Outward membrane currents activated in the plateau range of potentials in cardiac Purkinje fibres

1. The membrane currents in Purkinje fibres under voltage clamp conditions have been investigated in the range of potentials at which the action potential plateau occurs. The results show that in this range slow outward current changes occur which are quite distinct from the potassium current activated in the pace-maker range of potentials.2. The time course of current change in response to step voltage changes is non-exponential. At each potential the current changes may be analysed in terms of the sum of two exponential changes and this property has been used to dissect the currents into two components, i(x1) and i(x2), both of which have been found to obey kinetics of the Hodgkin-Huxley type.3. The first component, i(x1), is activated with a time constant of about 0.5 sec at the plateau. At more positive and more negative potentials the time constants are shorter. The steady-state degree of activation varies from 0 at about -50 mV to about 1 at +20 mV. The instantaneous current-voltage relation is an inward-going rectifier but shows no detectable negative slope. In normal Tyrode solution ([K](0) = 4 mM) the reversal potential is about -85 mV.4. The second component, i(x2), is activated extremely slowly and the time constant at the plateau is about 4 sec. The steady-state activation curve varies from 0 at about -40 mV to 1 at about +20 mV. The instantaneous current-voltage relation is nearly linear. The reversal potential occurs between -50 and -20 mV in different preparations.5. It is suggested that these currents are carried largely by K ions, but that some other ions (e.g. Na) also contribute so that the reversal potentials are positive to E(K).6. The relation of these results to previous work on delayed rectification in cardiac muscle is discussed.     

Membrane calcium current in ventricular myocardial fibres

1. A slow inward current in ventricular preparations of the dog heart can be measured by the voltage clamp method without interference from the initial rapid sodium current if the sodium system is inactivated by conditioning depolarization.2. The slow inward current is very sensitive to variation in [Ca](o). It occurs above the equilibrium potential of I(Na) immediately after changing the bathing fluid to a sodium-free solution and persists under this condition for a long time without much alteration, while I(Na) is rapidly abolished. Tetrodotoxin and [Mg](o) have no effect on this current component. These results strongly support the view that the slow inward current in cardiac tissue is carried by calcium ions.3. The threshold for initiation of the calcium current is around -35 mV in Tyrode solution and is shifted to more negative potentials by either increasing [Ca](o) or reducing [Na](o).4. Calcium sensitive inward current tails associated with repolarization are assumed to represent a proportional measure of calcium conductance activated during the preceding depolarization. Calcium conductance declines rapidly with time in the inside negative potential range and slowly at positive potentials. The time constants for this ;inactivation' process vary between 40 and 700 msec in the potential range -35 to +50 mV.5. By using instantaneous current-voltage relations the reversal potential of calcium current was estimated to be about +60 mV in normal Tyrode solution. As shown in the Appendix, however, the calcium equilibrium potential cannot be considered to be constant.6. The importance of the calcium current for the plateau of the cardiac action potential is discussed.     

Inward rectification in rat nucleus accumbens neurons

1. Intracellular recordings were made from neurons in slices cut from the rat nucleus accumbens septi. Membrane currents were measured with a single-electrode voltage-clamp amplifier in the potential range -50 to -140 mV. 2. In control conditions (2.5 mM potassium), the resting membrane potential of the neurons was -83.4 +/- 1.1 (SE) mV (n = 157). Steady state membrane conductance was voltage dependent, being 34.8 +/- 1.7 nS (n = 25) at -100 mV and 8.0 +/- 0.7 nS (n = 25) at -60 mV. 3. Barium (1 microM) markedly reduced the inward rectification and caused a small inward current (40.6 +/- 8.7 pA, n = 8) at the resting potential. These effects became larger with higher barium concentrations, and, in 100 microM barium, the current-voltage relation was straight. 4. The block of the inward current by barium (at -130 mV) occurred with an exponential time course; the time constant was approximately 1 s at 1 microM barium and less than 90 ms with 100 microM. Strontium had effects similar to those of barium, but 1000-fold higher concentrations were required. Cesium chloride (2 mM) and rubidium chloride (2 mM) also blocked the inward rectification; their action reached steady state within 50 ms. 5. It is concluded that the nucleus accumbens neurons have a potassium conductance with many features of a typical inward rectifier and that this contributes to the potassium conductance at the resting potential.     

Modulation of the endogenous electrical activity of the bursting neuron in the snail Helix pomatia--II. The membrane characteristics related to modulation of the endogenous activity of the neuron.

The relation of the amplitude of the slow inward current to the holding potential and the changes of membrane conductance during the development of the slow inward current under voltage clamp conditions were investigated on neurons RPa1 and RPa7. The amplitude of the slow inward current in neuron RPa1 linearly increased with membrane hyperpolarization from ?30 to ?90 mV. The equilibrium potential for the slow inward current found by extrapolation was about +45 mV. In neuron RPa1 rectangular hyperpolarizing pulses (7 mV, 2s), applied from a holding potential of ?60 mV, evoked inward currents before the development of the slow inward current and outward currents at the maximum of the slow inward current. These currents decreased with time. In the neuron RPa7 no relation of the amplitude of the slow inward current to holding potentials between ?58 and ?78 mV and no changes of membrane conductance during the development of the slow inward current were found.It is concluded that the slow inward current in neuron RPa1 is due to an increase of the membrane's sodium permeability. During the development of the slow inward current an additional potential- and time-dependent increase in permeability occurs upon hyperpolarization, obviously to potassium or chloride ions.     

Sarcotubular anomalous rectification of frog sartorius muscle.

The membrane site responsible for anomalous rectification was determined in frog sartorius muscle fibers. The total current-voltage relation of glycerol-treated fibers which represents mainly the properties of the sarcolemma was linear for membrane potentials between about -90 and -50 mV. Thus moderate depolarization-induced anomalous rectification in intact fibers represents a property of the sarcotubular system. The absence of slow hyperpolarization in glycerol-treated fibers was caused by the abolition of early conductance increase, and the sarcotubular system is responsible for the inward rectifier. Picrotoxin selectively inhibited both moderate depolarization-induced anomalous rectification and hyperpolarization-induced early conductance increase. This suggests that the same component in the sarcotubular system is responsible for these conductance changes. The inhibition with picrotoxin of moderate depolarization-induced anomalous rectification suggests the possibility that it is caused by an electrogenic effect rather than a decrease in K conductance. A sarcolemmal hyperpolarization-activated slow conductance increase was revealed.     

A quantitative analysis of the slow component of delayed rectification in frog atrium

1. The slow component of delayed rectification in the atrial muscle membrane of Rana ridibunda has been analysed quantitatively using a voltage-clamp technique.2. It is shown that the current is proportional to a variable which obeys first-order kinetics. At negative potentials the time constant of this process is very long (tau = 3 sec at -55 mV) but it becomes faster at positive potentials (tau = 1.85 sec at +25 mV).3. In the steady state the degree of activation is a sigmoid function of potential. The activation threshold varies substantially between preparations but is usually negative to -30 mV.4. The instantaneous current-voltage relation is nearly linear and its reversal potential most frequently lies between zero and -30 mV.5. The properties of this current system resemble those of the current system i(x2) found in mammalian Purkinje fibres.     

Potassium and calcium conductance in slow muscle fibres of the toad.

Slow muscle fibres in isotonic potassium sulphate saline could be easily repolarized to -90 mV. From this membrane potential a regenerative response could be elicited with short depolarizing pulses. 2. This response is blocked by TEA, suggesting that potassium is the main ion involved. 3. In the presence of TEA, a transient depolarization is recorded when the steady hyperpolarization is withdrawn. This anode break response is dependent upon the external calcium and is blocked by cobalt, suggesting that it is due to a calcium conductance. 4. The membrane conductance change was continuously recorded with short pulses at the end of the hyperpolarization. The membrane conductance decayed with at least two components with an average t1/2 of 1-2 and 6-8 sec. TEA blocked the slow component, and the fast one was dependent upon calcium and was blocked by cobalt.     

Participation of a chloride conductance in the subthreshold behavior of the rat sympathetic neuron.

The presence of a novel voltage-dependent chloride current, active in the subthreshold range of membrane potential, was detected in the mature and intact rat sympathetic neuron in vitro by using the two-microelectrode voltage-clamp technique. Hyperpolarizing voltage steps applied to a neuron held at -40/-50 mV elicited inward currents, whose initial magnitude displayed a linear instantaneous current-voltage (I-V) relationship; afterward, the currents decayed exponentially with a single voltage-dependent time constant (63.5 s at -40 mV; 10.8 s at -130 mV). The cell input conductance decreased during the command step with the same time course as the current. On returning to the holding potential, the ensuing outward currents were accompanied by a slow increase in input conductance toward the initial values; the inward charge movement during the transient ON response (a mean of 76 nC in 8 neurons stepped from -50 to -90 mV) was completely balanced by outward charge displacement during the OFF response. The chloride movements accompanying voltage modifications were studied by estimating the chloride equilibrium potential (E(Cl)) at different holding potentials from the reversal of GABA evoked currents. [Cl(-)](i) was strongly affected by membrane potential, and at steady state it was systematically higher than expected from passive ion distribution. The transient current was blocked by substitution of isethionate for chloride and by Cl(-) channel blockers (9AC and DIDS). It proved insensitive to K(+) channel blockers, external Cd(2+), intracellular Ca(2+) chelators [bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid (BAPTA)] and reduction of [Na(+)](e). It is concluded that membrane potential shifts elicit a chloride current that reflects readjustment of [Cl(-)](i). The cell input conductance was measured over the -40/-120-mV voltage range, in control medium, and under conditions in which either the chloride or the potassium current was blocked. A mix of chloride, potassium, and leakage conductances was detected at all potentials. The leakage component was voltage independent and constant at approximately 14 nS. Conversely, gCl decreased with hyperpolarization (80 nS at -40 mV, undetectable below -110 mV), whereas gK displayed a maximum at -80 mV (55.3 nS). Thus the ratio gCl/gK continuously varied with membrane polarization (2.72 at -50 mV; 0.33 at -110 mV). These data were forced in a model of the three current components here described, which accurately simulates the behavior observed in the "resting" neuron during membrane migrations in the subthreshold potential range, thereby confirming that active K and Cl conductances contribute to the genesis of membrane potential and possibly to the control of neuronal excitability.     

Local superfusion modifies the inward rectifying potassium conductance of isolated retinal horizontal cells

1. Horizontal cells were enzymatically and mechanically dissociated from the white perch (Roccus americana) retina and voltage clamped using patch electrodes. Steady-state current-voltage (I-V) relationships of solitary horizontal cells were determined by changing the membrane potential in a rampwise fashion. 2. The I-V curve of cells bathed in normal Ringer solution exhibited a large conductance increase at negative membrane potentials. This conductance activated near the K+ equilibrium potential, had no clear reversal potential, was enhanced by raising the extracellular concentration of K+, and was suppressed by external Cs+. These properties identify the conductance as the inward (anomalous) rectifier. 3. Continuous superfusion of the cells' local environment with drug-free Ringer reduced the magnitude of the inward rectifier current and shifted its activation point to more negative potentials. This effect developed over approximately 30 s, lasted as long as superfusion continued and was reversible upon cessation of superfusion. 4. Pressure ejection of drug-free Ringer solution onto cells bathed in the identical solution also reduced the magnitude of the inward rectifier current, although the effects were more rapid and more transient than those exerted by superfusion. Pressure ejection had little effect when cells were simultaneously superfused with Ringer, suggesting a common mode of action on the inward rectifier. 5. In the absence of superfusion, pressure ejection of Ringer containing 200 microM L-glutamate had a biphasic effect on membrane conductance. At potentials above -60 mV, glutamate caused a conductance increase with a reversal potential near +10 mV. At potentials below -60 mV, glutamate caused a conductance decrease whose reversal potential could not reliably be determined. The latter effect was similar to the suppression of the inward rectifier by application of Ringer alone, suggesting that it may represent an artifact of pressure ejection rather than a direct effect of glutamate. 6. In support of this interpretation, we found that pressure ejection of glutamate in the presence of external Cs+ (which blocks the inward rectifier) or during local superfusion with Ringer (which prevents attenuation of the inward rectifier by pressure ejection) did not cause a conductance decrease at negative potentials. Under these conditions, glutamate caused primarily a conductance increase with a reversal potential near +10 mV.(ABSTRACT TRUNCATED AT 400 WORDS)     

The effect of the tetraethylammonium ion on the delayed currents of frog skeletal muscle

1. A method which permitted control of the membrane potential near the end of a muscle fibre and measurement of an approximation of the membrane current was used to investigate the effects of the tetraethylammonium (TEA) ion on the delayed outward potassium current obtained on depolarizing.2. Assuming R(i) to be 250 Omega cm and a fibre diameter of 80 mu, the mean value for the maximum potassium conductance (g(K)) was 23.2 +/- 3.2 mmho.cm(-2).3. 58 mM-TEA, in two series of experiments, reduced g(K) by about 90%. A concentration-effect relation for TEA in its action on the delayed rectifier could be fitted by a curve for a drug-receptor complex assuming one molecule of TEA to combine reversibly with one receptor, and a dissociation constant of 8 x 10(-3)M.4. TEA tended to shift the threshold for delayed rectification to slightly more negative membrane potentials. TEA caused a similar shift in the relation between n(infinity) and membrane potential, but did not much alter the form of the relation.5. The relation between n(infinity) and membrane potential and between tau(n) (-1) and membrane potential were well fitted by the model of Adrian, Chandler & Hodgkin (1970a) assuming that the Q(10) was 2.5.6. TEA slowed the rate of onset of the delayed potassium currents, decreasing tau(n) (-1) (the reciprocal of the time constant of the fourth power function which described the current's development) by about 80%.7. The inactivation of the delayed current with time was shown to follow a complex time course. A fast phase decays with a time constant of 270 msec and a slow phase with a time constant of 2.3 sec at a membrane potential of + 10 mV.8. The fast phase of the delayed current is much more susceptible to the action of TEA than the slow phase, and these are interpreted in terms of different potassium channels. TEA has little effect on the time constant with which either the fast current or the slow current inactivates.     

Anomalous rectification in neurons from cat sensorimotor cortex in vitro

The ionic mechanisms underlying anomalous rectification in large neurons from layer V of cat sensorimotor cortex were studied in an in vitro brain slice. The anomalous rectification was apparent as an increase of slope conductance during membrane hyperpolarization, and the development of anomalous rectification during a hyperpolarizing current pulse was signaled by a depolarizing sag of membrane potential toward resting potential (RP). Voltage-clamp analysis revealed the time- and voltage-dependent inward current (IAR) that produced anomalous rectification. IAR reversal potential (EAR) was estimated to be approximately -50 mV from extrapolation of linear, instantaneous, current-voltage relations. The conductance underlying IAR (GAR) had a sigmoidal steady-state activation characteristic. GAR increased with hyperpolarization from -55 to -105 mV with half-activation at approximately -82 mV. The time course of both GAR and IAR during a voltage step was described by two exponentials. The faster exponential had a time constant (tau F) of approximately 40 ms; the slow time constant (tau S) was approximately 300 ms. Neither tau F nor tau S changed with voltage in the range -60 mV to -110 mV. The fast component constituted approximately 80% of IAR at each potential. Both IAR and GAR increased in raised extracellular potassium [( K+]o) and EAR shifted positive, but the GAR activation curve did not shift along the voltage axis. Solutions containing an impermeable Na+ substitute caused an initial transient decrease in IAR followed by a slower increase of IAR. Brain slices bathed in Na+-substituted solution developed a gradual increase in [K+]o as measured with K+-sensitive microelectrodes. We conclude that GAR is permeable to both Na+ and K+, but the full contribution of Na+ was masked by the slow increase of [K+]o that occurred in Na+ substituted solutions. Chloride did not appear to contribute significantly to IAR since estimates of EAR were similar in neurons impaled with microelectrodes filled with potassium chloride or methylsulfate, whereas, ECl (estimated from reversal of a GABA-induced ionic current) was approximately 30 mV more positive with the KCl-filled microelectrodes. Extracellular Cs+ caused a reversible dose- and voltage-dependent reduction of GAR, whereas intracellular Cs+ was ineffective. The parameters measured during voltage clamp were used to formulate a quantitative empirical model of IAR.(ABSTRACT TRUNCATED AT 400 WORDS)     

A time- and voltage-dependent potassium current in the rabbit sinoatrial node cell

Summary Voltage clamp experiments were conducted in the rabbit sinoatrial node (S-A node) using the double microelectrode technique. When the membrane was repolarized to the resting potential after depolarizing test pulses, the outward current slowly decayed to the steady level (outward current tail). The magnitude of the outward current tail was a sigmoid function of the amplitude of the preceding depolarization. The degree of activation of this current varied from 0 at about – 50 mV to 1 at about +20 mV. The time course of the current change was a simple exponential and was independent of the preceding depolarization. The reciprocal time constant appeared to be a U-shaped function of the membrane potential with a minimum value of about 3 s^–1 at –40 mV. The instantaneous current voltage relation was an inward-going rectifier, but showed no detectable negative slope. The reversal potential, obtained between 10 and 50 mM [K] _o , decreased with a slope of 58 mV for a 10-fold increase in [K] _o . These findings indicate that the outward current tail in the S-A node cell is attributable to a single component of K current (pacemaker current component). The pacemaker current component is mainly responsible for the slow diastolic depolarization.This work was supported by the Japanese Ministry of EducationSupported by the Matsunaga Research Grant     

The decline of potassium permeability during extreme hyperpolarization in frog skeletal muscle

1. The voltage-clamp technique was used to separate the effects of K depletion in the T-system from the decline in K permeability during hyperpolarization, and to characterize the time- and voltage-dependence of the latter.2. K permeability due to the inward rectifier can be described as being proportional to a parameter which diminishes when the membrane is hyperpolarized beyond -120 mV. The parameter obeys first-order kinetics. At 24 degrees C, it can change with a time constant of 49 msec at -150 mV and 25 msec at -65 mV. At -200 mV the fall in membrane conductance due to the permeability change is to 30% of its initial value. The Q(10) for the rate of conductance change at that potential is about 2.8.3. It is estimated that K inward current can lower the average K concentration in the T-system by more than 50%, and that, on the average, the space enclosed by the T-system should be less than 0.8% of the fibre volume. Assuming the T-system space to be 0.3% of the fibre volume, it is calculated that on the average, and during hyperpolarization to about -150 mV, no more than 20% of the initial current should flow across the surface membrane.     

Effect of acetylcholine on time-independent currents in sheep cardiac Purkinje fibers

Voltage-clamp experiments were carried out in sheep Purkinje fibers in order to find an explanation for the prolongation of the action potential, the positive shift of the plateau, the hyperpolarization of the maximum diastolic potential and the increase in rate of diastolic depolarization, occurring in the presence of acetylcholine (Ach).In the presence of Ach the instantaneous current-voltage relation is shifted in the inward direction for potentials positive to –75 mV, while the opposite shift is obtained for more negative potentials; the results suggest a decrease in background conductance.The contribution of K, Cl, Na and Ca to the Ach sensitive current was studied by varying K₀ concentration or adding 20 mmol·l^–1 Cs, by omitting Cl or Na, and by changing the Ca concentration.In 20 mmol·l^–1 Cs the apparent reversal potential of the Ach sensitive current is –50 mV, as compared to –75 mV in normal Tyrode. The component of the Ach sensitive current, which is suppressed by Cs, shows inward going rectification. In different K₀ concentrations the reversal potential of the Ach sensitive current is changed; the shift obeys the theoretical change in equilibrium potential of K. The results are consistent with a decrease in K background current by Ach (inward and outward rectifier).In Cl free media the Ach sensitive current is not decreased excluding a major contribution of Cl ions. The Ach effect also persists in Na free media; the reversal potential of the Ach sensitive current is slightly shifted in the hyperpolarizing direction. These results indicate that active electrogenic pumps (Na or Na–Ca) do not play an important role; they are in accord with a reduction in inward Na background current by Ach. The shift of the current-voltage relation by Ach was greater the lower the Ca_o concentration; the mechanism is not clear.The inward shift of the current at –40 mV was dependent on the Ach concentration. Half-maximum effect was obtained at 3·10^–7 mol·l^–1 Ach; the Hill coefficient was 1.12.It is concluded that Ach interacts in a one to one reaction with a muscarinic receptor and reduces the background current mainly carried by K (inward and outward rectifier), and less by Na (and probably Ca).Supported by F.G.W.O. Belgium 3.0087.74     

Voltage clamp experiments in striated muscle fibres

1. Membrane currents during step depolarizations were determined by a method in which three electrodes were inserted near the end of a fibre in the frog's sartorius muscle. The theoretical basis and limitations of the method are discussed.2. Measurements of the membrane capacity (C(M)) and resting resistance (R(M)) derived from the current during a step change in membrane potential are consistent with values found by other methods.3. In fibres made mechanically inactive with hypertonic solutions (Ringer solution plus 350 mM sucrose) step depolarizations produced ionic currents which resembled those of nerve in showing (a) an early transient inward current, abolished by tetrodotoxin, which reversed when the depolarization was carried beyond an internal potential of about +20 mV, (b) a delayed outward current, with a linear instantaneous current-voltage relation, and a mean equilibrium potential with a normal potassium concentration (2.5 mM) of -85 mV.4. The reversal potential for the early current appears to be consistent with the sodium equilibrium potential expected in hypertonic solutions.5. The variation of the equilibrium potential for the delayed current (V'(K)) with external potassium concentration suggests that the channel for delayed current has a ratio of potassium to sodium permeability of 30:1; this is less than the resting membrane where the ratio appears to be 100:1. V'(K) corresponds well with the membrane potential at the beginning of the negative after-potential observed under similar conditions.6. The variation of V'(K) with the amount of current which has passed through the delayed channel suggests that potassium ions accumulate in a space of between (1/3) and (1/6) of the fibre volume. If potassium accumulates in the transverse tubular system (T system) much greater variation in V'(K) would be expected.7. The delayed current is not maintained but is inactivated like the early current. The inactivation is approximately exponential with a time constant of 0.5 to 1 sec at 20 degrees C. The steady-state inactivation of the potassium current is similar to that for the sodium current, but its voltage dependence is less steep and the potential for half inactivation is 20 mV rate more positive.8. Reconstructions of ionic currents were made in terms of the parameters (m, n, h) of the Hodgkin-Huxley model for the squid axon, using constants which showed a similar dependence on voltage.9. Propagated action potentials and conduction velocities were computed for various conditions on the assumption that the T system behaves as if it were a series resistance and capacity in parallel with surface capacity and the channels for sodium, potassium and leak current. There was reasonable agreement with observed values, the main difference being that the calculated velocities and rates of rise were somewhat less than those observed experimentally.     

Slow changes in membrane permeability and long-lasting action potentials in axons perfused with fluoride solutions

1. Voltage clamp experiments were carried out on squid giant axons which were perfused internally with 300 mM-NaF + sucrose and placed in K-free artificial sea-water at 16-17 degrees C. On stepwise depolarization (V = -38.5 to 68 mV) the Na conductance g(Na) rapidly reached a peak value and then declined to a new level; this ;maintained' level was slowly inactivated in an exponential manner with a rate constant which varied from 0.3 to 1.1 sec(-1). This process was not influenced appreciably by replacing 50 mM-NaF with KF.2. On repolarization to a potential which varied between -73 and -101 mV the slow inactivation was removed with a rate constant of 0.11-0.73 sec(-1).3. Prolonged depolarization also produced a slow inactivation of the ability of the membrane to give a transient increase in g(Na). This effect developed at a rate about (1/3)-(1/2) that associated with the inactivation of the ;maintained' component; on repolarization, recovery of the peak g(Na) was 1-2 times as fast as recovery of the ;maintained' g(Na).4. In experiments on fibres perfused with 300 mM-KF after internal NaF had removed the usual delayed K conductance, depolarization resulted in an outward current which developed in an exponential manner with a time constant of a fraction of a second. The equilibrium potential for this component was more negative than -50 mV.5. Long-lasting action potentials were computed on the basis of the above slow changes and, except for the period of final repolarization, were found to be in satisfactory agreement with experimental records. The discrepancies suggest that there may be additional slow changes in permeability which could not be resolved in the present experiments.     

Block by the neuropeptide TRH of an apparently novel K+ conductance of rat motoneurones

Using a single electrode voltage clamp technique the actions of rapidly superfused thyrotropin-releasing hormone (TRH, 1 microM) on lumbar motoneurones of the isolated neonatal rat spinal cord were investigated. TRH induced a slowly developing inward current (associated with an input conductance fall) with slow recovery on washout. In the presence of TRH the normally linear current-voltage relations displayed strong inward rectification up to about -40 mV. The TRH-induced current peaked at -50 mV, reversed at -120 mV and was not blocked by Cs+, tetraethylammonium, 4-aminopyridine, Cd2+, or low Na+. Its reversal potential was sensitive to changes in extracellular K+. Ba2+ (0.2-1.5 mM) depressed the effects of TRH. It is suggested that in rat motoneurones TRH blocked an apparently novel K+ conductance (IK(T)) active at resting membrane potential.     

Slow inactivation of currents in cardiac Purkinje fibres

1. When membrane currents, associated with repolarization to the holding potential, were measured in cardiac Purkinje fibres after depolarizing voltage clamp steps to the plateau level of the action potential (-20 to -10 mV), tails of inward current were observed which turned into outward current. Amplitude and duration of the tails of inward current increased when the preceding depolarization was progressively shortened. This current could only be recorded in Na-containing solution which strongly supports the view that the inactivation of sodium conductance (g(Na)) has a slow and incomplete component.2. Initial outward current (most probably chloride current (I(Cl))) during strong depolarization (beyond -20 mV) could be inactivated either by increasing the frequency of depolarizations or by applying a conditioning depolarizing prepulse.3. In Na-free solution a small, slowly decreasing inward current was recorded. This is most probably carried by calcium ions and is at least partly responsible for the negative conductance of the current-voltage relation in the voltage range -60 to -30 mV.4. If a strong conditioning depolarization (V(1)) to the inside positive potential range preceded repolarizing clamp steps to different membrane potentials (V(2)) inward current (probably calcium current (I(Ca)) could be activated in Na-free and Na-containing solutions. This current depended on the duration of V(1) as well as on the potential level of V(2). After the maximum inward current, increasing outward current occurred when the duration of V(1) was increased. This current could be explained either by an inactivation of calcium conductance (g(Ca)) with time or by a very slow activation of an additional outward current.5. The main conclusions from these experiments are that (i) slow and incomplete inactivation of g(Na) are important features for the plateau of the action potential and (ii) calcium ions carry additional charge during this phase of the action potential. Chloride conductance (g(Cl)) should be largely inactivated at a heart rate of 90/min.     

Instantaneous inward rectification in the Mauthner cell: a postsynaptic booster for excitatory inputs

Single and double electrode voltage clamp techniques have been used to analyse the leakage conductance of the goldfish Mauthner cell. The results indicate that the input conductance of this neuron is maximal at the resting potential, having an average value of 4.68 microS. Approximately 50% of this conductance is voltage dependent and could be inactivated by large hyperpolarizing command pulses (30-50 mV magnitude) of 20-35 ms duration. The magnitude and apparent time constant of the inactivation are both functions of membrane potential, such that the relaxation of the inward current increased and occurred more rapidly for greater hyperpolarizations. The presence of instantaneous inward tail currents when the imposed voltage steps were terminated and membrane potential was returned to the resting level indicated that the conductance mechanism inactivated during hyperpolarization had generated a small outward current at the resting level. Thus, we propose that a major fraction of the Mauthner cell's input conductance is a voltage-dependent K+ conductance. Hyperpolarizing inactivation is a feature of an inward K+ rectifier in other cell types, and when the recording microelectrode was located in the Mauthner cell lateral dendrite, it was possible to demonstrate characteristics consistent with rectification, namely a low conductance for outward depolarizing currents and a high conductance for inward currents. The inward rectifier of the Mauthner cell is different from that of other neurons in that it is already maximally activated at the resting potential and therefore is a major determinant of that parameter. In addition, its activation and inactivation kinetics are quite fast.(ABSTRACT TRUNCATED AT 250 WORDS)     

The kinetics and rectifier properties of the slow potassium current in cardiac Purkinje fibres

1. The reversal potential of the slow outward current in Purkinje fibres varies with [K](o) in accordance with the expected potassium equilibrium potential. It is concluded that virtually all of this current is carried by potassium ions.2. The magnitude of the current is determined by two separable factors. The first factor is directly proportional to a variable obeying first-order voltage-dependent kinetics of the Hodgkin-Huxley type but with extremely long time constants. The time constants of this variable are extremely sensitive to temperature and the Q(10) over the range 26-38 degrees C is 6.3. The second factor shows inward-going rectification with a marked negative slope in the current-voltage relation beyond about 25 mV positive to the K equilibrium potential. The current-voltage relations measured at different values of [K](o) cross each other on the outward current side of the equilibrium potential.4. The changes in slow potassium current during pace-maker activity have been calculated. It is shown that the mechanism of the pace-maker potential differs in several important respects from that described by Noble's (1962) model. The negative slope in the current-voltage relation appears to be an important factor in generating the last phase of pace-maker depolarization.5. The role of the slow potassium current during the action potential and the consequences of the high temperature dependence of the kinetics are discussed.