Fast Na+ channels and slow Ca2+ current in smooth muscle from pregnant rat uterus

Summary Smooth muscle cells normally do not possess fast Na²⁺ channels, but inward current is carried through two types of Ca²⁺ channels: slow (L-type) Ca²⁺ channels and fast (T-type) Ca²⁺ channels. Using whole-cell voltage clamp of single smooth muscle cells isolated from the longitudinal layer of 18-day pregnant rat uterus, depolarizing pusles, applied from a holding potential of –90 mV, evoked two types of inward current, fast and slow [8]. The fast inward current decayed within 30 ms, depended on [Na]₀, and was inhibited by TTX (K_0.5 = 27 nM). The slow inward current decayed slowly, was dependent on [Ca]₀, and was inhibited by nifedipine. These results suggest that the fast inward current is a fast Na²⁺ channel current, and that the slow inward current is a Ca²⁺ channel current was not evident. Thus, the ion channels which generate inward currents in pregnant rat uterine cells are TTX-sensitive fast Na⁺ channels and dihudropuridine-sensitive slow Ca²⁺ channels. The number of fast Na⁺ channels increased during gestation [9]. 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 which possess fast Na²⁺ channels, and it suggested that the fast Na⁺ current may be involved in spread of excitation. The Ca²⁺ channel current density also was 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 may facilitate parturition. Isoproterenol (beta-agonist) did not affect either I_Ca(s) or I_Na(f), whereas Mg²⁺ (K_0.5 of 12 mM) and nifedipine (K_0.5 of 3.3 nM) depressed I_Ca(s). Oxytocin had no effect on I_Na(f) and actually depressed I_Ca(s) to a small extect. Therefore, the tocolytic action of beta-agonists cannot be explained by an inhibition of I_Ca(s), whereas that of Mg²⁺ can be so explained. The stimulating action of oxytocin on uterine contractions is not due to stimulation of I_Ca(s).     

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.     

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.     

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.     

Excitation contraction coupling in skeletal muscle: evidence for a role of slow Ca2+ channels using Ca2+ channel activators and inhibitors in the dihydropyridine series

Ca2+ current and tension have been simultaneously recorded from single twitch fibres of the semi-tendinosus of Rana esculenta in a medium containing a physiological Ca2+ concentration (1.8 mM). Under appropriate conditions it can be shown that tension develops in two phases. The first is rapid and reaches its maximum before activation of the inward Ca2+ current. The second phase is slower and with a time course which appears to be correlated with that of the inward current. Nifedipine, a specific Ca2+ channel inhibitor greatly reduced ICa2+ and the slower component of tension. Bay K8644 a Ca2+ channel activator, which has receptors on T-tubule, increased ICa2+ and the slow component of tension. These results indicate that a slow component of skeletal muscle contraction is related to the inward Ca2+ current flowing through dihydropyridine sensitive voltage-dependent Ca2+ channels.     

Divalent cations as modulators of neuronal excitability: emphasis on copper and zinc

Based on indirect evidence, a role for synaptically released copper and zinc as modulators of neuronal activity has been proposed. To test this proposal directly, we studied the effect of copper, zinc, and other divalent cations on voltage-dependent currents in dissociated toad olfactory neurons and on their firing rate induced by small depolarizing currents. Divalent cations in the nanomolar range sped up the activation kinetics and increased the amplitude of the inward sodium current. In the micromolar range, they caused a dose dependent inhibition of the inward Na+ and Ca2+ currents (INa and ICa) and reduced de amplitude of the Ca2+-dependent K+ outward current (ICa-K). On the other hand, the firing rate of olfactory neurons increased when exposed to nanomolar concentration of divalent cations and decreased when exposed to micromolar concentrations. This biphasic effect of divalent cations on neuronal excitability may be explained by the interaction of these ions with high and low affinity sites in voltage-gated channels. Our results support the idea that these ions are normal modulators of neuronal excitability.     

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.     

Macroscopic Ca2+ -Na+ and K+ currents in single heart and aortic cells

The whole-cell voltage clamp technique was used to study the slow inward currents and K+ outward currents in single heart cells of embryonic chick and in rabbit aortic cells. In single heart cells of 3-day-old chick embryo three types of slow inward Na+ currents were found. The kinetics and the pharmacology of the slow INa were different from those of the slow ICa in older embryos. Two types of slow inward currents were found in aortic single cells of rabbit; angiotensin II increased the sustained type and d-cAMP and d-cGMP decreased the slow transient component. Two types of outward K+ currents were found in both aortic and heart cells. Single channel analysis demonstrated the presence of a high single K+ channel conductance in aortic cells. In cardiac and vascular smooth muscles, slow inward currents do share some pharmacological properties, although the regulation of these channels by cyclic nucleotides and several drugs seems to be different.     

Sodium-conducting channels in cardiac membranes in low calcium.

With no Ca in the patch electrode, two kinds of channels conduct Na in spontaneously beating embryonic chick heart cells. One channel conducts Na primarily during the upstroke of the action potential and is blocked by tetrodotoxin (TTX). The other channel conducts Na primarily during the late plateau and early repolarization phase of the action potential, but only in Ca concentrations below 10(-6) M. This second channel is TTX-insensitive and has a conductance of 50 to 90 pS, depending upon the interpretation of open-channel flickering. These two Na-conducting channels correspond to the channels that normally carry the fast Na current (INa) and the slow Ca current (Isi).     

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.     

Two Ca current components of the receptor current in the electroreceptors of the marine catfish Plotosus

In the isolated sensory epithelium of the Plotosus electroreceptor, the receptor current has been dissected into inward Ca current, ICa, and superimposed outward transient of Ca-gated K current, IK(Ca). In control saline (170 mM/liter Na), with IK(Ca) abolished by K blockers, ICa declined in two successive exponential phases with voltage-dependent time constants. Double-pulse experiments revealed that the test ICa was partially depressed by prepulses, maximally near voltage levels for the control ICa maximum, which suggests current-dependent inactivation. In low Na saline (80 mM/liter), ICa declined in a single phase with time constants similar to those of the slower phase in control saline. The test ICa was then unaffected by prepulses. The implied presence of two Ca current components, the fast and slow ICa's, were further examined. In control saline, the PSP externally recorded from the afferent nerve showed a fast peak and a slow tonic phase. The double-pulse experiments revealed that IK(Ca) and the peak PSP were similarly depressed, i.e., secondarily to inactivation of the peak current. The steady inward current, however, was unaffected by prolonged prepulses that were stepped to 0 mV, the in situ DC level. Therefore, the fast ICa seems to initiate IK(Ca) and phasic release of transmitter, which serves for phasic receptor responses. The slow ICa may provide persistent active current, which has been shown to maintain tonic receptor operation.     

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.     

Conductance and permeation of monovalent cations through depletion-activated Ca2+ channels (ICRAC) in Jurkat T cells.

We studied monovalent permeability of Ca2+ release-activated Ca2+ channels (ICRAC) in Jurkat T lymphocytes following depletion of calcium stores. When external free Ca2+ ([Ca2+]o) was reduced to micromolar levels in the absence of Mg2+, the inward current transiently decreased and then increased approximately sixfold, accompanied by visibly enhanced current noise. The monovalent currents showed a characteristically slow deactivation (tau = 3.8 and 21.6 s). The extent of Na+ current deactivation correlated with the instantaneous Ca2+ current upon readdition of [Ca2+]o. No conductance increase was seen when [Ca2+]o was reduced before activation of ICRAC. With Na+ outside and Cs+ inside, the current rectified inwardly without apparent reversal below 40 mV. The sequence of conductance determined from the inward current at -80 mV was Na+ > Li+ = K+ > Rb+ >> Cs+. Unitary inward conductance of the Na+ current was 2.6 pS, estimated from the ratios delta sigma2/delta Imean at different voltages. External Ca2+ blocked the Na+ current reversibly with an IC50 value of 4 microM. Na+ currents were also blocked by 3 mM Mg2+ or 10 microM La3+. We conclude that ICRAC channels become permeable to monovalent cations at low levels of external divalent ions. In contrast to voltage-activated Ca2+ channels, the monovalent conductance is highly selective for Na+ over Cs+. Na+ currents through ICRAC channels provide a means to study channel characteristics in an amplified current model.     

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.     

Voltage-dependent inactivation of slow calcium channels in intact twitch muscle fibers of the frog

Inactivation of slow Ca2+ channels was studied in intact twitch skeletal muscle fibers of the frog by using the three-microelectrode voltage-clamp technique. Hypertonic sucrose solutions were used to abolish contraction. The rate constant of decay of the slow Ca2+ current (ICa) remained practically unchanged when the recording solution containing 10 mM Ca2+ was replaced by a Ca2+-buffered solution (126 mM Ca-maleate). The rate constant of decay of ICa monotonically increased with depolarization although the corresponding time integral of ICa followed a bell-shaped function. The replacement of Ca2+ by Ba2+ did not result in a slowing of the rate of decay of the inward current nor did it reduce the degree of steady-state inactivation. The voltage dependence of the steady-state inactivation curve was steeper in the presence of Ba2+. In two-pulse experiments with large conditioning depolarizations ICa inactivation remained unchanged although Ca2+ influx during the prepulse greatly decreased. Dantrolene (12 microM) increased mechanical threshold at all pulse durations tested, the effect being more prominent for short pulses. Dantrolene did not significantly modify ICa decay and the voltage dependence of inactivation. These results indicate that in intact muscle fibers Ca2+ channels inactivate in a voltage-dependent manner through a mechanism that does not require Ca2+ entry into the cell.     

Effects of benzocaine on the kinetics of normal and batrachotoxin-modified Na channels in frog node of Ranvier.

The effects of benzocaine (0.5-1 mM) on normal Na currents, and on Na current and gating charge movement (Q) of batrachotoxin (BTX)-modified Na channels were analyzed in voltage-clamped frog node of Ranvier. Without BTX treatment the decay of Na current during pulses to between -40 and 0 mV could be decomposed into two exponential components both in the absence and in the presence of benzocaine. Benzocaine did not significantly alter the inactivation time constant of either component, but reduced both their amplitudes. The amplitude of the slow inactivating component was more decreased by benzocaine than the amplitude of the fast one, leading to an apparently faster decline of the overall Na current. After removal of Na inactivation and charge movement immobilization by BTX, benzocaine decreased the amplitude of INa with no change in time course. INa, QON, and QOFF were all reduced by the same factor. The results suggest that the rate of reaction of benzocaine with its receptor is slow compared to the rates of channel activation and inactivation. The differential effects of benzocaine on the two components of Na current inactivation in normal channels can be explained assuming two types of channel with different rates of inactivation and different affinities for the drug.     

Interaction of Ca2+ agonist and depolarization on Ca2+ channel current in guinea pig detrusor cells [published erratum appears in J Gen Physiiol 1996 Jun;107(6):755]

The interaction of large depolarization and dihydropyridine Ca2+ agonists, both of which are known to enhance L-type Ca2+ channel current, was examined using a conventional whole-cell clamp technique. In guinea pig detrusor cells, only L-type Ca2+ channels occur. A second open state (long open state: O2) of the Ca2+ channels develops during large depolarization (at +80 mV, without Ca2+ agonists). This was judged from lack of inactivation of the Ca2+ channel current during the large depolarizing steps (5 s) and slowly deactivating inward tail currents (= 10-15 ms) upon repolarization of the cell membrane to the holding potential (-60 mV). Application of Bay K 8644 (in 2.4 mM Ca(2+)- containing solutions) increased the amplitude of the Ca2+ currents evoked by simple depolarizations, and made it possible to observe inward tail currents (= 2.5-5 ms at -60 mV). The open state induced by large depolarization (O2*) in the Bay K 8644 also seemed hardly to inactivate. After preconditioning with large depolarizing steps, the decay time course of the inward tail currents upon repolarization to the holding potential (-60 mV) was significantly slowed, and could be fitted reasonably with two exponentials. The fast and slow time constants were 10 and 45 ms, respectively, after 2 s preconditioning depolarizations. Qualitatively the same results were obtained using Ba2+ as a charge carrier. Although the amplitudes of the inward currents observed in the test step and the subsequent repolarization to the holding potential were decreased in the same manner by additional application of nifedipine (in the presence of Bay K 8644), the very slow deactivation time course of the tail current was little changed. The additive enhancement by large depolarization and Ca2+ agonists of the inward tail current implies that two mechanisms separately induce long opening of the Ca2+ channels: i.e., that there are four open states.     

A role for the carbohydrate portion of ginsenoside Rg3 in Na+ channel inhibition

We showed recently that ginsenosides inhibit the activity of various types of ion channel. Here we have investigated the role of the carbohydrate component of ginsenoside Rg3 in the inhibition of Na+ channels. The channels were expressed in Xenopus oocytes by injecting cRNAs encoding rat brain Nav1.2 alpha and beta1 subunits, and analyzed by the two-electrode voltage clamp technique. Treatment with Rg3 reversibly inhibited the inward Na+ peak current (INa) with an IC50 of 32.2 +/- 4.5 microM, and the inhibition was voltage-dependent. To examine the role of the sugar moiety, we prepared a straight chain form of the second glucose and a conjugate of this glucose with 3-(4-hydroxyphenyl) propionic acid hydrazide (HPPH). Neither derivative inhibited INa. Treatment with the carbohydrate portion of ginsenoside Rg3, sophorose [beta-D-glucopyranosyl (1-->2)- beta-glucopyranoside], or the aglycone (protopanaxadiol), on their own or in combination had no effect on INa. These observations indicate that the carbohydrate portion of ginsenoside Rg3 plays an important role in its effect on the Na+ channel.     

Is inward calcium current in crayfish muscle membrane constituted of one or two components?

Two inward currents were observed in crayfish muscle membrane during depolarization steps by the method described by Adrian et al. (1970). Under voltage clamp conditions, hyperpolarization steps elicited a large current (leak current If), associated with an inward voltage dependent current. This inward current was inhibited by niflumic acid (NA), a drug known to block Cl---HCO-3 exchange (Cousin et Motais 1982; Br?lè et al. 1983b). Dynamic outward currents triggered by depolarizing steps were inhibited to a great extent by TEA, the not inhibited portion disappearing when procaine (2 mmol/l) was added to external solution. In the presence of TEA, procaine and NA, it was thus possible to dissect the regenerative calcium current (ICa) into two components: a "fast component" (ICa1) and a "slow component" (ICa2). The reversal potential of ICa was 65 mV (for [Ca]0 = 2.8 mmol/l), and [Ca]i could be calculated to be 1.6 X 10(-5) mol/l. This value of [Ca]i is the same as calculated from values reported by Hencek and Zachar (1977). ICa1 was triggered at a threshold membrane potential of -45 mV and ICa2 at -30 mV. Moreover, the inactivation kinetics for ICa1 was faster than that for ICa2. Our results are in perfect agreement with those obtained by Zahradník and Zachar (1982) who postulated two populations of calcium channels.     

Calcium-dependent transient potassium outward current in the marine ciliate Euplotes vannus

In the marine hypotrichous ciliate Euplotes vannus, the transient K+ outward current, IK fast, was studied by use of a single-microelectrode voltage-clamp equipment. Activation and inactivation kinetics, and steady-state inactivation are comparable to the properties of A-currents. Not typical for this type of current is its insensitivity to either 4-AP or 3,4-AP and its Ca2+ dependence which was derived from its inhibition by either extracellular Cd2+, La3+, D-600, or by intracellular BAPTA. Actual amplitudes of IK fast were obtained from a composite current, by subtraction of early parts of a slowly activating K+ current, IK slow, and of the early, transient Ca2+ inward current, ICa fast, that is typical for ciliates. IK fast counteracts ICa fast during the first milliseconds after onset of depolarization such that the composite current is purely outward directed.