Separation of sodium and calcium currents in the somatic membrane of mollusc neurones

Calcium-dependent action potentials in mouse spinal cord neurons in cell culture

Following blockade of membrane potassium conductance with tetraethylammonium ions or 3-aminopyridine, long-duration action potentials were recorded from mouse spinal cord neurons in primary dissociated cell culture. The action potentials were calcium-dependent since they: (1) were not blocked by the sodium-channel blocker tetrodotoxin, (2) could be recorded in sodium-free, calcium-containing medium (3) could not be evoked in sodium-containing, calcium-free medium, (4) were blocked by calcium channel blockers manganese and cobalt and (5) had overshoot amplitudes that varied linearly with the log of the extracellular calcium concentration (slope of 27.5 mV/decade change in calcium concentration).

Potassium and caffeine contractures in fast and slow muscles of the chicken

1 K+ contractures, caffeine contractures and electrical properties were studied in slow (posterior latissimus dorsi; p.l.d.) and fast (anterior latissimus dorsi; a.l.d.) chicken muscles. 2. P.l.d. K+ contractures show a transient increase of tension that relaxes spontaneously. Contractures in a.l.d. show an initial component followed by a maintained tension. 3. A.l.d. K+ contractures of similar amplitude and time course were reproduced at 4 min intervals. In p.l.d., the interval needed for full recovery is about 30 min. In Cl-free saline p.l.d. and a.l.d. K+ contractures can be reproduced at 4 min intervals. 4. The time course of repolarization after a short exposure to 160 mM-KCl was much slower in p.l.d. than in a.l.d. In Cl-free saline the time course of repolarization becomes faster in p.l.d. 5. The membrane resistance was not modified in a.l.d. and was increased in p.l.d. by Cl-free saline. The calculated Cl- conductance in p.l.d. was about 70% of the total membrane conductance. 6. In a.l.d., Mn2+, D600 and external Ca2+ reduction greatly diminishes the maintained phase of the K+ contracture leaving the initial phase almost unmodified. Under similar conditions p.l.d. K+ contractures were slightly reduced. 7. P.l.d. caffeine contractures (10-40 mM) were not maintained and they were not modified by Ca-free saline, Cd2+, Co2+, Mn2+ and D600. 8. A.l.d. caffeine contractures (2-15 mM) were maintained and were highly dependent on external Ca2+. In addition they were greatly reduced by Cd2+, Co2+, Mn2 and D600. 9. It is suggested that caffeine contractures of a.l.d. are elicited by a Ca2+ entry into the muscle from the external fluid.

The time courses of intracellular free calcium and related electrical effects after injection of CaCl2 into neurons of the snail, Helix pomatia

Controlled quantities of 100 mM aqueous CaCl2 solutions were pressure injected into voltage-clamped neurons with a resolution of 10(-11) 1. Ca2+-selective microelectrodes monitored the time course of changes in [Ca2+]i. At a membrane potential of -50 mV CaCl2 quantities in the range of 1% of the cell volume induced an inward current, associated with a conductance increase and having an equilibrium potential between -20 and +20 mV, which accompanied the rise in [Ca2+]i. An artifactual origin of the inward current by the injection procedure or by calcium screening of membrane sites could be excluded. The calcium-induced hyperpolarizing conductance, producing an outward current at -50 mV, followed the inward current and reached maximum during the late decline in [Ca2+]i. In most cases its development was separated from the inward current by an intermediate relative decrease of the membrane conductance. Neither of the two transient conductance increases showed a particular dependence on voltage. Renewed Ca2+ injection quickly decreased the calcium-induced hyperpolarizing conductance for several seconds. Ca2+ injections below 0.05% of the cell volume mostly produced pure outward currents or hyperpolarizing responses. Partial substitution of extracellular CaCl2 by NiCl2 decreased the hyperpolarizing response but not the initial inward current. The immediate effects of increased [Ca2+]i are activation of a depolarizing conductance and the partial block of the late hyperpolarizing conductance. The latter is probably produced through intermediate steps after increasing [Ca2+]i.

Improved patch-clamp techniques for high-resolution current recording from cells and cell-free membrane patches

1. The extracellular patch clamp method, which first allowed the detection of single channel currents in biological membranes, has been further refined to enable higher current resolution, direct membrane patch potential control, and physical isolation of membrane patches. 2. A description of a convenient method for the fabrication of patch recording pipettes is given together with procedures followed to achieve giga-seals i.e. pipette-membrane seals with resistances of 10(9) - 10(11) omega. 3. The basic patch clamp recording circuit, and designs for improved frequency response are described along with the present limitations in recording the currents from single channels. 4. Procedures for preparation and recording from three representative cell types are given. Some properties of single acetylcholine-activated channels in muscle membrane are described to illustrate the improved current and time resolution achieved with giga-seals. 5. A description is given of the various ways that patches of membrane can be physically isolated from cells. This isolation enables the recording of single channel currents with well-defined solutions on both sides of the membrane. Two types of isolated cell-free patch configurations can be formed: an inside-out patch with its cytoplasmic membrane face exposed to the bath solution, and an outside-out patch with its extracellular membrane face exposed to the bath solution. 6. The application of the method for the recording of ionic currents and internal dialysis of small cells is considered. Single channel resolution can be achieved when recording from whole cells, if the cell diameter is small (less than 20 micrometer). 7. The wide range of cell types amenable to giga-seal formation is discussed.

Potassium current suppression by quinidine reveals additional calcium currents in neuroblastoma cells

Quinine and quinidine have been evaluated with regard to their effects on the electrical activity of neuroblastoma cells. Under voltage-clamp conditions, we have found that quinine and quinidine block both the voltage-dependent and Ca2+-dependent K+ conductances. Blockage of the voltage-dependent K+ channel is manifest as an increase in the amplitude and in the duration of the action potential. Blockage of the Ca2+-dependent K+ channel in Na+-free (replaced by Tris) solutions containing 6.8 mM Ca2+ and tetraethylammonium ion or 4-aminopyridine (to block the voltage-dependent K+ current) is seen as a further prolongation of the Ca2+ action potential and diminution of the after-hyperpolarization. A critical role of the Ca2+-dependent K+ conductance in modulation of the rate and duration of trains of Ca2+ action potentials is shown by the use of low concentrations (5-40 microM) of quinine or quinidine, which diminish the Ca2+-dependent K+ conductance in a graded manner. After complete blockade of K+ currents, the peak Ca2+ currents are enhanced at all voltages, especially at values more positive than -30 mV, where a steady-state inward current appears as well. In this same voltage range, the decay of the Ca2+ current exhibits two time constants--that of the transient inward current, which is about 20 msec, and a much slower (approximately 2000 msec) component. It is suggested that neuroblastoma cells have two types of calcium channels--one which generates the Ca2+ action potential and a second, distinguished by activation at more depolarized levels and by a slow rate of inactivation, which underlies the calcium entry necessary to activate the Ca2+-dependent K+ conductance.

Soma spike of neuroendocrine bag cells of Aplysia californica

Soma action potentials of the neuroendocrine bag cells of Aplysia californica were studied with intracellular recording and current injection. Spikes in artificial sea water (ASW) were either graded with increasing depolarizing current pulses, or had a well-defined threshold. The latter spikes typically had faster rise times with larger overshoots and hyperpolarizing afterpotentials. Repetitive stimulation led to spike potentiation (SP), manifested as an increase in overshoot amplitude and duration of successive spikes in a train. SP was usually detectable at 0.5 Hz, and maximal between 0.8 and 4 Hz. Concomitant accommodation occurred rapidly at greater than or equal to 5 Hz. The increase in spike duration during SP resulted from a progressive enhancement of an inflection on the repolarizing phase. The inflection was dependent on membrane potential; small depolarizations (5-10 mV) enhanced it; hyperpolarization (less than 35 mV) reduced it. Solutions with O--Na+ (Tris-substituted) or O--Ca2+ (1 mM EGTA) revealed mixed Na+/Ca2+ spikes with variable degrees of Na+ versus Ca2+ dominance. Cd2+, Co2+, and Mn2+ reversibly abolished the inflection on the repolarizing phase, indicating that it is Ca2+ mediated; the spike was reduced irreversibly at higher concentrations. SP was generally reduced only if the spike was severely attenuated. It is proposed that SP results primarily from a voltage- and time-dependent potassium inactivation which then unmasks a calcium current. SP may play a role in augmenting the release of egg-laying hormone.

Calcium channels in the somatic membrane of the rat dorsal root ganglion neurons, effect of cAMP

Isolated rat dorsal root ganglion neurons have been perfused with potassium-free solutions containing cAMP, ATP and Mg2+ ions. In these conditions stable inward calcium currents can be recorded in the somatic membrane of all investigated cells. The kinetics of these currents can be approximated by a modified Hodgkin-Huxley equation using a square power of the m-variable; its inactivation is extremely slow. The corresponding channels pass Ba2+ ions about twice more effective than Ca2+.

Electrophysiology of mammalian inferior olivary neurones in vitro. Different types of voltage-dependent ionic conductances

The electrophysiological properties of guinea-pig inferior olivary (I.O.) cells have been studied in an in vitro brain stem slice preparation. 1. Intracellular recordings from 185 neurones in this nucleus reveal that antidromic, orthodromic or direct stimulation generates action potentials consisting of a fast spike followed by an after-depolarizing potential (ADP). The ADP had an amplitude of 49 +/- 8 mV (mean +/- S.D.) and a duration which varied over a wide range with the level of depolarization. This ADP is followed by an after-hyperpolarizing potential (AHP) having an amplitude of 12 +/- 3 mV (mean +/- S.D.) from rest and lasting up to 250 msec. The AHP shows a rebound depolarization wave. 2. Synaptic activation may be obtained by peri-olivary stimulation with a bipolar electrode located in the immediate vicinity of the I.O. nucleus. These potentials are a mixture of depolarizing and hyperpolarizing synaptic events which can be reversed by direct membrane polarization. 3. Addition of tetrodotoxin (TTX) to the bath, or removal of extracellular Na, abolishes the fast initial action potential but does not modify the ADP or the AHP. Blockage of Ca conductance by Co, Mn, Cd or D600, or replacement of Ca by Mg, abolishes the ADP--AHP sequence. 4. Hyperpolarization of the neurone uncovers a low-threshold Ca conductance which is inactivated at rest and has similar pharmacological properties to the ADP. This low-threshold spike plays a central role in the rebound potential following the AHP. 5. Simultaneous impalement of I.O. neurone pairs demonstrated the presence of electrotonic coupling between neurones, which is especially prominent in the medial accessory olive.

Separation of potassium and calcium channels in the nerve cell soma membranes

Investigation of isolated neurons of Helix pomatia during intracellular dialysis revealed differences in the sensitivity of the channels for the outward potassium and inward calcium currents to changes in pH of the external medium. As a result of this difference, considerable separation of the regions of activation of the currents was obtained along the potential axis in solutions with low pH and the characteristics of the inward and outward currents could be studied during their minimal application. Channels for the outward current were shown to have some permeability for tris ions (PTris : PK=0.05), which is the reason why it is impossible to block this current completely by replacing the intracellular potassium by Tris. Channels for the inward calcium current are characterized by slow inactivation, with a first-order kinetics; their momentary voltage-current characteristic curve reveals significant Goldman's rectification. The selectivity of the calcium channels for other bivalent cations is : Ba:Sr:Ca:Mg=2.8:2.6:1.0:0.2.

Calcium-mediated inactivation of the calcium conductance in caesium-loaded giant neurones of Aplysia californica

1. The intracellular potassium in giant neurones of Aplysia californica was replaced with caesium by a method utilizing the ionophore nystatin. Because caesium ions have low permeability through potassium channels outward currents during voltage-clamp depolarization were strongly curtailed after the caesium loading procedure and the subsequent wash-out of the ionophore. 2. The calcium current elicited by a test voltage-clamp depolarization (pulse 2) was depressed following the entry of calcium elicited by a prior depolarization (pulse 1). 3. The percentage depression of the test current was a linear function of the pulse 1 current-time integral, and thus appears to be related linearly to the amount of calcium carried into the cell during pulse 1. This linear relation was maintained when calcium entry was varied by changes in external calcium concentration, by altered pulse 1 amplitude and altered pulse 1 duration. Depression was substantially reduced by injection of EGTA, and by substitution of barium for extracellular calcium. 4. The calcium current was unaffected by prior hyperpolarization of the membrane or by prior depolarizations to about ECa. Depression of the current was not altered by addition of extracellular 50 mM-TEA or by a strong hyperpolarization between the conditioning and test pulses. 5. The rate relaxation of the inward current during a given depolarization depended on the rate of entry and accumulation of free calcium. Relaxation under a given command potential became slower when calcium was partially replaced with magnesium so as to produce a smaller calcium current, or when accumulation of intracellular free calcium was retarded by injected EGTA or by barium substitution for extracellular calcium. 6 Evidence is considered that accumulation of calcium ions at the cytoplasmic surface of the membrane leads to inactivation through an action upon the calcium conductance. Reduced driving force and intracellular surface-charge neutralization do not adequately account for the observed depression of the calcium current resulting from intracellular accumulation of calcium ions.

Ionic mechanism of a voltage-dependent current elicited by cyclic AMP

Intracellular pressure injection of cyclic AMP induces a slow voltage-dependent inward current in some neurons of Aplysia californica. The time course, voltage dependence, and ionic sensitivities of this response are nearly identical to those of the voltage-dependent calcium current induced by serotonin in the same preparation. The response to cyclic AMP is unaffected by changes in the extracellular concentration of chloride or potassium. The current is slowly but minimally reduced by a sodium-free solution. The calcium channel blocker, cadmium, blocks the current elicited by injection of cyclic AMP. The data presented here suggest that cyclic AMP can induce a voltage-dependent calcium current.

Presynaptic calcium currents in squid giant synapse

A voltage clamp study has been performed in the presynaptic terminal of the squid stellate ganglion. After blockage of the voltage-dependent sodium and potassium conductances, an inward calcium current is demonstrated. Given a step-depolarization pulse, this voltage- and time-dependent conductance has an S-shaped onset. At the "break" of the voltage step, a rapid tail current is observed. From these results a kinetic model is generated which accounts for the experimental results and predicts for the time course and amplitude a possible calcium entry during presynaptic action potentials.

Slow calcium and potassium currents across frog muscle membrane: measurements with a vaseline-gap technique

1. A vaseline-gap voltage-clamp technique was used to record slow Ca2+ and K+ currents from frog skeletal muscle fibres loaded with the Ca2+ chelator EGTA. 2. K+ currents were increased when Mg2+ replaced external Ca2+, and they were abolished when internal K+ was replaced by tetraethylammonium (TEA+). Ca2+ currents could be studied in isolation in fibres loaded with (TEA)2EGTA. 3. Under maintained depolarization, Ca2+ currents slowly increase (half-time of 35 msec or more at 25 mV) and then decline to a steady value. Decline under repolarization is rapid (half-time of 6-7 msec) and complete. During an action potential, the Ca2+ influx through this system is probably less than the influx observed with tracers. 4. Ba2+, Sr2+, Ca2+, Mn2+ and Mg2+ can carry current across the membrane; Ni2+ and Co2+ cannot. Ca2+ currents are weakly blocked by external Mg2+.

Calcium depletion in frog muscle tubules: the decline of calcium current under maintained depolarization

1. Ca2+ currents in frog skeletal muscle fibres were studied with a voltage-clamp technique. Under membrane depolarization maintained for several seconds, Ca2+ current was found to decline with time constants of 0.2-2 sec when [Ca2+]o = 10 mM. 2. Ca2+ currents are diminished by nifedipine, D-600, tetracaine and Ni2+. 3. When peak current is diminished by making the membrane potential positive, by block with drugs or by substituting the relatively less permeant Mn2+ for Ca2+ then the rate of decline is diminished also. When peak current is increased by recording at relatively negative membrane potentials or by substituting for Ca2+ the more permeant ions Ba2+ or Sr2+, then the rate of decline is increased in proportion. Evidently, the size of the current determines the rate of decline. 4. Decline of current is greatly slowed in isotonic Ca2+ saline or when the [Ca2+]o is buffered by the organic anion malate. These findings indicate that the decline of current arises from Ca2+ depletion in an extracellular compartment, most probably the transverse tubules. On this basis, an analysis of Ca2+ current decline and recovery leads to the following conclusions. 5. Ca2+ current flows almost entirely across the membranes of the transverse tubules. 6. After allowing for the tortuosity of the tubular network, the apparent diffusion coefficient for Ca2+ in the transverse tubules is about 2.6 X 10(-6) cm2/sec, three times less than the diffusion coefficient for K+ in the transverse tubules and about three times less than the diffusion coefficient for Ca2+ in free solution. 7. The transverse tubule lumen does not appear to have a large Ca2+-buffering capacity in the millimolar range. At [Ca2+]o = 10 mM, the tubule lumen binds less than 0.6 dissociable Ca2+ ions for every free ion.

Who do barium ions imitate acetylcholine?

The action of 1-4 mM barium on bullfrog sympathetic ganglion cells was studied using voltage clamp. Barium imitated the action of muscarine, causing depolarization, increased input resistance, tendency to repetitive firing, and a specific inhibition of a slow small outward current termed the M current. Barium did not produce significant short-term inhibition of three other outward currents; the delayed rectifier, the calcium-dependent K current, and the A current.

The action of high hydrostatic pressure on the membrane currents of Helix neurones

1. The actions of high hydrostatic pressure (10.4, 20.8 MPa) on the membrane currents of Helix neurones were examined under voltage clamp. 2. High hydrostatic pressure (20.8 MPa) reduced the maximum inward current to 0.78 and the delayed outward current, measured at the inward current reversal potential, to 0.75 of their value at atmospheric pressure. 3. High hydrostatic pressure shifted the curve relating the inward current conductance to membrane potential to more positive values but the maximum conductance was altered. 4. The rates of activation of the inward and delayed outward currents were slowed by pressure. 5. The steady-state level and time course of inactivation of the inward current was unaffected by high pressure over the investigated range. 6. The effects of high hydrostatic pressure on the fast outward current identified in gastropod neurones by Connor & Stevens (1971) were also examined. 20.8 MPa reduced the current measured at -30 mV to 0.71 of its control value. 7. The rate of activation of the fast outward current was slowed by high pressure but the time constant of inactivation was unchanged. 8. The majority of the effects of high hydrostatic pressure were completely reversible upon decompression. 9. These results are discussed with reference to the known effects of high hydrostatic pressure on the action potential and discharge frequency of gastropod neurones. Possible sites and mechanisms of pressure action on the excitable cell are briefly discussed.

Voltage-dependent inactivation of a calcium channel

The inactivation of Ca currents in unfertilized eggs of the marine polychaete Neanthes arenaceodentata was investigated by using a voltage clamp technique. These Ca currents do not appear to be masked by other currents in the voltage range studied. Inactivation increased monotonically with increasing depolarization and occurred at potentials more negative than the inward Ca current. Currents elicited by depolarization at different Ca concentrations had approximately the same time course of inactivation, even though the size of the currents varied by almost an order of magnitude. There was complete inactivation in solutions containing Ca, Sr, or Ba (all permeant) for depolarizations beyond -30 mV absolute; the time course of the inactivation process was similar for all three permeant ions. Increasing depolarizations toward the expected equilibrium potential for Ca, Ba, or Sr did not produce any lessening of the inactivation. Thus, it appears that the inactivation seen in Neanthes eggs is a purely voltage-dependent phenomenon.

Calcium current-dependent and voltage-dependent inactivation of calcium channels in Helix aspersa

1. Inactivation of the Ca channels has been examined in isolated nerve cell bodies of Helix aspersa using the suction pipette method for voltage clamp and internal perfusion.2. Satisfactory suppression of outward currents was essential. This was achieved over most of the voltage range by substitution of Cs ion for K ion and by the use of TEA intra- and extracellularly and 4-AP extracellularly. A small time- and voltage-dependent non-specific current remained at potentials above +60 mV.3. In these solutions, Ca current approaches E(Ca) but cannot be detected in the outward direction. The Ca channel appears to be impermeable to Cs and Tris ions.4. Inactivation of Ca currents occurs as a bi-exponential process. The faster rate is 10-20 times the slower rate and is about one twentieth the rate of activation. The development of inactivation during a single voltage-clamp step and the onset of inactivation produced by prepulses followed after brief intervals by a test pulse, have roughly similar time courses.5. The rates of inactivation increase monotonically at potentials more positive than about -25 mV. The amount of steady-state inactivation increases with membrane depolarizations to potentials of about +50 mV. At more positive potentials, steady-state inactivation is reduced.6. Intracellular EGTA slows the faster rate of inactivation of I(Ca) and reduces the amount of steady-state inactivation measured with a standard two pulse protocol. The effect is specifically related to Ca chelation and hydrogen ions are not involved. This component of inactivation is referred to as Ca current-dependent inactivation and is consistent with observations that increased Ca(i) inactivates the Ca channel. The process does not depend upon current flow alone since Ba currents of comparable or greater magnitude have smaller initial rates of inactivation. Furthermore, application of Ba ion intracellularly in large concentrations has no effect on steady-state inactivation.7. The bi-exponential inactivation process that persists in the presence of EGTA(i) is similar to that occurring when extracellular Ba ion carries current through the Ca channel. Steady-state inactivation also persists and is similar in the two cases. Therefore it is concluded that inactivation is voltage-dependent as well as Ca current-dependent.8. Diffusion models that included reasonable values for the effect of binding on diffusion, even when combined with declining influxes, did not account for this ;mixed' form of calcium- and voltage-dependent inactivation. A compartmental model in which the particular kinetic model of voltage-dependent inactivation was not critical described the Ca current-dependent inactivation.

Ion conductance and ion selectivity of potassium channels in snail neurones

Delayed potassium channels were studied in internally perfused neurone somata from land snails. Relaxation and fluctuation analysis of this class of ion channels revealed Hodgkin-Huxley type K channels with an average single channel conductance (gamma K) of 2.40 +/- 0.15 pS. The conductance of open channels is independent of voltage and virtually all K channels seem to be open at maximum K conductance (gk) of the membrane. Voltage dependent time constants of activation of gK, calculated from K current relaxation and from cut-off frequencies of power spectra, are very similar indicating dominant first-order kinetics. Ion selectivity of K channels was studied by ion substitution in the external medium and exhibited the following sequence: Tl+ greater than K+ greater than Rb+ greater Cs+ greater than NH4+ greater Li+ greater than Na+. The sequence of the alkali cations does not conform to any of the sequences predicted by Eisenman's theory. However, the data are well accommodated by a new theory assuming a single rate-limiting barrier that governs ion movement through the channel.

Current recorded from a cut-open giant axon under voltage clamp

Squid giant axons were cut open and the resulting membrane sheet was positioned in a chamber separating two compartments. Under voltage clamp, normal sodium, potassium, and gating currents could be recorded for several hours. This preparation allows direct access to the internal side of the axonal membrane and should prove useful for recording of ionic currents, gating currents, and channel-induced current fluctuations in the same membrane sheet.

Calcium action potentials in single freshly isolated smooth muscle cells

The ionic basis of the action potential was investigated using intracellular microelectrodes in single smooth muscle cells freshly isolated from the stomach of the toad Bufo marinus. When [Ca2+]0 was elevated (> 8mM), action potentials were readily elicited, which had similar characteristics to those found in many tissue preparations of visceral smooth muscle. There was a decrease in membrane resistance at the peak of the action potential and during the undershoot. The following evidence indicated that the inward current is carried by Ca2+: 1) Raising [Ca2+]0 from 15 to 49.6 mM in the presence of 18.2 mM tetraethylammonium chloride (TEA) increased the maximum rate of rise and the overshoot amplitude, the latter by 15 mV, i.e., 29.5 mV/10-fold change in [Ca2+]0. Changing [Na2+]0 from 11.8 to 81.8 mM had no significant effect on the maximum rate of rise or the overshoot. 2) The action potentials were blocked by 8 mM Mn2+ ([Ca2+]0 = 14.6 mM) but not by 14.3 microM tetrodotoxin (TTX) ([Na2+]0 = 100 mM). 3) Action potentials could be elicited when [Ba2+]0 or [Sr2+]0 were present in high concentrations ([Ca2+]0 less than or equal to 31 microM,[Na2+]0 = 11.8 mM). Both the maximum rate of rise and overshoot amplitude of the action potential increased as the membrane potential became more negative, suggesting increased activation of the inward current. Both TEA and Ba2+ prolonged the action potential, suggesting that a K+ current is responsible for repolarization. Action potentials could also be elicited on anode break at elevated [K+]0 (91 mM).

Calcium independence of slow currents underlying spike frequency adaptation

This study assessed the role of calcium in the activation of the slow potassium current responsible for spike frequency adaptation in molluscan neurons. Inward calcium currents were eliminated by using Co2+, Cd2+, or OCa2+ EGTA in the bathing solution. In each case adaptation was found to persist, as did the slow current believed to be responsible for adaptation. Injection of EGTA into neurons was also found not to block adaptation. This potassium current provides an example of a slow voltage-dependent potassium process which is independent of calcium influx.

Single channel currents from excised patches of muscle membrane

The currents through single acetylcholine-activated channels were measured on membrane fragments that had been torn from rat muscle myotubes with patch pipettes. The membrane fragments were sealed into the pipette by using the "gigohm-seal" technique of Neher, which also permitted voltage clamp of the membrane via the patch electrode. Membrane patches were excited by sudden withdrawal of the electrode from the cell. Substitution of fluoride for chloride ions in the bathing solution could prevent or reverse the tendency for the membrane at the electrode tip to seal over into a closed vesicle. The single membrane layer at the electrode tip could remain functional for up to 30 min. The apparent single channel conductance was minimally affected by excision. The current-voltage relationships for the single channel currents show that the inside (i.e., cytoplasmic surface) of the membrane fragment was exposed to the bathing solution. In symmetric Na solutions the current-voltage curve was nearly linear and reversed at approximately 0 mV. In other bathing solutions from 40 to 500 mM NaF, the observed zero current potential was close to that predicted by the Nernst equation. We present evidence that internal Na interacts with the channel, causing both saturation of outward current and block of inward current. At + 100 mV the apparent dissociation constant for internal Na was 138 mM.

Electrophysiological properties of in vitro Purkinje cell dendrites in mammalian cerebellar slices

1. Intradendritic recordings from Purkinje cells in vitro indicate that white matter stimulation produces large synaptic responses by the activation of the climbing fibre afferent, but antidromic potentials do not actively invade the dendritic tree. 2. Climbing fibre responses may be reversed in a manner similar to that observed at the somatic level. However, the reversal does not show the biphasicity often seen at somatic level. 3. Input resistance of these dendrites was found to range from 15 to 30 M omega. The non-linear properties seen at the somatic level for depolarizing currents are also encountered here. However, there seems to be less anomalous rectification. 4. Detailed analysis of repetitive firing of Purkinje cells elicited by outward DC current shows that, as in the case of the antidromic invasion, the fast somatic potentials (s.s.) do not invade the dendrite actively. However, the dendritic spike bursts (d.s.b.s) interposed between the s.s. potentials are most prominent at dendritic level. 5. Two types of voltage-dependent Ca responses were observed. At low stimulus level a plateau-like depolarization is accompanied by a prominent conductance change; further depolarization produces large dendritic action potentials. These two classes of response are TTX-resistant but are blocked by Cd, Co, Mn or D600, or by the removal of extracellular Ca. 6. Following blockage of the Ca conductance, plateau potentials produced by a non-inactivating Na conductance are observed mainly near the soma indicating that this voltage-dependent conductance is probably associated with the somatic membrane. 7. Spontaneous firing in Purkinje cell dendrites is very similar to that observed at the soma. However, the amplitude of these bursts is larger at dendritic level. It is further concluded that these TTX-insensitive spikes are generated at multiple sites along the dendritic tree. 8. Six ionic conductances seem to be involved in Purkinje cell electroresponsiveness: (a) an inactivating and (b) a non-inactivating Na conductance at or near the soma, (c) a spike- and (d) a plateau-generating Ca conductance, and (e) voltage-dependent and (f) Ca-dependent K currents. 9. The possible role of these conductances in Purkinje cell integration is discussed.

Trypsin-induced masking of tetrodotoxin receptor of the sodium channels in mollusc neurons

At the early stage of trypsin treatment of mollusc neurons tetrodotoxin cannot block the Na+ current. In the course of further exposure of neurones to trypsin, tetrodotoxin-sensitivity is restored completely, so its temporal loss results from shielding rather than destruction of the tetrodotoxin-binding site. Pronase and papain do not affect the tetrodotoxin action on the Na+ current.

Acetylcholine-induced current in perfused rat myoballs

Spherical "myoballs" were grown under tissue culture conditions from striated muscle of neonatal rat thighs. The myoballs were examined electrophysiologically with a suction pipette which was used to pass current and perfuse internally. A microelectrode was used to record membrane potential. Experiments were performed with approximately symmetrical (intracellular and extracellular) sodium aspartate solutions. The resting potential, acetylcholine (ACh) reversal potential, and sodium channel reversal potential were all approximately 0 mV. ACh-induced currents were examined by use of both voltage jumps and voltage ramps in the presence of iontophoretically applied agonist. The voltage-jump relaxations had a single exponential time-course. The time constant, tau, was exponentially related to membrane potential, increasing e-fold for 81 mV hyperpolarization. The equilibrium current-voltage relationship was also approximately exponential, from -120 to +81 mV, increasing e-fold for 104 mV hyperpolarization. The data are consistent with a first-order gating process in which the channel opening rate constant is slightly voltage dependent. The instantaneous current-voltage relationship was sublinear in the hyperpolarizing direction. Several models are discussed which can account for the nonlinearity. Evidence is presented that the "selectivity filter" for the ACh channel is located near the intracellular membrane surface.

Intracellular calcium accumulation during depolarization in a molluscan neurone

1. The bursting pacemaker neurone R-15 of Aplysia was injected with the Ca2+ sensitive dye arsenzo III. Changes in absorbance were measured with a differential spectrophotometer to monitor changes in free intracellular Ca2+ during membrane depolarization under voltage clamp conditions. 2. Dye absorbance increased linearly for depolarizing pulse durations up to 100 msec and approximately linearly between 100 and 300 msec, but for longer durations the absorbance change decreased. 3. The absorbance change vs. voltage relation increased steeply between -20 and 0 mV (e-fold per 8.5 mV), peaked at +36 mV and declined non-linearly to an estimated null or suppression potential of about +139 mV. 4. TTX (5 x 10(-5 M) had no effect on the change in dye absorbance produced by brief or long duration stimuli whereas Ca2+ free ASW abolished all changes in dye absorbance. 5. The absorbance change saturated with increasing external Ca2+ concentrations. The relation between dye absorbance and external Ca2+ concentration was hyperbolic and for a small range of external Ca2+ concentration and membrane potentials could be fitted by a Michaelis--Menten expression where the dissociation constant and the maximum absorbance change are voltage dependent. 6. The absorbance change was reduced by external divalent ions which block the Ca2+ channel (e.g. Cd2+ and Ni2+). The suppression of dye absorbance was increased by membrane depolarization and suggests that there is a voltage dependent site within the Ca2+ channel which binds divalent ions. 7. The decline of the absorbance--voltage relation from its peak to the suppression potential showed a greater nonlinearity when longer duration voltage clamp pulses were used. The non-linearity can be explained if the accumulation of Ca2+ ions next to the inner surface of the membrane during depolarization reduces the driving force on Ca2+ ions and thus decreases Ca2+ ion influx. 8. The suppression potential estimated from the absorbance--voltage relation increased 29 mV per tenfold change in the external Ca2+ concentration and thus can be used to estimate the Ca2+ equilibrium potential. 9. The change in dye absorbance produced by brief depolarizing voltage clamp steps was inactivated at positive holding potentials (50% inactivation at about -14 mV). Our results suggest that the slow decrease in dye absorbance during prolonged depolarization is caused by inactivation of the Ca2+ channel.

Voltage sensitive calcium entry in frog motoneurones

1. The electrical properties of motoneurone membrane were investigated in the isolated and hemisected spinal cord of frogs, using intracellular recording techniques. 2. TTX (1 x 10(-6) g/ml.) blocked action potentials produced either by intracellular depolarizing current pulses or ventral root stimuli. Voltage--current relations from these cells showed a diminishing slope for depolarizing current pulses of increasing intensity. 3. If TEA (5--10 mM) was added to the media containing TTX, intracellular depolarizing pulses elicited prolonged regenerative depolarizations characterized by a peak of variable amplitude and a repolarizing phase preceded by a prolonged plateau of variable duration. 4. During the plateau of the response, the membrane conductance was increased above its resting value. 5. The response was shortened during repetitive stimulation and could be curtailed by applying a hyperpolarizing pulse during the plateau. 6. The response depended on the presence of external Ca2+ and increased in size and duration with increasing Ca2+ concentration. Sr2+ substituted effectively for Ca2+. Sr2+-dependent responses were considerably longer than the Ca2+-dependent ones. Ca2+ or Sr2+ dependent responses persisted in Na+-free media containing isotonic TEA, and were abolished by addition of Co2+. 7. Ca2+ or Sr2+-dependent regenerative responses were followed by a hyperpolarization which could last several seconds. The current responsible for this after-hyperpolarization was TTX and TEA resistant. 8. It is concluded that the TTX-resistant regenerative response is probably generated in the soma-dendritic membrane, and is due to influx of Ca2+ or Sr2+ through voltage sensitive channels different to those through which Na+ permeates during generation of 'normal' action potentials. In addition it is shown that the hyperpolarization following 'Ca spikes', and which might be due to an increase in K+ conductance can also be triggered by Sr2+.

Potassium conductance and internal calcium accumulation in a molluscan neurone

1. The Aplysia neurone R-15 was injected with the Ca(2+) sensitive dye arsenazo III. Changes in dye absorbance were measured with a differential spectrophotometer to monitor changes in the free internal Ca(2+) concentration, [Ca](i), during membrane depolarization and during intracellular Ca(2+) ion injection under voltage clamp conditions.2. The absorbance change, and thus [Ca](i), increases linearly with Ca(2+) injection intensity at constant duration. The absorbance change produced by a constant intensity Ca(2+) injection also increases with injection duration, but this increase is asymptotic.3. The Ca(2+) activated K(+) current, I(K, Ca), increases linearly with the increase in [Ca](i) and its rise and decay follows closely the time course of the absorbance change produced by internal Ca(2+) injection.4. The Ca(2+) activated K(+) conductance increases exponentially with membrane depolarization. The increase in K(+) conductance activated by a constant intensity and duration Ca(2+) injection is on average e-fold for a 25.3 mV change in membrane potential.5. The difference in net outward K(+) current measured during depolarizing pulses to different membrane potentials in normal and in Ca(2+) free ASW was used as an index of I(K, Ca). Its time course was approximately linear for the first 50-100 msec of depolarization, but for longer times the relation approached a maximum. Simultaneous measurements of the arsenazo III absorbance changes were broadly consistent with the activation of I(K, Ca) being brought about by the rise in [Ca](i) during a pulse.6. The relation between Ca(2+) activated K(+) conductance and membrane potential is bell shaped and resembles the absorbance vs. potential curve, but its maximum is displaced to more positive membrane potentials. The shift in the two curves on the voltage axis can be explained by the potential dependence of G(K, Ca).7. The net outward K(+) current measured with depolarizing voltage pulses in normal and in Ca(2+) free ASW is increased when [Ca](i) is elevated by internal Ca(2+) injection. With large and prolonged Ca(2+) injections the net outward current is depressed following the decline of [Ca](i).8. The time and frequency dependent depression of the net outward K(+) current which occurs during repetitive stimulation is shown to have no obvious temporal relation to the increase in [Ca](i). The depression is relieved by an increase in [Ca](i) caused by internal Ca(2+) injection.9. The net outward K(+) current measured with brief depolarizing pulses which approach the estimated Ca(2+) equilibrium potential and therefore do not cause Ca(2+) influx and accumulation is facilitated by a previous depolarizing pulse which causes a rise in [Ca](i)..10. The facilitation experiments also suggest that the activation of I(K, Ca) by [Ca](i) has a significant time constant. During a depolarizing pulse, the rise in [Ca](i) next to the membrane, and hence I(K, Ca) is expected to follow the square root of time, but a delay in the activation of I(K, Ca) by [Ca](i) could explain why the observed time course of I(K, Ca) is initially almost linear.11. The potential dependence of the Ca(2+) activated K(+) conductance can be explained if the internal Ca(2+) binding site is about half way through the membrane.

Zinc-dependent action potentials in giant neurons of the snail, Euhadra quaestia

In giant neurons of subesophageal ganglion of the Japanese land snail, Euhadra quaestia Deshayes, permeation of Zn ions through Ca channels were investigated with a conventional current clamp method. All-or-none action potentials of long duration (90 to 120 sec) were evoked in 24 mM Zn containing salines. The overshoots were about +10 mV and the maximum rate of rises (MRRs) was about 2.9 V/sec. The amplitudes and the MRRs of the action potentials depended on external Zn ion concentrations. The action potentials were suppressed by specific Ca-channel inhibitors such as Co2+, La3+ and Verapamil, but they were resistant to Na-channel inhibitor, tetrodotoxin, even at 30 microM. It is concluded that these action potentials are generated by Zn ions permeating Ca channels in snail neuronal membrane. On the basis of Hagiwara and Takahashi's (S. Hagiwara & K. Takahashi, 1967, J. Gen. Physiol. 50:583) model of Ca channels, it is inferred that Zn ions are 5 to 10 times stronger in affinity to Ca channels than Ca ions, but 10 to 20 times less permeable.

The calcium current and the activation of a slow potassium conductance in voltage-clamped mouse neuroblastoma cells

1. The Ca2+ inward current (ICa) and a slow outward current in differentiated cells of mouse neuroblastoma clone N1E-115 have been studied under voltage-clamp conditions. 2. ICa shows voltage- and time-dependent inactivation when evoked by step-wise depolarizations in Na+-free solution containing high [Ca2+] (20 nM) and tetraethylammonium (TEA, 25 mM). Ba2+ and Sr2+ can substitute for Ca2+. 3. Holding potentials below -70 mV maximal activate ICa. Half inactivation occurs at -56 mV and ICa is completely inactivated beyond holding levels of -30 mV. Maximum peak currents are of the order of 10(-4) A/cm2 and the reversal potential ranges from +40 to +60 mV. The ICa inactivation time course follows first-order kinetics with a voltage-depedent time constant ranging from 25 to 100 msec. 4. The striking resemblance between ICa and the Ca2+ current in the unfertilized mouse oocyte (Okamoto, Takahashi & Yamashita, 1977) is discussed. 5. A slow outward current with a rise time of several seconds is recorded on voltage steps beyond -20 mV in high [Ca2+] solutions. It is carried primarily by K+ on account of the value of the reversal potential and its dependence on [K]0. This K+ current is TEA-insensitive and is blocked by Ca2+ antagonists. 6. The slow K+ current (IK(Ca)) is suggested to be mediated by Ca2+ influx, but the voltage-dependence of the underlying conductance (GK(Ca)) differs significantly from the ICa voltage-dependence. 7. The results are consistent with the hypothesis that IK(Ca) depends both on ICa and on membrane potential. An alternative hypothesis is briefly discussed.

Effects of internal potassium and sodium on the anomalous rectification of the starfish egg as examined by internal perfusion

1. The effects of alterations of the intracellular ionic composition on the properties of anomalous (or inward) rectification of the egg membrane of the starfish, Mediaster aequalis, were studied by using an intracellular perfusion technique. The following results were obtained, analysing the membrane current with the voltage-clamp technique. 2. The inward rectification of the K conductance depends only on the membrane potential, V, when the K equilibrium potential, VK, is altered by changing the internal K+ concentration at a fixed external K+ concentration, while it depends on V-VK when VK is altered by changing the external K+ at a fixed internal K+ concentration. 3. From the above the conclusion is reached that the gating of the K channel of the inward rectification depends on V and external but not internal K+ concentration. 4. The conductance of the K channel at a given voltage is roughly proportional to the square root of [K+]i when the latter is altered at a fixed external K+ concentration. 5. Since the conductance is proportional to the square root of [K+]o when this is altered at a fixed internal K+ concentration, the final conclusion is that this conductance is proportional to the geometric mean of the external and internal K+ concentrations. 6. Intracellular Na+ ions are necessary for the activation of inward rectification; the K conductance increases sharply with internal Na+ concentration, reaching saturation at 200 mM. 7. A similar potentiating effect is found for Li+, although it is weaker. Rb+, Cs+ and organic cations such as arginine+ do not have this effect.

Sodium and calcium gating currents in an Aplysia neurone

1. Currents generated by depolarizing the hyperpolarizing voltage pulses were recorded at temperatures of 4--12 degrees C in the voltage-clamped soma of R15 in aplysia abdominal ganglia exposed to solutions which suppressed ionic currents. 2. Subtraction of linear capacitive and leakage currents from current generated by voltage pulses to levels more positive than -20mV revealed non-linear transient outward displacement currents at the onset of the clamp step (on-current) and transient inward displacement currents after the membrane potential returned to the holding potential (off-current). Only on-currents were studied. 3. Pulses to membrane potentials of -20 to 0 mV generated a displacement current with rapid onset and exponential decay. At membrane potentials more positive than o mV a second displacement current with a much slower onset and slower exponential decay was seen. Because the different threshold potentials for the two displacement currents were close to the different threshold potentials for Na and Ca ion currents, the two displacement currents were called Na and Ca 'gating' currents. 4. The amount of charge transfer during Ca gating currents increased sigmoidally with increasing depolarization, reaching a maximum at +30 to +40 mV. Half-maximum charge transfer occurred at +15 mV. 5. Total charge movement during Ca gating currents was maximal with holding potentials of -30 to -40 mV. More positive or more negative holding potentials produced a decrease in charge movement. 6. The time course of the gating currents, but not the total charge displaced, was very sensitive to temperature. The time constant of decay of Ca gating currents had a Q10 of about 3, whereas the total amount of charge displaced had a Q10 of 1.2. 7. The charge transfer during both Na and Ca gating currents and the amplitude of Na and Ca (but not K) ionic currents were reduced in solutions containing 1 mm-n-octanol.

Calcium-dependent potentials in the mammalian sympathetic neurone

1. Intracellular recordings from post-ganglionic neurones of the rat superior cervical ganglion revealed two non-synaptic potentials dependent upon Ca2+, a hyperpolarizing afterpotential (h.a.p.) and a tetrodotoxin (TTX)-insensitive spike. 2. The h.a.p. followed regeneration discharge of the membrane potential in normal and TTX-containing Locke solution. 3. The h.a.p. appeared to arise from an increased K+ conductance because it was associated with a decrease in input resistance, reversed at -90 mV, and was proportional in magnitude to the extracellular K+ concentration. 4. Tetraethylammonium (TEA) and 4-aminopyridine (4-AP) apparently antagonized a voltage-sensitive K+ conductance because they broadened the action potential. However, these substances reduced only slightly the peak amplitude and earliest phases of the h.a.p. 5. The TTX-insensitive spike was most apparent when TEA was present and was invariably followed by an h.a.p. with a magnitude proportional to that of the spike. 6. The magnitude of the h.a.p. and the TTX-insensitive spike was directly proportional to the external Ca2+ concentration and was antagonized by Co2+ and Mn2+ in a dose-dependent fashion. 7. In normal Locke solution, Ba2+ antagonized the h.a.p. and allowed the neurone to sustain discharge during prolonged depolarization. In Locke solution containing TTX and TEA, Ba2+ reduced the magnitude of the h.a.p. but greatly increased the duration of the TTX-insensitive spike. 8. The h.a.p. was not significantly affected by altering external Cl- concentration and the TTX-insensitive spike was not reduced by altering external Na+ concentration. 9. It is concluded that the post-ganglionic neurone supports a regenerative Ca2+ conductance mechanism which in turn triggers an increased K+ conductance. The h.a.p. appears to result from outward K+ current in both a Ca2+ and voltage-dependent fashion.