巨孔匙(虫戚)(Megathura)未受精卵细胞膜离子流的特征

用双微电极电压钳技术在巨孔匙(虫戚)(Megathura)未受精卵细胞膜上记录到多种离子流。主要有一种内向的两价离子流和几种钾离子流:包括钡离子激活的钾离子流,迅速激活又迅速失活的钾离子流(类似于I_A)和异常整流钾离子流。不同细胞的离子流大小不同。在一些卵可能会缺少其中某一种离子流。此外,还观察到浴槽溶液中氯和钠离子浓度改变对膜电位及膜电导的影响。     

Mechanisms of generation of long action potentials in barium solutions

Under voltage clamp conditions ionic currents of neurons of the molluskHelix were studied in solutions containing barium ions. Replacement of the calcium ions in the normal external solution by barium ions led to displacement of the potassium conductivity versus membrane potential curve along the voltage axis toward more positive potentials and also to a decrease in the limiting value of the potassium conductance of the membrane. In sodium- and calcium-free solutions containing barium ions two fractions of the inward current are recorded: quickly (I) and slowly (II) inactivated. The rates of activation of these fractions are comparable. Barium ions are regarded as carriers of both fractions of the inward current. It is postulated that both fractions of the barium current are carried along the calcium channels of the membrane.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 9, No. 4, pp. 408–414, July–August, 1977.     

机械分离的果蝇幼虫中枢神经元全细胞钾电流的特性

培养的果蝇胚胎及幼虫中枢神经元已被广泛用于细胞膜离子通道,突触传递和胞内信使调节等电生理学研究,在本实验中,利用机械震荡分离方法获得了大量的果蝇幼虫中枢神经元,其中大部分为Ⅱ型神经元,运用膜片钳技术,鉴定了Ⅱ型神经元上五种具有不同动力学特性的全细胞钾电流,其中E型电流表型表现出与其它四种电流完全不同的“钟形”激活特性,进一步的研究还表明该类型电流具有明显的钙依赖性,而且它具有与其它四种电流不同的衰减特性。     

Study of a neuronal potassium current in different culture conditions

The potassium current of neurons in explants cultured from quail mesencephalic neural crest were studied in voltage clamp, using the whole cell recording technique. Two voltage-dependent potassium currents were identified; they differed in their sensitivity to blocking agents and to sustained depolarizing voltages. The potassium current component most sensitive to 4-aminopyridine had fast activation kinetics and inactivated quickly at sustained depolarized voltages. By analogy with a current described in other preparations, this current was called IA. The current component most sensitive to tetraethylammonium had slower activation kinetics and inactivated more slowly than IA at sustained depolarized voltages. This current was called IK. The properties of IA and IK were examined in neurons cultured in a defined medium and in neurons co-cultured with striated muscle. The rate of inactivation of IA appeared to be increased when neural crest neurons were cultured in the presence of striated muscle. The change in the properties of IA could be due to a direct effect of the co-culture with muscle on the membrane current; another possibility could be that co-culture favors the survival of a neuronal population that does not survive well when cultured in a defined medium.     

Fluid Recirculation in Necturus Intestine and the Effect of Alanine

Previous studies in our laboratory have shown that Na absorption across the porcine endometrium is stimulated by PGF_2α and cAMP-dependent activation of a barium-sensitive K channel located in the basolateral membrane of surface epithelial cells. In this study, we identify and characterize this basolateral, barium-sensitive K conductance. Porcine uterine tissues were mounted in Ussing chambers and bathed with KMeSO₄ Ringer solution. Amphotericin B (70 μm) was added to the luminal solution to permeabilize the apical membrane and determine the current-voltage relationship of the basolateral K conductance after activation by 100 μm CPT-cAMP. An inwardly rectifying current was identified which possessed a reversal potential of −53 mV when standard Ringer solution was used to bathe the serosal surface. The K:Na selectivity ratio was calculated to be 12:1. Administration of 5 mm barium to the serosal solution completely inhibited the current activated by cAMP under these conditions. In addition to these experiments, amphotericin-perforated whole cell patch clamp recordings were obtained from primary cultures of porcine surface endometrial cells. The isolated cells displayed an inwardly rectifying current under basal conditions. This current was significantly stimulated by CPT-cAMP and blocked by barium. These results together with our previous studies demonstrate that cAMP increases Na absorption in porcine endometrial epithelial cells by activating an inwardly rectifying K channel present in the basolateral membrane. Similar patch clamp experiments were conducted using cells from a human endometrial epithelial cell line, RL95-2. An inwardly rectifying current was also identified in these cells which possessed a reversal potential of −56 mV when the cells were bathed in standard Ringer solution. This current was blocked by barium as well as cesium. However, the current from the human cells did not appear to be activated by cAMP, indicating that distinct subtypes of inwardly rectifying K channels are present in endometrial epithelial cells from different species. Received: 6 February 1997/Revised: 10 July 1997     

Electrophysiological and Theoretical Analysis of Depolarization-Dependent Outward Currents in the Dendritic Membrane of an Identified Nonspiking Interneuron in Crayfish

Depolarization-dependent outward currents were analyzed using the single-electrode voltage clamp technique in the dendritic membrane of an identified nonspiking interneuron (LDS interneuron) in situ in the terminal abdominal ganglion of crayfish. When the membrane was depolarized by more than 20 mV from the resting potential (65.0 ± 5.7 mV), a transient outward current was observed to be followed by a sustained outward current. Pharmacological experiments revealed that these outward currents were composed of 3 distinct components. A sustained component (I _s) was activated slowly (half rise time > 5 msec) and blocked by 20 mM TEA. A transient component (I _t1) that was activated and inactivated very rapidly (peak time < 2.5 msec, half decay time < 1.2 msec) was also blocked by 20 mM TEA. Another transient component (I _t2) was blocked by 100 M 4AP, activated rapidly (peak time < 10.0 msec) and inactivated slowly (half decay time > 131.8 msec). Two-step pulse experiments have revealed that both sustained and transient components are not inactivated at the resting potential: the half-maximal inactivation was attained at –21.0 mV in I _t1, and –38.0 mV in I _t2. I _s showed no noticeable inactivation. When the membrane was initially held at the resting potential level and clamped to varying potential levels, the half-maximal activation was attained at –36.0 mV in I _s, –31.0 mV in I _t1 and –40.0 mV in I _t2. The activation and inactivation time constants were both voltage dependent. A mathematical model of the LDS interneuron was constructed based on the present electrophysiological records to simulate the dynamic interaction of outward currents during membrane depolarization. The results suggest that those membrane conductances found in this study underlie the outward rectification of the interneuron membrane as well as depolarization-dependent shaping of the excitatory synaptic potential observed in current-clamp experiments.     

Gating of O2-sensitive K+ channels of arterial chemoreceptor cells and kinetic modifications induced by low PO2

We have studied the kinetic properties of the O2-sensitive K+ channels (KO2 channels) of dissociated glomus cells from rabbit carotid bodies exposed to variable O2 tension (PO2). Experiments were done using single-channel and whole-cell recording techniques. The major gating properties of KO2 channels in excised membrane patches can be explained by a minimal kinetic scheme that includes several closed states (C0 to C4), an open state (O), and two inactivated states (I0 and I1). At negative membrane potentials most channels are distributed between the left-most closed states (C0 and C1), but membrane depolarization displaces the equilibrium toward the open state. After opening, channels undergo reversible transitions to a short-living closed state (C4). These transitions configure a burst, which terminates by channels either returning to a closed state in the activation pathway (C3) or entering a reversible inactivated conformation (I0). Burst duration increases with membrane depolarization. During a maintained depolarization, KO2 channels make several bursts before ending at a nonreversible, absorbing, inactivated state (I1). On moderate depolarizations, KO2 channels inactivate very often from a closed state. Exposure to low PO2 reversibly induces an increase in the first latency, a decrease in the number of bursts per trace, and a higher occurrence of closed-state inactivation. The open state and the transitions to adjacent closed or inactivated states seem to be unaltered by hypoxia. Thus, at low PO2 the number of channels that open in response to a depolarization decreases, and those channels that follow the activation pathway open more slowly and inactivate faster. At the macroscopic level, these changes are paralleled by a reduction in the peak current amplitude, slowing down of the activation kinetics, and acceleration of the inactivation time course. The effects of low PO2 can be explained by assuming that under this condition the closed state C0 is stabilized and the transitions to the absorbing inactivated state I1 are favored. The fact that hypoxia modifies kinetically defined conformational states of the channels suggests that O2 levels determine the structure of specific domains of the KO2 channel molecule. These results help to understand the molecular mechanisms underlying the enhancement of the excitability of glomus cells in response to hypoxia.     

Voltage- and time-dependent K+ channel currents in the basolateral membrane of villus enterocytes isolated from guinea pig small intestine

Patch-clamp studies were carried out in villus enterocytes isolated from the guinea pig proximal small intestine. In the whole-cell mode, outward K+ currents were found to be activated by depolarizing command pulses to -45 mV. The activation followed fourth order kinetics. The time constant of K+ current activation was voltage-dependent, decreasing from approximately 3 ms at -10 mV to 1 ms at +50 mV. The K+ current inactivated during maintained depolarizations by a voltage- independent, monoexponential process with a time constant of approximately 470 ms. If the interpulse interval was shorter than 30 s, cumulative inactivation was observed upon repeated stimulations. The steady state inactivation was voltage-dependent over the voltage range from -70 to -30 mV with a half inactivation voltage of -46 mV. The steady state activation was also voltage-dependent with a half- activation voltage of -22 mV. The K+ current profiles were not affected by chelation of cytosolic Ca2+. The K+ current induced by a depolarizing pulse was suppressed by extracellular application of TEA+, Ba2+, 4-aminopyridine or quinine with half-maximal inhibitory concentrations of 8.9 mM, 4.6 mM, 86 microM and 26 microM, respectively. The inactivation time course was accelerated by quinine but decelerated by TEA+, when applied to the extracellular (but not the intracellular) solution. Extracellular (but not intracellular) applications of verapamil and nifedipine also quickened the inactivation time course with 50% effective concentrations of 3 and 17 microM, respectively. Quinine, verapamil and nifedipine shifted the steady state inactivation curve towards more negative potentials. Outward single K+ channel events with a unitary conductance of approximately 8.4 pS were observed in excised inside-out patches of the basolateral membrane, when the patch was depolarized to -40 mV. The ensemble current rapidly activated and thereafter slowly inactivated with similar time constants to those of whole-cell K+ currents. It is concluded that the basolateral membrane of guinea pig villus enterocytes has a voltage-gated, time-dependent, Ca(2+)-insensitive, small-conductance K+ channel. Quinine, verapamil, and nifedipine accelerate the inactivation time course by affecting the inactivation gate from the external side of the cell membrane.     

Calcium-dependent inactivation of the dihydropyridine-sensitive calcium channels in GH3 cells

The inactivation of calcium channels in mammalian pituitary tumor cells (GH3) was studied with patch electrodes under voltage clamp in cell-free membrane patches and in dialyzed cells. The calcium current elicited by depolarization from a holding potential of -40 mV passed predominantly through one class of channels previously shown to be modulated by dihydropyridines and cAMP-dependent phosphorylation (Armstrong and Eckert, 1987). When exogenous calcium buffers were omitted from the pipette solution, the macroscopic calcium current through those channels inactivated with a half time of approximately 10 ms to a steady state level 40-75% smaller than the peak. Inactivation was also measured as the reduction in peak current during a test pulse that closely followed a prepulse. Inactivation was largely reduced or eliminated by (a) buffering free calcium in the pipette solution to less than 10(-8) M; (b) replacing extracellular calcium with barium; (c) increasing the prepulse voltage from +10 to +60 mV; or (d) increasing the intracellular concentration of cAMP, either 'directly' with dibutyryl-cAMP or indirectly by activating adenylate cyclase with forskolin or vasoactive intestinal peptide. Thus, inactivation of the dihydropyridine-sensitive calcium channels in GH3 cells only occurs when membrane depolarization leads to calcium ion entry and intracellular accumulation.     

Cation Activation of the Basolateral Sodium-Potassium Pump in Turtle Colon

The current generated by electrogenic sodium-potassium exchange at the basolateral membrane of the turtle colon can be measured directly in tissues that have been treated with serosal barium (to block the basolateral potassium conductance) and mucosal amphotericin B (to reduce the cation selectivity of the apical membrane). We studied the activation of this pump current by mucosal sodium and serosal potassium, rubidium, cesium, and ammonium. The kinetics of sodium activation were consistent with binding to three independent sites on the cytoplasmic side of the pump. The pump was not activated by cellular lithium ions. The kinetics of serosal cation activation were consistent with binding to two independent sites with the selectivity Rb > K > Cs > NH₄. The properties and kinetics of the basolateral Na/K pump in the turtle colon are at least qualitatively similar to those ofthe well-characterized Na/K-ATPase of the human red blood cell .     

Membrane currents in taste cells of the rat fungiform papilla. Evidence for two types of Ca currents and inhibition of K currents by saccharin

Taste buds were isolated from the fungiform papilla of the rat tongue and the receptor cells (TRCs) were patch clamped. Seals were obtained on the basolateral membrane of 281 TRCs, protruding from the intact taste buds or isolated by micro-dissection. In whole-cell configuration 72% of the cells had a TTX blockable transient Na inward current (mean peak amplitude 0.74 nA). All cells had outward K currents. Their activation was slower than for the Na current and a slow inactivation was also noticeable. The K currents were blocked by tetraethylammonium, Ba, and 4-aminopyridine, and were absent when the pipette contained Cs instead of K. With 100 mM Ba or 100 mM Ca in the bath, two types of inward current were observed. An L-type Ca current (ICaL) activated at -20 mV had a mean peak amplitude of 440 pA and inactivated very slowly. At 3 mM Ca the activation threshold of ICaL was near -40 mV. A transient T-type current (ICaT) activated at -50 mV had an average peak amplitude of 53 pA and inactivated with a time constant of 36 ms at -30 mV. ICaL was blocked more efficiently by Cd and D600 than ICaT. ICaT was blocked by 0.2 mM Ni and half blocked by 200 microM amiloride. In whole-cell voltage clamp, Na-saccharin caused (in 34% of 55 cells tested) a decrease in outward K currents by 21%, which may be expected to depolarize the TRCs. Also, Na-saccharin caused some taste cells to fire action potentials (on-cell, 7 out of 24 cells; whole-cell, 2 out of 38 cells responding to saccharin) of amplitudes sufficient to activate ICaL. Thus the action potentials will cause Ca inflow, which may trigger release of transmitter.     

Hormone-induced parthenogenetic activation of mature starfish oocytes

1-Methyladenine, which has been previously shown to be the hormone responsible for meiosis reinitiation in starfish oocytes, triggers parthenogenetic activation when applied to matured starfish oocytes after emission of the second polar body and formation of the pronucleus. In Marthasterias glacialis and Asterias rubens oocytes parthenogenetic activation includes elevation of a fertilization membrane, cleavage and the formation of normal bipinnaria larvae. Activation is likely to result from 1-methyladenine interaction with the category of stereospecific membrane receptors involved in meiosis reinitiation, since structural requirements of this compound are identical for both biological responses. Appearance of oocyte responsiveness to 1-MeAde after, but not before emission of the second polar body cannot be accounted for by their increased sensitivity to intracellular Ca²⁺ at that time, although it is shown that Ca²⁺ mediates hormone effect in inducing parthenogenetic activation. Pretreatment of immature oocytes with the free hormone in excess strongly inhibits the 1-methyladenine-induced parthenogenetic activation of the oocytes when they have completed maturation.It is suggested that reappearance of 1-MeAde sensitivity when oocytes form a pronucleus depends either upon recruitment or new receptor units or on the reactivation of pre-existing inactivated receptors at this stage of oocyte maturation.     

Ionic currents in dispersed chemoreceptor cells of the mammalian carotid body

Ionic currents of enzymatically dispersed type I and type II cells of the carotid body have been studied using the whole cell variant of the patch-clamp technique. Type II cells only have a tiny, slowly activating outward potassium current. By contrast, in every type I chemoreceptor cell studied we found (a) sodium, (b) calcium, and (c) potassium currents. (a) The sodium current has a fast activation time course and an activation threshold at approximately -40 mV. At all voltages inactivation follows a single exponential time course. The time constant of inactivation is 0.67 ms at 0 mV. Half steady state inactivation occurs at a membrane potential of approximately -50 mV. (b) The calcium current is almost totally abolished when most of the external calcium is replaced by magnesium. The activation threshold of this current is at approximately -40 mV and at 0 mV it reaches a peak amplitude in 6-8 ms. The calcium current inactivates very slowly and only decreases to 27% of the maximal value at the end of 300-ms pulses to 40 mV. The calcium current was about two times larger when barium ions were used as charge carriers instead of calcium ions. Barium ions also shifted 15-20 mV toward negative voltages the conductance vs. voltage curve. Deactivation kinetics of the calcium current follows a biphasic time course well fitted by the sum of two exponentials. At -80 mV the slow component has a time constant of 1.3 +/- 0.4 ms whereas the fast component, with an amplitude about 20 times larger than the slow component, has a time constant of 0.16 +/- 0.03 ms. These results suggest that type I cells have predominantly fast deactivating calcium channels. The slow component of the tails may represent the activity of a small population of slowly deactivating calcium channels, although other possibilities are considered. (c) Potassium current seems to be mainly due to the activity of voltage-dependent potassium channels, but a small percentage of calcium-activated channels may also exist. This current activates slowly, reaches a peak amplitude in 5-10 ms, and thereafter slowly inactivates. Inactivation is almost complete in 250-300 ms. The potassium current is reversibly blocked by tetraethylammonium. Under current-clamp conditions type I cells can spontaneously fire large action potentials. These results indicate that type I cells are excitable and have a variety of ionic conductances. We suggest a possible participation of these conductances in chemoreception.     

Depolarization-induced automaticity in rat ventricular cardiomyocytes is based on the gating properties of L-type calcium and slow Kv channels

Depolarization-induced automaticity (DIA) of cardiomyocytes is the property of those cells to generate pacemaker cell-like spontaneous electrical activity when subjected to a depolarizing current. This property provides a candidate mechanism for generation of pathogenic ectopy in cardiac tissue. The purpose of this study was to determine the biophysical mechanism of DIA in terms of the ion conductance properties of the cardiomyocyte membrane. First, we determined, by use of the conventional whole-cell patch-clamp technique, the membrane conductance and DIA properties of ventricular cardiomyocytes isolated from adult rat heart. Second, we reproduced and analysed DIA properties by using an adapted version of the experimentally based mathematical cardiomyocyte model of Pandit et al. (Biophys J 81:3029–3051 2001, Biophys J 84:832–841 2003) and Padmala and Demir (J Cardiovasc Electrophysiol 14:990–995 2003). DIA in 23 rat cardiomyocytes was a damped membrane potential oscillation with a variable number of action potentials and/or waves, depending on the strength of the depolarizing current and the particular cell. The adapted model was used to reconstruct the DIA properties of a particular cardiomyocyte from its whole-cell voltage-clamp currents. The main currents involved in DIA were an L-type calcium current (I _CaL) and a slowly activating and inactivating Kv current (I _ss), with linear (I _B) and inward rectifier (I _K1) currents acting as background currents and I _Na and I _t as modulators. Essential for DIA is a sufficiently large window current of a slowly inactivating I _CaL combined with a critically sized repolarizing current I _ss. Slow inactivation of I _ss makes DIA transient. In conclusion, we established a membrane mechanism of DIA primarily based on I _CaL, I _ss and inward rectifier properties; this may be helpful in understanding cardiac ectopy and its treatment.     

Macroscopic Na+ Currents in the “Nonconducting” Shaker Potassium Channel Mutant W434F

C-type inactivation in Shaker potassium channels inhibits K⁺ permeation. The associated structural changes appear to involve the outer region of the pore. Recently, we have shown that C-type inactivation involves a change in the selectivity of the Shaker channel, such that C-type inactivated channels show maintained voltage-sensitive activation and deactivation of Na⁺ and Li⁺ currents in K⁺-free solutions, although they show no measurable ionic currents in physiological solutions. In addition, it appears that the effective block of ion conduction produced by the mutation W434F in the pore region may be associated with permanent C-type inactivation of W434F channels. These conclusions predict that permanently C-type inactivated W434F channels would also show Na⁺ and Li⁺ currents (in K⁺-free solutions) with kinetics similar to those seen in C-type-inactivated Shaker channels. This paper confirms that prediction and demonstrates that activation and deactivation parameters for this mutant can be obtained from macroscopic ionic current measurements. We also show that the prolonged Na⁺ tail currents typical of C-type inactivated channels involve an equivalent prolongation of the return of gating charge, thus demonstrating that the kinetics of gating charge return in W434F channels can be markedly altered by changes in ionic conditions.     

Inositol trisphosphate activates a voltage-dependent calcium influx in Xenopus oocytes

Injection of inositol trisphosphate (IP3) into oocytes of Xenopus laevis induces the appearance of a transient inward (Tin) current on hyperpolarization of the membrane. This current is carried largely by chloride ions, but is shown to depend on extracellular calcium, because it is abolished by removal of calcium in the bathing fluid or by addition of manganese. Recordings with aequorin as an intracellular calcium indicator show that a calcium influx is activated by hyperpolarization after intracellular injection of IP3 as well as after activation of neurotransmitter receptors thought to mediate a rise in IP3. Furthermore, by substituting barium for calcium in the bathing solution, inward barium currents can be recorded during hyperpolarization. We conclude that intracellular IP3 modulates the activity of a class of calcium channels, so as to allow an influx of calcium on hyperpolarization. In normal Ringer solution this then leads to the generation of a chloride current, because of the large numbers of calcium-dependent chloride channels in the oocyte membrane.     

Cardiac Na currents and the inactivating, reopening, and waiting properties of single cardiac Na channels

Tetrodotoxin (TTX)-sensitive Na currents were examined in single dissociated ventricular myocytes from neonatal rats. Single channel and whole cell currents were measured using the patch-clamp method. The channel density was calculated as 2/micron 2, which agreed with our usual finding of four channels per membrane patch. At 20 degrees C, the single channel conductance was 20 pS. The open time distributions were fit by a single-exponential function with a mean open time of approximately 1.0 ms at membrane potentials from -60 to -40 mV. Averaged single channel and whole cell currents were similar when scaled and showed both fast and slow rates of inactivation. The inactivation and activation gating shifted quickly to hyperpolarized potentials for channels in cell-attached as well as excised patches, whereas a much slower shift occurred in whole cells. Slowly inactivating currents were present in both whole cell and single channel current measurements at potentials as positive as -40 mV. In whole cell measurements, the potential range could be extended, and slow inactivation was present at potentials as positive as -10 mV. The curves relating steady state activation and inactivation to membrane potential had very little overlap, and slow inactivation occurred at potentials that were positive to the overlap. Slow inactivation is in this way distinguishable from the overlap or window current, and the slowly inactivating current may contribute to the plateau of the rat cardiac action potential. On rare occasions, a second set of Na channels having a smaller unit conductance and briefer duration was observed. However, a separate set of threshold channels, as described by Gilly and Armstrong (1984. Nature [Lond.]. 309:448), was not found. For the commonly observed Na channels, the number of openings in some samples far exceeded the number of channels per patch and the latencies to first opening or waiting times were not sufficiently dispersed to account for the slowly inactivating currents: the slow inactivation was produced by channel reopening. A general model was developed to predict the number of openings in each sample. Models in which the number of openings per sample was due to a dispersion of waiting times combined with a rapid transition from an open to an absorbing inactivated state were unsatisfactory and a model that was more consistent with the results was identified.     

Mechanism of action of nitric oxide donors on voltage-activated calcium channels in vascular smooth muscle cells

The effects of nitric oxide (NO) donors on inward barium current (I _Ba) in freshly isolated smooth muscle cells (SMC) of the guinea pig mesenteric artery and on inward calcium current (I _Ca) in SMC of the canine coronary artery were studied using a patch-clamp recording technique in whole-cell configuration. The inward current in SMC of the guinea pig artery was shown to flow through a single type of calcium channels, which have characteristics of high-threshold slowly inactivated channels of L-type. Nitroglycerin (NG) and sodium nitroprusside (NP) reversibly inhibitedI _Ba in a dose-dependent manner. Effects of NO donors onI _Ba were related to the changes in voltage-dependent properties of calcium channels. In particular, NG and NP accelerated the current inactivation, and their blocking effects increased with the membrane depolarization. Methylene blue, the guanylate cyclase inhibitor, decreased the inhibitory action of NG onI _Ba by a factor of 5. 8-Bromo-cGMP, the membrane-permeant cGMP analog, evokedI _Ba inhibition similar to that caused by NO donors. In the canine coronary artery, NO donors also inhibitedI _Ca flowing through the L-type calcium channels. It has been concluded that NO originating from NG and NP inhibits activity of L-type calcium channels in vascular SMC; it is possible that cGMP-dependent processes are involved.Neirofiziologiya/Neurophysiology, Vol. 28, No. 6, pp. 296–304, November–December, 1996.     

Kinetic and steady-state characteristics of sodium channels modified by aconitine

Kinetic and steady-state characteristics of sodium channels of the Ranvier node membrane, modified by aconitine, were investigated by the voltage clamp method. Channels modified by aconitine were shown to be only partially inactivated. Dependence of the fraction of uninactivated channels on voltage can be described by a model of a channel with three states: closed, open, and inactivated. It is suggested that aconitine does not significantly change the parameters of the inactivated state. Repolarization of the membrane to between –70 and –110 mV after a long (10 msec) test shift of potential induces initially a rapid decline of the current ("tail"), followed by a slow rise to the steady-state value. To explain the kinetics of this current it is necessary to postulate two or more open states of the channel.Institute of Cytology, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 12, No. 6, pp. 612–618, November–December, 1980.     

Fast-deactivating calcium channels in chick sensory neurons

Whole-cell Ca and Ba currents were studied in chick dorsal root ganglion (DRG) cells kept 6-10 in culture. Voltage steps with a 15-microseconds rise time were imposed on the membrane using an improved patch-clamp circuit. Changes in membrane current could be measured 30 microseconds after the initiation of the test pulse. Currents through Ca channels were recorded under conditions that eliminate Na and K currents. Tail currents, associated with Ca channel closing, decayed in two distinct phases that were very well fitted by the sum of two exponentials. The time constants tau f and tau s were near 160 microseconds and 1.5 ms at -80 mV, 20 degrees C. The tail current components, called FD and SD (fast-deactivating and slowly deactivating), are Ca channel currents. They were greatly reduced when Mg2+ replaced all other divalent cations in the bath. The SD component inactivated almost completely as the test pulse duration was increased to 100 ms. It was suppressed when the cell was held at membrane potentials positive to -50 mV and was blocked by 100-200 microM Ni2+. This behavior indicates that the SD component was due to the closing of the low-voltage-activated (LVA) Ca channels previously described in this preparation. The FD component was fully activated with 10-ms test pulses to +20 mV at 20 degrees C, and inactivated to approximately 30% during 500-ms test pulses. It was reduced in amplitude by holding at -40 mV, but was only slightly reduced by micromolar concentrations of Ni2+. Replacement of Ca2+ with Ba2+ increased the FD tail current amplitudes by a factor of approximately 1.5. The deactivation kinetics did not change (a) as channels inactivated during progressively longer pulses or (b) when the degree of activation was varied. Further, tau f was affected neither by changing the holding potential nor by varying the test pulse amplitude. Lowering the temperature from 20 to 10 degrees C decreased tau f by a factor of 2.5. In all cases, the FD component was very well fitted by a single exponential. There was no indication of an additional tail component of significant size. Our findings indicate that the FD component is due to closing of a single class of Ca channels that coexist with the LVA Ca channel type in chick DRG neurons.