Calcium-dependent Action Potentials Research Articles

Spontaneous and CRH-Induced Excitability and Calcium Signaling in Mice Corticotrophs Involves Sodium, Calcium, and Cation-Conducting Channels.

Transgenic mice expressing the tdimer2(12) form of Discosoma red fluorescent protein under control of the proopiomelanocortin gene's regulatory elements are a useful model for studying corticotrophs. Using these mice, we studied the ion channels and mechanisms controlling corticotroph excitability. Corticotrophs were either quiescent or electrically active, with a 22-mV difference in the resting membrane potential (RMP) between the 2 groups. In quiescent cells, CRH depolarized the membrane, leading to initial single spiking and sustained bursting; in active cells, CRH further facilitated or inhibited electrical activity and calcium spiking, depending on the initial activity pattern and CRH concentration. The stimulatory but not inhibitory action of CRH on electrical activity was mimicked by cAMP independently of the presence or absence of arachidonic acid. Removal of bath sodium silenced spiking and hyperpolarized the majority of cells; in contrast, the removal of bath calcium did not affect RMP but reduced CRH-induced depolarization, which abolished bursting electrical activity and decreased the spiking frequency but not the amplitude of single spikes. Corticotrophs with inhibited voltage-gated sodium channels fired calcium-dependent action potentials, whereas cells with inhibited L-type calcium channels fired sodium-dependent spikes; blockade of both channels abolished spiking without affecting the RMP. These results indicate that the background voltage-insensitive sodium conductance influences RMP, the CRH-depolarization current is driven by a cationic conductance, and the interplay between voltage-gated sodium and calcium channels plays a critical role in determining the status and pattern of electrical activity and calcium signaling.

Action potentials drive body wall muscle contractions in Caenorhabditis elegans

The sinusoidal locomotion exhibited by Caenorhabditis elegans predicts a tight regulation of contractions and relaxations of its body wall muscles. Vertebrate skeletal muscle contractions are driven by voltage-gated sodium channel-dependent action potentials. How coordinated motor outputs are regulated in C. elegans, which does not have voltage-gated sodium channels, remains unknown. Here, we show that C. elegans body wall muscles fire all-or-none, calcium-dependent action potentials that are driven by the L-type voltage-gated calcium and Kv1 voltage-dependent potassium channels. We further demonstrate that the excitatory and inhibitory motoneuron activities regulate the frequency of action potentials to coordinate muscle contraction and relaxation, respectively. This study provides direct evidence for the dual-modulatory model of the C. elegans motor circuit; moreover, it reveals a mode of motor control in which muscle cells integrate graded inputs of the nervous system and respond with all-or-none electrical signals.

Calcium- and Otoferlin-Dependent Exocytosis by Immature Outer Hair Cells

Immature cochlear outer hair cells (OHCs) make transient synaptic contacts (ribbon synapses) with type I afferent nerve fibers, but direct evidence of synaptic vesicle exocytosis is still missing. We thus investigated calcium-dependent exocytosis in murine OHCs at postnatal day 2 (P2)-P3, a developmental stage when calcium current maximum amplitude was the highest. By using time-resolved patch-clamp capacitance measurements, we show that voltage step activation of L-type calcium channels triggers fast membrane capacitance increase. Capacitance increase displayed two kinetic components, which are likely to reflect two functionally distinct pools of synaptic vesicles, a readily releasable pool (RRP; tau = 79 ms) and a slowly releasable pool (tau = 870 ms). The RRP size and maximal release rate were estimated at approximately 1200 vesicles and approximately 15,000 vesicles/s, respectively. In addition, we found a linear relationship between capacitance increase and calcium influx, like in mature inner hair cells (IHCs). These results give strong support to the existence of efficient calcium-dependent neurotransmitter release in immature OHCs. Moreover, we show that immature OHCs, just like immature IHCs, are able to produce regenerative calcium-dependent action potentials that could trigger synaptic exocytosis in vivo. Finally, the evoked membrane capacitance increases were abolished in P2-P3 OHCs from mutant Otof-/- mice defective for otoferlin, despite normal calcium currents. We conclude that otoferlin, the putative major calcium sensor at IHC ribbon synapses, is essential to synaptic exocytosis in immature OHCs too.

Calcium and Cardiac Arrhythmias: DADs, EADs, and Alternans

Rapid progress has been made in understanding the molecular mechanisms by which calcium ions mediate certain cardiac arrhythmias. Principal advances include imaging of cytosolic calcium in isolated cells and in intact tissues, use of fluorescent indicators and monophasic action potentials to record membrane potentials in isolated tissue, and sequencing of the genes that encode critical ion channel proteins. In this review, five types of arrhythmias are discussed where calcium ion currents, or currents controlled by calcium, appear to be responsible for arrythmogenesis. These include: (1) the delayed afterpotential that occurs in conditions of intracellular calcium overload such as digitalis toxicity; (2) the early afterdepolarization that occurs when action potential duration is prolonged; (3) the slowly conducted calcium-dependent action potential (the slow response) in the SA and AV nodes; (4) the phenomenon of calcium transient alternans during ischemia, which is related to action potential duration alternans and t-wave alternans; (5) catecholamine-induced cardiac arrhythmias in families with mutations of the sarcoplasmic reticulum calcium-release channel. For each type of arrhythmia, the clinical implications of emerging knowledge are discussed. An especially important issue is whether ventricular fibrillation during acute coronary artery occlusion is due to calcium transient alternans. Ventricular fibrillation due to acute ischemia is an important subset of the 400,000 sudden cardiac deaths that occur annually in the U.S. Certain drugs, including beta blockers, fish oils, verapamil, and diltiazem, seem to specifically prevent ventricular fibrillation in this setting, and in most cases an effect of the drug on cytosolic calcium appears to be involved.Referee: David S. Rosenbaum, Heart and Vascular Center, Metro Health Campus, Case Western Reserve University, 2500 Metro Health Drive, Hamann 3, Cleveland, OH 44109-1998. Tel: 216-778-2005. Fax: 216-778-4294

Developmental changes in the electrophysiological properties and response characteristics of Manduca antennal-lobe neurons.

Using whole cell patch-clamp recordings, we have examined changes in the electrophysiological properties and response characteristics of antennal lobe (AL) neurons associated with the metamorphic adult development of the sphinx moth, Manduca sexta. Whole cell current profiles and electrical excitability were examined in dispersed AL neurons in vitro, and in medial-group AL neurons in situ in semi-intact brain preparations. Around stages 2-4 of the 18 stages of metamorphic adult development, whole cell current profiles were dominated by large outward (K+) currents. Calcium-dependent action potentials could be elicited at this stage, but only a small percentage of cells exhibited sodium spikes. From stages 3 to 10, there was a rapid increase in the proportion of AL neurons exhibiting rapidly activating, transient sodium currents, and many cells in vitro exhibited spontaneous bursts of spike activity at this time. As development progressed, action-potential waveforms became shorter in duration and larger in amplitude. Cell-type-specific differences in the prevalence of spontaneous activity, and in the electrophysiological properties and response characteristics of AL neurons, were most apparent late in metamorphosis. While removal of antennal sensory input to the ALs early (stage 1-2) in metamorphosis had no detectable effect on the development of cell excitability, a significantly higher percentage of neurons in vitro from stage 4 pupae exhibited sodium-based action potentials following the addition of serotonin to the culture medium. Characteristic forms of electrical excitability in developing Manduca AL neurons, and their modulation by serotonin, seem likely to play a central role in the functional development of the ALs.

Activity-dependent neuronal differentiation prior to synapse formation: the functions of calcium transients

Spinal cord neurons become excitable prior to synapse formation, and generate spontaneous calcium transients that regulate aspects of their differentiation before neuronal networks are established. Calcium spikes, generated by calcium-dependent action potentials and calcium-induced calcium release (CICR), regulate transcription. Growth cone calcium transients, produced by calcium influx through unidentified channels that triggers CICR, control the rate of axon outgrowth in response to environmental cues. Filopodial calcium transients, generated by calcium influx through channels activated by β1 integrins, signal information about the molecular identity of the substrate and regulate growth cone turning. All three classes of calcium transients appear to use a frequency code to implement their effects. Oscillations of second messengers in embryonic neurons and perhaps more generally in other differentiating cells may behave like a kinetic quilt, demonstrating patchy fluctuations in concentrations that orchestrate the complex processes of development.

Initiation and propagation of calcium-dependent action potentials in a coupled network of olfactory interneurons.

Coherent oscillatory electrical activity and apical-basal wave propagation have been described previously in the procerebral (PC) lobe, an olfactory center of the terrestrial slug Limax maximus. In this study, we investigate the physiological basis of oscillatory activity and wave propagation in the PC lobe. Calcium green dextran was locally deposited in the PC lobe; this led to cellular uptake and transport of dye by bursting and nonbursting neurons of the PC lobe. The change of intracellular calcium concentration was measured at several different positions in neurites of individual bursting neurons in the PC lobe with a two-photon laser-scanning microscope. Fluorescence measurements were also made from neurons intracellularly injected with calcium green-1. Two different morphological classes of bursting neurons were found, varicose (VB) and smooth (SB). Our results from concurrent optical and intracellular recordings suggest that Ca2+ is the major carrier for the inward current during action potentials of bursting neurons. Intracellular recordings from bursting neurons with nystatin perforated-patch electrodes made while simultaneously recording the local field potential (LFP) with extracellular electrodes indicate that the burster spikes are precisely phase-locked to the periodic LFP events. By referencing successive calcium measurements to the common LFP signal, we could therefore accurately determine the relative timing of calcium transients at different points along a neurite. Measuring the relation of temporal to spatial differences allowed us to estimate the velocity of action potential propagation, which was 4.3 +/- 0.2 (SE) mm/s in VBs, and 1.3 +/- 0.2 mm/s in SB.

Cholesterol inhibits spontaneous action potentials and calcium currents in guinea pig gallbladder smooth muscle.

Elevated cholesterol decreases agonist-induced contractility and enhances stone formation in the gallbladder. The current study was conducted to determine if and how the electrical properties and ionic conductances of gallbladder smooth muscle are altered by elevated cholesterol. Cholesterol was delivered as a complex with cyclodextrin, and effects were evaluated with intracellular recordings from intact gallbladder and whole cell patch-clamp recordings from isolated cells. Cholesterol significantly attenuated the spontaneous action potentials of intact tissue. Furthermore, calcium-dependent action potentials and calcium currents were reduced in the intact tissue and in isolated cells, respectively. However, neither membrane potential hyperpolarizations induced by the ATP-sensitive potassium channel opener, pinacidil, nor voltage-activated outward potassium currents were affected by cholesterol. Hyperpolarizations elicited by calcitonin gene-related peptide were reduced by cholesterol enrichment, indicating potential changes in receptor ligand binding and/or second messenger interactions. These data indicate that excess cholesterol can contribute to gallbladder stasis by affecting calcium channel activity, whereas potassium channels remained unaffected. In addition, cholesterol enrichment may also modulate receptor ligand behavior and/or second messenger interactions.

Optical identification of calcium-dependent action potentials transiently expressed in the embryonic rat brainstem

Using multiple-site optical recording of transmembrane potential changes, we have found a new type of calcium-dependent action potential expressed transiently in the embryonic rat dorsal motor nucleus of the vagus nerve. Slice preparations with vagus nerve fibers attached were dissected from 12- to 16-day-old embryonic (E12–E16) rat brainstems, and they were stained with a voltage-sensitive merocyanine–rhodanine dye (NK2761). Electrical activities in response to vagal stimuli were optically recorded simultaneously from many sites using 1020- or 128-element photodiode array measuring systems. In brainstem preparations, two types of action potential-related optical signals were identified. One was detected from the dorsolateral region, and was related to sensory nerve activity (Type I). The other was detected from the dorsomedial region, and corresponded to the action potential in the dorsal motor nucleus of the vagus nerve (Type II). We found a difference in the ionic basis of the Type I vs Type II signals. The Type I signal was not altered in Ca 2+-free bathing solution and was eliminated by tetrodotoxin, suggesting that the sensory nerve activity is mediated by Na + currents. The Type II signal at early developmental stages (E12–E13, and some preparations in E14) was also independent of Ca 2+. However, the Type II signal in later developmental stages (E15–E16, and some preparations in E14) did depend upon Ca 2+: it was eliminated in Ca 2+-free Ringer's solution, blocked by Cd 2+, Ni 2+ or Mn 2+, and elicited in Sr 2+-containing Ringer's solution, where CaCl 2 was replaced with SrCl 2. These results suggest that the cation which dominates the motoneuron action potential changes from Na + to Ca 2+ during development, and this change occurs around E14. With pharmacological analysis using Ca 2+ channel blockers, we show that the Ca 2+ channel mediating the motoneuron action potential is distinct from T-, L-, N-, P- or Q-type channels. Because the vagal action potential in adult mammals is mainly mediated by Na +, we suggest that a Ca 2+ action potential mediated by a new type of Ca 2+ channel is expressed transiently in the rat dorsal motor nucleus of the vagus nerve at particular stages of development.

Na+-Dependent neuritic spikes initiate Ca2+-dependent somatic plateau action potentials in insect dorsal paired median neurons.

The origin of plateau action potentials was studied in short-term cultures of dorsal paired median (DPM) neurons dissociated from the terminal abdominal ganglion of the cockroach, Periplaneta americana. Spontaneous plateau action potentials were recorded by intracellular microelectrodes in cell bodies that had neurite stumps. These action potentials featured a fast initial depolarization followed by a plateau. However, only fast spikes of short duration were observed when the cell was hyperpolarized from the resting membrane potential. These two different components of the action potentials could be separated by applying depolarizing current pulses from a hyperpolarized holding potential. Application of 200 nM tetrodotoxin (TTX) abolished both fast and slow phases, but depolarization to the original resting potential by steady current injection triggered slow monophasic action potentials that could be blocked by 3 mM CoCl2. In contrast, DPM neurons without neurites were not spontaneously active. In these cells, calcium-dependent slow monophasic action potentials were only recorded immediately after impalement or with current pulse stimulation. Immunocytochemical observations showed that dorsal unpaired median (DUM) neuron cell bodies, which are known to exhibit spontaneous sodium-dependent action potentials, reacted with an antibody directed against a synthetic peptide corresponding to the SP19 segment of voltage-activated sodium channels. In contrast, the antibody did not stain DPM neuron cell bodies but gave intense, patchy staining only in the neurite. Whole cell patch-clamp experiments performed on isolated DPM neuron cell bodies without a neurite revealed the presence of an inward current that did not inactivate completly within the duration of the test pulse. This current was insensitive to both 100 nM TTX and sodium-free saline. It was defined as a high-voltage-activated calcium current according to its high threshold of activation (-30 mV) and its sensitivity to 1 mM CdCl2 and 100 nM omega-conotoxin GVIA. Our findings demonstrate that spontaneous sodium-dependent spikes arising from the neurite are required to initiate slow somatic calcium-dependent action potentials in DPM neurons.

Coordination of Neuronal Activity in Developing Visual Cortex by Gap Junction-Mediated Biochemical Communication

During brain development, endogenously generated coordinated neuronal activity regulates the precision of developing synaptic circuits (Shatz and Stryker, 1988; Weliky and Katz, 1997). In the neonatal neocortex, a form of endogenous coordinated activity is present as locally restricted intercellular calcium waves that are mediated by gap junctions (Yuste et al., 1992). As in other neuronal and non-neuronal systems, these coordinated calcium fluctuations may form the basis of functional cell assemblies (for review, seeWarner, 1992; Peinado et al., 1993b). In the present study, we investigated the cellular mechanisms that mediate the activation of neuronal domains and the propagation of intercellular calcium waves in slices from neonatal rat neocortex. The occurrence of neuronal domains did not depend on intercellular propagation of regenerative electrical signals because domains persisted after blockade of sodium and calcium-dependent action potentials. Neuronal domains were elicited by intracellular infusion of inositol trisphosphate (IP3) but not of calcium, indicating the involvement of IP3-related second-messenger systems. Pharmacological stimulation of metabotropic glutamate receptors, which are linked to the production of IP3, elicited similarly coordinated calcium increases, whereas pharmacological blockade of metabotropic glutamate receptors dramatically reduced the number of neuronal domains. Therefore, the propagating cellular signal that causes the occurrence of neuronal domains seems to be inositol trisphosphate but not calcium. Because coordination of neuronal calcium changes by gap junctions is independent of electrical signals, the function of gap junctions between neocortical neurons is probably to synchronize biochemical rather than electrical activity.

Development of HVA and LVA calcium currents in pyramidal CA1 neurons in the hippocampus of the rat

High voltage activated (HVA) and low voltage activated (LVA) calcium currents were recorded in acutely dissociated CA1 hippocampal pyramidal neurons of the rat during the first three postnatal weeks as well as in adults. Measured in whole cell voltage clamp the amplitude of the HVA calcium current increased steadily and reached adult values after 20 postnatal days (P20). Using the perforated patch configuration with amphotericin B the amplitude of the HVA component was more than five times smaller, but the time course of development was the same. The LVA component also increased with age but reached adult values already around P13. The amplitude and developmental pattern of this component were not different when measured with the perforated patch technique. The results indicate a different role for intracellular modulators on these calcium currents, but exclude them as important factors in the developmental pattern. The fast development of the LVA component could lead to calcium dependent action potentials (and calcium spikes) in immature cells. The complex developmental pattern of the relative amplitude of the two currents will either lead to specific variations in the intracellular calcium homeostasis or will have to be accompanied by an adequate developmental pattern of buffering and extrusion mechanisms.

Induction of calcium-dependent action potentials by 4-aminopyridine in potassium depolarized guinea-pig papillary muscle.

Exposure of guinea-pig papillary muscle to 22 mM K+ resulted in loss of electromechanical excitability accompanied by an elevation of mean electrical threshold by 8.96 V and a membrane depolarization of about 47.3 mV. After setting the threshold to a new sensitive level for stimulation, 4-AP (1.25-5 x 10(-3) M) could restore both the propagated action potentials typical for Ca2+ (e.g. resting potential: -46 mV; action potential amplitude: 74.25 mV; upstroke velocity: 11.4 V/sec; abolition by Ca(2+)-channel blokers) and contractions to papillary muscle cells, with further elevation of resting potential from -51.2 mV to -46 mV. Restoration of electromechanical activity by 4-AP was dose-dependent and susceptible to blockade with D600 (5 x 10(-7) M). From these data, it was concluded that 4-AP restored electrical and mechanical excitability by increasing membrane conductance to Ca2+ (gCa2+).

Effects of riluzole on rat cortical neurones: an in vitro electrophysiological study.

1. The electrophysiological effects of riluzole on rat prefrontal and frontal cortical neurones were investigated by using both extracellular (field) and intracellular recording techniques in brain slices. 2. Bath applied riluzole (3-200 microM) depressed the cortico-cortical stimulus-evoked field potential in a concentration-related manner (EC50 = 29.5 microM). 3. Riluzole (3-100 microM) reduced the tonic firing of the neocortical neurones which was caused by intracellular current injection, while it did not have any effect on the resting membrane potential and apparent input resistance of these cells. 4. In the presence of tetrodotoxin (1 microM) and tetraethylammonium (30 mM), the injection of a depolarizing current step generated a calcium spike in the neocortical neurones. Riluzole (30 microM) abolished this calcium-dependent action potential. However, when the amount of the depolarizing current was increased the calcium-dependent regenerative potential was evoked again. 5. The depolarization of the membrane (10-20 mV) caused by brief (8-15 s) bath applications of glutamate (300 microM-1 mM) were not changed in the presence of riluzole (30 microM). 6. It is concluded that riluzole has direct actions on rat neocortical neurones: (a) it blocks the repetitive discharge of sodium action potentials and (b) it increases the threshold for the generation of the calcium spike. These two cellular mechanisms might at least in part account for the depression of the cortico-cortical field potential caused by this drug.

A role for a background sodium current in spontaneous action potentials and secretion from rat lactotrophs.

We have used the perforated-patch variation of whole cell patch-clamp techniques, measurements of cytosolic calcium with use of fura 2, and secretion measurements with use of the reverse-hemolytic plaque assay to address the role of depolarizing background currents in maintaining spontaneous action potentials and spontaneous secretion from rat lactotrophs in primary culture. Replacement of bath sodium with tris(hydroxymethyl)aminomethane or N-methyl-D-glucamine caused a dramatic hyperpolarization of the cells, a cessation of spontaneous action potentials, and an increase in input resistance of cells. Tetrodotoxin had no effect on spontaneous action potentials, and removal of bath calcium stopped spiking but did not hyperpolarize the cells. The hyperpolarization in response to removal of bath sodium was associated with a decrease in cytosolic calcium levels. Finally, removal of bath sodium caused a decrease in spontaneous secretion of prolactin from lactotrophs. These data suggest that a background sodium current is essential to drive the membrane to threshold for firing spontaneous calcium-dependent action potentials in lactotrophs. This, in turn, results in elevated intracellular calcium, which supports spontaneous secretion of prolactin from these cells.

Electrophysiological study of nucleus gigantocellularis neurons in guinea-pig brainstem slices.

Gigantocellular neurons of the medullary nucleus gigantocellularis represent a major source of reticulospinal pathways. Among other roles, they have been involved in the processing of vestibular information. The aim of the present study was to describe the major intrinsic membrane properties of these cells in guinea-pig brainstem slices. We found nucleus gigantocellularis neurons to be segregated in two cell types. Type A nucleus gigantocellularis neurons were characterized by the presence of a single large afterhyperpolarization and a potent transient 4-aminopyridine-sensitive rectification likely due to the presence of a transient outward potassium current. In contrast, type B nucleus gigantocellularis neurons had a narrower and faster rising action potential followed by an early fast and a delayed slower after-hyperpolarization. In contrast to type A neurons, type B neurons were, in addition, endowed with subthreshold tetrodotoxin-sensitive sodium-dependent plateau potentials. Whereas both cell types were endowed with high-threshold calcium-dependent action potentials, only type B nucleus gigantocellularis neurons also displayed long-lasting calcium-dependent plateau potentials. These results show that nucleus gigantocellularis neurons can be segregated by their intrinsic membrane properties it two cell types which are very similar to those that we have previously described in the medial vestibular nucleus. The possibility that these differences between type A and B neurons might play a role in the segregation between tonic and kinetic cells is discussed.

Spontaneous calcium transients regulate neuronal plasticity in developing neurons.

Calcium ions play critical roles in neuronal differentiation. We have recorded transient, repeated elevations of calcium in embryonic Xenopus spinal neurons over periods of 1 h in vitro and in vivo, confocally imaging fluo 3-loaded cells at 5 s intervals. Calcium spikes and calcium waves are found both in neurons in culture and in the intact spinal cord. Spikes rise rapidly to approximately 400% of baseline fluorescence and have a double exponential decay, whereas waves rise slowly to approximately 200% of baseline fluorescence and decay slowly as well. Imaging of fura 2-loaded neurons indicates that intracellular calcium increases from 50 to 500 nM during spikes. Both spikes and waves are abolished by removal of extracellular calcium. Developmentally, the incidence and frequency of spikes decrease, whereas the incidence and frequency of waves are constant. Spikes are generated by spontaneous calcium-dependent action potentials and also utilize intracellular calcium stores. Waves are produced by a mechanism that does not involve classic voltage-dependent calcium channels. Spikes are required for expression of the transmitter GABA and for potassium channel modulation. Waves in growth cones are likely to regulate neurite extension. The results demonstrate the roles of a novel signaling system in regulating neuronal plasticity, that operates on a time scale 10(4) times slower than that of action potentials.

Spontaneous neuronal calcium spikes and waves during early differentiation

Calcium ions play critical roles in neuronal development, but the factors that govern spontaneous fluctuations in intracellular calcium are not well understood. Transient, repeated elevations of calcium in embryonic Xenopus spinal neurons have been recorded over periods of 1 hr in vitro and in vivo, confocally imaging fluo-3-loaded cells at 5 sec intervals. Calcium spikes and calcium waves are found both in neurons in culture and in the intact spinal cord. Spikes rise rapidly to approximately 400% of baseline fluorescence and have a characteristic double exponential decay, while waves rise slowly to approximately 200% of baseline fluorescence and decay slowly as well. Imaging of fura-2-loaded neurons indicates that intracellular calcium increases from 50 to 500 nM during spikes. Both spikes and waves are abolished by removal of extracellular calcium. Developmentally, the incidence and frequency of spikes decrease while the incidence and frequency of waves are constant. Spikes are generated by spontaneous calcium-dependent action potentials that can be triggered by low-threshold, T-type calcium current and are eliminated by agents that block voltage-dependent calcium channels. They can be elicited by depolarization, are generated in an all-or-none manner, and are rapidly and bidirectionally propagated. Spikes also utilize intracellular calcium stores, since blocking release from stores substantially reduces their amplitude. Waves are not elicited by depolarization nor by activation of glutamate receptors, and are propagated at a rate consistent with diffusion of calcium. Waves are blocked by Ni2+ at a higher concentration than required to block classical voltage-dependent calcium channels. Previous work now suggests that spikes are required for expression of the transmitter GABA and for potassium channel modulation. The present study indicates that waves in growth cones are likely to regulate neurite extension.

Classification and mechanism of action of antiarrhythmic drugs.

The present paper reviews classification and mode of action of agents that suppress extrasystoles and tachyarrhythmias. These are classified according to their electrophysiological effects observed in isolated cardiac tissues in vitro (Vaughan Williams, 1989). Fast sodium channel blockers (class I) which reduce the upstroke velocity of the action potential are usually subclassified into three groups, class I A-C, according to their effect on the action potential duration. Beta-adrenergic antagonists (class II) exert their effects by antagonizing the electrophysiological effects of beta-adrenergic catecholamines. Class III antiarrhythmic agents (eg amiodarone) prolong the action potential and slow calcium channel blockers (class IV) suppress the calcium inward current and calcium-dependent action potentials. The classification of antiarrhythmic drugs is still under debate. This particularly applies to agents of class I and III. The effect of class I agents is frequency-dependent because the binding affinity of these drugs to the sodium channel is modulated by the state of the channel (modulated receptor hypothesis). Class I agents bind to the channel in the activated and inactivated state and dissociate from the channel in the rested state. This occurs at a drug-specific rate so that class I agents can be subclassified into only two groups, namely in those of the slow- and fast-recovery type respectively (time constant of reactivation greater or smaller than 1 s). Slow-recovery class I agents affect regular action potentials at normal heart rates which can more easily lead to a lengthening of the QRS duration in the ECG, to conduction disturbances and hence to pro-arrhythmic effects.(ABSTRACT TRUNCATED AT 250 WORDS)

A background sodium conductance is necessary for spontaneous depolarizations in rat pituitary cell line GH3.

The role of Na+ in the expression of membrane potential activity in the clonal rat pituitary cell line GH3 was investigated using the perforated patch variation of patch-clamp electrophysiological techniques. It was found that replacing bath Na+ with choline, tris(hydroxymethyl)aminomethane (Tris), or N-methyl-D-glucamine (NMG) caused the cells to hyperpolarize 20-30 mV. Tetrodotoxin had no effect. The effects of the Na+ substitutes could not be explained by effects on potassium or calcium currents. Although all three Na+ substitutes suppressed voltage-dependent calcium current by 10-20%, block of voltage-dependent calcium current by nifedipine or Co2+ did not result in hyperpolarization of the cells. There was no effect of the Na+ substitutes on voltage-dependent potassium currents. In contrast, all three Na+ substitutes influenced calcium-activated potassium currents [IK(Ca)], but only at depolarized potentials. Choline consistently suppressed IK(Ca), whereas Tris and NMG either had no effect or slightly increased IK(Ca). These effects on IK(Ca) also cannot explain the hyperpolarization induced by removing bath Na+. Choline always hyperpolarized cells yet suppressed IK(Ca). Furthermore, removing bath Na+ caused an increase in cell input resistance, an observation consistent with the loss of a membrane conductance as the basis of the hyperpolarization. Direct measurement of background currents revealed a 12-pA inward current at -84 mV that was lost upon removing bath Na+. These results suggest that this background sodium conductance provides the depolarizing drive for GH3 cells to reach the threshold for firing calcium-dependent action potentials.