Subcellular Distribution of Persistent Sodium Conductance in Cortical Pyramidal Neurons

Cortical pyramidal neurons possess a persistent Na+ current (I NaP), which, in contrast to the larger transient current, does not undergo rapid inactivation. Although relatively quite small, I NaP is active at subthreshold voltages and therefore plays an important role in neuronal input-output processing. The subcellular distribution of channels responsible for I NaP and the mechanisms that render them persistent are not known. Using high-speed fluorescence Na+ imaging and whole-cell recordings in brain slices obtained from mice of either sex, we reconstructed the I NaP elicited by slow voltage ramps in soma and processes of cortical pyramidal neurons. We found that in all neuronal compartments, the relationship between persistent Na+ conductance and membrane voltage has the shape of a Boltzmann function. Although the density of channels underlying I NaP was about twofold lower in the axon initial segment (AIS) than in the soma, the axonal channels were activated by ∼10 mV less depolarization than were somatic channels. This difference in voltage dependence explains why, at functionally critical subthreshold voltages, most I NaP originates in the AIS. Finally, we show that endogenous polyamines constrain I NaP availability in both somatodendritic and axonal compartments of nondialyzed cortical neurons.SIGNIFICANCE STATEMENT The most salient characteristic of neuronal sodium channels is fast inactivation. However, a fraction of the sodium current does not inactivate. In cortical neurons, persistent current (I NaP) plays a prominent role in many important functions. Its subcellular distribution and generation mechanisms are, however, elusive. Using high-speed fluorescence Na+ imaging and electrical recordings, we reconstructed the I NaP in soma and processes of cortical pyramidal neurons. We found that at near-threshold voltages I NaP originates predominately from the axon, because of the distinctive voltage dependence of the underlying channels and not because of their high density. Finally, we show that the presence of endogenous polyamines significantly constrains I NaP availability in all compartments of nondialyzed cortical neurons.

Functional implications of axon initial segment cytoskeletal disruption in stroke

Axon initial segment (AIS) is the proximal part of the axon, which is not covered with a myelin sheath and possesses a distinctive, specialized assembly of voltage-gated ion channels and associated proteins. AIS plays critical roles in synaptic integration and action potential generation in central neurons. Recent evidence shows that stroke causes rapid, irreversible calpain-mediated proteolysis of the AIS cytoskeleton of neurons surrounding the ischemic necrotic core. A better understanding of the molecular mechanisms underlying this "non-lethal" neuronal damage might provide new therapeutic strategies for improving stroke outcome. Here, we present a brief overview of the structure and function of the AIS. We then discuss possible mechanisms underlying stroke-induced AIS damage, including the roles of calpains and possible sources of Ca(2+) ions, which are necessary for the activation of calpains. Finally, we discuss the potential functional implications of the loss of the AIS cytoskeleton and ion channel clusters for neuronal excitability.

The Outwardly Rectifying Current of Layer 5 Neocortical Neurons that was Originally Identified as "Non-Specific Cationic" Is Essentially a Potassium Current

In whole-cell patch clamp recordings from layer 5 neocortical neurons, blockade of voltage gated sodium and calcium channels leaves a cesium current that is outward rectifying. This current was originally identified as a “non-specific cationic current”, and subsequently it was hypothesized that it is mediated by TRP channels. In order to test this hypothesis, we used fluorescence imaging of intracellular sodium and calcium indicators, and found no evidence to suggest that it is associated with influx of either of these ions to the cell body or dendrites. Moreover, the current is still prominent in neurons from TRPC1^-/- and TRPC5^-/- mice. The effects on the current of various blocking agents, and especially its sensitivity to intracellular tetraethylammonium, suggest that it is not a non-specific cationic current, but rather that it is generated by cesium-permeable delayed rectifier potassium channels.

Long-term GnRH-induced gonadotropin secretion in a novel hypothalamo-pituitary slice culture from tilapia brain

Organotypic cultures, prepared from hypothalamo-pituitary slices of tilapia, were developed to enable long-term study of secretory cells in the pituitary of a teleost. Values of membrane potential at rest were similar to those recorded from acute slices, and cells presented similar spontaneous spikes and spikelets. Some cells also exhibited slow spontaneous oscillations in membrane potential, which may be network-driven. Long-term (6days) continuous exposure to GnRH induced increases in LH and FSH secretion. FSH levels reached the highest levels after 24h of exposure to GnRH, and the highest secretion of LH was observed in days 4 and 5 of the experiment. Since slices were viable for several weeks in culture, maintaining the original cytoarchitecture, electrical membrane properties and the ability to secrete hormones in response to exogenous GnRH, this technique is ideal for studying the mechanisms regulating cell-to-cell communication under conditions resembling the in vivo tissue organization.

Na+ imaging reveals little difference in action potential-evoked Na+ influx between axon and soma

In cortical pyramidal neurons, the axon initial segment (AIS) is pivotal in synaptic integration. It has been asserted that this is because there is a high density of Na(+) channels in the AIS. However, we found that action potential-associated Na(+) flux, as measured by high-speed fluorescence Na(+) imaging, was about threefold larger in the rat AIS than in the soma. Spike-evoked Na(+) flux in the AIS and the first node of Ranvier was similar and was eightfold lower in basal dendrites. At near-threshold voltages, persistent Na(+) conductance was almost entirely axonal. On a time scale of seconds, passive diffusion, and not pumping, was responsible for maintaining transmembrane Na(+) gradients in thin axons during high-frequency action potential firing. In computer simulations, these data were consistent with the known features of action potential generation in these neurons.

Endogenous polyamines regulate cortical neuronal excitability by blocking voltage-gated Na+ channels

Because the excitable properties of neurons in the neocortex depend on the characteristics of voltage-gated Na(+) channels, factors which regulate those characteristics can fundamentally modify the dynamics of cortical circuits. Here, we report on a novel neuromodulatory mechanism that links the availability of Na(+) channels to metabolism of polyamines (PAs) in the cerebral cortex. Using single channel and whole-cell recordings, we found that products of PA metabolism, the ubiquitous aliphatic polycations spermine and spermidine, are endogenous blockers of Na(+) channels in layer 5 pyramidal cells. Because the blockade is activity-dependent, it is particularly effective against Na(+) channels which fail to inactivate rapidly and thus underlie the persistent Na(+) current. At the level of the local cortical circuit, pharmacological depletion of PAs led to increased spontaneous spiking and periods of hypersynchronous discharge. Our data suggest that changes in PA levels, whether associated with normal brain states or pathological conditions, profoundly modify Na(+) channel availability and thereby shape the integrative behavior of single neurons and neocortical circuits.

Bolus injection of acetylcholine terminates atrial fibrillation in rats

It is well established that a tonic increase in the availability of the atrial muscarinic K(+) channels, either by enhanced vagal tone or by steady infusion of a low-dose of cholinergic or adenosine receptor agonists, promotes the genesis of atrial fibrillation. Here, we aimed to test the hypothesis that bolus administration of a muscarinic receptor agonist would destabilize and terminate atrial arrhythmia by uniformly and transiently activating K(+) channels throughout the atria, and that if the agonist was rapidly hydrolysable, it would dissipate before the more tonic, pro-arrhythmic effects could take hold. The episodes of untreated atrial fibrillation, induced in anesthetized rats by programmed electrical stimulation via trans-esophageal bipolar catheter, lasted on average 8.6+/-2.2 min (n=32). Intravenous injection of a model hydrolysable muscarinic agonist, acetylcholine (0.2 mg/kg body weight), converted atrial fibrillation into sinus rhythm within 8.4+/-1.9 s (n=10, P<0.05). The termination of an atrial fibrillation episode was always accompanied by transient bradycardia; the sinus rhythm gradually accelerated and reached pre-atrial fibrillation values within 10-20 s of injection. In conclusion, our evidence indicates that bolus administration of rapidly hydrolysable muscarinic agonist could be an effective way to pharmacologically terminate atrial fibrillation and restore sinus rhythm.

Persistent sodium current in layer 5 neocortical neurons is primarily generated in the proximal axon

In addition to the well described fast-inactivating component of the Na+ current [transient Na+ current (INaT)], neocortical neurons also exhibit a low-voltage-activated, slowly inactivating "persistent" Na+ current (INaP), which plays a role in determining neuronal excitability and synaptic integration. We investigated the Na+ channels responsible for INaP in layer 5 pyramidal cells using cell-attached and whole-cell recordings in neocortical slices. In simultaneous cell-attached and whole-cell somatic recordings, no persistent Na+ channel activity was detected at potentials at which whole-cell INaP operates. Detailed kinetic analysis of late Na+ channel activity in cell-attached patches at 36 degrees C revealed that somatic Na+ channels do not demonstrate "modal gating" behavior and that the probability of single late openings is extremely low (<1.4 x 10(-4) or <0.02% of maximal open probability of INaT). Ensemble averages of these currents did not reveal a sustained component whose amplitude and voltage dependence could account for INaP as seen in whole-cell recordings. Local application of TTX to the axon blocked somatically recorded INaP, whereas somatic and dendritic application had little or no effect. Finally, simultaneous current-clamp recordings from soma and apical dendrite revealed that Na+ plateau potentials originate closer to the axon. Our data indicate that the primary source of INaP is in the spike initiation zone in the proximal axon. The focal axonal presence of regenerative subthreshold conductance with voltage and time dependence optimal to manipulate integration of synaptic input, spike threshold, and the pattern of repetitive firing provides the layer 5 pyramidal neuron with a mechanism for dynamic control of its gain.

NMDA receptors in layer 4 spiny stellate cells of the mouse barrel cortex contain the NR2C subunit

In layer 4 of the somatosensory cortex, the glutamatergic synapses that interconnect spiny stellate (SpS) neurons, which are the major targets of thalamocortical input, differ from most other neocortical excitatory synapses in that they have an extremely large NMDA receptor (NMDAR)-mediated component that is relatively insensitive to voltage-dependent Mg2+ blockade. We now report that this unique feature of the NMDA response reflects the distinctive subunit composition of the underlying receptors. We studied NMDAR-mediated miniature EPSCs (mEPSCs) and NMDA channel currents in tangential brain slices of mouse barrel cortex, which exclusively contain layer 4. NMDAR-mediated mEPSCs in SpS neurons were prominent at negative membrane potentials, and NMDA channels in outside-out patches excised from the somata of the same neurons had relatively low conductance and reduced susceptibility to Mg2+ block. These are characteristic features of heteromeric NMDAR assemblies that contain the NR2C subunit. Some patches also contained NMDA channels with higher conductance and a greater sensitivity to Mg2+. In the neocortex of transgenic mice in which a beta-galactosidase (lacZ) indicator gene was controlled by the NR2C promoter, the lacZ indicator was densely expressed in layer 4. In current-clamp recordings, blockade of NMDARs caused hyperpolarization and an increase in apparent input resistance. Our data demonstrate that the SpS neurons of layer 4 functionally express NR2C subunits; this is the likely explanation for their ability to generate large NMDAR-mediated EPSPs that are effective at resting potential, without previous depolarization.

Electrotonic coupling in the anterior pituitary of a teleost fish

The anterior pituitary of teleost fish contains a variety of endocrine cells, which, under control from the hypothalamus, release trophic hormones and thereby play a major role in reproduction, social behavior, and growth. In fish, hypothalamic fibers directly innervate the pituitary. The hypothalamic hormones released from these fibers bind to membrane receptors on pituitary cells, triggering action potentials, a rise in cytosolic calcium, and exocytosis. It is unclear whether these activities are confined to the stimulated cell or propagate to adjacent cells. We addressed this issue using whole cell and perforated patch-clamp techniques in a novel, hypothalamo-pituitary slice preparation from the tilapia fish (Oreochromis niloticus). Pituitary cells at rest generated occasional spontaneous spikes and sharp depolarizations of lower amplitude. The latter probably represented spikes in neighboring, electrotonically coupled cells. The presence of electrotonic communication, probably mediated by gap junctions, was also supported by the finding that Lucifer Yellow diffuses between cells. To quantify this connectivity, we performed simultaneous recording from pairs of adjacent cells. Thirty-three percent of the cells exhibited strong reciprocal coupling. Coupling coefficients ranged between 0.18 and 0.31, and coupling resistances ranged between 16 and 39 GOhm. The electrical junctions were effective low pass filters, attenuating action potentials much more than low frequency waveforms. We conclude that electrical activities of anterior pituitary cells in teleost fish are synchronized by coupling through gap junctions. Regulation of this coupling may play a critical role in determining complex patterns of pituitary hormone secretion.

Enhanced spontaneous transmitter release is the earliest consequence of neocortical hypoxia that can explain the disruption of normal circuit function

After the onset of an acute episode of arrested circulation to the brain and consequent cerebral hypoxia, EEG changes and modifications of consciousness ensue within seconds. This in part reflects the rapid effect of hypoxia on the neocortex, where oxygen deprivation leads to impaired neuronal excitability and abnormal synaptic transmission. To identify the cellular mechanisms responsible for the earliest changes in neocortical function and to determine their time course, we have used patch-in-slice recording techniques to investigate the effects of acute hypoxia on the synaptic and intrinsic properties of layer 5 neurons. Coronal slices of mouse somatosensory cortex were maintained at 37 degrees C and challenged with episodes of hypoxia (3-4 min of exposure to 95% N(2), 5% CO(2)). In recordings with cell-attached patch electrodes, activation of ATP-sensitive potassium channels first became detectable 211 +/- 11 sec (range, 185-240 sec; n = 6 patches) after the onset of hypoxia. Similar recording techniques revealed no alterations in the properties of Na(+) currents in the first 4 min after the onset of hypoxia. The earliest hypoxia-induced disturbance was a marked increase in the frequency of spontaneous EPSCs and IPSCs, which began within 15-30 sec of the removal of oxygen. This rapid synaptic effect was not sensitive to TTX and was present in Ca(2+)-free perfusate, indicating that the hypoxia had a direct influence on the vesicular release mechanisms. The incoherent, massive increase in miniature PSCs would be expected to deplete the readily releasable pool of vesicles in cortical terminals, and to thereby markedly distort the neuronal interactions that underlie normal circuit function.

Autoantibodies to the glutamate receptor kill neurons via activation of the receptor ion channel

Antibodies to the glutamate/AMPA receptor subunit 3 (GluR3), are found in a human epilepsy, Rasmussen's encephalitis [RE], and were hypothesized as the major cause for the neuronal loss, chronic inflammatory changes and epileptic seizures characteristic of the disease. To establish the pathogenic potential and mechanism of action of such antibodies, we raised murine antibodies against specific peptides of the GluR3 protein and studied their ability to bind, activate, and kill neurons. Mice were immunized with two GluR3 specific peptides: GluR3A (amino acids 245-274) and GluR3B (amino acids 372-395), and with a scrambled GluR3B peptide for control. High levels of antibodies to each of these peptides were obtained, with no cross reactivity between them. Antibodies to the GluR3B peptide were found to bind to cultured neurons, evoke GluR ion channel activity, and kill neurons. In contrast, antibodies against GluR3A peptide bound to neurons but failed to activate the receptor or kill neurons. Anti-scrambled-GluR3B antibodies had no effect. Both the activation of the GluRs and the neuronal death induced by anti-GluR3B antibodies were blocked by CNQX, a specific glutamate/AMPA receptor antagonist; killing was independent of complement. This indicates a mechanism of excitotoxicity-neuronal death due to over-activation of the receptor, a phenomenon known to be caused by excess of glutamate. Purified anti-GluR3B IgGs retained the neuronal killing capacity, and killing was completely and specifically blocked by preincubation with the GluR3B peptide. Excitotoxic neuronal death induced by anti-GluR3B antibodies took place primarily via apoptosis. Taken together, these results show that antibodies to a specific peptide of the GluR can kill neurons by an excitotoxic mechanism, thus mimicking the effects of excess of glutamate. This is the first example that antibodies can lead to neuronal death in a non-classical complement-independent manner, via activation of a membranal neurotransmitter receptor.

Functionally distinct NMDA receptors mediate horizontal connectivity within layer 4 of mouse barrel cortex

In sensory areas of neocortex, thalamocortical afferents project primarily onto the spiny stellate neurons of Layer 4. Anatomical evidence indicates that these cells receive most of their excitatory input from other cortical neurons, including other spiny stellate cells. Although this local network must play an important role in sensory processing, little is known about the properties of the neurons and synapses involved. We have produced a slice preparation of mouse barrel cortex that isolates Layer 4. We report that excitatory interaction between spiny stellate neurons is largely via N-methyl-D-aspartate receptors (NMDARs) and that a given neuron contains more than one type of NMDAR, as distinguished by voltage dependence. Thus, spiny stellate cells act as effective integrators of powerful and persistent NMDAR-mediated recurrent excitation.

Activation of protein kinase C increases neuronal excitability by regulating persistent Na+ current in mouse neocortical slices

Effects of the protein kinase C activating phorbol ester, phorbol 12-myristate 13-acetate (PMA), were studied in whole cell recordings from layer V neurons in slices of mouse somatosensory neocortex. PMA was applied intracellularly (100 nM to 1 microM) to restrict its action to the cell under study. In current-clamp recordings, it enhanced neuronal excitability by inducing a 10- to 20-mV decrease in voltage threshold for action-potential generation. Because spike threshold in neocortical neurons critically depends on the properties of persistent Na+ current (INaP), effects of PMA on this current were studied in voltage clamp. After blocking K+ and Ca2+ currents, INaP was revealed by applying slow depolarizing voltage ramps from -70 to 0 mV. Intracellular PMA induced a decrease in INaP at very depolarized membrane potentials. It also shifted activation of INaP in the hyperpolarizing direction, however, such that there was a significant increase in persistent inward current at potentials more negative than -45 mV. When tetrodotoxin (TTX) was added to the bath, blocking INaP and leaving only an outward nonspecific cationic current (Icat), PMA had no apparent effect on responses to voltage ramps. Thus PMA did not affect Icat, and it did not induce any additional current. Intracellular application of the inactive PMA analogue, 4 alpha-PMA, did not affect INaP. The specific protein kinase C inhibitors, chelerythrine (20 microM) and calphostin C (10 microM), blocked the effect of PMA on INaP. The data suggest that PMA enhances neuronal excitability via a protein kinase C-mediated increase in INaP at functionally critical subthreshold voltages. This novel effect would modulate all neuronal functions that are influenced by INaP, including synaptic integration and active backpropagation of action potential from the soma into the dendrites.

Kinetics of slow inactivation of persistent sodium current in layer V neurons of mouse neocortical slices

1. In whole cell recordings from layer V neurons in slices of mouse somatosensory neocortex, tetrodotoxin (TTX)-sensitive persistent Na+ current (INaP) was studied by blocking K+ currents with intracellular Cs+ and Ca2+ currents with extracellular Cd2+. During slow voltage ramps, INaP began to activate at around -60 mV, and attained a peak at around -25 mV. The peak amplitude of INaP varied widely from cell to cell (range 60-3,160 pA; median 308 pA, n = 77). At potentials more positive than -35 mV, INaP in all cells was superimposed on a large, TTX-resistant outward current. 2. In hybrid clamp experiments, INaP was significantly reduced by a preceding high-frequency train of spikes. 3. INaP underwent pronounced slow inactivation, which was revealed by systematically varying the ramp speed between 233 and 2.33 mV/s, or varying the duration of a depolarizing prepulse between 0.1 and 10 s. 4. Onset of slow inactivation at +20 mV was monoexponential with tau = 2.06 s (n = 17 cells). Recovery from slow inactivation was voltage dependent. It followed a monoexponential time course with tau = 2.31 s (n = 6) at -70 mV and tau = 1.10 s (n = 4) at -90 mV. These values are not significantly different than values previously reported for slow inactivation of fast-inactivating INa. 5. Slow inactivation of neocortical INaP will influence all neuronal functions in which this current plays a role, including spike threshold determination, synaptic integration, and active propagation in dendrites. The kinetics of slow inactivation suggest that it may be a factor not only during extremely intense spiking, but also during periods of "spontaneous" activity.

Slow inactivation of Na+ current and slow cumulative spike adaptation in mouse and guinea-pig neocortical neurones in slices

1. Spike adaptation of neocortical pyramidal neurones was studied with sharp electrode recordings in slices of guinea-pig parietal cortex and whole-cell patch recordings of mouse somatosensory cortex. Repetitive intracellular stimulation with 1 s depolarizing pulses delivered at intervals of < 5 s caused slow, cumulative adaptation of spike firing, which was not associated with a change in resting conductance, and which persisted when Co2+ replaced Ca2+ in the bathing medium. 2. Development of slow cumulative adaptation was associated with a gradual decrease in maximal rates of rise of action potentials, a slowing in the post-spike depolarization towards threshold, and a positive shift in the threshold voltage for the next spike in the train; maximal spike repolarization rates and after-hyperpolarizations were unchanged. 3. The data suggested that slow adaptation reflects use-dependent removal of Na+ channels from the available pool by an inactivation process which is much slower than fast, Hodgkin-Huxley-type inactivation. 4. We therefore studied the properties of Na+ channels in layer II-III mouse neocortical cells using the cell-attached configuration of the patch-in-slice technique. These had a slope conductance of 18 +/- 1 pS and an extrapolated reversal potential of 127 +/- 6 mV above resting potential (Vr) (mean +/- S.E.M.; n = 5). Vr was estimated at -72 +/- 3 mV (n = 8), based on the voltage dependence of the steady-state inactivation (h infinity) curve. 5. Slow inactivation (SI) of Na+ channels had a mono-exponential onset with tau on between 0.86 and 2.33 s (n = 3). Steady-state SI was half-maximal at -43.8 mV and had a slope of 14.4 mV (e-fold)-1. Recovery from a 2 s conditioning pulse was bi-exponential and voltage dependent; the slow time constant ranged between 0.45 and 2.5 s at voltages between-128 and -68 mV. 6. The experimentally determined parameters of SI were adequate to simulate slow cumulative adaptation of spike firing in a single-compartment computer model. 7. Persistent Na+ current, which was recorded in whole-cell configuration during slow voltage ramps (35 mV s-1), also underwent pronounced SI, which was apparent when the ramp was preceded by a prolonged depolarizing pulse.

Paired-pulse facilitation of IPSCs in slices of immature and mature mouse somatosensory neocortex

1. Whole cell recordings from layer V neurons of mouse somatosensory cortex were made with the use of a "blind" patch-clamp technique. In slices from immature [postnatal days 6 to 11 (P6-P11)] and juvenile (P18-P21) animals, inhibitory postsynaptic currents (IPSCs) were evoked in all cells by extracellular stimulation at the layer V-VI border. Monosynaptic IPSCs, with latency < 2 ms, were isolated pharmacologically by blockade of ionotropic glutamatergic transmission. IPSCs were blocked by bicuculline methiodide and reversed near the predicted equilibrium potential for Cl-. 2. IPSC characteristics were not different for the two age groups. At 1.5-2 times threshold intensity (0.2 Hz), they fluctuated in amplitude with occasional failures. At -70 or -80 mV, mean amplitudes were -202 +/- 20 (SE) pA and -207 +/- 32 pA for immature (39 cells) and juvenile (13 cells) cortex, respectively. Half rise times were 0.74 +/- 0.03 ms (n = 7 cells) in neonates and 0.67 +/- 0.04 ms (n = 7 cells) in juveniles. Decays were biexponential with tau 1 = 14.8 +/- 1.3 ms and tau 2 = 59.0 +/- 7.4 ms (n = 7 cells) in neonates, and tau 1 = 11.9 +/- 1.1 ms and tau 2 = 55.5 +/- 4.2 ms (n = 7 cells) in juveniles. 3. Pairs of stimuli elicited paired-pulse facilitation (PPF) when delivered at brief interstimulus intervals (ISI), and paired-pulse depression (PPD) at long ISI. PPF, which was evident in 64% of immature cells and 38% of juvenile cells, was maximal (38 +/- 4% greater than the conditioning response) at 20-40 ms. PPD, which was evident in 82% of immature cells and 87% of juvenile cells, was maximal (29 +/- 2% smaller than the conditioning response) by 300 ms. In each age group, some animals showed PPF without PPD.(ABSTRACT TRUNCATED AT 250 WORDS)

Inward sodium current at resting potentials in single cardiac myocytes induced by the ischemic metabolite lysophosphatidylcholine

To investigate possible ionic current mechanisms underlying ischemic arrhythmias, we studied single Na+ channel currents in rat and rabbit cardiac myocytes treated with the ischemic metabolite lysophosphatidylcholine (LPC) using the cell-attached and excised inside-out patch-clamp technique at 22 degrees C. LPC has been reported previously to reduce open probability and to induce sustained open channel activity at depolarized potentials. We now report two new observations for Na+ currents in LPC-treated patches: 1) The activation-voltage relation of the peak of the ensemble currents is shifted in the negative (hyperpolarizing) direction by approximately 20 mV compared with control currents. This effect was observed in all patches for depolarizations from a holding potential of -150 mV to different test potentials. 2) In some LPC-treated patches, Na+ channels exhibited sustained bursting activity at potentials as negative as -150 mV, giving a nondecaying inward current. This bursting activity was accompanied by double and triple simultaneous openings and closings, suggesting tight cooperativity in channel gating. These LPC-modified channels were identified as Na+ channels, because their unitary conductance was the same as Na+ channels in control solutions, because the single channel current-voltage relation was extrapolated to reverse at the Na+ Nernst potential, and because the current was blocked by the local anesthetic QX-222. This novel depolarizing current may play a role in the electrophysiological abnormalities in ischemia, including abnormal automaticity and reentrant arrhythmias, and could be a target for antiarrhythmic drugs.

Modulation of cardiac sodium channel gating by lysophosphatidylcholine

The effects of lysophosphatidylcholine (LPC) on Na channels have been investigated in inside-out patches of adult rat ventricular cells using the patch-clamp technique. Applying LPC (9-25 microM) to the inner side of the membrane reduced peak Na current (INa) and prolonged the time course of INa inactivation. Both effects increased with duration of LPC exposure. Analysis of single-channel behavior revealed that after 15-20 min of exposure to LPC, Na channels displayed 2 types of gating behavior. One type consisted of normal kinetics and the other consisted of long-lasting burst (LB) of Na channel openings (up to the 300 ms bursts of the test pulse). This modification in gating resulted in the initial appearance of a slowly decaying and later a noninactivating component of INa in the ensemble average current. The slope conductance and reversal potential of these modified channels remained unchanged from control. Overall open time distribution for long bursting kinetics channels was biexponential, indicating existence of two long-lasting bursting modes, one with fast kinetics (LB-f, mean open time 1.2 ms) and one with slow kinetics (LB-s, mean open time 13.7 ms). These data indicate that exposure to LPC results in the slow interconversion of Na channels between several modes of activity, including those in which inactivation occurs slowly or not at all.

Ischemic poison lysophosphatidylcholine modifies heart sodium channels gating inducing long-lasting bursts of openings

The effects of lysophosphatidylcholine (LPC) on Na channels in inside-out patches of adult rat ventricular cells using the patch-clamp technique have been investigated. Application of LPC (9-25 microM) from the inner side of membrane for 4-15 min caused a reduction of averaged Na current (INa) peak and prolonged the time course of inactivation in the potential range of -50 to -10 mV. Analysis of single channel behaviour revealed that after 30-50 min of exposure, in addition to normally functioning Na channels with short openings, LPC induced long-lasting bursts of Na channel openings (up to the 300 ms duration of the test pulses). This resulted in an appearance of noninactivated component of INa. The slope conductance of these modified channels remained the same as in control (11.3 pS - control; 11.6 pS - LPC-treated). The dwell time for modified channels increased significantly.

Single channel sodium current in rat cardiomyocytes: use-dependent block by ethacizin

Ethacizin is a phenothiazine derivative antiarrhythmic agent that blocks sodium current. The cell-attached patch clamp of single adult rat ventricular cells was used to investigate mechanisms of use-dependent block of sodium current. Under control conditions peak open probability, first latency, fraction of null sweeps, mean open time and single channel current amplitude were not different at both 1 and 4 Hz. Ethacizin (5 microM) caused a significant decrease in the peak open probability, a significant increase in the first latency and an increase in the fraction of null sweeps at 4 Hz compared with 1 Hz; mean open time and single channel current amplitude were unchanged. These observations support a model of antiarrhythmic action which proposes complete block of single channel conductance resulting from drug binding. A "runs analysis" revealed nonrandom clustering of null traces in the presence of ethacizin and no clustering in control patches. Increasing stimulation frequency makes this nonrandom behavior more apparent. We conclude that the relatively slow cycling of a few channels between blocked and unblocked states induces null sweeps clustering. The implications of these findings for mechanisms of drug block of the Na channel are discussed.

Some mechanisms of nonspecific antiarrhythmic action of phosphocreatine in acute myocardial ischemia

Using isotope-labeled microspheres (diameter 15 microns) it was shown that phosphocreatine at a dose of 300 mg/kg does not affect the myocardial blood flow in the ischemic zone during acute occlusion (5 min) of the left anterior descending coronary artery (LAD) in dogs. Intravenous administration of NaCl hypertonic solution which contained the same amount of Na+ as 300 mg/kg of phosphocreatinine disodium salt prevented the development of ventricular fibrillation during acute LAD occlusion in dogs. Under these conditions excitation conduction velocity significantly increased. Experiments in isolated intraventricular rabbit septum have showed that the addition of phosphocreatine or phosphocreatinine to the perfusion medium at a concentration of 10 mmole/liter increased excitation conduction velocity in ischemic myocardium. However, when changes in perfusate Na+ and Ca2+ concentration produced by addition of phosphocreatine or phosphocreatinine were compensated, these compounds do not affect excitation conduction velocity. On the other hand, the alterations similar to those produced by the addition of phosphocreatine or phosphocreatinine led to the same increase of excitation conduction velocity. The results obtained indicate an important role of the changes of blood plasma ionic composition on intravenous administration of phosphocreatine in electrophysiological and antiarrhythmic effects of this substance during acute myocardial ischemia.