Loop Diuretics Inhibit Ischemia-Induced Intracellular Ca2+ Overload in Neurons via the Inhibition of Voltage-Gated Ca2+ and Na+ Channels

One consequence of ischemic stroke is disruption of intracellular ionic homeostasis. Intracellular overload of both Na⁺ and Ca²⁺ has been linked to neuronal death in this pathophysiological state. The etiology of ionic imbalances resulting from stroke-induced ischemia and acidosis includes the dysregulation of multiple plasma membrane transport proteins, such as increased activity of sodium-potassium-chloride cotransporter-1 (NKCC-1). Experiments using NKCC1 antagonists, bumetanide (BMN) and ethacrynic acid (EA), were carried out to determine if inhibition of this cotransporter affects Na⁺ and Ca²⁺ overload observed following in vitro ischemia-acidosis. Fluorometric Ca²⁺ and Na⁺ measurements were performed using cultured cortical neurons, and measurements of whole-cell membrane currents were used to determine target(s) of BMN and EA, other than the electroneutral NKCC-1. Both BMN and EA depressed ischemia-acidosis induced [Ca²⁺]_i overload without appreciably reducing [Na⁺]_i increases. Voltage-gated Ca²⁺ channels were inhibited by both BMN and EA with half-maximal inhibitory concentration (IC₅₀) values of 4 and 36 μM, respectively. Similarly, voltage-gated Na⁺ channels were blocked by BMN and EA with IC₅₀ values of 13 and 30 μM, respectively. However, neither BMN nor EA affected currents mediated by acid-sensing ion channels or ionotropic glutamatergic receptors, both of which are known to produce [Ca²⁺]_i overload following ischemia. Data suggest that loop diuretics effectively inhibit voltage-gated Ca²⁺ and Na⁺ channels at clinically relevant concentrations, and block of these channels by these compounds likely contributes to their clinical effects. Importantly, inhibition of these channels, and not NKCC1, by loop diuretics reduces [Ca²⁺]_i overload in neurons during ischemia-acidosis, and thus BMN and EA could potentially be used therapeutically to lessen injury following ischemic stroke.

Activation of Sigma Receptors With Afobazole Modulates Microglial, but Not Neuronal, Apoptotic Gene Expression in Response to Long-Term Ischemia Exposure

Stroke continues to be a leading cause of death and serious long-term disability. The lack of therapeutic options for treating stroke at delayed time points (≥6 h post-stroke) remains a challenge. The sigma receptor agonist, afobazole, an anxiolytic used clinically in Russia, has been shown to reduce neuronal and glial cell injury following ischemia and acidosis; both of which have been shown to play important roles following an ischemic stroke. However, the mechanism(s) responsible for this cytoprotection remain unknown. Experiments were carried out on isolated microglia from neonatal rats and cortical neurons from embryonic rats to gain further insight into these mechanisms. Prolonged exposure to in vitro ischemia resulted in microglial cell death, which was associated with increased expression of the pro-apoptotic protein, Bax, the death protease, caspase-3, and reduced expression in the anti-apoptotic protein Bcl-2. Incubation of cells with afobazole during ischemia decreased the number of microglia expressing both Bax and caspase-3, and increased cells expressing Bcl-2, which resulted in a concomitant enhancement in cell survival. In similar experiments, incubation of neurons under in vitro ischemic conditions resulted in higher expression of Bax and caspase-3, while at the same time expression of Bcl-2 was decreased. However, unlike observations made in microglial cells, afobazole was unable to modulate the expression of these apoptotic proteins, but a reduction in neuronal death was still noted. The functional state of surviving neurons was assessed by measuring metabolic activity, resting membrane potential, and responses to membrane depolarizations. Results showed that these neurons maintained membrane potential but had low metabolic activity and were unresponsive to membrane depolarizations. However, while these neurons were not fully functional, there was significant protection by afobazole against long-term ischemia-induced cell death. Thus, the effects of sigma receptor activation on microglial and neuronal responses to ischemia differ significantly.

Structural basis for the antiarrhythmic blockade of a potassium channel with a small molecule

The acetylcholine-activated inward rectifier potassium current ( I_KACh) is constitutively active in persistent atrial fibrillation (AF). We tested the hypothesis that the blocking of I_KACh with the small molecule chloroquine terminates persistent AF. We used a sheep model of tachypacing-induced, persistent AF, molecular modeling, electrophysiology, and structural biology approaches. The 50% inhibition/inhibitory concentration of I_KACh block with chloroquine, measured by patch clamp, was 1 μM. In optical mapping of sheep hearts with persistent AF, 1 μM chloroquine restored sinus rhythm. Molecular modeling suggested that chloroquine blocked the passage of a hydrated potassium ion through the intracellular domain of Kir3.1 (a molecular correlate of I_KACh) by interacting with residues D260 and F255, in proximity to I228, Q227, and L299. ¹H ¹⁵N heteronuclear single-quantum correlation of purified Kir3.1 intracellular domain confirmed the modeling results. F255, I228, Q227, and L299 underwent significant chemical-shift perturbations upon drug binding. We then crystallized and solved a 2.5 Å X-ray structure of Kir3.1 with F255A mutation. Modeling of chloroquine binding to the mutant channel suggested that the drug's binding to the pore becomes off centered, reducing its ability to block a hydrated potassium ion. Patch clamp validated the structural and modeling data, where the F255A and D260A mutations significantly reduced I_KACh block by chloroquine. With the use of numerical and structural biology approaches, we elucidated the details of how a small molecule could block an ion channel and exert antiarrhythmic effects. Chloroquine binds the I_KACh channel at a site formed by specific amino acids in the ion-permeation pathway, leading to decreased I_KACh and the subsequent termination of AF.-Takemoto, Y., Slough, D. P., Meinke, G., Katnik, C., Graziano, Z. A., Chidipi, B., Reiser, M., Alhadidy, M. M., Ramirez, R., Salvador-Montañés, O., Ennis, S., Guerrero-Serna, G., Haburcak, M., Diehl, C., Cuevas, J., Jalife, J., Bohm, A., Lin,Y.-S., Noujaim, S. F. Structural basis for the antiarrhythmic blockade of a potassium channel with a small molecule.

Modulation of mesenteric collecting lymphatic contractions by σ1-receptor activation and nitric oxide production

Recently, it has been reported that a σ-receptor antagonist could reduce inflammation-induced edema. Lymphatic vessels play an essential role in removing excess interstitial fluid. We tested the hypothesis that activation of σ-receptors would reduce or weaken collecting lymphatic contractions. We used isolated, cannulated rat mesenteric collecting lymphatic vessels to study contractions in response to the σ-receptor agonist afobazole in the absence and presence of different σ-receptor antagonists. We used RT-PCR and Western blot analysis to investigate whether these vessels express the σ1-receptor and immunofluorescence confocal microscopy to examine localization of the σ1-receptor in the collecting lymphatic wall. Using N-nitro-l-arginine methyl ester (l-NAME) pretreatment before afobazole in isolated lymphatics, we tested the role of nitric oxide (NO) signaling. Finally, we used 4-amino-5-methylamino-2',7'-difluorofluorescein diacetate fluorescence as an indicator to test whether afobazole increases NO release in cultured lymphatic endothelial cells. Our results show that afobazole (50-150 µM) elevated end-systolic diameter and generally reduced pump efficiency and that this response could be partially blocked by the σ1-receptor antagonists BD 1047 and BD 1063 but not by the σ2-receptor antagonist SM-21. σ1-Receptor mRNA and protein were detected in lysates from isolated rat mesenteric collecting lymphatics. Confocal images with anti-σ1-receptor antibody labeling suggested localization in the lymphatic endothelium. Blockade of NO synthases with l-NAME inhibited the effects of afobazole. Finally, afobazole elicited increases in NO production from cultured lymphatic endothelial cells. Our findings suggest that the σ1-receptor limits collecting lymphatic pumping through a NO-dependent mechanism.NEW & NOTEWORTHY Relatively little is known about the mechanisms that govern contractions of lymphatic vessels. σ1-Receptor activation has been shown to reduce the fractional pump flow of isolated rat mesenteric collecting lymphatics. The σ1-receptor was localized mainly in the endothelium, and blockade of nitric oxide synthase inhibited the effects of afobazole.

Kvβ1.1 (AKR6A8) senses pyridine nucleotide changes in the mouse heart and modulates cardiac electrical activity

The present study investigates the physiological role of Kvβ1 subunit for sensing pyridine nucleotide (NADH/NAD+) changes in the heart. We used Kvβ1.1 knockout (KO) or wild-type (WT) mice and established that Kvβ1.1 preferentially binds with Kv4.2 and senses the pyridine nucleotide changes in the heart. The cellular action potential duration (APD) obtained from WT cardiomyocytes showed longer APDs with lactate perfusion, which increases intracellular NADH levels, while the APDs remained unaltered in the Kvβ1.1 KO. Ex vivo monophasic action potentials showed a similar response, in which the APDs were prolonged in WT mouse hearts with lactate perfusion; however, the Kvβ1.1 KO mouse hearts did not show APD changes upon lactate perfusion. COS-7 cells coexpressing Kv4.2 and Kvβ1.1 were used for whole cell patch-clamp recordings to evaluate changes caused by NADH (lactate). These data reveal that Kvβ1.1 is required in the mediated inactivation of Kv4.2 currents, when NADH (lactate) levels are increased. In vivo, isoproterenol infusion led to increased NADH in the heart along with QTc prolongation in wild-type mice; regardless of the approach, our data show that Kvβ1.1 recognizes NADH changes and modulates Kv4.2 currents affecting AP and QTc durations. Overall, this study uses multiple levels of investigation, including the heterologous overexpression system, cardiomyocyte, ex vivo, and ECG, and clearly depicts that Kvβ1.1 is an obligatory sensor of NADH/NAD changes in vivo, with a physiological role in the heart.NEW & NOTEWORTHY Cardiac electrical activity is mediated by ion channels, and Kv4.2 plays a significant role, along with its binding partner, the Kvβ1.1 subunit. In the present study, we identify Kvβ1.1 as a sensor of pyridine nucleotide changes and as a modulator of Kv4.2 gating, action potential duration, and ECG in the mouse heart.

Activation of σ1 and σ2 receptors by afobazole increases glial cell survival and prevents glial cell activation and nitrosative stress after ischemic stroke

Activation of sigma receptors at delayed time points has been shown to decrease injury following ischemic stroke. The mixed σ1/σ2 receptor agonist, 5-ethoxy-2-[2-(morpholino)-ethylthio]benzimidazole (afobazole), provides superior long-term outcomes compared to other σ ligands in the rat middle cerebral artery occlusion (MCAO) stroke model. Experiments using the MCAO model were carried out to determine the molecular mechanism involved in the beneficial effects of afobazole. Administration of afobazole (3 mg/kg) at delayed time points post-stroke significantly increased the number of microglia and astrocytes detected in the ipsilateral hemisphere at 96 h post-surgery. Morphological analysis of the microglia indicated that a greater number of these cells were found in the ramified resting state in MCAO animals treated with afobazole relative to MCAO vehicle controls. Similarly, fewer reactive astrocytes were detected in the injured hemisphere of afobazole-treated animals. Both the enhanced survival and reduced activation of glial cells were abolished by co-application of either a σ1 (BD-1063) or a σ2 (SM-21) receptor antagonist with afobazole. To gain further insight into the mechanisms by which afobazole lessens stroke injury, we probed the brain sections for markers of neuroinflammation (tumor necrosis factor α) and nitrosative stress (S-nitrosocysteine). Data show that afobazole significantly reduces S-nitrosocysteine levels, but does not alter tumor necrosis factor α expression 96 h after an ischemic stroke. Taken together our data indicate that afobazole acting via both σ1 and σ2 receptors decreases stroke injury by enhancing glial cell survival, blocking ischemia-induced glial cell activation, and decreasing nitrosative stress.

σ-1 Receptor Inhibition of ASIC1a Channels is Dependent on a Pertussis Toxin-Sensitive G-Protein and an AKAP150/Calcineurin Complex

ASIC1a channels play a major role in various pathophysiological conditions including depression, anxiety, epilepsy, and neurodegeneration following ischemic stroke. Sigma-1 (σ-1) receptor stimulation depresses the activity of ASIC1a channels in cortical neurons, but the mechanism(s) by which σ-1 receptors exert their influence on ASIC1a remains unknown. Experiments were undertaken to elucidate the signaling cascade linking σ-1 receptors to ASIC1a channels. Immunohistochemical studies showed that σ-1 receptors, ASIC1a and A-kinase anchoring peptide 150 colocalize in the plasma membrane of the cell body and processes of cortical neurons. Fluorometric Ca(2+) imaging experiments showed that disruption of the macromolecular complexes containing AKAP150 diminished the effects of the σ-1 on ASIC1a, as did application of the calcineurin inhibitors, cyclosporin A and FK-506. Moreover, whole-cell patch clamp experiments showed that σ-1 receptors were less effective at decreasing ASIC1a-mediated currents in the presence of the VIVIT peptide, which binds to calcineurin and prevents cellular effects dependent on AKAP150/calcineurin interaction. The coupling of σ-1 to ASIC1a was also disrupted by preincubation of the neurons in the G-protein inhibitor, pertussis toxin (PTX). Taken together, our data reveal that σ-1 receptor block of ASIC1a function is dependent on activation of a PTX-sensitive G-protein and stimulation of AKAP150 bound calcineurin.

Afobazole modulates neuronal response to ischemia and acidosis via activation of sigma-1 receptors

Afobazole is an anxiolytic medication that has been previously shown to be neuroprotective both in vitro and in vivo. However, the mechanism(s) by which afobazole can enhance neuronal survival remain poorly understood. Experiments were carried out to determine whether afobazole can decrease intracellular calcium overload associated with ischemia and acidosis and whether the effects of afobazole are mediated via interaction of the compound with σ receptors. Fluorometric Ca(2+) imaging was used to resolve how application of afobazole affects intracellular Ca(2+) handling in cortical neurons. Application of afobazole significantly depressed, in a concentration-dependent and reversible manner, the intracellular Ca(2+) overload resulting from in vitro ischemia and acidosis. The IC(50) for afobazole inhibition of ischemia-evoked intracellular Ca(2+) overload was considerably less than that for the inhibition of [Ca(2+)](i) increases induced by acidosis. However, afobazole maximally inhibited only 70% of the ischemia-evoked intracellular Ca(2+) overload but effectively abolished intracellular Ca(2+) increases produced by acidosis. The effects of afobazole on ischemia- and acidosis-induced intracellular Ca(2+) dysregulation were inhibited by preincubating the neurons in the irreversible, pan-selective σ-receptor antagonist, metaphit. Moreover, the effects of afobazole on intracellular Ca(2+) increases triggered by acidosis and ischemia were blocked by the selective σ-1-receptor antagonists, BD 1063 and BD 1047, respectively. Experiments examining the effects of afobazole on neuronal survival in response to ischemia showed that afobazole was neuroprotective. Taken together, these data suggest that afobazole regulates intracellular Ca(2+) overload during ischemia and acidosis via activation of σ-1 receptors. This mechanism is probably responsible for afobazole-mediated neuroprotection.

Sigma receptor activation inhibits voltage-gated sodium channels in rat intracardiac ganglion neurons

Sigma (σ) receptors have been shown to regulate multiple ion channel types in intracardiac ganglion neurons, including voltage-gated calcium and potassium channels. However, the inhibition of these channels alone cannot fully account for σ receptor-induced changes in neuronal excitability previously reported. Whole-cell patch clamp experiments were conducted under current-clamp mode in isolated intracardiac neurons from neonatal rats to assess the effects of σ receptor activation on the active membrane properties of these cells. Bath application of the pan-selective σ receptor agonist, 1,3-Di-o-tolylguanidine (DTG), and the σ-1-selective agonist, (+)-pentazocine, significantly increased the action potential latency and decreased action potential overshoot in response to depolarizing current ramps, which suggests inhibition of voltage-gated sodium channels. Whole-cell voltage clamp experiments showed that these σ agonists reversibly decrease depolarization-activated Na(+) currents in these cells at all potentials tested. The peak currents generated by membrane depolarizations were decreased in a dose dependent manner with IC(50) values for DTG and (+)-pentazocine of 32 μM and 49 μM, respectively. The σ-1 receptor-selective antagonist, BD 1063 (100 nM), inhibited DTG (30 μM) block of Na(+) currents by ∼ 50%, suggesting that the effects are mediated by activation of σ-1 receptors. DTG also shifted the steady-state inactivation curve of Na(+) channels to more negative potentials, with the membrane potential of half-activation shifting from -49 mV to -63 mV in the absence and presence of 30 μM DTG, respectively. Taken together, these results suggest that σ-1 receptor activation decreases intracardiac ganglion neuron excitability by modulating voltage-gated Na(+) channels.

Cyclic AMP-dependent protein kinase A and protein kinase C phosphorylate alpha4beta2 nicotinic receptor subunits at distinct stages of receptor formation and maturation

Neuronal nicotinic receptor alpha4 subunits associated with nicotinic alpha4beta2 receptors are phosphorylated by cyclic AMP-dependent protein kinase (PKA) and protein kinase C (PKC), but the stages of receptor formation during which phosphorylation occurs and the functional consequences of kinase activation are unknown. SH-EP1 cells transfected with DNAs coding for human alpha4 and/or beta2 subunits were incubated with (32)Pi, and PKA or PKC was activated by forskolin or phorbol 12,13-dibutyrate, respectively. Immunoprecipitation and immunoblotting of proteins from cells expressing alpha4beta2 receptors or only alpha4 subunits were used to identify free alpha4 subunits, and alpha4 subunits present in immature alpha4beta2 complexes and mature alpha4beta2 pentamers containing complex carbohydrates. In the absence of kinase activation, phosphorylation of alpha4 subunits associated with mature pentamers was three times higher than subunits associated with immature complexes. PKA and PKC activation increased phosphorylation of free alpha4 subunits on different serine residues; only PKC activation phosphorylated subunits associated with mature alpha4beta2 receptors. Activation of both PKA and PKC increased the density of membrane-associated receptors, but only PKC activation increased peak membrane currents. PKA and PKC activation also phosphorylated beta2 subunits associated with mature alpha4beta2 receptors. Results indicate that activation of PKA and PKC leads to the phosphorylation alpha4beta2 receptors at different stages of receptor formation and maturation and has differential effects on the expression and function of human alpha4beta2 receptors.

sigma-1 receptor modulation of acid-sensing ion channel a (ASIC1a) and ASIC1a-induced Ca2+ influx in rat cortical neurons

Acid-sensing ion channels (ASICs) are proton-gated cation channels found in peripheral and central nervous system neurons. The ASIC1a subtype, which has high Ca2+ permeability, is activated by ischemia-induced acidosis and contributes to the neuronal loss that accompanies ischemic stroke. Our laboratory has shown that activation of sigma receptors depresses ion channel activity and [Ca2+](i) dysregulation during ischemia, which enhances neuronal survival. Whole-cell patch-clamp electrophysiology and fluorometric Ca2+ imaging were used to determine whether sigma receptors regulate the function of ASIC in cultured rat cortical neurons. Bath application of the selective ASIC1a blocker, psalmotoxin1, decreased proton-evoked [Ca2+](i) transients and peak membrane currents, suggesting the presence of homomeric ASIC1a channels. The pan-selective sigma-1/sigma-2 receptor agonists, 1,3-di-o-tolyl-guanidine (100 microM) and opipramol (10 microM), reversibly decreased acid-induced elevations in [Ca2+](i) and membrane currents. Pharmacological experiments using sigma receptor-subtype-specific agonists demonstrated that sigma-1, but not sigma-2, receptors inhibit ASIC1a-induced Ca2+ elevations. These results were confirmed using the irreversible sigma receptor antagonist metaphit (50 microM) and the selective sigma-1 antagonist BD1063 (10 nM), which obtunded the inhibitory effects of the sigma-1 agonist, carbetapentane. Activation of ASIC1a was shown to stimulate downstream Ca2+ influx pathways, specifically N-methyl-D-aspartate and (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid/kainate receptors and voltage-gated Ca2+ channels. These subsequent Ca2+ influxes were also inhibited upon activation of sigma-1 receptors. These findings demonstrate that sigma-1 receptor stimulation inhibits ASIC1a-mediated membrane currents and consequent intracellular Ca2+ accumulation. The ability to control ionic imbalances and Ca2+ dysregulation evoked by ASIC1a activation makes sigma receptors an attractive target for ischemic stroke therapy.

Sigma-1 receptor activation prevents intracellular calcium dysregulation in cortical neurons during in vitro ischemia

Sigma receptors are putative targets for neuroprotection following ischemia; however, little is known on their mechanism of action. One of the key components in the demise of neurons following ischemic injury is the disruption of intracellular calcium homeostasis. Fluorometric calcium imaging was used to examine the effects of sigma receptor activation on changes in intracellular calcium concentrations ([Ca(2+)](i)) evoked by in vitro ischemia in cultured cortical neurons from embryonic rats. The sigma receptor agonist, 1,3-di-o-tolyl-guanidine (DTG), was shown to depress [Ca(2+)](i) elevations observed in response to ischemia induced by sodium azide and glucose deprivation. Two sigma receptor antagonists, metaphit [1-(1-(3-isothiocyanatophenyl)-cyclohexyl)-piperidine] and BD-1047 (N-[2-3,4-dichlorophenyl)-ethyl]-N-methyl-2-(dimethylamino)ethylamine), were shown to blunt the ability of DTG to inhibit ischemia-evoked increases in [Ca(2+)](i), revealing that the effects are mediated by activation of sigma receptors and not via the actions of DTG on nonspecific targets such as N-methyl-d-aspartate receptors. DTG inhibition of ischemia-induced increases in [Ca(2+)](i) was mimicked by the sigma-1 receptor-selective agonists, carbetapentane, (+)-pentazocine and PRE-084 [2-(4-morpholinethyl) 1-phenylcyclohexanecarboxylate hydrochloride], but not by the sigma-2-selective agonist, ibogaine, showing that activation of sigma-1 receptors is responsible for the effects. In contrast, DTG, carbetapentane, and ibogaine blocked spontaneous, synchronous calcium transients observed in our preparation at concentrations consistent with sigma receptor-mediated effects, indicating that both sigma-1 and sigma-2 receptors regulate events that affect [Ca(2+)](i) in cortical neurons. Our studies show that activation of sigma receptors can ameliorate [Ca(2+)](i) dysregulation associated with ischemia in cortical neurons and, thus, identify one of the mechanisms by which these receptors may exert their neuroprotective properties.

P2Y purinoceptor activation mobilizes intracellular Ca2+ and induces a membrane current in rat intracardiac neurones

1. The mobilization of Ca2+ by purinoceptor activation and the relative contributions of intra- and extracellular sources of Ca2+ were investigated using microfluorimetric measurements of fura-2 loaded in cultured neurones from rat intracardiac ganglia. 2. Reverse transcriptase-polymerase chain reaction (RT-PCR) revealed expression of mRNA for the G protein-coupled P2Y2 and P2Y4 receptors. 3. Brief application of either 300 microM ATP or 300 microM UTP caused transient increases in [Ca2+]i of 277 +/- 22 nM and 267 +/- 39 nM, respectively. Removal of external Ca2+ did not significantly reduce these [Ca2+]i responses. 4. The order of purinoceptor agonist potency for [Ca2+]i increases was ATP = UTP > 2-MeSATP > ADP >> adenosine, consistent with the profile for P2Y2 purinoceptors. ATP- and UTP-induced rises in [Ca2+]i were completely and reversibly blocked by 10 microM PPADS (a P2 purinoceptor antagonist) and partially inhibited by 100 microM suramin (a relatively non-specific purinoceptor antagonist). 5. In the presence of the endoplasmic reticulum Ca2+-ATPase inhibitor cyclopiazonic acid (10 microM) in Ca2+-free media, the [Ca2+]i responses evoked by ATP were progressively decreased and abolished. 6. ATP- and UTP-induced [Ca2+]i rises were insensitive to pertussis toxin, caffeine (5 mM) and ryanodine (10 microM) but were significantly reduced by U-73122, a phospholipase C (PLC) inhibitor. 7. In fura-2-loaded cells, perforated patch whole-cell recordings show that ATP and UTP evoked slow outward currents at -60 mV, concomitant with the rise in [Ca2+]i, in approximately 30 % of rat intracardiac neurones. 8. In conclusion, these results suggest that in r intracardiac neurones, ATP binds to P2Y2 purinoceptors to transiently raise [Ca2+]i and activate an outward current. The signalling pathway appears to involve a PTX-insensitive G protein coupled to PLC generation of IP3 which triggers the release of Ca2+ from a ryanodine-insensitive Ca2+ store(s).

ACh- and caffeine-induced Ca2+ mobilization and current activation in rabbit arterial endothelial cells

Fura 2 microfluorometry and perforated-patch whole cell recording were carried out simultaneously to investigate the relationship between intracellular free Ca2+ concentration ([Ca2+]i) and membrane current activation in response to ACh and caffeine in freshly dissociated arterial endothelial cells. ACh and caffeine evoked transient increases in [Ca2+]i. The initial increase in [Ca2+]i was accompanied by a transient outward current, which caused membrane hyperpolarization. The amplitudes of the [Ca2+]i transient and outward current were dependent on caffeine concentration (EC50 approximately 1 mM). Cyclopiazonic acid raised resting [Ca2+]i levels by >/=50 nM and failed to completely block caffeine- or ACh-induced [Ca2+]i transients but slowed [Ca2+]i recovery fourfold. The reversal potential of caffeine-induced currents was dependent on external K+ and Cl- concentrations. Caffeine-induced current amplitudes, but not [Ca2+]i responses, were attenuated by external tetraethylammonium, Zn2+, and La3+. A consistent temporal relationship between agonist-activated membrane current and [Ca2+]i increases was not observed, and, in some cases, time differences were greater than expected for simple diffusion of Ca2+ throughout the cell. These results suggest that Ca2+-dependent current activation monitors local [Ca2+]i changes adjacent to the plasmalemma, whereas single-cell photometry provides a measure of global changes in [Ca2+]i.

Characterization of ATP-sensitive potassium channels in freshly dissociated rabbit aortic endothelial cells

ATP-sensitive potassium (KATP) channels represent a class of K+ channel regulated by intracellular ATP and serve to transduce changes in cell metabolism into changes in membrane potential. The presence of an KATP conductance has recently been demonstrated in freshly dissociated endothelial cells from rabbit arteries. In the present study, the single-channel activity underlying the KATP conductance in rabbit aortic endothelial cells was examined. Unitary currents were evoked in response to lowering intracellular ATP concentration or application of the K(+)-channel activator levcromakalim and were inhibited by the sulfonylurea drug glibenclamide. Exposure of the cytoplasmic face of an inside-out membrane patch to a solution containing 0.1 mM ATP produced single-channel events with unitary conductances of approximately 150 and approximately 25 pS that were inhibited by either 6 mM ATP or 10 microM glibenclamide. A small conductance channel was also activated in cell-attached patches by bath-applied levcromakalim (25 microM). Activation of endothelial cell KATP channels, and subsequent membrane hyperpolarization, may contribute to endothelium-dependent regulation of vascular smooth muscle tone in response to changes in levels of intracellular metabolites.

An ATP-sensitive potassium conductance in rabbit arterial endothelial cells

1. Whole-cell patch clamp recording was used to study an ATP-sensitive, sulphonylurea-inhibitable potassium (K+) conductance in freshly dissociated endothelial cells from rabbit arteries. 2. The ATP-sensitive K+ conductance was activated by micromolar concentrations of the K+ channel opener, levcromakalim, and by metabolic inhibition of endothelial cells using dinitrophenol and iodoacetic acid. The current-voltage (I-V) relationship obtained in isotonic K+ solutions was linear between -150 and -50 mV and had a slope conductance of approximately 1 nS. 3. The permeability of the ATP-sensitive K+ conductance determined from reversal potential measurements exhibited the following ionic selectivity sequence: Rb+ > K+ > Cs+ >> Na+ > NH4+ > Li+. 4. Membrane currents activated by either levcromakalim or metabolic inhibition were inhibited by the sulphonylurea drugs, glibenclamide and tolbutamide, with half-maximal inhibitory concentrations of 43 nM and 224 microM and Hill coefficients of 1.1 and 1.2, respectively. Levcromakalim-induced currents were also inhibited by millimolar concentrations of Ba2+ or tetraethylammonium ions in the external solution. 5. Levcromakalim (3 microM) and metabolic inhibition hyperpolarized endothelial cells by approximately 10-15 mV in normal physiological salt solutions. The hyperpolarization induced by levcromakalim or metabolic inhibition was inhibited by bath application of 10 microM glibenclamide. 6. Internal perfusion of the cytosol of whole-cell voltage-clamped endothelial cells with an ATP-free pipette solution activated a membrane current which was reversibly inhibited by internal perfusion with a 3 mM MgATP pipette solution. This current was insensitive to other adenine and guanine nucleotides in the pipette solution. The inward current evoked in a nominally ATP-free internal solution was further increased by bath application of levcromakalim. 7. Levcromakalim (25 microM) did not induce a change in the intracellular Ca2+ concentration of fura-2-loaded endothelial cells, whereas metabolic inhibition caused a slow and sustained increase in intracellular Ca2+ concentration, which was attenuated by 10 microM glibenclamide applied externally.(ABSTRACT TRUNCATED AT 400 WORDS)

Characterization of the 1,25-dihydroxycholecalciferol-stimulated calcium influx pathway in CaCo-2 cells

The present studies were conducted to investigate the mechanisms underlying the 1,25-dihydroxycholecalciferol (1,25(OH)2D3)-induced increase in intracellular Ca2+ ([Ca2+]i) in individual CaCo-2 cells. In the presence of 2 mM Ca2+, 1,25(OH)2D3-induced a rapid transient rise in [Ca2+]i in Fura-2-loaded cells in a concentration-dependent manner, which decreased, but did not return to baseline levels. In Ca(2+)-free buffer, this hormone still induced a transient rise in [Ca2+]i, although of lower magnitude, but [Ca2+]i then subsequently fell to baseline. In addition, 1,25(OH)2D3 also rapidly induced 45Ca uptake by these cells, indicating that the sustained rise in [Ca2+]i was due to Ca2+ entry. In Mn(2+)-containing solutions, 1,25(OH)2D3 increased the rate of Mn2+ influx which was temporally preceded by an increase in [Ca2+]i. The sustained rise in [Ca2+]i was inhibited in the presence of external La3+ (0.5 mM). 1,25(OH)2D3 did not increase Ba2+ entry into the cells. Moreover, neither high external K+ (75 mM), nor the addition of Bay K 8644 (1 microM), an L-type, voltage-dependent Ca2+ channel agonist, alone or in combination, were found to increase [Ca2+]i. 1,25(OH)2D3 did, however, increase intracellular Na+ in the absence, but not in the presence of 2 mM Ca2+, as assessed by the sodium-sensitive dye, sodium-binding benzofuran isophthalate. These data, therefore, indicate that CaCo-2 cells do not express L-type, voltage-dependent Ca2+ channels. 1,25(OH)2D3 does appear to activate a La(3+)-inhibitable, cation influx pathway in CaCo-2 cells.

Platelet activating factor-induced increase in cytosolic calcium and transmembrane current in human macrophages

Platelet-activating factor (PAF) is synthesized and secreted by macrophages in responding to inflammatory stimuli. When exogenously applied to human monocyte derived macrophages (HMDMs), PAF induces a rapid rise in cytosolic free calcium (Cai) believed to be an early triggering event in macrophage activation. We investigated PAF-induced Ca2+ signaling in HMDMs using the calcium indicator Fura-2, combining single cell ratio fluorimetry and digital video imaging with whole-cell recording techniques. Application of PAF (20 ng/ml) to adherent macrophages induced transient increases in Cai that were biphasic, consisting of an initial phase that could be observed in Ca(2+)-free solutions and a second phase that was critically dependent upon Ca2+ entry. When Mn2+ was applied to cells in the presence and absence of Ca2+, PAF increased the rate of Mn2+ entry rate only when Ca2+ was absent. PAF increased the rate of Ba2+ entry even when measured in the presence of external Ca2+. Ca2+ entry was reversibly inhibited in the presence of external La3+ (1 mM). Data obtained from simultaneous voltage-clamp/microfluorimetry experiments demonstrated the activation of a nonselective cation current which closely paralleled the rising phase of the Cai transient. We investigated whether the non-selective cation conductance provided for the bulk of the agonist-induced Ca2+ influx. Changes in Cai following removal of extracellular Ca2+ (Cao) during the agonist-induced Cai response were not associated with changes in whole-cell current. The inability to detect whole-cell current changes correlated with a decrease in Cao suggests that the bulk of the Ca2+ influx was not through the nonselective conductance and either does not occur through a conductance pathway or occurs via a parallel pathway consisting of channels which are both low conductance and highly Ca2+ selective.

Electric fields induce reversible changes in the surface to volume ratio of micropipette-aspirated erythrocytes

Micropipette-aspirated erythrocytes exhibit reversible changes in sphericity (surface-to-volume ratio) in response to applied electric fields. The potentials were applied between the shaft of the pipette and the bathing medium using Ag-AgCl electrodes and current clamping electronics. The change in surface-to-volume ratio is evidenced as a reversible change in the length of the cell projection in the pipette at constant aspiration pressure and changing voltage. The magnitude of the changes decreased in proportion to the inverse of the solute concentration indicating that the change in sphericity was due to a change in cell volume. Reversible changes in projection length equivalent to a 10% change in cell volume were observed to occur over times on the order of 10 s. The magnitude and time course of the effect were not affected by the removal of intracellular hemoglobin or inhibition of anion exchange. The effect was reduced by the presence of lanthanum and other multivalent cations in the suspending solution, suggesting that surface charge may play a role in mediating the effect.

Alterations of the apparent area expansivity modulus of red blood cell membrane by electric fields

Red blood cell membrane exhibits a large resistance to changes in surface area. This resistance is characterized by the area expansivity modulus K, which relates the isotropic membrane force resultant, T, to the fractional change in membrane surface area delta A/Ao. The experimental technique commonly used to determine K is micropipette aspiration. Using this method, E. A. Evans and R. Waugh (1977, Biophys. J. 20:307-313) obtained a value of 450 dyn/cm for the modulus. In the present report, it is shown that the value of K, as determined using this method, is affected by electric potential differences applied across the tip of the pipette. Using Ag-AgCl electrodes and current clamping electronics, we obtained values for K ranging from 150 dyn/cm with -1.0 V applied, to 1,500 dyn/cm with 1.0 V applied. At 0.0 V the modulus obtained was approximately 500 dyn/cm. A reversible, voltage- and pressure-dependent change in the cell volume probably accounts for the effect of the voltage on the calculated value of the modulus. The use of lanthanum chloride or increasing the extra- and intracellular solute concentrations reduced the voltage dependence of the measurements. It was also found that when dissimilar metals were used to "ground" the pipette to the chamber to prevent lysis of cells by static charge, values for K ranged from 121 to 608 dyn/cm. Based on measurements made at zero applied volts, in the presence of 0.4 mM lanthanum and at high solute concentration, we conclude that the true value of the modulus is approximately 500 dyn/cm.