Single Ca-activated cation channels in bursting neurons of Helix

Hydrogen Peroxide Gates a Voltage-Dependent Cation Current in Aplysia Neuroendocrine Cells

Nonselective cation channels promote persistent spiking in many neurons from a diversity of animals. In the hermaphroditic marine-snail, Aplysia californica, synaptic input to the neuroendocrine bag cell neurons triggers various cation channels, causing an ∼30 min afterdischarge of action potentials and the secretion of egg-laying hormone. During the afterdischarge, protein kinase C is also activated, which in turn elevates hydrogen peroxide (H₂O₂), likely by stimulating nicotinamide adenine dinucleotide phosphate oxidase. The present study investigated whether H₂O₂ regulates cation channels to drive the afterdischarge. In single, cultured bag cell neurons, H₂O₂ elicited a prolonged, concentration- and voltage-dependent inward current, associated with an increase in membrane conductance and a reversal potential of ∼+30 mV. Compared with normal saline, the presence of Ca²⁺-free, Na⁺-free, or Na⁺/Ca²⁺-free extracellular saline, lowered the current amplitude and left-shifted the reversal potential, consistent with a nonselective cationic conductance. Preventing H₂O₂ reduction with the glutathione peroxidase inhibitor, mercaptosuccinate, enhanced the H₂O₂-induced current, while boosting glutathione production with its precursor, N-acetylcysteine, or adding the reducing agent, dithiothreitol, lessened the response. Moreover, the current generated by the alkylating agent, N-ethylmaleimide, occluded the effect of H₂O₂ The H₂O₂-induced current was inhibited by tetrodotoxin as well as the cation channel blockers, 9-phenanthrol and clotrimazole. In current-clamp, H₂O₂ stimulated burst firing, but this was attenuated or prevented altogether by the channel blockers. Finally, H₂O₂ evoked an afterdischarge from whole bag cell neuron clusters recorded ex vivo by sharp-electrode. H₂O₂ may regulate a cation channel to influence long-term changes in activity and ultimately reproduction.SIGNIFICANCE STATEMENT Hydrogen peroxide (H₂O₂) is often studied in a pathological context, such as ischemia or inflammation. However, H₂O₂ also physiologically modulates synaptic transmission and gates certain transient receptor potential channels. That stated, the effect of H₂O₂ on neuronal excitability remains less well defined. Here, we examine how H₂O₂ influences Aplysia bag cell neurons, which elicit ovulation by releasing hormones during an afterdischarge. These neuroendocrine cells are uniquely identifiable and amenable to recording as individual cultured neurons or a cluster from the nervous system. In both culture and the cluster, H₂O₂ evokes prolonged, afterdischarge-like bursting by gating a nonselective voltage-dependent cationic current. Thus, H₂O₂, which is generated in response to afterdischarge-associated second messengers, may prompt the firing necessary for hormone secretion and procreation.

TRPM4

TRPM4 is a Ca(2+)-activated nonselective cation channel. The channel is activated by an increase of intracellular Ca(2+) and is regulated by several factors including temperature and Pi(4,5)P2. TRPM4 allows Na(+) entry into the cell upon activation, but is completely impermeable to Ca(2+). Unlike TRPM5, its closest relative in the transient receptor potential family, TRPM4 proteins are widely expressed in the body. Currents with properties that are reminiscent of TRPM4 have been described in a variety of tissues since the advent of the patch clamp technology, but their physiological role is only beginning to be clarified with the increasing characterization of knockout mouse models for TRPM4. Furthermore, mutations in the TRPM4 gene have been associated with cardiac conduction disorders in human patients. This review aims to overview the currently available data on the functional properties of TRPM4 and the current understanding of its physiological role in healthy and diseased tissue.

Regulation of cerebral artery smooth muscle membrane potential by Ca²⁺-activated cation channels

Arterial tone is dependent on the depolarizing and hyperpolarizing currents regulating membrane potential and governing the influx of Ca²⁺ needed for smooth muscle contraction. Several ion channels have been proposed to contribute to membrane depolarization, but the underlying molecular mechanisms are not fully understood. In this review, we will discuss the historical and physiological significance of the Ca²⁺-activated cation channel, TRPM4, in regulating membrane potential of cerebral artery smooth muscle cells. As a member of the recently described transient receptor potential super family of ion channels, TRPM4 possesses the biophysical properties and upstream cellular signaling and regulatory pathways that establish it as a major physiological player in smooth muscle membrane depolarization.

Dopamine-induced oscillations of the pyloric pacemaker neuron rely on release of calcium from intracellular stores

Endogenously bursting neurons play central roles in many aspects of nervous system function, ranging from motor control to perception. The properties and bursting patterns generated by these neurons are subject to neuromodulation, which can alter cycle frequency and amplitude by modifying the properties of the neuron's ionic currents. In the stomatogastric ganglion (STG) of the spiny lobster, Panulirus interruptus, the anterior burster (AB) neuron is a conditional oscillator in the presence of dopamine (DA) and other neuromodulators and serves as the pacemaker to drive rhythmic output from the pyloric network. We analyzed the mechanisms by which DA evokes bursting in the AB neuron. Previous work showed that DA-evoked bursting is critically dependent on external calcium (Harris-Warrick RM, Flamm RE. J Neurosci 7: 2113-2128, 1987). Using two-photon microscopy and calcium imaging, we show that DA evokes the release of calcium from intracellular stores well before the emergence of voltage oscillations. When this release from intracellular stores is blocked by antagonists of ryanodine or inositol trisphosphate (IP(3)) receptor channels, DA fails to evoke AB bursting. We further demonstrate that DA enhances the calcium-activated inward current, I(CAN), despite the fact that it significantly reduces voltage-activated calcium currents. This suggests that DA-induced release of calcium from intracellular stores activates I(CAN), which provides a depolarizing ramp current that underlies endogenous bursting in the AB neuron.

The non-selective monovalent cationic channels TRPM4 and TRPM5

Transient Receptor Potential (TRP) proteins are non-selective cationic channels with a consistent Ca(2+)-permeability, except for TRPM4 and TRPM5 that are not permeable to this ion. However, Ca(2+) is a major regulator of their activity since both channels are activated by a rise in internal Ca(2+). Thus TRPM4 and TRPM5 are responsible for most of the Ca(2+)-activated non-selective cationic currents (NSC(Ca)) recorded in a large variety of tissues. Their activation induces cell-membrane depolarization that modifies the driving force for ions as well as activity of voltage gated channels and thereby strongly impacts cell physiology. In the last few years, the ubiquitously expressed TRPM4 channel has been implicated in insulin secretion, the immune response, constriction of cerebral arteries, the activity of inspiratory neurons and cardiac dysfunction. Conversely, TRPM5 whose expression is more restricted, has until now been mainly implicated in taste transduction.

Physiological roles of the TRPM4 channel extracted from background currents

Calcium-activated nonselective cationic currents have been known for 30 years, but their physiological implications have remained unresolved until the recent cloning of the TRPM4 ion channel. Since then, TRPM4 has been identified as a key modulator of numerous calcium-dependent mechanisms such as the immune response, insulin secretion, cerebral artery constriction, respiratory rhythm, and cardiac conduction.

Mitochondrial Ca2+ Activates a Cation Current in Aplysia Bag Cell Neurons

Ion channels may be gated by Ca2+ entering from the extracellular space or released from intracellular stores—typically the endoplasmic reticulum. The present study examines how Ca2+ impacts ion channels in the bag cell neurons of Aplysia californica. These neuroendocrine cells trigger ovulation through an afterdischarge involving Ca2+ influx from Ca2+ channels and Ca2+ release from both the mitochondria and endoplasmic reticulum. Liberating mitochondrial Ca2+ with the protonophore, carbonyl cyanide-4-trifluoromethoxyphenyl-hydrazone (FCCP), depolarized bag cell neurons, whereas depleting endoplasmic reticulum Ca2+ with the Ca2+-ATPase inhibitor, cyclopiazonic acid, did not. In a concentration-dependent manner, FCCP elicited an inward current associated with an increase in conductance and a linear current/voltage relationship that reversed near −40 mV. The reversal potential was unaffected by changing intracellular Cl−, but left-shifted when extracellular Ca2+ was removed and right-shifted when intracellular K+ was decreased. Strong buffering of intracellular Ca2+ decreased the current, although the response was not altered by blocking Ca2+-dependent proteases. Furthermore, fura imaging demonstrated that FCCP elevated intracellular Ca2+ with a time course similar to the current itself. Inhibiting either the V-type H+-ATPase or the ATP synthetase failed to produce a current, ruling out acidic Ca2+ stores or disruption of ATP production as mechanisms for the FCCP response. Similarly, any involvement of reactive oxygen species potentially produced by mitochondrial depolarization was mitigated by the fact that dialysis with xanthine/xanthine oxidase did not evoke an inward current. However, both the FCCP-induced current and Ca2+ elevation were diminished by disabling the mitochondrial permeability transition pore with the alkylating agent, N-ethylmaleimide. The data suggest that mitochondrial Ca2+ gates a voltage-independent, nonselective cation current with the potential to drive the afterdischarge and contribute to reproduction. Employing Ca2+ from mitochondria, rather than the more common endoplasmic reticulum, represents a diversification of the mechanisms that influence neuronal activity.

Calcium-dependent fast depolarizing afterpotentials in vasopressin neurons in the rat supraoptic nucleus

Oxytocin (OT) and vasopressin (VP) synthesizing magnocellular cells (MNCs) in the supraoptic nucleus (SON) display distinct firing patterns during the physiological demands for these hormones. Depolarizing afterpotentials (DAPs) in these neurons are involved in controlling phasic bursting in VP neurons. Our whole cell recordings demonstrated a Cs(+)-resistant fast DAP (fDAP; decay tau = approximately 200 ms), which has not been previously reported, in addition to the well-known Cs(+)-sensitive slower DAP (sDAP; decay tau = approximately 2 s). Immunoidentification of recorded neurons revealed that all VP neurons, but only 20% of OT neurons, expressed the fDAP. The activation of the fDAP required influx of Ca(2+) through voltage-gated Ca(2+) channels as it was strongly suppressed in Ca(2+)-free extracellular solution or by bath application of Cd(2+). Additionally, the current underlying the fDAP (I(fDAP)) is a Ca(2+)-activated current rather than a Ca(2+) current per se as it was abolished by strongly buffering intracellular Ca(2+) with BAPTA. The I-V relationship of the I(fDAP) was linear at potentials less than -60 mV but showed pronounced outward rectification near -50 mV. I(fDAP) is sensitive to changes in extracellular Na(+) and K(+) but not Cl(-). A blocker of Ca(2+)-activated nonselective cation (CAN) currents, flufenamic acid, blocked the fDAP, suggesting the involvement of a CAN current in the generation of fDAP in VP neurons. We speculate that the two DAPs have different roles in generating after burst discharges and could play important roles in determining the distinct firing properties of VP neurons in the SON neurons.

Ca2+-dependent regulation of a non-selective cation channel from Aplysia bag cell neurones

Ca2+-activated, non-selective cation channels feature prominently in the regulation of neuronal excitability, yet the mechanism of their Ca2+ activation is poorly defined. In the bag cell neurones of Aplysia californica, opening of a voltage-gated, non-selective cation channel initiates a long-lasting afterdischarge that induces egg-laying behaviour. The present study used single-channel recording to investigate Ca2+ activation in this cation channel. Perfusion of Ca2+ onto the cytoplasmic face of channels in excised, inside-out patches yielded a Ca2+ activation EC50 of 10 microm with a Hill coefficient of 0.66. Increasing Ca2+ from 100 nm to 10 microm caused an apparent hyperpolarizing shift in the open probability (Po) versus voltage curve. Beyond 10 microm Ca2+, additional changes in voltage dependence were not evident. Perfusion of Ba2+ onto the cytoplasmic face did not alter Po; moreover, in outside-out recordings, Po was decreased by replacing external Ca2+ with Ba2+ as a charge carrier, suggesting Ca2+ influx through the channel may provide positive feedback. The lack of Ba2+ sensitivity implicated calmodulin in Ca2+ activation. Consistent with this, the application to the cytoplasmic face of calmodulin antagonists, calmidazolium and calmodulin-binding domain, reduced Po, whereas exogenous calmodulin increased Po. Overall, the data indicated that the cation channel is activated by Ca2+ through closely associated calmodulin. Bag cell neurone intracellular Ca2+ rises markedly at the onset of the afterdischarge, which would enhance channel opening and promote bursting to elicit reproduction. Cation channels are essential to nervous system function in many organisms, and closely associated calmodulin may represent a widespread mechanism for their Ca2+ sensitivity.

Calcium-dependent subthreshold oscillations determine bursting activity induced by N-methyl-D-aspartate in rat subthalamic neurons in vitro

We used whole-cell patch recordings in current clamp to investigate the ionic dependence of burst firing induced by N-methyl-d-aspartate (NMDA) in neurons of the subthalamic nucleus (STN) in slices of rat brain. NMDA (20 microm) converted single-spike firing to burst firing in 87% of STN neurons tested. NMDA-induced bursting was blocked by AP5 (50 microm), and was not mimicked by the non-NMDA receptor agonist AMPA (0.6 microm). Tetrodotoxin (1 microm) converted bursts to oscillations of membrane potential, which were most robust when oscillations ranged between -50 and -70 mV. The NMDA bursts were blocked by an elevated extracellular concentration of Mg(2+), but superfusate containing no added Mg(2+) either reduced or increased burst firing, depending upon the amount of intracellular current injection. Block of K(+) conductances by apamin and tetraethylammonium prolonged burst duration, but iberiotoxin had no effect. NMDA-induced burst firing and membrane oscillations were completely blocked by superfusate containing no added Ca(2+), and they were significantly reduced when patch pipettes contained BAPTA. Selective antagonists for T-type (mibefradil, 10 microm), L-type (nifedipine, 3 microm), and N-type (omega-conotoxin GVIA, 1 micro m) Ca(2+) channels had no effect on NMDA burst firing. Superfusate containing a low concentration of Na(+) (20 mm) completely abolished NMDA-induced burst firing. Flufenamic acid (10 microm), which blocks current mediated by Ca(2+)-activated nonselective cation channels (I(CAN)), reversibly abolished NMDA-depended bursting. These results are consistent with the hypothesis that NMDA-induced burst firing in STN neurons requires activation of either an I(CAN) or a Na(+)-Ca(2+) exchanger.

Mefenamate, an agent that fails to attenuate experimental cerebral infarction

BACKGROUND

Blockade of nonselective cation channels is a potential therapeutic approach that has not been attempted in cerebral ischemia, in spite of the ability of these channels to allow cellular calcium influx into neurons. Fenamates are a class of molecules that block these channels, and many congeners are also anti-inflammatory and free radical scavenging. These three mechanisms may contribute to brain damage in ischemia.

METHODS

Pretreatment or posttreatment with mefenamate (30 mg/kg) was evaluated in a temperature-controlled rat transient focal ischemia model. Quantitative histopathology on 26 coronal sections allowed determination of tissue necrosis and tissue atrophy at one week survival.

RESULTS

Neither pre- nor postischemic administration of a dose previously shown effective in preventing epileptic neuronal necrosis was found to reduce necrosis in cortex, nor in any subcortical structures.

CONCLUSIONS

We conclude that nonselective cation channel blockade with mefenamate affords no neuroprotection in this model. Publication bias against negative studies exists in the literature, but we here report negative findings due to the multiple potentially positive actions of the drug. Closer examination of the effects of the molecule, however, reveals several potentially negative effects as well. We conclude there may be inherent weakness in pharmacologic monotherapy, even with molecules having protean potentially beneficial effects. This conclusion seems to have been borne out by the results of recent clinical trials.

Calcium-activated cationic channel in rat sensory neurons

Ion channels in sensory neurons are molecular sensors that detect external stimuli and transduce them to neuronal signals. Although Ca2+-activated nonselective cation (CAN) channels were found in many cell types, CAN channels in mammalian sensory neurons are not yet identified. In the present study, we describe an ion channel that is activated by intracellular Ca2+ in cultured rat sensory neurons. Half-maximal concentration of Ca2+ in activating the CAN channel was approximately 780 micro m. The current-voltage relationship of this channel was linear with a unit conductance of 28.8 +/- 0.4 pS at -60 mV in symmetrical 140 mm Na+ solution. The CAN channel was permeable to monovalent cations such as Na+, K+, Cs+, and Li+, but poorly permeable to Ca2+. The CAN channel in mammalian sensory neurons was reversibly blocked by intracellular adenine nucleotides, such as ATP, ADP, and AMP. Interestingly, single-channel currents activated by Ca2+ were blocked by fenamates, such as flufenamic acid, a class of nonsteroidal anti-inflammatory drugs. Thus, these results suggest that CAN channels in mammalian sensory neurons would participate in modulating nociceptive neural transmission in response to ever-changing intracellular Ca2+ in the local microenvironment.

Action potential afterdepolarization mediated by a Ca2+-activated cation conductance in myenteric AH neurons

We investigated the nature of afterdepolarizing potentials in AH neurons from the guinea-pig duodenum using whole-cell patch-clamp recordings in intact myenteric ganglia. Afterdepolarizing potentials were minimally activated following action-potential firing under normal conditions, but after application of charybdotoxin (40 nM) or tetraethyl ammonium (TEA; 10-20 mM) to the bathing solution, prominent afterdepolarizing potentials followed action potentials. The whole-cell current underlying afterdepolarizing potentials (I(ADP)) in the presence of TEA (10-20 mM) reversed at -38 mV and was not voltage-dependent. Reduction of NaCl in the bathing (Krebs) solution to 58 mM shifted the reversal potential of the I(ADP) to -58 mV, suggesting that the current underlying the afterdepolarizing potential was carried by a mixture of cations. The relative contributions of Na(+) and K(+) to this current were estimated to be about 1:5. Substitution of external Na(+) with N-methyl D-glucamine blocked the current while replacement of internal Cl(-) with gluconate did not block the I(ADP). The I(ADP) was also inhibited when CsCl-filled patch pipettes were used. The I(ADP) was blocked or substantially decreased in amplitude in the presence of N-type Ca(2+) channel antagonists, omega-conotoxin GVIA and omega-conotoxin MVIIC, respectively, and was eliminated by external Cd(2+), indicating that it was dependent on Ca(2+) entry. The I(ADP) was also inhibited by ryanodine (10-20 microM), indicating that Ca(2+)-induced Ca(2+) release was involved in its activation. Niflumic acid consistently inhibited the I(ADP) with an IC(50) of 63 microM. Using antibodies against the pore-forming subunits of L-, N- and P/Q-type voltage-gated Ca(2+) channels, we have demonstrated that myenteric AH neurons express N- and P/Q, but not L-type voltage-gated Ca(2+) channels. We conclude that the ADP in myenteric AH neurons, in the presence of an L-type Ca(2+)-channel blocker, is generated by the opening of Ca(2+)-activated non-selective cation channels following action potential-mediated Ca(2+) entry mainly through N-type Ca(2+) channels. Ca(2+) release from ryanodine-sensitive stores triggered by Ca(2+) entry contributes significantly to the activation of this current.

Contribution of a calcium-activated non-specific conductance to NMDA receptor-mediated synaptic potentials in granule cells of the frog olfactory bulb

We studied granule cells (GCs) in the intact frog olfactory bulb (OB) by combining whole-cell recordings and functional two-photon Ca(2+) imaging in an in vitro nose-brain preparation. GCs are local interneurones that shape OB output via distributed dendrodendritic inhibition of OB projection neurones, the mitral-tufted cells (MTCs). In contrast to MTCs, GCs exhibited a Ca(2+)-activated non-specific cation conductance (I(CAN)) that could be evoked through strong synaptic stimulation or suprathreshold current injection. Photolysis of the caged Ca(2+) chelator o-nitrophenol-EGTA resulted in activation of an inward current with a reversal potential within the range -20 to +10 mV. I(CAN) in GCs was suppressed by the intracellular Ca(2+) chelator BAPTA (0.5-5.0 mM), but not by EGTA (up to 5 mM). The current persisted in whole-cell recordings for up to 1.5 h post-breakthrough, was observed during perforated-patch recordings and was independent of ionotropic glutamate and GABA(A) receptor activity. In current-clamp mode, GC responses to synaptic stimulation consisted of an initial AMPA-mediated conductance followed by a late-phase APV-sensitive plateau (100-500 ms). BAPTA-mediated suppression of I(CAN) resulted in a selective reduction of the late component of the evoked synaptic potential, consistent with a positive feedback relationship between NMDA receptor (NMDAR) current and I(CAN). I(CAN) requires Ca(2+) influx either through voltage-gated Ca(2+) channels or possibly NMDARs, both of which have a high threshold for activation in GCs, predicting a functional role for this current in the selective enhancement of strong synaptic inputs to GCs.

Ca2+ store-dependent potentiation of Ca2+-activated non-selective cation channels in rat hippocampal neurones in vitro

1. Potentiation of calcium-activated non-selective cation (CAN) channels was studied in rat hippocampal neurones. CAN channels were activated by IP3-dependent Ca2+ release following metabotropic glutamate receptor (mGluR) stimulation either by Schaffer collateral input to CA1 neurones in brain slices in which ionotropic glutamate and GABAA receptors, K+ channels, and the Na+-Ca2+ exchanger were blocked or by application of the mGluR antagonist ACPD in cultured hippocampal neurones. 2. The CAN channel-dependent depolarization (DeltaVCAN) was potentiated when [Ca2+]i was increased in neurones impaled with Ca2+-containing microelectrodes. 3. Fura-2 measurements revealed a biphasic increase in [Ca2+]i when 200 microM ACPD was bath applied to cultured hippocampal neurones. This increase was greatly attenuated in the presence of Cd2+. 4. Thapsigargin (1 microM) caused marked potentiation of DeltaVCAN in CA1 neurones in the slices and of the CAN current (ICAN) measured in whole cell-clamped cultured hippocampal neurones. 5. Ryanodine (20 microM) also led to a potentiation of DeltaVCAN while neurones pretreated with 100 microM dantrolene failed to show potentiation of DeltaVCAN when impaled with Ca2+-containing microelectrodes. 6. The mitochondrial oxidative phosphorylation uncoupler carbonyl cyanide m-chlorophenyl hydrazone (2 microM) also caused a potentiation of DeltaVCAN. 7. CAN channels are subject to considerable potentiation following an increase in [Ca2+]i due to Ca2+ release from IP3-sensitive, Ca2+-sensitive, or mitochondrial Ca2+ stores. This ICAN potentiation may play a crucial role in the 'amplification' phase of excitotoxicity.

Muscarinic receptors regulate two different calcium-dependent non-selective cation currents in rat prefrontal cortex

Pyramidal neurons of layer V in rat prefrontal cortex display a prominent fast afterdepolarization (fADP) and a muscarinic-induced slow afterdepolarization (sADP). We have shown previously that both of these ADPs are produced by the activation of calcium-dependent non-selective cation currents. In the present report we examine whether they represent two distinct currents. In most pyramidal neurons recorded with caesium gluconate-based intracellular solution, a calcium spike is followed by a fast decaying inward aftercurrent (IfADP). The decay of IfADP is monoexponential with a time constant (t) of approximately 35 ms. Administration of carbachol (10-30 microm) increases the time constant of this decay by approximately 80% and induces the appearance of a much slower inward aftercurrent (IsADP). IfADP recorded in control conditions and in the presence of carbachol increases linearly with membrane hyperpolarization. In contrast, the carbachol-induced IsADP decreases with membrane hyperpolarization. When the sodium driving force across the cell membrane was reduced, IfADP was found to reverse at around -40 mV whereas IsADP remain inward over the same voltage range tested. Finally, bath administration of flufenamic acid (100 microm-1 mm) selectively blocks the carbachol-induced IsADP without a significant effect on the amplitude of IfADP. These differences in the electrical and pharmacological properties of IfADP and IsADP suggest that they were mediated by two distinct non-selective cation currents.

Ca2+ influx and activation of a cation current are coupled to intracellular Ca2+ release in peptidergic neurons of Aplysia californica

1. Stimulation of inputs to bag cell neurons in the abdominal ganglion of Aplysia californica causes an increase in their intracellular Ca2+ concentration ([Ca2+]i). We have used thapsigargin, a specific inhibitor of the endoplasmic reticulum Ca2+ pump, to analyse the effects of Ca2+ released from intracellular stores on the electrophysiological responses of bag cell neurons. 2. Using digital imaging of fura-2-loaded isolated bag cell neurons we found that thapsigargin rapidly evoked an increase in [Ca2+]i in somata, with smaller increases in neurites. Thapsigargin-induced elevation of [Ca2+]i peaked at about 1 microM within 5-10 min and then decayed to basal levels by 30 min. 3. Placement of an extracellular vibrating Ca(2+)-selective microelectrode to within 1 micron of somata revealed a relatively large steady-state Ca2+ efflux. Thapsigargin produced a rapid increase in Ca2+ influx. Changes in Ca2+ flux were not detected at neurites. 4. Thapsigargin produced a small depolarization in isolated bag cell neurons in artificial sea water (ASW). Sometimes enhanced depolarizations were observed when extracellular Na+ was replaced by TEA or Tris, but not N-methyl-D-glucamine (NMDG). The depolarization was not blocked by 100 microM tetrodotoxin (TTX), removal of extracellular Ca2+ (0.5 mM EGTA) or addition of 10 mM Co2+ to the bath solution. 5. In voltage-clamp experiments, thapsigargin induced an inward current (ITg) that was recorded in Ca(2+)-free media containing TEA or Tris substituted for Na+. The apparent reversal potential of ITg was -16.8 +/- 1.2 mV in TEA-ASW. Induction of ITg was inhibited in neurons that were microinjected with the Ca2+ chelator BAPTA-Dextran70 or treated with the membrane-permeant analogue BAPTA AM. Activation of ITg was not observed when Na+ was replaced with NMDG. Manipulation of [Na+]o and [K+]o produced shifts in the reversal potential of ITg consistent with the underlying channels being permeable to both Na+ and K+. 6. Thapsigargin did not alter the amplitude or kinetics of voltage-activated Ba2+ currents, but in some experiments it did increase the amplitude of a component of outward K+ current. 7. Thapsigargin neither induced bag cell neurons within the intact ganglion to depolarize and fire spontaneously, nor did it alter the frequency or duration of firing of an electrically stimulated bag cell after-discharge. 8. We conclude that thapsigargin-sensitive Ca2+ pools are present predominantly in the somata of bag cell neurons. Ca2+ that is released from thapsigargin-sensitive Ca2+ stores activates a non-selective cation current that may help sustain depolarization of the somata, but does not by itself trigger an after-discharge.

Identification and characterization of a Ca(2+)-sensitive nonspecific cation channel underlying prolonged repetitive firing in Aplysia neurons

The afterdischarge of Aplysia bag cell neurons has served as a model system for the study of phosphorylation-mediated changes in neuronal excitability. The nature of the depolarization generating the afterdischarge, however, has remained unclear. We now have found that venom from Conus textile triggers a similar prolonged discharge, and we have identified a slow inward current and corresponding channel, the activation of which seems to contribute to the onset of the discharge. The slow inward current is voltage-dependent and Ca(2+)-sensitive, reverses at potentials slightly positive to O mV, exhibits a selectivity of K approximately equal to Na >> Tris > N-methyl-D-glucamine (NMDG), and is blocked by high concentrations of tetrodotoxin. Comparison of these features with those observed in channel recordings provides evidence that a Ca(2+)-sensitive, nonspecific cation channel is responsible for a slow inward current that regulates spontaneous repetitive firing and suggests that modulation of the cation channel underlies prolonged changes in neuronal response properties.

Mechanism of action of the non-steroidal anti-inflammatory drug flufenamate on [Ca2+]i and Ca(2+)-activated currents in neurons

We have shown previously that the non-steroidal anti-inflammatory drug flufenamate (FFA) causes a maintained increase in [Ca2+]i and transient increases in a Ca(2+)-activated nonselective cation current (ICAN) and a Ca(2+)-activated slow, outward Cl- current (lo-slow) in molluscan neurons [Shaw T., Lee R.J., Partridge L.D. Action of diphenylamine carboxylate derivatives, a family of non-steroidal anti-inflammatory drugs, on [Ca2+]i and Ca(2+)-activated channels in neurons. Neurosci Lett 1995; 190:121-124]. Here we demonstrate that pretreatment of neurons with 10 microM thapsigargin eliminates the FFA-induced increase in [Ca2+]i and substantially reduces both ICAN and Io-slow supporting the hypothesis that the FFA-induced increase in [Ca2+]i results primarily from Ca2+ release from a thapsigargin-sensitive intracellular store. The [Ca2+]i response appears to be sustained, not by influx of extracellular Ca2+, but by inhibitory effects of FFA on Ca2+ removal from the cytosol. Inhibition of Ca2+ efflux may be an important component of the FFA-induced activation of both ICAN and Io-slow, as Ca2+ release by thapsigargin alone is not sufficient to activate either current. Our data also demonstrate that the effects of FFA on [Ca2+]i, ICAN and Io-slow are reversible and suggest that protein phosphorylation as well as an increase in [Ca2+]i are involved in the FFA-induced activation of Io-slow. Effects on neuronal Ca2+ handling as well as activation of ICAN or Io-slow may partially explain the analgesic effects of FFA.

Action of diphenylamine carboxylate derivatives, a family of non-steroidal anti-inflammatory drugs, on [Ca2+]i and Ca(2+)-activated channels in neurons

Ca(2+)-activated channels, including Ca(2+)-activated non-selective (CAN) channels and Ca(2+)-activated Cl- channels play important roles in regulating the electrical activity of neurons. No blockers of neuronal CAN channels have been previously reported. We used 2-electrode voltage clamping to measure membrane currents and fura-2 fluorescence imaging to measure [Ca2+]i in molluscan neurons. We show that the diphenylamine carboxylate derivative flufenamate (FFA), but not mefenamate or the parent compound, cause a transient increase in ICAN and a slow outward current, and a maintained increase in [Ca2+]i. We interpret this as a FFA-dependent release of Ca2+ from intracellular stores and Ca2+ influx, [Ca2+]i-dependent activation of the CAN and slow outward currents, and slow FFA-dependent channel block.

Calcium-activated non-selective channels in the nervous system

In the decade, since the first description of calcium-activated non-selective (CAN) channels in cardiac myocytes, pancreatic acini and neuroblastoma, this type of channel has been shown to have a ubiquitous distribution across a variety of tissues. Recently, their role in the function of cells of the nervous system has become better delineated. Because CAN channels pass depolarizing current, respond to cytoplasmic Ca2+ activity and do not inactivate, they are capable of producing maintained depolarization of neurons. This property endows upon CAN channels an important role in both physiological functions and pathological processes of the nervous system.

Cytoplasmic Ca2+ activity regulation as measured by a calcium-activated current

Calcium-activated non-selective cation (CAN) currents were activated by quantitative injections of Ca2+ into voltage clamped bursting neurons of the snails Helix aspersa or Helix pomatia. Membrane potential was held at the potassium equilibrium potential and CAN currents were fit with a rising and falling exponential function. Ca2+ transporters and pumps of the cell membrane, endoplasmic reticulum, and mitochondria were selectively blocked with pharmacological agents. Bath solutions containing 0 Na Ringers, chlorpromazine, Na3VO4, or thapsigargin did not significantly change the CAN current decay constants from those measured in Ringers. External 2,4-dinitrophenol or internal ruthenium red, however, significantly lengthened the CAN current decay constant. It is concluded that mitochondria are the most important sink for sub-membrane Ca2+ activity in the range necessary to effectively activate CAN currents.

A calcium-activated nonselective cationic channel in the basolateral membrane of outer hair cells of the guinea-pig cochlea

The patch-clamp technique was used to investigate ion channels in the basolateral perilymph-facing membrane of freshly isolated outer hair cells (OHCs) from the guinea-pig cochlea. These sensory cells probably determine, via their motile activity, the fine tuning of sound frequencies and the high sensitivity of the inner ear. A Ca(2+)-activated nonselective cationic channel was found in excised inside-out membrane patches. The current/voltage relationship was linear with a unit conductance of 26.3 +/- 0.3 pS (n = 15) under symmetrical inger conditions. The channel excluded anions (PNa/PCl = 18 where PNa/PCl denotes the relative permeability of Na to Cl); it was equally permeant to the Na+ and K+ ions and exhibited a low permeability to N-methyl-D-glucamine and Ba2+ or Ca2+. Channel opening required a free Ca2+ concentration of about 10(-6) mol/l on the internal side of the membrane and the open probability (Po) was maximal at 10(-3) mol/l (Po = 0.72 +/- 0.06, n = 12). Adenosine 5'mono-, tri- and di-phosphate reduced Po to 29 +/- 14 (n = 5), 42 +/- 10 (n = 8) and 51 +/- 12 (n = 5) % of control Po, respectively, when they were added at a concentration of 10(-3) mol/l to the internal side. The channel was partially blocked by flufenamic acid (10(-4) mol/l) and 3',5'-dichlorodiphenylamine-2-carboxylic acid (DCDPC, 10(-5) mol/l). This type of channel, together with Ca(2+)-activated K+ channels, might participate in the control of membrane potential and modulate the motility of OHCs.

Mode-switching of a voltage-gated cation channel is mediated by a protein kinase A-regulated tyrosine phosphatase

Tyrosine kinases and tyrosine phosphatases are abundant in central nervous system tissue, yet the role of these enzymes in the modulation of neuronal excitability is unknown. Patch-clamp studies of an Aplysia voltage-gated cation channel now demonstrate that a tyrosine phosphatase endogenous to excised patches determines both the gating mode of the channel and the response of the channel to protein kinase A. Moreover, a switch in gating modes similar to that triggered by the phosphatase occurs at the onset of a prolonged change in the excitability of Aplysia bag cell neurons.

Calcium buffering in bursting Helix pacemaker neurons

Bursting pacemaker neurons of the snail Helix pomatia were voltage-clamped and Ca currents in response to depolarizing steps were recorded. Simultaneously, changes in intracellular Ca concentrations were measured using the fluorescent dye fura-2 and a highly sensitive digital camera. Ca influx through voltage-gated channels induced a spatially non-uniform increase in intracellular Ca. The Ca signals decayed with a time constant of about 5 s. By increasing the concentration of the indicator dye, its Ca-buffering capacity was enhanced and Ca transients in response to depolarization were diminished. Thereby, the endogenous Ca buffer capacity could be determined and was calculated to be about 480 buffered ions for every free Ca ion. The buffer capacity did not vary significantly with the amount of Ca influx within the range tested, suggesting that the buffer is not saturated at Ca concentrations of up to 1 microM.

Activation and modulation of calcium-activated non-selective cation channels from embryonic chick sensory neurons

We have shown that calcium-activated non-selective (CAN) channels from embryonic chick sensory neurons are permeable to both Na+ and K+ and are not blocked by TTX, TEA, or 4-AP. These neuronal CAN channels are activated by sub-micromolar cytoplasmic Ca2+ with negative cooperativity. The effect of Ca2+ is to decrease the closed times of the channel with little effect on the time the channel remains open. Isolated neuronal CAN channels can be phosphorylated by cAMP-dependent protein kinase (PKA). The effect of phosphorylation is to shorten channel open time and to minimize the effect of Ca2+ on channel closed time.

[Role of voltage-dependent ion channels in epileptogenesis]

The aim of this review is to gather information in favour of the involvement of voltage-dependent ion channels in epileptogenesis. Although, up to now, no study has shown that epilepsy is accompanied by a modification in the activity to these channels, the recently acquired knowledge of their physiology allows to presume would favor their involvement in epileptogenesis. The results from electrophysiological studies are as follows: a persistent sodium current increases neuronal excitability whereas potassium currents have an inhibitory role. In particular, calcium-dependent potassium current are involved in the post-hyperpolarization phases which follows PDS. Calcium currents are also involved in the genesis of the "bursting pacemaker" activity displayed by the neurons presumed to be inducers of the epileptic activity. Biochemical data has shown that as a consequence of epileptic activity, sodium and calcium channels are down regulated. This down-regulation could be a way to reduces neuronal hyperexcitability. Pharmacological data demonstrate the drugs which activate calcium channels or which inhibit potassium channels have a convusilvant effect. On the contrary, agents which block calcium or sodium channels or which properties. Among the latter ones, some antiepileptic drugs can be found. In summary situations which lead to increase in calcium and sodium currents and/or to an inhibition in potassium currents are potentially epileptogenic.

Carbachol potentiates Q current and activates a calcium-dependent non-specific conductance in rat hippocampus in vitro

Intracellular recordings were made from CA1 neurons in rat hippocampal slices maintained in vitro. When Na+ currents were blocked with tetrodotoxin and K+ conductances known to be sensitive to suppression by muscarinic agonists were blocked by 2 mM Ba2+, CA1 cells were depolarized by carbachol (3-10 microM) with an attendant conductance increase, whereas prior to Ba2+ the agonist produced a decrease or no change in conductance. Under voltage clamp at approximately -60 mV and in the presence of tetrodotoxin and Ba2+, carbachol (3-10 microM) induced a variable-latency biphasic inward current of up to 380 pA associated with a conductance increase of approximately 50%. The first phase was associated with an increase (more than 2-fold) of the Cs(+)-sensitive, hyperpolarization-activated cationic current, IQ. Carbachol also accelerated the kinetics of IQ at -100 mV with an average 24% reduction in its activation time constant. The second phase reflected an additional inward current that was Cs(+)-resistant, displayed little apparent voltage sensitivity and had a mean extrapolated reversal potential, determined in the presence of external Cs+ (< or = 5 mM), of approximately -20 mV. In a small proportion of cells the second phase of inward current was followed (or overlapped) by an outward current, also associated with a conductance increase, which reversed at approximately -70 mV. These carbachol actions were prevented by extracellular 300 microM Cd2+ and 2 mM Mn2+, by high levels (> 5 mM) of extracellular Mg2+ or Ca2+, and by omission of Ca2+ or reduction of extracellular Na+ to 25 mM by substitution of NaCl with Tris or N-methyl-D-glucamine. Carbachol action was not mimicked by oxotremorine (< or = 60 microM), but was irreversibly blocked by this drug. Likewise, atropine (100 nM) irreversibly and gallamine (10 microM) reversibly antagonized carbachol's action. The action of carbachol was blocked shortly after prior exposure of slices to 2-5 mM caffeine. Chronic or acute incubation of slices with 2 mM Li+ potentiated (between 1- and 2-fold) carbachol responses. The data indicate that muscarinic activation increases cationic flux by a calcium-dependent potentiation of IQ and activation of a non-selective conductance. The probability that inositol phospholipid metabolism is involved in triggering these events is discussed.

Nonselective cation channels in exocrine gland cells

The nonselective cation channel has been described in a wide variety of nonexcitable cells. However even in such closely related tissues as the pancreatic acinar cell and the lacrimal acinar cell, which both possess a superficially similar channel, recent work has shown fundamental differences in channel regulation (Sasaki and Gallacher, 1992; Thorn and Petersen, 1992). These differences are a reflection of a diverse function of the nonselective channel in different tissues.

Calcium-activated currents in cultured neurones from rat dorsal root ganglia

1. Voltage-activated Ca2+ currents and caffeine (1 to 10 mM) were used to increase intracellular Ca2+ in rat cultured dorsal root ganglia (DRG) neurones. Elevation of intracellular Ca2+ resulted in activation of inward currents which were attenuated by increasing the Ca2+ buffering capacity of cells by raising the concentration of EGTA in the patch solution to 10 mM. Low and high voltage-activated Ca2+ currents gave rise to Cl- tail currents in cells loaded with CsCl patch solution. Outward Ca2+ channel currents activated at very depolarized potentials (Vc + 60 mV to + 100 mV) also activated Cl- tail currents, which were enhanced when extracellular Ca2+ was elevated from 2 mM to 4 mM. 2. The Ca(2+)-activated Cl- tail currents were identified by estimation of tail current reversal potential by use of a double pulse protocol and by sensitivity to the Cl- channel blocker 5-nitro 2-(3-phenyl-propylamino) benzoic acid (NPPB) applied at a concentration of 10 microM. 3. Cells loaded with Cs acetate patch solution and bathed in medium containing 4 mM Ca2+ also had prolonged Ca(2+)-dependent tail currents, however these smaller tail currents were insensitive to NPPB. Release of Ca2+ from intracellular stores by caffeine gave rise to sustained inward currents in cells loaded with Cs acetate. Both Ca(2+)-activated tail currents and caffeine-induced inward currents recorded from cells loaded with Cs acetate were attenuated by Tris based recording media, and had reversal potentials positive to 0 mV suggesting activity of Ca(2+)-activated cation channels.4. Our data may reflect (a) different degrees of association between Ca2+-activated channels with voltage-gated Ca2+ channels, (b) distinct relationships between channels and intracellular Ca2" stores and Ca2+ homeostatic mechanisms, (c) regulation of Ca2+-activated channels by second messengers, and (d) varying channel sensitivity to Ca2 , in the cell body of DRG neurones.

Effects of acid Ca2+ Ringer on passive electrical properties and intracellular ion activities in leech Retzius neuron

1. A significant drop in effective input resistance of the free membrane and an increase in effective coupling resistance in acid Ca2+ Ringer (complete replacement of Na+ with Ca2+, pH 4) compared to control medium has been obtained in leech Retzius neurons. 2. In neutral Ca2+ Ringer (pH 7.2), effective input resistance increased while effective coupling resistance did not change. In acid sodium, leech Ringer (pH 4) effective input resistance increased while coupling resistance decreased. 3. Ten millimolar manganese and 10 mmol tetraethylammonium did not block conductance changes obtained in acid Ca2+ Ringer. 4. Intracellular activity of Na+ decreased, cellular activity of Cl- increased and intracellular K+ activity was unchanged in both acid and neutral Ca2+ Ringer. 5. The main difference was intracellular acidification in acid Ca2+ Ringer while intracellular pH was unchanged in neutral Ca2+ Ringer. 6. We discuss the possibility that in acid Ca2+ Ringer, intracellular acidification in leech neurons may be responsible for accompanying conductive changes.

Characterization of ion channels on the surface membrane of adult rat skeletal muscle

The channels present on the surface membrane of isolated rat flexor digitorum brevis muscle fibers were surveyed using the patch clamp technique. 85 out of 139 fibers had a novel channel which excluded the anions chloride, sulfate, and isethionate with a permeability ratio of chloride to sodium of less than 0.05. The selectivity sequence for cations was Na+ = K+ = Cs+ greater than Ca++ = Mg++ greater than N-Methyl-D-Glucamine. The channel remained closed for long periods, and had a large conductance of approximately 320 pS with several subconductance states at approximately 34 pS levels. Channel activity was not voltage dependent and the reversal potential for cations in muscle fibers of approximately 0 mV results in the channel's behaving as a physiological leakage conductance. Voltage activated potassium channels were present in 65 of the cell attached patches and had conductances of mostly 6, 12, and 25 pS. The voltage sensitivity of the potassium channels was consistent with that of the delayed rectifier current. Only three patches contained chloride channels. The scarcity of chloride channels despite the known high chloride conductance of skeletal muscle suggests that most of the chloride channels must be located in the transverse tubular system.

Regulation of Cl- channels by multifunctional CaM kinase

cAMP kinase has been shown to mediate the cAMP pathway for regulation of Cl- channels in lymphocytes, but the mediator of an alternative, Ca2+ pathway has not been identified. We show here that Ca2+ ionophore activates Cl- currents in cell-attached and whole-cell patch-clamp recordings of Jurkat T lymphocytes, but this activation is not direct. The effect of Ca2+ ionophore on whole-cell Cl- currents is inhibited by a specific peptide inhibitor of multifunctional Ca2+/calmodulin-dependent protein kinase (CaM kinase). Furthermore, Cl- channels are activated in excised patches by purified CaM kinase in a fashion that mimics the effect of Ca2+ ionophore in cell-attached recordings. These results suggest that CaM kinase mediates the Ca2+ pathway of Cl- channel activation.

Cobalt-dependent potentiation of net inward current density in Helix aspersa neurons

A low concentration of transition metal ions Co2+ and Ni2+ increases the inward current density in neurons from the land snail Helix aspersa. The currents were measured using a single electrode voltage-clamp/internal perfusion method under conditions in which the external Na+ was replaced by Tris+, the predominant external current carrying cation was Ca2+, and the internal perfusate contained 120 mM Cs+/0 K+; 30 mM tetraethylammonium (TEA) was added externally to block K+ current. In the presence of Co2+ (3 mM) or Ni2+ (0.5 mM) inward Ca2+ currents were stimulated normally by voltage-dependent activation of Ca2+ channels. There was a 5-10% decrease in the rate of rise of the inward current. The principal effect of Co2+ and Ni2+ in increasing the current density seems to be a decrease in the rate at which the inward currents decline during a depolarizing voltage pulse. The results may be due to a decrease in a voltage-dependent or Ca(2+)-dependent outward current and/or an inhibition of Ca2+ channel inactivation. Outward current under these conditions (zero internal K+) was significant and most likely due to Cs+ efflux through the voltage-activated or Ca(2+)-activated nonspecific cation channels. Co2+ is an extremely effective blocker of this outward current. These results are not an artifact of internal perfusion or the special ionic conditions. Intracellular recording of unperfused neurons in normal Helix Ringer's solution showed that the Ca(2+)-dependent action potential duration was increased significantly by low concentrations of Co2+. This result is consistant with the Co(2+)-dependent increase in inward (depolarizing) current seen in voltage-clamp experiments.

Non-selective cation channel on pancreatic duct cells

We have identified a non-selective cation channel on pancreatic duct cells. These epithelial cells secrete the bicarbonate ions found in pancreatic juice; a process controlled by the hormone secretin, which uses cyclic AMP as an intracellular messenger. The non-selective channel is located on both apical and basolateral plasma membranes of the duct cell, is equally permeable to sodium and potassium, and has a linear I/V relationship with a single-channel conductance of about 25 pS. Channel opening requires the presence of 1 microM Ca2+ on the cytoplasmic face of the membrane, and is also increased by depolarization. Intracellular ATP, ADP, magnesium, and a rise in pH all decreased channel activity. The channel was not affected by 10 mM TEA, 1 mM Ba2+ or 0.5 mM decamethonium applied to the cytoplasmic face of the membrane, but 0.5 mM quinine caused a flickering block which was more pronounced at depolarizing potentials. We observed the channel only rarely in cell-attached patches on unstimulated duct cells, and acute exposure to stimulants did not cause channel activation. However, after prolonged stimulation, the proportion of cell-attached patches containing active channels was increased 9-fold. The role of this channel in pancreatic duct cell function remains to be elucidated.

Modulation of calcium-activated non-specific cation currents by cyclic AMP-dependent phosphorylation in neurones of Helix

1. Currents through calcium-activated non-specific cation (CAN) channels were studied in the fast burster neurone of Helix aspersa and Helix pomatia. CAN currents were activated by reproducible intracellular injections of small quantities of Ca2+ utilizing a fast, quantitative pressure injection technique. 2. External application of forskolin (10-25 microM), an activator of adenylate cyclase, caused the endogenous bursting activity of the cells to be replaced by beating activity. These same concentrations of forskolin reduced CAN currents reversibly to about 50%. 3. External application of IBMX (3-isobutyl-1-methylxanthine, 100 microM), an inhibitor of phosphodiesterase, the enzyme which breaks down cyclic AMP, reduced CAN currents reversibly to about 40%. 4. External application of the membrane-permeable cyclic AMP analogues 8-bromo-cyclic AMP and dibutyryl-cyclic AMP (100 microM) caused almost complete block of the CAN current. A marked reduction in the CAN current was also observed following quantitative injections of cyclic AMP (internal concentrations up to 50 microM) directly into the cells from a second pressure injection pipette. 5. Similar results were obtained with quantitative injections of the catalytic subunit (C-subunit) of the cyclic AMP-dependent protein kinase (internal concentrations 10(-4) units of enzyme) directly into the cells from a second pressure injection pipette. 6. Injection of the non-hydrolysable GTP analogue, GTP-gamma-S (internal concentrations 100 microM), which stimulates G-proteins, produced a prolonged increase in CAN current amplitude by as much as 300%. 7. External application of serotonin (100-200 microM) caused a transition from bursting to beating activity of the neurones and mimicked cyclic AMP's effects on CAN currents. Two other neurotransmitters, dopamine and acetylcholine, were not significantly effective in reducing CAN currents. 8. Injection of a peptide inhibitor of cyclic AMP-dependent protein kinase suppressed serotonin's action on bursting and on CAN current. 9. Our results indicate that CAN currents in Helix burster neurones are modulated by cyclic AMP-dependent membrane phosphorylation. They suggest that the physiological transmitter that induces this second messenger action is serotonin. The dual control of CAN channels by two second messengers, namely Ca2+ and cyclic AMP, has important functional implications. While Ca2+ activates these channels which generate the pacemaker current in these neurones, cyclic AMP-dependent phosphorylation down-regulates them, thereby resulting in modulation of neuronal bursting activity.

Characterization of a 25-pS nonselective cation channel in a cultured secretory epithelial cell line

We have studied a 25-pS nonselective cation channel from the apical membranes of cell line ST885, derived from neonatal mouse mandibular glands. Its Cl- permeability was not significantly different from zero. The permeabilities (relative to Na+) for inorganic cations were NH4+ (1.87) greater than K+ (1.12) greater than Li+ (1.02) greater than Na+ (1) greater than Rb+ (0.81) greater than Mg2+ (0.07) greater than Ca2+ (0.002), and for organic cations, guanidinium (1.61) greater than ethanolamine (0.70) greater than 4-aminopyridine (0.66) greater than diethylamine (0.54) greater than piperazine (0.25) greater than Tris (0.18) greater than N-methylglucamine (0.12). The Tris and N-methylglucamine permeabilities differed significantly from zero. Fitting the Renkin equation indicated that the channel had an equivalent pore radius of 0.49 nm. The channel was activated by Ca2+ on the cytosolic surface (greater than 0.1 mmol/liter) with a Hill coefficient of 1.2; it was also activated by depolarization. Open- and closed-time histograms indicated that it had at least two open and two closed states. The channel was blocked by cytosolic AMP or ATP (0.1 mmol/liter). It was also blocked by the Cl- channel blocker, diphenylamine-2-carboxylate (DPC; 0.1 mmol/liter), applied to the extracellular but not the cytosolic surface. 4-Amino-pyridine, which permeated the channel when applied to the extracellular surface, blocked it when applied in low concentrations (5 mmol/liter) to the cytosolic surface. Quinine (0.1 mmol/liter) blocked from both the extracellular and cytosolic surfaces, blockade from either side being enhanced by depolarization. The channel was held open by application of SITS (0.1 mmol/liter) to the cytosolic surface. The channel shows striking similarities to the nicotinic acetylcholine receptor channel, viz., both channel types are abnormally permeable to 4-aminopyridine applied externally, and their selectivity sequences for inorganic ions are similar and for organic cations are identical.

Ca2(+)-activated K+ current involvement in neuronal function revealed by in situ single-channel analysis in Helix neurones

1. The properties of single calcium-activated potassium channels (or C-channels) were studied in cell-attached patches using the patch-clamp technique. Experiments were performed on identified Ca2(+)-dependent U cells in juvenile specimens (1-2 months old) of Helix aspersa. 2. The criteria used to identify C-channels were based on comparison between macroscopic C-currents and currents reconstructed from unitary recordings. Both currents had a slow activation rate at large positive potentials which turned into fast activation after large Ca2+ entries. Both currents were blocked by intracellularly injected EGTA. 3. The unitary conductance in normal (5 mM) or reduced (0.5 mM) [K+]o ranged from 24 to 65 pS (mean +/- S.D., 48 +/- 13; n = 64). With 85-110 mM [K+]o, which is approximately equal to the internal [K+], the conductance was 64 pS and the reversal potential was approximately 0 mV. 4. C-channels in U cells were distributed in clusters of three to ten channels (mean 5.05 channels in seventy-five patches). Calcium channels were present in patches containing clustered C-channels. C-channels within clusters behaved independently. 5. With patch electrode containing 8 mM-calcium, C-channels opened transiently upon patch depolarization. Reopenings in quiescent depolarized patches were induced by whole-cell spikes triggered by current pulses applied to an intracellular electrode. Apparent inactivation of C-channels in depolarized patches was in fact due to a decrease in [Ca2+]i resulting from inactivation of Ca2+ channels. 6. Calcium-free saline solutions in the patch electrodes prevented C-channels from opening upon patch depolarization. Entry of calcium through the surrounding membrane induced delayed openings in the patch. Peak opening probability Po occurred 330 +/- 30 ms after a brief Ca2+ entry with a lag period of 50-80 ms. With patch electrodes filled with Ca2(+)-containing saline solutions and under conditions which maximized C-channel opening, peak Po was reached in 20-50 ms. The same value was observed for the whole-cell C-current. 7. The peak Po at a given patch potential and in response to a whole-cell spike was not altered by a previous long-lasting patch depolarization, or by producing several successive Ca2+ entries. Thus, C-channels did not appear to be inactivated by depolarization or increase in [Ca2+]i. 8. C-channels were found to be relatively highly voltage dependent, with an e-fold increase in Po per 14.9 mV increase in potential.(ABSTRACT TRUNCATED AT 400 WORDS)

Permeability of the non-selective channel in brown adipocytes to small cations

The non-selective channel for monovalent cations of cultured brown adipocytes was studied concerning its permeability to alkali metal ions, NH4+, Tris+, Ca2+, and Ba2+. Experiments were done by means of the patch clamp technique using inside-out patches. With symmetrically increasing sodium concentrations the ion fluxes saturated. They are described by a dissociation constant (KNa) of 155 mmol/l and a maximum single channel conductance of 50 pS. Permeabilities were determined in relation to those for sodium yielding values of 0.80 for potassium and 1.55 for ammonium. The complete permeability sequence for ammonium and the alkali metals is: NH4+ greater than Na+ greater than Li+ greater than K+ greater than or equal to Rb+ congruent to Cs+ . Ca2+ and Ba2+ as well as the buffer ion Tris+ are not able to pass the channel measurably. It is shown that the conductance behaviour of the non-selective channel is not sufficiently described by the Goldman-Hodgkin-Katz theory. Deviations from independence are saturation with increased activity of the permeant ion and non-linear current voltage relations in symmetrical solutions. A simple two barrier model with one binding site in the center of the electric field is shown to be more appropriate.

Calcium-activated chloride current in rat vascular smooth muscle cells in short-term primary culture

Isolated cells from rat portal vein smooth muscle in short-term primary culture were studied using patch-clamp technique (whole-cell configuration). In order to study a calcium-activated chloride current, the potassium currents were blocked by intracellular cesium diffusion. Without EGTA in the pipette solution, depolarizing voltage pulses from a holding potential of -70 mV to positive potentials activated an early inward and a late outward current. The latter persisted as a long-lasting inward tail current when the membrane was repolarized to -70 mV. The outward current measured at the end of the pulse and the tail current were blocked by extracellular cobalt, after replacement of external calcium with barium, after removal of external calcium, and when the calcium concentration of the pipette solution was less than 0.5 microM, suggesting that they were calcium-dependent. The tail current decay was voltage sensitive, becoming faster with hyperpolarization. The reversal potential of the calcium-activated current was near the equilibrium potential for chloride ions, and was shifted as predicted by the Nernst equation when the extracellular or intracellular chloride concentration was changed. The calcium-activated current was blocked by adding micromolar concentrations of niflumic acid or millimolar concentrations of DIDS. This effect of compounds known to interfere with chloride channels together with the data on the equilibrium potential for chloride ions indicated above suggested the existence of a calcium-activated chloride current in vascular smooth muscle cells.

Voltage-independent barium-permeable channel activated in Lymnaea neurons by internal perfusion or patch excision

Isolated nerve cells from Lymnaea stagnalis were studied using the internal-perfusion and patch-clamp techniques. Patch excision frequently activated a voltage-independent Ba2+-permeable channel with a slope conductance of 27 pS at negative potentials (50 mM Ba2+). This channel is not seen in patches on healthy cells and, unlike the voltage-dependent Ca channel, is not labile in isolated patches. The activity of the channel in inside-out patches is unaffected by intracellular ATP, Ca2+ below 1 mM or the catalytic subunit of cAMP-dependent protein kinase but is reversibly blocked by millimolar intracellular Ca2+ or Ba2+. The channel can be activated in on-cell patches by either internal perfusion with high Ca2+ or the long-term internal perfusion of low Ca2+ solutions not containing ATP. These channels may carry the inward Ca2+ current which causes a regenerative increase in intracellular Ca+ when snail neurons are perfused with high Ca2+ solutions. High internal Ca2+, or long periods of internal perfusion with ATP-free solutions, induces an increase in a resting (-50 mV) whole-cell Ba2+ conductance. This conductance can be turned off by returning the intracellular perfusate to a low Ca2+ solution containing ATP and Mg2+. The activity of this channel appears to have an opposite dependence on intracellular conditions to that of the voltage-dependent Ca channel.

The nonselective cation channel in the basolateral membrane of rat exocrine pancreas. Inhibition by 3',5-dichlorodiphenylamine-2-carboxylic acid (DCDPC) and activation by stilbene disulfonates

Nonselective Ca2+-sensitive cation channels in the basolateral membrane of isolated cells of the rat exocrine pancreas were investigated with the patch clamp technique. With 1.3 mmol/l Ca2+ on the cytosolic side, the mean open-state probability Po of one channel was about 0.5. In inside-out oriented cell-excised membrane patches the substances diphenylamine-2-carboxylic acid (DPC), 5-nitro-2-(3-phenelpropylamino)-benzoic acid (NPPB) and 3',5-dichlorodiphenylamine-2-carboxylic acid (DCDPC) were applied to the cytosolic side. These compounds inhibited the nonselective cation channels by increasing the mean channel closed time (slow block). 100 mumol/l of NPPB or DPC decreased Po from 0.5 (control conditions) to 0.2 and 0.04, respectively, whereas 100 mumol/l of DCDPC blocked the channel completely. All effects were reversible. 1 mmol/l quinine also reduced Po, but in contrast to the above mentioned substances, it induced fast flickering. Ba2+ (70 mmol/l) and tetraethylammonium (TEA+; 20 mmol/l) had no effects. We investigated also the stilbene disulfonates 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid (SITS), 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) and 4,4'-dinitro-2,2'-stilbenedisulfonate (DNDS). 10 mumol/l SITS applied to the cytosolic side increased Po from 0.5 to 0.7 and with 100 mumol/l SITS the channels remained nearly permanently in its open state (Po approximately equal to 1). A similar activation of the channels was also observed with DIDS and DNDS. These effects were poorly reversible. The stilbene disulfonates acted by increasing the channel mean open time. When the channel was inactivated by decreasing bath Ca2+ concentration to 0.1 mumol/l, addition of 100 mumol/l of SITS had no effect. Similarly, reducing bath Ca2+ concentration from 1.3 mmol/l in presence of 100 mumol/l SITS (channels are maximally activated) to 0.1 mumol/l, inactivated the channels completely. These results demonstrate, that SITS can only activate the channels in the presence of Ca2+. SITS had no effects, when applied to the extracellular side in out-side out patches.(ABSTRACT TRUNCATED AT 400 WORDS)

Direct ion channel gating: a new function for intracellular messengers

1. There is widespread belief that intracellular messengers [e.g., Ca2+, cyclic AMP, cyclic GMP, inositol-1,4,5-triphosphate (IP3)] assert their actions primarily through activation of protein kinases. 2. In studies of excitable cells protein kinase activation has been shown to alter membrane ionic conductance, presumably through phosphorylation of ion channels (see Levitan, 1985). However, recent reports from several laboratories indicate that intracellular messengers can also affect membrane ionic conductances directly without invoking protein kinase activation. 3. In this article we examine those examples of direct activation of ionic conductances by intracellular messengers which are supported by single-channel studies of isolated membrane patches. The list of cell types displaying this kind of response is growing and includes cells of neuronal as well as nonneuronal origin.