was determined to be -8 mV. The reversal potential (Erev) for IK, measured using deactivating tail currents, was -72 mV in the presence of 140 mM internal and 5 mM external [K+] solutions. Changes in external [K+] caused Erev to shift in a manner expected for a potassium-selective channel. In addition, increasing external [K+] from 5 to 50 mM caused the slope conductance of the tail currents to increase twofold. The chondrocyte IK was inhibited by the potassium-channel blocker 4-aminopyridine (4-AP) at concentrations of 0.5-4 mM and by the scorpion venom toxin charybdotoxin (CTX; 10 nM) but was unaffected by 10 mM tetraethylammonium (TEA). Addition of 20 microM ZnCl2 reduced IK in a voltage-dependent manner with the greatest inhibition found to occur at potentials near the threshold for current activation. Reduction of IK by ZnCl2 was accompanied by a slowing in the kinetics of IK activation. On the basis of the gating and pharmacological properties of this current, it is suggested that the chondrocyte channel belongs to a superfamily of K+ channels found in bone and immune system cells. The chondrocyte K+ channel may contribute to the unusually high [K+] found in the extracellular fluid of growth plate cartilage.

Voltage-dependent currents in isolated cells of the turtle retinal pigment epithelium

The electrophysiological properties of isolated turtle retinal pigment epithelial cells (RPE cells) were investigated using the whole-cell patch-clamp technique. Most RPE cells exhibited a voltage-dependent outward current activated by depolarization beyond about -43 mV that inactivated during a 500-ms voltage step. Tail current measurements indicated that the conductance underlying this current was potassium selective. This current inactivated with prolonged depolarization and was abolished or reduced by extracellular quinidine, barium, tetraethylammonium (TEA) and 4-aminopyridine (4-AP). Steady-state inactivation of the voltage-dependent outward current revealed a time-independent outwardly rectifying current/voltage relationship in many cells. In addition, many cells had an outward current that activated slowly upon depolarization beyond about +40 mV and appeared to reverse near 0 mV in both 3 mM KCl and 30 mM KCl external solutions. This current was often observed in the presence of potassium channel blockers. Hyperpolarizing pulses commonly evoked inward currents that activated slowly and did not inactivate. These currents were commonly observed when fluoride was absent from the pipette, and only occasionally when fluoride was the major pipette anion. Tail current measurements indicated that this current was somewhat anion selective. These currents may play important roles in the homeostatic and phagocytic functions of RPE cells in their interactions with the neural retina.

Characterization and functional expression of genomic DNA encoding the human lymphocyte type n potassium channel

Voltage-gated potassium channels play important functional roles in the development and maintenance of human lymphocyte functions. One such channel, known as the type n channel, has been well defined in human T cells and exhibits unique functional properties that distinguish it from other species of potassium channels. We report the characterization of a human genomic DNA clone, HGK5, encoding a 523-amino-acid potassium channel protein encoded by an open reading frame on a single exon. RNA transcribed in vitro from HGK5 genomic DNA directs expression of functional voltage-dependent potassium currents in Xenopus oocytes. The functional characteristics of the expressed channels are strikingly similar to those of the type n channel on human T lymphocytes. This, together with the presence of significant levels of HGK5 mRNA in human T lymphocytes, supports the notion that HGK5 encodes the human type n voltage-gated potassium channel. The effects of concanavalin A treatment on HGK5 mRNA levels in cultured human T lymphocytes was also examined. Mitogenic concentrations of concanavalin A induced a time-dependent decrease in HGK5 mRNA levels, suggesting that previously observed increases in potassium current density following concanavalin A treatment of human T lymphocytes are not due to increased transcriptional activity of the type n potassium channel gene.

Lipopolysaccharide induction of outward potassium current expression in human monocyte-derived macrophages: lack of correlation with secretion

Although an outwardly rectifying K+ conductance (IK,A) is prominently expressed in human alveolar macrophages, the expression of this conductance in human monocyte-derived macrophages (HMDMs) is rare. We have analyzed the induction of the expression of IK,A in voltage-clamped, in vitro differentiated HMDMs by a number of stimuli which produce either priming or activation of macrophages. Cultures were stimulated with lipopolysaccharide (LPS, 2 micrograms/ml), interleukin 2 (IL-2, 100 U/ml), or combinations of LPS and either recombinant interferon-gamma (gamma-IFN, 10 U/ml), phorbol myristate acetate (PMA, 0.01 or 1 microgram/ml) and platelet activating factor (PAF, 20 ng/ml) for periods of up to 24 hr. Treatment of the cells with either LPS or IL-2 greatly enhanced the frequency of current expression. Treatment with either PMA or gamma-IFN alone did not induce current expression; treatment of the cells with a combination of LPS and either PMA, gamma-IFN, or PAF did not enhance current expression over that observed with LPS alone. The expression of the outwardly rectifying K+ current was observed in 36% (n = 321) of the cells for cultures treated with LPS and 33% (n = 55) of the cells for cultures treated with IL-2. The inactivating outward K+ current was absent in cells which were not treated with either LPS or IL-2. The kinetics of current activation and inactivation appeared identical to that previously described for the transient-inactivating outward current of the human alveolar macrophage. Cycloheximide (1 microgram/ml), an inhibitor of protein synthesis, completely suppressed LPS-induced current expression. No correlation was found between peak current amplitude and cell size in LPS-activated cells expressing the outwardly rectifying K+ current, indicating that current density was not held constant from cell to cell. The coupling of ion channel expression and secretion in individual HMDMs was studied using the reverse hemolytic plaque assay. Although an enhancement of K+ current expression was observed following either LPS or IL-2 treatment, a quantitatively similar and uniform increase in the percentage of either IL-1 or lysozyme-secreting cells was not observed. The frequency of current expression in cells identified as secreting tumor necrosis factor-alpha (TNF-alpha), interleukin 1 (IL-1), or lysozyme was the same or decreased over that observed for nonsecreting cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Voltage-gated currents expressed by rat microglia in culture

Using the whole-cell patch-clamp technique, at least three types of voltage-gated currents expressed by cultured rat microglia were identified: an inward rectifier K+ current, a delayed rectifier K+ current (IK), and a Na+ current activated by depolarization. The inward rectifier conductance was activated by hyperpolarization to potentials more negative than -80 mV, depended on the external K+ concentration, and declined over time during whole cell recording, as the cell was internally dialyzed. The delayed rectifier current was activated by depolarization to potentials more positive than -40 mV and the rates of activation and deactivation showed a voltage-dependence similar to such currents seen in other preparations. An inward current possibly carried by Na+ was seen in a small percentage of cells. Recordings had been made from two morphological cell types, namely process-bearing ("ramified") and non-process-bearing ("ameboid"). Each of these currents was present in microglia of both morphological types. However, microglial morphology, which is thought to represent different states of activation, was significantly related to the types of combinations of currents expressed in a given cell.

A subpopulation of bone marrow-derived macrophage-like cells shares a unique ion channel pattern with microglia

Rat microglia share a number of antigenic, functional, and morphological similarities with macrophages from other tissues, but are characterized by a distinctly different pattern of ion channels in the cellular membrane (Kettenmann et al., J Neurosci Res 26:278-287, 1990). Macrophages typically express outward and inward K+ currents. In contrast, microglia lack outward currents and only show inwardly rectifying K+ currents, regardless of the isolation or cultivation method employed for microglia. In this study we demonstrate that a subpopulation of bone marrow-derived macrophage-like cells possesses inward rectifier K+ currents, but no outward currents and thus with regard to the electrophysiological characteristics closely resembles microglia. A second population of bone marrow-derived macrophage-like cells shows the usual channel pattern described for other body macrophages. Our results strengthen the hypothesis that in the bone marrow distinct pools of precursor cells exist, possibly reflecting an early differential lineage determination for body and brain macrophages, i.e., microglia.

Voltage-dependent K+ channels in the sarcolemma of mouse skeletal muscle

The voltage-dependent K+ channels of the mammalian sarcolemma were studied with the patch-clamp technique in intact, enzymatically dissociated fibres from the toe muscle of the mouse. With a physiological solution (containing 2.5 mM K+) in the pipette, depolarizing pulses imposed on a cell-attached membrane patch activated K+ channels with a conductance of about 17 pS. No channel activity was observed when the pipette solution contained 2 mM tetraethylammonium (TEA), or 2 mM 4-aminopyridine (4-AP). Whole cell recordings from these very small muscle fibres showed the well-known delayed rectifier K+ outward current with a threshold of about -40 mV. The whole-cell current was completely blocked by 2 mM TEA in the bath, suggesting that the TEA-sensitive channels in the patch were also delayed rectifier channels. The inactivation properties of the channels were studied in the cell-attached mode. Averaged single-channel traces showed at least two types of channels discernible by their inactivation time course at a test potential of 60 mV. The fast type inactivated with a time constant of about 150 ms, the slow type with a time constant of about 400 ms. A little channel activity always remained during pulses lasting several minutes, indicating either the presence of a very slowly inactivating third type of K+ channel, or the tendency of the fast inactivating channels to re-open at constant voltage. No difference was seen in the single-channel amplitudes of the different types of K+ channels. The well characterized adenosine-5'-triphosphate-(ATP)-sensitive and Ca(2+)-dependent K+ channels, although present, were not active under the conditions used.(ABSTRACT TRUNCATED AT 250 WORDS)

Voltage-gated K+ channels in the mouse interleukin 3-dependent cell line, FDC-P2

The electrical properties of a mouse interleukin (IL)-3-dependent cell line, FDC-P2, were examined using the tight-seal whole-cell clamp technique. Under current clamp conditions with 140 mM K+ in the pipette, the cells had a resting potential of approximately -30 mV. Under voltage-clamp conditions, a transient outward current was elicited upon depolarization from a holding potential of -80 mV. The current was activated at potentials more positive than -10 mV and had a delayed-rectifying property. It showed rapid activation and slow inactivation during command steps. The current was abolished by Cs+ in the pipette, indicating that K+ is the charge carrier. The K+ current was suppressed by tetraethylammonium with Ki of less than 0.1 mM and was not affected by scorpion toxin. Recovery from inactivation was steeply voltage dependent: As the holding potential was more hyperpolarized, the recovery became faster. Thus, with a holding potential of -80 mV, the current showed slight use-dependent inactivation, while the current decreased prominently by repetitive depolarization at a holding potential of -40 mV. These properties of the K+ current are similar to those of the l-type K+ channel current in mature T lymphocytes. The K+ current in FDC-P2 cells was dramatically reduced after culture in the IL-3-free medium for 1-2 days. When IL-3 was re-added to the medium, the current was re-expressed. These observations suggest that expression of the K+ current depends on extracellular IL-3, and that the current may play some roles in proliferation of these cells.

On the sodium and potassium currents of a human neuroblastoma cell line

1. The patch-clamp method was applied to the study of ionic currents activated by depolarization of undifferentiated IMR-32 human neuroblastoma cells. Whole-cell sodium and potassium currents and single potassium ion channel currents from cell-attached patches were investigated. 2. Cells had a mean resting potential of -38 mV and mean input resistance of 1.6 G omega. Single action potentials were evoked under current clamp during the injection of depolarizing currents. 3. A voltage-dependent inward sodium current was observed which reversed at +44 mV. A Boltzmann fit to the activation curve gave a half-maximal activation voltage of -41.6 mV and a 'slope' of 3.9 mV. The steady-state inactivation curve had a half-maximal inactivation voltage of -81 mV and a 'slope' of 9.7 mV. 4. The time-dependent activation and inactivation of the current displayed classical Hodgkin-Huxley kinetics. Values for the time constants tau m and tau h of 0.16 and 0.63 ms were calculated for a voltage jump from -80 to -10 mV; tau m and tau h decreased as the step potential was changed from -30 to +20 mV. 5. Outward currents were activated in bathing solutions substantially free of anions and could thus be attributed to potassium ions. The tail current reversed in direction on repolarization to -60 mV when the potassium concentration in the bathing solution was increased from 6 to 30 mM. When the bathing solution contained 145 mM-potassium, and the patch pipette, 95 mM, a depolarization to -10 mV from a holding potential of -60 mV evoked an inward current. 6. Outward currents were examined by using voltage pulses which depolarized the cell to -20 mV, or more positive values, from a holding potential of -80 mV and by pulses which depolarized the cell to 0 mV, or to positive values, from a holding potential of -30 mV. A Boltzmann fit of typical activation data gave a half-maximal activation voltage of 17 mV and a 'slope' of 14 mV. 7. The time course of the rising phase of the current was described by a function of the form A(1-exp[-(t-delta t)/tau]), where delta t varied between 1 and 4 ms and tau varied between 4 and 27 ms, decreasing with increasing depolarization. There was no evidence for a fast transient component. 8. The amplitude of outward currents was reduced by extracellular calcium ions, cobalt ions, tetraethylammonium and 4-aminopyridine.(ABSTRACT TRUNCATED AT 400 WORDS)

Sodium and potassium channels in epithelial cells from thymus glands and thymomas of myasthenia gravis patients

In both normal and neoplastic epithelial cells from human thymus glands and thymomas, respectively, we found voltage-gated sodium and potassium channels that resemble the adult-type Na channel and the delayed outward rectifier K channel, respectively, of human skeletal muscle and mammalian nervous system. These voltage-gated ion channels might be part of a communication system between epithelial cells and other components of the microenvironment of the thymus.

Voltage-gated calcium and potassium currents in megakaryocytes dissociated from guinea-pig bone marrow

1. The electrophysiological properties of the cell membrane of guinea-pig megakaryocytes were studied using the whole-cell patch-clamp technique. The megakaryocytes (diameter, 17-42 microns) were dissociated mechanically from the bone marrow of adult guinea-pigs. 2. In a proportion of cells, spike-like action potentials were generated in response to depolarization when the cells were immersed in standard saline containing 10 mM-Ca2+. Under voltage clamping, a transient inward current followed by a slowly Ca2+. Under voltage clamping, a transient inward current followed by a slowly developing outward current was produced when the membrane potential was made more positive than -55 mV. 3. The inward currents were identified as Ca2(+)-carried current, since the amplitude depended distinctly on external Ca2+ concentration and since replacement of external Ca2+ with Mn2+ reversibly diminished the current. The Ca2+ channels involved are most probably of the transient type (T-type). 4. The reversal potential of the outward current changed from -87 to -46 and -7 mV when the external K+ concentration was raised from 5 to 25 and 125 mM. 5. The outward current was insensitive to chelation of internal Ca2+ but was blocked by external application of quinine, 4-aminopyridine and tetraethylammonium, and was thus very probably a membrane potential-dependent K+ current. The dependence of the current activation and inactivation on the membrane potential was consistent with that of a delayed K+ rectifier. 6. The amplitudes of the Ca2+ currents and K+ currents showed considerable intercell variation. However, the density of the Ca2+ current showed a tendency to increase with megakaryocyte size, presumably accompanying maturation. The roles of these currents in cellular function remain to be elucidated.

Voltage-dependent currents in isolated cells of the frog retinal pigment epithelium

1. Retinal pigment epithelial (RPE) cells were isolated enzymatically from bullfrog retinae. The patch-clamp technique was employed to investigate whole-cell currents under voltage-clamp conditions. 2. Isolated RPE cells were columnar or cuboidal in form, often with long processes protruding from the apical surface. Distinct apical and basal membrane domains were maintained for several hours following isolation. 3. The mean membrane capacitance was 62 pF. The resting potential averaged -30 mV, but it was as high as -75 mV in some cells. 4. Three voltage-dependent currents were observed: a time-independent and inwardly rectifying current and two time-dependent outwardly rectifying currents that had distinct kinetic properties. 5. Voltage pulses from a holding potential of -70 mV to potentials ranging from -30 to -120 mV produced membrane currents that were essentially time independent. The I-V relationship in this voltage range depended on the resting potential. It was usually inwardly rectifying in cells with resting potentials negative to about -50 mV, but tended to be linear in cells with more positive potentials. Three observations strongly suggested that the inwardly rectifying current is carried by K+. First, increasing the extracellular K+ concentration [( K+]) from 2 to 112 mM shifted the zero-current potential of the I-V relationship in the positive direction from an average value of -60 mV to 0 mV. Second, the addition of the K+ channel blockers Ba2+ (2 mM) or Cs+ (5 mM) to the extracellular solution inhibited a major component of the inwardly rectifying current. Finally, the reversal potential (Vr) of the Ba2(+)-sensitive current averaged -90 mV, near the K+ equilibrium potential (EK). 6. In approximately 50% of the cells, depolarizing voltage pulses to potentials more negative than -30 mV evoked an outward current that resembled the delayed rectifier present in other non-excitable cells. It activated with sigmoidal kinetics in less than 100 ms following a brief delay and then declined exponentially with a time constant of approximately 1 s. The peak chord conductance associated with this current was half-maximal at +14 mV. Several observations indicated that this outwardly rectifying current is carried primarily by K+: its Vr closely matched EK over a wide range of extracellular [K+]; it was inhibited 80% by exposure to the K+ channel blockers 4-aminopyridine (1 mM) and tetraethylammonium (20 mM); and it was abolished by intracellular dialysis with a K(+)-free solution.(ABSTRACT TRUNCATED AT 400 WORDS)

Potassium and chloride conductances in rat Leydig cells: effects of gonadotrophins and cyclic adenosine monophosphate

1. The effects of gonadotrophins (luteinizing hormone and human chorionic gonadotrophin) and cyclic AMP on ionic conductances were investigated using the tight-seal whole-cell recording technique in Leydig cells freshly isolated from nature rat testis by enzymatic treatment. 2. In resting cells, the predominant ionic conductance is a voltage-dependent K+ conductance resembling the delayed rectifier K+ conductance of T-lymphocytes. This conductance is characterized by: (1) a time-dependent inactivation for potentials more positive than +20 mV, (2) a reversal potential near -65 mV, (3) a sensitivity to intracellular Cs+, and (4) a sensitivity to extracellular TEA and 4-aminopyridine. 3. A Cl- conductance is also present resembling the Cl- background conductance in squid axons and heart cells. In resting cells, this conductance contributes only a small component of the total outward current obtained with depolarizing pulses. 4. Gonadotrophins (human chorionic gonadotrophin, porcine luteinizing hormone and ovine luteinizing hormone) have little effect on the K+ conductance. They transiently increase a Cl- conductance after a delay of up to 30 s. This response does not occur if the hormones are applied late in the whole-cell recording. Gonadoliberine (GnRH) does not affect the Cl- or K+ conductance. 5. Internal cyclic AMP (100 microM) mimics all these effects while internal application of a GTP-ATP mixture induces a similar response, which is, however, sustained rather than transient. 6. The Cl- conductance was studied quantitatively with a GTP-ATP internal solution. This conductance is activated by depolarizing voltage steps to test potentials of -40 mV or more. Under these conditions, the instantaneous current observed as soon as the depolarizing pulse is applied displays outward rectification and reverses near ECl. During the pulses, a strong inactivation is observed for potentials greater than +40 mV. This conductance is independent of external and internal calcium. 7. It is concluded that the gonadotrophins act through a cyclic AMP-dependent process to activate a Cl- conductance. This conductance is different to the hyperpolarization-activated Cl- conductance and the calcium-activated Cl-conductance also present in the membrane of resting cells.