Tail Current Decay Research Articles

Potassium current activated by depolarization of dissociated neurons from adult guinea pig hippocampus.

Currents were generated by depolarizing pulses in voltage-clamped, dissociated neurons from the CA1 region of adult guinea pig hippocampus in solutions containing 1 microm tetrodotoxin. When the extracellular potassium concentration was 100 mM, the currents reversed at -8.1 +/- 1.6 mV (n = 5), close to the calculated potassium equilibrium potential of -7 mV. The currents were depressed by 30 mM tetraethylammonium in the extracellular solution but were unaffected by 4-aminopyridine at concentrations of 0.5 or 1 mM. It was concluded that the currents were depolarization-activated potassium currents. Instantaneous current-voltage curves were nonlinear but could be fitted by a Goldman-Hodgkin-Katz equation with PNa/PK = 0.04. Conductance-voltage curves could be described by a Boltzmann-type equation: the average maximum conductance was 65.2 +/- 15.7 nS (n = 9) and the potential at which gK was half-maximal was -4.8 +/- 3.9 mV (mean +/- 1 SEM, n = 10). The relationship between the null potential and the extracellular potassium concentration was nonlinear and could be fitted by a Goldman-Hodgkin-Katz equation with PNa/PK = 0.04. The rising phase of potassium currents and the decay of tail currents could be fitted with exponentials with single time constants that varied with membrane potential. Potassium currents inactivated to a steady level with a time constant of approximately 450 ms that did not vary with potential. The currents were depressed by substituting cobalt or cadmium for extracellular calcium but similar effects were not obtained by substituting magnesium for calcium.

Voltage-clamp analysis of the ionic conductances in a leech neuron with a purely calcium-dependent action potential

1. The purely calcium-dependent action potential of the anterior lateral giant (ALG) cell in the leech Haementeria was examined under voltage clamp. 2. Analysis with ion substitutions showed that the ALG cell action potential is generated by only two time- and voltage-dependent conductance systems, an inward Ca-dependent current (ICa) and an outward Ca-dependent K current IK(Ca). 3. The kinetic properties of the inward current were examined both in Cs-loaded neurons with Ca as the current carrier as well as in Ba-containing Ringer solutions with Ba as the current carrier, since Ba effectively blocked all time- and voltage-dependent outward current. 4. During a maintained depolarization, Ba and Ca currents activated with a time constant tau m, they then inactivated with the decay following a single exponential time course with a time constant tau h. The time constants for decay of both Ba and Ca currents were comparable, suggesting that the mechanism of inactivation of ICa in the ALG cell is largely voltage dependent. In the range of potentials from 5 to 45 mV, tau m varied from 8 to 2 ms and tau h varied from 250 to 125 ms. 5. The activation of currents carried by Ba, after correction for inactivation, could be described reasonably well by the expression I'Ba = I'Ba(infinity) [1--exp(-t/tau m)]. 6. The steady-state activation of the Ba-conductance mBa(infinity) increased sigmoidally with voltage and was approximated by the equation mBa(infinity) = (1 + exp[(Vh-6)/3])-1. The steady-state inactivation hBa(infinity) varied with holding potential and could be described by the equation hBa(infinity) = [1 + exp(Vh + 10/7)]-1. Recovery from inactivation of IBa was best described by the sum of two exponential time courses with time constants of 300 ms and 1.75 s, respectively. 7. The outward current IK(Ca) developed very slowly (0.5-1 s to half-maximal amplitude) and did not inactivate during a 20-s depolarizing command pulse. Tail current decay of IK(Ca) followed a single exponential time course with voltage-dependent time constants of between 360 and 960 ms. The steady-state activation n infinity of IK(Ca) increased sigmoidally with depolarization as described by the equation n infinity = [1 + exp(Vh-13.5)/-8)]-1. 8. The reversal potentials of IK(Ca) tail currents were close to the expected equilibrium potential for potassium and they varied linearly with log [K]o with a slope of 51 mV. These results suggest a high selectivity of the conductance for K ions.(ABSTRACT TRUNCATED AT 400 WORDS)

A calcium-dependent potassium current is increased by a single-gene mutation inParamecium

The membrane currents of wild type Paramecium tetraurelia and the behavioral mutant teaA were analyzed under voltage clamp. The teaA mutant was shown to have a greatly increased outward current which was blocked completely by the combined use of internally delivered Cs+ and external TEA+. This, along with previous work (Satow, Y., Kung, C., 1976, J. Exp. Biol. 65:51-63) identified this as a K+ current. It was further found to be a calcium-activated K+ current since this increased outward K+ current cannot be elicited when the internal calcium is buffered with injected EGTA. The mutation pwB, which blocks the inward calcium current, also blocks this increased outward K+ current in teaA. This shows that this mutant current is activated by calcium through the normal depolarization-sensitive calcium channel. While tail current decay kinetic analysis showed that the apparent inactivation rates for this calcium-dependent K+ current are the same for mutant and wild type, the teaA current activates extremely rapidly. It is fully activated within 2 msec. This early activation of such a large outward current causes a characteristic reduction in the amplitude of the action potential of the teaA mutant. The teaA mutation had no effect on any of the other electrophysiological parameters examined. The phenotype of the teaA mutant is therefore a general decrease in responsiveness to depolarizing stimuli because of a rapidly activating calcium-dependent K+ current which prematurely repolarizes the action potential.

Toxins That Modulate the Sodium Channel Gating Mechanisma

A variety of toxins and chemicals has been shown to modulate the gating kinetics of the sodium channel. Studies of batrachotoxin, grayanotoxins and pyrethroids are summarized here as examples. Batrachotoxin and grayanotoxins eliminate the sodium channel inactivation thereby causing a prolonged, steady-state sodium current to flow during a depolarizing step. The sodium channel activation kinetics are not affected markedly. Batrachotoxin appears to bind to a site in the sodium channel to which the inactivation gate normally binds, thus causing an inhibition of sodium inactivation. Single channel recording experiments have shown that the mean open time of individual sodium channels is greatly prolonged by batrachotoxin. It appears that individual sodium channels are modified by batrachotoxin in an all-or-none manner. Pyrethroids which are synthetic derivatives of pyrethrins also modify the kinetics of sodium channels in a very drastic manner. In the presence of type I pyrethroids which lack a cyano group at the alpha position (e.g., allethrin and tetramethrin), a large steady-state sodium current appears during a step depolarization and a large slowly decaying sodium tail current appears upon repolarization. Thus both the activation and inactivation kinetics are slowed. Type II pyrethroids which contain an alpha-cyano group (e.g., deltamethrin, cyphenothrin, and fenvalerate) exert effects on sodium channels qualitatively similar to those of type I pyrethroids. However, the amplitudes of the steady-state sodium current and sodium tail current are smaller and the time constant of tail current decay is much longer. The mean open time of single sodium channels is greatly prolonged by the pyrethroids, and the effect is much more pronounced in type II than in type I pyrethroids. A high degree of stereospecificity has been found among four isomers of tetramethrin, (+)-trans and (+)-cis isomers being highly active and (-)-trans and (-)-cis isomers almost totally inactive. The inactive isomers bind to the sodium channel sites, thus preventing the action of the active isomers. Because of the unique action of pyrethroids in modulating the sodium channels, they are becoming useful tools for channel physiology and pharmacology.

A calcium‐activated chloride current generates the after‐depolarization of rat sensory neurones in culture.

Neurones from the dorsal root ganglia of 1-day-old rat pups were grown in dissociated culture and voltage clamped using patch electrodes for whole-cell recording. The pipettes were filled with either 140 mM-KCl or CsCl. Depolarizing voltage jumps activated net inward calcium currents in all neurones, which in a subpopulation of 46% were followed by slowly decaying inward tail currents accompanied by large increases in membrane conductance. During voltage jumps to membrane potentials more positive than 0 mV the inward calcium current was contaminated by a slow outward relaxation only in those neurones with slow inward tail currents. The availability curve for the slow inward tail current was U shaped, with a peak at approximately +5 mV in medium containing 2.5 mM-Ca2+; further depolarization reduced the amplitude of the tail current. During perfusion with calcium-free solution, or in the presence of the calcium-channel blockers cadmium or cobalt, or on substitution of barium for calcium, both the slow inward tail currents and outward relaxations were reversibly blocked. The reversal potential of the slow inward tail current, measured using a twin-pulse protocol, was approximately -10 mV. Replacement of sodium by tetraethylammonium (TEA) did not reduce the slow inward tail current, nor change its reversal potential. Reduction of the extracellular chloride activity produced a large increase in the amplitude of the slow inward tail current suggesting an increase in permeability to anions. This conductance, which behaves as though activated by prior or concurrent calcium entry triggered by membrane potential depolarization, is referred to as ICl(Ca). The activation and deactivation kinetics of ICl(Ca) are complex: envelope experiments measuring peak tail current amplitude revealed activation to be described by a single exponential function, of time constant approximately 100 ms at -10 to +8 mV. The integral of the tail currents increased with the duration of depolarizing pre-pulses suggesting accumulation of intracellular calcium. The decay of tail currents activated by short depolarizing voltage jumps was described by a single exponential function of time constant approximately 200 ms at -60 mV; more complex decay kinetics were recorded following activation by voltage jumps of duration greater than 60 ms. Tail current decay was voltage sensitive, becoming faster with hyperpolarization and increasing e-fold per 120 mV change in membrane potential.

Two components of Ca-dependent potassium current in identified neurones ofAplysia californica

Outward tail currents measured in Aplysia neurones after termination of depolarizing voltage-clamp pulses consist of rapidly decaying voltage-dependent K currents and slow tail currents of much slower time course. The rapidly decaying voltage-dependent tail currents were blocked with aminopyridines, and measurements of the slow tail currents were made following decay of any residual rapid tail currents. The slow tail current exhibited two components of differing sensitivity to externally applied tetraethylammonium (TEA) ions. In some neurones of the abdominal ganglion (L-2, L-4), virtually all of the slow tail current was resistant to blockage by TEA, while in others (L-3, L-6) 80% or more of the slow tail current was blocked by low TEA concentrations (KD less than 1 mM), the remaining slow tail current being resistant to TEA. This TEA-resistant slow tail current was identified as a K current because it reversed near the K equilibrium potential (EK), the reversal potential was shifted by changes in the external K concentration, and it could be blocked by injection of Cs+. It was abolished by replacement of external Ca2+ by Co2+ or Ba2+, by addition of Cd2+, or by injection of EGTA, and thus determined to be a Ca-dependent current. Intracellular injection of TEA or external application of aminopyridine or apamine had little or no effect on the TEA-resistant slow tail current. Quinidine reduced the TEA-sensitive, but not the TEA-resistant current. Both the TEA-sensitive and the TEA-resistant components of the slow tail current exhibited similar time courses of decay.(ABSTRACT TRUNCATED AT 250 WORDS)

Facilitation of Ca2+-channel currents in bovine adrenal chromaffin cells.

Ca2+-channel currents in primary cultures of bovine adrenal chromaffin cells were studied using the whole-cell patch-clamp method. Parameters of a double-pulse protocol were systematically varied to characterize facilitation by a prepulse (P1) of Ca2+-channel current during a test pulse (P2). The pulses were usually separated by 30 msec, an interval sufficient for decay of any measurable P1 tail currents. The Ca2+-channel current amplitude during P2 increased when P1 voltage was more positive than 0 mV. The effect became progressively greater with more positive P1 voltage. With a 60-msec P1 to +80 mV, the current amplitude typically increased by 25%-35% during a 60-msec P2. Comparison of facilitated and control inward Ca2+-channel current I(V) curves showed that facilitation was also strongly dependent on P2 test voltage. Facilitation of Ca2+-channel currents is a voltage-dependent phenomenon and is not dependent on Ca2+ entry. When short repetitive voltage-clamp pulses were applied, the Ca2+-channel current amplitude increased with each pulse. This suggests that Ca2+-channel facilitation could enhance release of catecholamines from chromaffin cells during a train of action potentials.

Slow sodium inactivation in nerve after exposure to sulhydryl blocking reagents.

Exposure to N-ethylmaleimide (NEM), a reagent that binds covalently to protein sulfhydryl groups, results in a specific reduction in sodium conductance in crayfish axons. Resting potential, the delayed rise in potassium conductance, and the selectivity of the sodium channel are unaffected. Sodium currents are only slightly increased by hyperpolarizing prepulses of up to 50 ms duration, but can be restored to about 70% of their value before treatment if this duration is increased to 300-800 ms. The time to peak sodium current and the time constant of decay of sodium tail currents are unaffected by NEM, suggesting that the sodium activation system remains unaltered. Kinetic studies suggest that NEM reacts with a "slow" sodium inactivation system that is present in normal axons and that may be seen after depolarization produced by lowered the holding potential or increasing the external potassium concentration. NEM also perturbs the fast h inactivation system, and in a potential-dependent manner. At small depolarizations tauh is decreased, while at strong depolarizations it is increased over control values. Experiments with structural analogs of NEM suggest that sulfhydryl block is involved, but do not rule out an action similar to that of local anesthetics, p-Chloromercuriphenylsulfonic acid (PCMBS), another reagent with high specificity for SH groups, also blocks sodium currents, but restoration with prolonged hyperpolarizations is not possible.