Bethanidine increased Na+ and Ca2+ currents and caused a positive inotropic effect in heart cells

The effects of bethanidine sulphate, a pharmacological analog of the cardiac antibrillatory drug, bretylium tosylate, were studied on action potentials (APs) and K+, Na+, and Ca2+ currents of single cultured embryonic chick heart cells using the whole-cell current clamp and voltage clamp technique. Extracellular application of bethanidine (3 X 10(-4) M) increased the overshoot and the duration of the APs and greatly decreased the outward K+ current (IK) and potentiated the inward fast Na+ currents (INa) and the inward slow calcium current (ICa). However, intracellular introduction of bethanidine (10(-4) M) blocked INa. In isolated atria of rat, bethanidine increased the force of contraction in a dose-dependent manner. These findings suggest that when applied extracellularly, bethanidine exerts a potentiating effect on the myocardial fast Na+ current and slow Ca2+ current and an inhibitory effect of IK. The positive inotropic effect of bethanidine could be due, at least in part, to an increase of Ca2+ influx via the slow Ca2+ channel and the Na-Ca exchange. It is suggested that the decrease of IK by bethanidine may account for its antifibrillatory action.     

T-type Ca2+ channel lowers the threshold of spike generation in the newt olfactory receptor cell

Mechanisms underlying action potential generation in the newt olfactory receptor cell were investigated by using the whole-cell version of the patch-clamp technique. Isolated olfactory cells had a resting membrane potential of -70 +/- 9 mV. Injection of a depolarizing current step triggered action potentials under current clamp condition. The amplitude of the action potential was reduced by lowering external Na+ concentration. After a complete removal of Na+, however, cells still showed action potentials which was abolished either by Ca2+ removal or by an application of Ca2+ channel blocker (Co2+ or Ni2+), indicating an involvement of Ca2+ current in spike generation of newt olfactory receptor cells. Under the voltage clamp condition, depolarization of the cell to -40 mV from the holding voltage of -100 mV induced a fast transient inward current, which consisted of Na+ (INa) and T-type Ca2+ (ICa.T) currents. The amplitude of ICa,T was about one fourth of that of INa. Depolarization to more positive voltages also induced L-type Ca2+ current (ICa,L). ICa,L was as small as a few pA in normal Ringer solution. The activating voltage of ICa,T was approximately 10 mV more negative than that of INa. Under current clamp, action potentials generated by a least effective depolarization was almost completely blocked by 0.1 mM Ni2+ (a specific T-type Ca2+ channel blocker) even in the presence of Na+. These results suggest that ICa,T contributes to action potential in the newt olfactory receptor cell and lowers the threshold of spike generation.     

Prolonged high-pressure treatments in mammalian skeletal muscle result in loss of functional sodium channels and altered calcium channel kinetics

Activation and inactivation of ion channels involve volume changes from conformational rearrangements of channel proteins. These volume changes are highly susceptible to changes in ambient pressure. Depending on the pressure level, channel function may be irreversibly altered by pressure. The corresponding structural changes persist through the post-decompression phase. High-pressure applications are a useful tool to evaluate the pressure dependence as well as pressure limits for reversibility of such alterations. Mammalian cells are only able to tolerate much lower pressures than microorganisms. Although some limits for pressure tolerance in mammalian cells have been evaluated, the mechanisms of pressure-induced alteration of membrane physiology, in particular of channel function, are unknown. To address this question, we recorded fast inward sodium (I(Na)) and slowly activating L-type calcium (I(Ca)) currents in single mammalian muscle fibers in the post-decompression phase after a prolonged 3-h, high-pressure treatment of up to 20 MPa. I(Na) and I(Ca) peak amplitudes were markedly reduced after pressure treatment at 20 MPa. This was not from a general breakdown of membrane integrity as judged from in situ high-pressure fluorescence microscopy. Membrane integrity was preserved even for pressures as high as 35 MPa at least for pressure applications of shorter durations. Therefore, the underlying mechanisms for the observed amplitude reductions have to be determined from the activation (time-to-peak [TTP]) and inactivation (tau(dec)) kinetics of I(Na) and I(Ca). No major changes in I(Na) kinetics, but marked increases, both in TTP and tau(dec) for I(Ca), were detected after 20 MPa. The apparent molecular volume changes (activation volumes) deltaV(double dagger) for the pressure-dependent irreversible alteration of channel gating approached zero for Na+ channels. For Ca2+ channels, deltaV(double dagger) was very large, with approx 2.5-fold greater values for channel activation than inactivation (approx 210 A3). We conclude, that in skeletal muscle, high pressure differentially and irreversibly affects the gating properties and the density of functional Na+ and Ca2+ channels. Based on these results, a model of high pressure-induced alterations to the channel conformation is proposed.     

Selective block of calcium current by lanthanum in single bullfrog atrial cells

A single suction microelectrode voltage-clamp technique was used to study the actions of lanthanum ions (La3+) on ionic currents in single cells isolated from bullfrog right atrium. La3+, added as LaCl3, blocked the "slow" inward Ca2+ current (ICa) in a dose-dependent fashion; 10(-5) M produced complete inhibition. This effect was best fitted by a dose-response curve that was calculated assuming 1:1 binding of La3+ to a site having a dissociation constant of 7.5 x 10(-7) M. La3+ block was reversed (to 90% of control ICa) following washout and, in the presence of 10(-5) M La3+, was antagonized by raising the Ca2+ concentration from 2.5 to 7.5 mM (ICa recovered to 56% of the control). However, the latter effect took approximately 1 h to develop. Concentrations of La3+ that reduced ICa by 12-67%, 0.1-1.5 x 10(-6) M, had no measurable effect upon the voltage dependence of steady state ICa inactivation, which suggest that at these concentrations there are no significant surface-charge effects of La3+ on this gating mechanism. Three additional findings indicate that doses of La3+ that blocked ICa failed to produce nonspecific effects: (a) 10(-5) M La3+ had no measurable effect on the time-independent inwardly rectifying current, IK1; (b) the same concentration had no effect on the kinetics, amplitude, or voltage dependence of a time- and voltage-dependent K+ current, IK; and (c) 10(-4) M La3+ did not alter the size of the tetrodotoxin-sensitive inward Na+ current, INa, or the voltage dependence of its steady state inactivation. Higher concentrations (0.5-1.0 mM) reduced both IK1 and IK, and shifted the steady state activation curve for IK toward more positive potentials, presumably by reducing the external surface potential. Our results suggest that at a concentration of less than or equal to 10(-5) M, La3+ inhibits ICa selectively by direct blockade of Ca channels rather than by altering the external surface potential. At higher concentrations, La3+ exhibits nonspecific effects, including neutralization of negative external surface charge and inhibition of other time- and voltage-dependent ionic currents.     

Maturation involves suppression of voltage-gated currents in the frog oocyte

Voltage- and time-dependent currents having slow kinetics have been studied in plasma membranes of immature oocytes of the european frog, Rana esculenta. IK, corresponding to an outward flow of K+, is activated at potentials more positive than about -40 mV, and subserves outward rectification; Iir, corresponding to an outward flow of Cl-, is activated at potentials more negative than about -80 mV and subserves inward rectification. Such currents can act as negative feedback mechanisms in the control of membrane potential in the immature oocyte and limit to a somewhat restricted range its possible deviations from resting values. Besides IK, membrane depolarizations to potentials more positive than about +30 mV are capable of activating INa, corresponding to outflow of Na+. By contrast, the frog mature egg-cell has a single voltage- and time-dependent current, IM, activated at potentials more positive than +30 mV, with properties similar to INa. The disappearance of IK and Iir along with remarkable reduction in leakage lowers impedance in the egg membrane. It seems reasonable to suggest that the observed changes in membrane permeability reflect changes which have taken place along the maturation process and are of importance for successful fertilization.     

Ionic currents underlying the action potential of Rana pipiens oocytes

Ionic currents in immature, ovulated Rana pipiens oocytes (metaphase I) were studied using the voltage-clamp technique. At this stage of maturity the oocyte can produce action potentials in response to depolarizing current or as an "off response" to hyperpolarizing current. Reducing external Na+ to 1/10 normal (choline substituted) eliminated the action potentials and both the negative-slope region and zero-crossing of the I-V relation. Reducing external Cl- to 1/10 or 1/100 normal (methanesulfonate substituted) lengthened the action potential. The outward current was reduced and a net inward current was revealed. By changing external Na+, Cl-, and K+ concentrations and using blocking agents (SITS, TEA), three voltage- and time-dependent currents were identified, INa, IK and ICl. The Na+ current activated at about 0 mV and reversed at very positive values which decreased during maturation. Inward Na+ current produced the upstroke of the action potential. During each voltage-clamp step the Na+ current activated slowly (seconds) and did not inactivate within many minutes. The Na+ current was not blocked by TTX at micromolar concentrations. The K+ current was present only in the youngest oocytes. Because IK was superimposed on a large leakage current, it appeared to reverse at the resting potential. When leakage currents were subtracted, the reversal potential for IK was more negative than -110 mV in Ringer's solution. IK was outwardly rectifying and strongly activated above -50 mV. The outward K+ current produced an after hyperpolarization at the end of each action potential. IK was blocked completely and reversibly by 20 mM external TEA. The Cl- current activated at about +10 mV and was outwardly rectifying. ICl was blocked completely and reversibly by 400 microM SITS added to the bathing medium. This current helped repolarize the membrane following an action potential in the youngest oocytes and was the only repolarizing current in more mature oocytes that had lost IK. The total leakage current had an apparently linear I-V relation and was separated into two components: a Na+ current (IN) and a smaller component carried by as yet unidentified ions.     

A model of the electrophysiological properties of nucleus reticularis thalami neurons.

A model of the electrophysiological properties of rodent nucleus reticularis thalami (NRT) neurons of the dorsal lateral thalamus was developed using Hodgkin-Huxley style equations. The model incorporated voltage-dependent rate constants and kinetics obtained from recent voltage-clamp experiments in vitro. The intrinsic electroresponsivity of the model cell was found to be similar to several empirical observations. Three distinct modes of oscillatory activity were identified: 1) a pattern of slow rhythmic burst firing (0.5-7 Hz) usually associated with membrane potentials negative to approximately -70 mV which resulted from the interplay of ITs and IK(Ca); 2) at membrane potentials from approximately -69 to -62 mV, rhythmic burst firing in the spindle frequency range (7-12 Hz) developed and was immediately followed by a tonic tail of single spike firing after several bursts. The initial bursting rhythm resulted from the interaction of ITs and IK(Ca), with a slow after-depolarization due to ICAN which mediated the later tonic firing; 3) with further depolarization of the membrane potential positive to approximately -61 mV, sustained tonic firing appeared in the 10-200-Hz frequency range depending on the amplitude of the injected current. The frequency of this firing was also dependent on the maximum conductance of the leak current, IK(leak), and an interaction between the fast currents involved in generating action potentials, INa(fast) and IK(DR), and the persistent Na+ current, INa(P). Transitions between different firing modes were identified and studied parametrically.     

Sodium current in voltage clamped internally perfused canine cardiac Purkinje cells.

Study of the excitatory sodium current (INa) intact heart muscle has been hampered by the limitations of voltage clamp methods in multicellular preparations that result from the presence of large series resistance and from extracellular ion accumulation and depletion. To minimize these problems we voltage clamped and internally perfused freshly isolated canine cardiac Purkinje cells using a large bore (25-microns diam) double-barreled flow-through glass suction pipette. Control of [Na+]i was demonstrated by the agreement of measured INa reversal potentials with the predictions of the Nernst relation. Series resistance measured by an independent microelectrode was comparable to values obtained in voltage clamp studies of squid axons (less than 3.0 omega-cm2). The rapid capacity transient decays (tau c less than 15 microseconds) and small deviations of membrane potential (less than 4 mV at peak INa) achieved in these experiments represent good conditions for the study of INa. We studied INa in 26 cells (temperature range 13 degrees-24 degrees C) with 120 or 45 mM [Na+]o and 15 mM [Na+]i. Time to peak INa at 18 degrees C ranged from 1.0 ms (-40 mV) to less than 250 microseconds (+ 40 mV), and INa decayed with a time course best described by two time constants in the voltage range -60 to -10 mV. Normalized peak INa in eight cells at 18 degrees C was 2.0 +/- 0.2 mA/cm2 with [Na+]o 45 mM and 4.1 +/- 0.6 mA/cm2 with [Na+]o 120 mM. These large peak current measurements require a high density of Na+ channels. It is estimated that 67 +/- 6 channels/micron 2 are open at peak INa, and from integrated INa as many as 260 Na+ channels/micron2 are available for opening in canine cardiac Purkinje cells.     

Sodium and calcium currents in neuroblastoma x glioma hybrid cells before and after morphological differentiation by dibutyryl cyclic AMP

Sodium and calcium inward currents (INa and ICa) were measured in neuroblastoma X glioma hybrid cells of clones 108CC5 and 108CC15 by a single suction pipette method for internal perfusion and voltage clamp. Morphologically undifferentiated, exponentially growing cells were compared with cells differentiated by cultivation with 1 mmol/l dibutyryl cyclic AMP. Outward currents were eliminated by perfusing the cells with a K+-free solution. Voltage dependence and ion selectivity as well as steady state inactivation characteristics of INa and ICa resembled those of differentiated mouse neuroblastoma cells, clone N1E-115 (Moolenaar and Spector 1978, 1979). These parameters were identical in undifferentiated and differentiated cells of both clones. After differentiation the average density of the peak sodium and calcium currents was increased two and four-fold, respectively, in both cell lines. Our data indicate that exponentially growing, morphologically undifferentiated 108CC5 and 108CC15 neuroblastoma X glioma hybrid cells possess functional Na+ and Ca2+ channels undistinguishable from those of non-proliferating cells of these clones differentiated morphologically by treatment with dibutyryl cyclic AMP. That Na+ and Ca2+ spikes were not detected by other authors in these cells prior to morphological differentiation by dibutyryl cyclic AMP may be attributed to the fact that at the low resting membrane potential measured the Na+ and Ca2+ channels are inactivated.     

Inositol trisphosphate regulation of photoreceptor membrane currents.

In previous studies elevation of intracellular Ca2+ was shown to cause prolonged reduction of two voltage-dependent K+ currents (IA and ICa2+-K+) across the membrane of the isolated Hermissenda photoreceptor, the type B cell (Alkon et al., 1982b; Alkon and Sakakibara, 1985). Here we show that iontophoretic injection of inositol trisphosphate (IP3), but not inositol monophosphate, also caused prolonged reduction of IA and ICa2+-K+. IP3 injection also caused reduction of a light-induced K+ current (also ICa2+-K+) but did not affect the voltage-dependent Ca2+ current, ICa2+, or the light-induced inward current, INa+, of the type B cell. IP3 injection caused similar effects on the K+ currents of the other type of Hermissenda photoreceptor, the type A cell. INA+ of the type A cell, unlike that of the type B cell, was, however, markedly increased following IP3 injection. The differences of IP3 effects on the two types of photoreceptors may be related to differences in regulation of ionic currents by endogenous IP3 as reflected by clear differences (before injection) in the magnitude of IA, ICa2+-K+, and INa+ between the two cell types.     

Effects of capsaicin on molluscan neurons: a voltage clamp study

The effects of capsaicin (CAP) on membrane ionic currents of identified and non-identified neurons were investigated by use of the single electrode clamp (SEC). CAP (300 microM, 22 degrees C, pH 7.4) caused a 25-50% reduction of the inward current and a 50-80% reduction of the outward current in normal or Na-free (Tris) solution. The Na current (INa) was moderately decreased (about 10%) in LPa2 neuron, but a 50% reduction of the peak Ca current (ICa) was observed. The action of CAP on ICa varied from cell to cell but an enhanced inactivation of the fast calcium current was found in all neurons studied. CAP (150 microM, 10 min) highly attenuated the long-lasting component of the inward current in LPa2 recorded in Na-free (TEA) Ba solutions. CAP attenuated the fast outward current (IA) and voltage-dependent outward current (IK) in 100 and 300 microM concentrations for the half blocking dose (ID50) in LPa2 neuron, respectively. CAP decreased the slow outward tail currents but hardly influenced the leakage current (IL). We suggest that the acute action of CAP coupled with a series of events in the neuronal membrane can modify the conductance via electrically excitable calcium, potassium and sodium channels differentially.     

Effects of KC 3791 on sodium and potassium channels in frog node of Ranvier

Effects of a new antiarrhytmic compound KC 3791 on sodium (INa) and potassium (IK) currents were studied in frog myelinated nerve fibres under voltage clamp conditions. When applied externally to the node of Ranvier, KC 3791 (KC) at concentrations of 10(-5)-10(-4) mol.l-1 produced both tonic and cumulative (use-dependent) inhibition of INa. An analysis of the frequency-, voltage- and time dependence of cumulative block by KC suggested that this block resulted from a voltage-dependent interaction of the drug with open Na channels. The progressive decrease in INa during repetitive pulsing was due to accumulation of Na channels in the resting-blocked state: closing of the activation gate after the end of each depolarizing pulse stabilized the KC-"receptor" complex. To unblock these channels a prolonged washing of the node had to be combined with a subsequent repetitive stimulation of the membrane; this suggested that channel could not become cleared of the blocker unless the activation gate has opened. KC also proved to be capable of blocking open K channels at outwardly directed potassium currents (IK). This block increased during membrane depolarization. Unblocking of K channels after the end of a depolarizing pulse proceeded much faster than unblocking of Na channels under identical conditions. Cumulative inhibition of outward IK during high-frequency membrane stimulation was therefore readily reversible upon a decrease in pulsing frequency.     

Electrical properties and cell-to-cell communication of the salivary gland cells of the snail, Helix pomatia

The aim of the present study was to assess the cellular mechanism of secretion in the salivary gland of the snail, Helix pomatia, using electrophysiological, electron microscopic and immunohistochemical techniques. A homogeneously distributed membrane potential (-56.6 +/- 9.8 mV) was determined mainly by a K+ -electrochemical gradient and partly by the contribution of the electrogenic Na+ -pump and Cl- conductance. Low resistance electrical coupling sites were identified physiologically. Transmission electron microscopy and innexin 2 antibody revealed the presence of gap-junction-like membrane structures between gland cells. It is suggested that gap-junctions are sites of electrotonic intercellular communication, which integrate the gland cells into a synchronized functional unit in the acinus. Stimulation of the salivary nerve elicited secretory potentials (depolarization) which could be mimicked by local application of acetylcholine, dopamine or serotonin. In voltage-clamp experiments four major conductances were identified: a delayed rectifier (IK), a transient (IA) and a Ca2+ -activated outward K+ current (IK(Ca)) and Ca2+ -inward currents (ICa). It is suggested that one or more of these conductances may give rise to a stimulus activated secretory potential leading to excitation-secretion coupling and subsequent the release of the mucus from the gland cells.     

Reactivation processes of three inward current systems involved in the rising phase of the action potentials in embryonic chick hearts

In embryonic chick hearts during development, there are three inward current systems which are involved in the rising phases of the action potentials (APs): fast INa, slow ICa, and tetrodotoxin-insensitive slow INa. To assess reactivation processes for these three types of inward current channels (fast Na+, slow Ca2+, and slow Na+ channels), diastolic recovery of Vmax was examined in embryonic chick hearts using a paired-pulse protocol. In all cases, the diastolic recoveries were approximated by single exponential functions. The time constants of recovery (tau(V)) and T90% (the diastolic interval which allows 90% recovery of Vmax of the premature AP) were, respectively, 53.1 +/- 5.2 and 61.5 +/- 8.6 ms for Na+-dependent fast AP (n = 10), 376.9 +/- 49.3 and 659.2 +/- 113.1 ms for the Ca2+-dependent slow AP (n = 10), and 40.7 +/- 5.3 and 45.6 +/- 12.0 ms for the Na+-dependent slow AP (n = 10). In the presence of lidocaine, the recovery kinetics also appeared to be single exponentials for diastolic intervals up to 500 ms (fast APs) or 250 ms (slow APs). The reactivation processes for the Na+-dependent fast and slow channels were significantly slowed by 100 microM lidocaine. In addition, in the presence of 100 microM lidocaine, Vmax was depressed in a frequency-dependent manner; the higher the stimulation frequency, the greater the depression. Hence, the fast Na+ channels and the slow Na+ channels had the following similarities: rapid reactivation, reactivation slowed by lidocaine, and frequency-dependent depression in the presence of lidocaine.     

Voltage-dependent block of cardiac inward-rectifying potassium current by monovalent cations

The inward-rectifying K+ current (IK1) in cat ventricular myocytes, like inward-rectifying K+ currents in many other preparations, exhibited a negative slope conductance region at hyperpolarized membrane potentials that was time-dependent. This was evident as an inactivation of inward current elicited by hyperpolarizing voltage-clamp pulses resulting in a negative slope region of the steady-state current-voltage relationship at potentials negative to -140 mV. Removing extracellular Na+ prevented the development of the negative slope in this voltage region, suggesting that Na+ can block IK1 channels in a time- and voltage-dependent manner. The time and voltage dependence of Cs+-induced block of IK1 was also examined. Cs+ blocked inward current in a manner similar to that of Na+, but the former was much more potent. The fraction of current blocked by Cs+ in the presence of Na+ was reduced in a time- and voltage-dependent manner, which suggested that these blocking ions compete for a common or at least similar site of action. In the absence of Na+, inactivation of IK1 could also be induced by both Cs+ and Li+. However, Li+ was less potent than Na+ in this respect. Calculation of the voltage sensitivity of current block by each of these ions suggests that the mechanism of block by each is similar.     

Tamoxifen inhibits Na+ and K+ currents in rat ventricular myocytes

Tamoxifen is an estrogen receptor antagonist used in the treatment of breast cancer. However, tamoxifen has been shown to induce QT prolongation of the electrocardiogram, thereby potentially causing life-threatening polymorphic ventricular arrhythmias. The purpose of the present study was to elucidate the electrophysiological mechanism(s) that underlie the arrhythmogenic effects of tamoxifen. We used standard ruptured whole cell and perforated patch-clamping techniques on rat ventricular myocytes to investigate the effects of tamoxifen on cardiac action potential (AP) waveforms and the underlying K+ currents. Tamoxifen (3 micromol/l) markedly prolonged AP duration, decreased maximal rate of depolarization, and decreased resting membrane potential. At this concentration, tamoxifen significantly depressed the Ca2+-independent transient outward K+ current (Ito), sustained outward delayed rectifier K+ current (Isus), inward rectifier K+ current (IK1), and Na+ current (INa) in the myocytes. Lower concentrations of tamoxifen (1 micromol/l) also decreased the resting membrane potential and significantly depressed IK1 to 79 +/- 5% (n = 5; at -120 mV) of pretreatment values. The results of this study indicate that inhibition of Ito, Isus, and IK1 by tamoxifen may underlie AP prolongation in cardiac myocytes and thereby contribute to prolonged QT interval observed in patients.     

The blocking action of platelet-activating factor (PAF) on the calcium currents of the atrial fibers in the frog and guinea pig

The effects of platelet-activating factor (PAF) on the myocardial cell membrane Ca-current (ICa) and Ca-action potential (Ca-AP) were investigated. In double sucrose-gap voltage-clamped frog atrial trabeculae PAF (2 X 10(-7) M) reduced ICa-amplitude to 40-50%; at the same time the IK-amplitude was increased to the same value. These changes of ICa and IK amplitudes were protected by simultaneous action of PAF and PAF antagonist BN 52021 (4 X 10(-6) M). In the partially depolarized (K+0 = 15-20 mM) of the guinea pig myocardial auricles PAF decreased Ca-AP amplitude and Vmax of its upstroke and shortened the Ca-AP duration (intracellular microelectrodes) like the isometric tension responses. These effects were prevented by PAF antagonist U-66985. Histamine was also able to protect from the PAF-induced changes of Ca-AP and tension responses. Our data demonstrated both by direct and by indirect methods of ICa registration in myocardia membrane that PAF induces reversed blocking of ICa. Because the blocking effects of PAF on frog and guinea pig myocardium are identical, these results imply that the mechanisms of PAF action on cold- and warm-blooded animals are similar in principle. The coupling of ICa and IK changes confirm our earlier supposition that PAF-induced Ca-AP shorting can be explained by IK augmentation.     

SUN 1165: a new antiarrhythmic Na current blocker in ventricular myocytes of guinea-pig

1. We compared the effect of a new antiarrhythmic compound, SUN 1165, on Na and Ca channels in papillary muscles and enzymatically dispersed single ventricular cells of guinea-pig. Action potential and contractile force in papillary muscle were measured by the conventional microelectrode technique and a strain gauge. The membrane currents were measured in internally perfused and voltage clamped cells by a single suction pipette technique. 2. In papillary muscles, SUN 1165 depressed the maximum rate of rise of action potential (Vmax) in a concentration dependent manner (IC30 = 1.7 X 10(-5) M) more markedly (about six times) than the contractile force. 3. In single ventricular cells, the Na current (INa) was reduced by the drug in a concentration dependent manner (IC30 = 9.1 X 10(-6) M). 4. It showed frequency-dependent block and the steady-state inactivation curve was shifted to more negative potentials. 5. The recovery of INa from inactivation was prolonged by SUN 1165. 6. The Ca current (ICa) was also blocked by the drug in a concentration dependent manner but much less than INa (IC30 = 5.5 X 10(-5) M). 7. These results suggested that SUN 1165 causes a selective inhibition of Na channels in guinea-pig ventricular cells at the antiarrhythmic concentrations.     

Fast Na+ channels in smooth muscle from pregnant rat uterus.

Smooth muscle cells normally do not possess fast Na+ channels, but inward current is carried through two types of Ca2+ channels: slow (L type) Ca2+ channels and fast (T type) Ca2+ channels. Whole-cell voltage clamp was done on single smooth muscle cells isolated from the longitudinal layer of the 18-day pregnant rat uterus. Depolarizing pulses, applied from a holding potential of -90 mV, evoked two types of inward current, fast and slow. The fast inward current decayed within 30 ms, depended on [Na]o, and was inhibited by tetrodotoxin (TTX) (K0.5 = 27 nM). The slow inward current decayed slowly, was dependent on [Ca]o (or Ba2+), and was inhibited by nifedipine. These results suggest that the fast inward current is a fast Na+ channel current and that the slow inward current is a Ca2+ slow channel current. A fast-inactivating Ca2+ channel current was not evident. We conclude that the ion channels that generate inward currents in pregnant rat uterine cells are TTX-sensitive fast Na+ channels and dihydropyridine-sensitive slow Ca2+ channels. The number of fast Na+ channels increased during gestation. The averaged current density increased from 0 on day 5, to 0.19 on day 9, to 0.56 on day 14, to 0.90 on day 18, and to 0.86 pA/pF on day 21. This almost linear increase occurs because of an increase in the fraction of cells that possess fast Na+ channels. The Ca2+ channel current density was also higher during the latter half of gestation. These results indicate that the fast Na+ channels and Ca2+ slow channels in myometrium become more numerous as term approaches, and we suggest that the fast Na+ current may be involved in spread of excitation. Isoproterenol (beta-agonist) did not affect either ICa(s) or INa(f), whereas Mg2+ (K0.5 = 12 mM) and nifedipine (K0.5 = 3.3 nM) depressed ICa(s). Oxytocin had no effect on INa(f) and actually depressed ICa(s) to a small extent. Therefore, the tocolytic action of beta-agonists cannot be explained by an inhibition of ICa(s), whereas that of Mg2+ can be so explained. The stimulating action of oxytocin on uterine contractions cannot be explained by a stimulation of ICa(s).