mV Change Research Articles

Induction and disappearance of excitability in the oocyte of Xenopus laevis: a voltage-clamp study.

Electrically excitable, sodium-selective channels are induced in the membrane of the oocytes of Xenopus laevis when it is submitted to prolonged positive potentials (Kado, Marcher & Ozon, 1979; Baud, Kado & Marcher, 1982). Under a long positive voltage-clamp step, the membrane current, initially outward, becomes inward with a sigmoidal time course. The mean time to half-maximal inward current (t 1/2) is about 18 s at 16 degrees C when stepping the membrane potential to +55 mV. The rate of channel induction was very temperature dependent (Q10 about 5). In an Arrhenius plot, the t 1/2 for induction at temperatures between 5 and 22 degrees C showed a single slope. The rate of induction was dependent on the membrane potential, increasing exponentially with positive membrane potential (e-fold for a 20 mV change). When the membrane was maintained at resting potential after induction, the ability to produce inward currents with short depolarizing steps slowly disappeared with a t 1/2 of 4 min at 16 degrees C. The temperature dependence for disappearance was larger than that found for induction (Q10 about 7). The rate of disappearance was not dependent on holding the membrane potential in the range -30 to -100 mV. Induction proceeded in calcium-free medium. Cycloheximide, a potent protein synthesis inhibitor had no effect (100 micrograms/ml) on the induction rate. Isobutylmethylxanthine (IBMX) or theophyllin (phosphodiesterase inhibitors) applied externally (10(-4) M) did not affect the induction or disappearance rates. From the present results, mechanisms such as protein synthesis or a second messenger (such as calcium or cyclic AMP) do not appear to be involved. During the depolarization necessary to obtain induction, another conductance was also activated. It was more slowly established, appeared to be non-saturable and had a reversal potential between zero and -10 mV. It was found to be very much reduced at temperatures below about 16 degrees C.

Isolation and kinetic analysis of inward currents in neuroblastoma cells

The suction pipette method for combined voltage clamp and intracellular dialysis was applied to isolate the two components of voltage-gated inward current across membranes of NIE-115 neuroblastoma cells. In order to anlayze the kinetic behavior of the Na + and Ca 2+ channels responsible for generating these components, current through K + channels was effectively blocked by substituting impermeant Cs + for internal and external K +. Block was confirmed independently by examining the effects of the application of external tetraethylammonium or Cd 2+; and comparing the time course of Ca 2+ tail currents with the decay of current during a maintained depolarization. Na + currents studied at 8–10°C, developed as a fourth order process giving a maximum e-fold conductance change for a 3 mV depolarization, with half activation occurring at −10 mV. The instantaneous current-voltage relationship was linear. Time constants of the activation parameter ( m) varied from 0.5ms (−50 mV) to 3–4 ms (−10 to −40 mV) at 10°C. Inactivation ( h) was a first order process having a time constant between 4 ms (+10 to +60mV) and 225ms (−60 mV). Steady-state inactivation for Na + channels attained a value of 0.5 at −50 mV. A slow inactivation process, however, also is involved in gating of Na + channels, and has a time course at least two orders of magnitude slower than that for h. The temperature sensitivity of Na + currents was found to be similar to that found for other preparations. Ca 2+ currents were studied at 24–29°C in the presence of internal ethyleneglycolbis-(aminoethylether)-tetra-acetate (EGTA) and an external Ca 2+ concentration of 20 mM. Ca 2+ channel activation could also be described by a fourth order process giving an e-fold conductance change for a 5–6 mV change in potential and the half activation potential of −13 mV. Internal EGTA (20 mM) did not abolish inactivation of Ca 2+ currents and no recovery from inactivation caused by a prepulse could be measured as the prepulse potential approached the null potential for Ca 2+ influx. Time constants of both activation and inactivation of Ca 2+ channels were measured between −20 and +50 mV. Currents through K + channels could be completely eliminated by substitution of K + with Cs +, although a residual non-linear leakage current remained, in addition to currents through the Na + and Ca 2+ channels. Na + channels of these mammalian neuronal cell bodies were found to have kinetics most similar to those of neurons in gastropods, and were much slower than axonal Na + channel kinetics in either mammals or poikilothermic animals, when compared at the same temperature. Measurements of the time course of both Ca 2+ channel inactivation and activation also point to a similarity between mammalian and gastropod neuronal cell bodies. Kinetics of Ca 2+ channels were found to be 20 times slower than those for Na + channels in this preparation when the latter were scaled to the same temperature. This difference explains the generation of the late depolarization phase of the action potential in these cells by Ca 2+ currents. Under the conditions described, inactivation in these cells appears to be primarily a voltagedependent process rather than one dependent on the concentration of intracellular Ca 2+.

The control of tonic tension by membrane potential and intracellular sodium activity in the sheep cardiac Purkinje fibre.

Intracellular Na activity (aiNa) was measured with recessed-tip, Na-selective micro-electrodes in voltage-clamped sheep cardiac Purkinje fibres. Tension was measured simultaneously. aiNa was increased reversibly either by exposing the preparation to K-free, Rb-free solution of by adding the cardioactive steroid strophanthidin. An increase of aiNa produced an increase of tonic tension which was larger at depolarized membrane potentials. At sufficiently negative membrane potentials, changes of aiNa (over the range 6-30 mM) had no effect on tonic tension. Therefore, both an increase of aiNa and a depolarization are required to increase tonic tension. It is concluded that either a low level of aiNa or a large negative membrane potential is sufficient to maintain a low intracellular Ca concentration. Tonic tension was measured as a function of aiNa. At a given membrane potential the relationship can be described empirically by an equation of the form: tonic tension = b(aiNa)y, where y is a constant and b depends on membrane potential. In five experiments y was found to be 3.7 +/- 0.7 (mean +/- S.E.M.) over a range of potentials from -60 to -10 mV. Tonic tension was measured as a function of membrane potential. At a given aiNa the relationship can be described approximately as: tonic tension = k exp (aV), where a is a constant and k depends on aiNa. In five experiments a was found to be 0.06 +/- 0.01 mV-1 (mean +/- S.E.M.). A depolarization of 10 mV increases tonic tension by the same amount as does an increase of aiNa that is equivalent to a 3.7 mV change of the Na equilibrium potential, ENa. Hence ENa is nearly 3 times more effective than membrane potential in controlling tonic tension. During a prolonged depolarization (several minutes) the initial increase of tonic tension decays gradually. This is associated with a fall of aiNa. The relationship between tonic tension and aiNa is similar to that seen when aiNa is increased by inhibiting the Na pump. It is concluded that the fall of aiNa is responsible for the decay of tonic tension. The changes of tonic tension reported in this paper are consistent with the effects of aiNa and membrane potential on a voltage-dependent Na-Ca exchange. The possibility that a voltage-dependent Ca channel contributes to tonic tension is also discussed.

Oscilloscopes: The digital alternative: The digital scope's capability in measurement, transient capture, and data storage is a significant improvement over its analog counterpart

As with its analog counterpart, the primary function of a digital oscilloscope is to graphically display the voltage and time characteristics of an analog waveform. The front panel controls are also very similar, including amplifier, triggering, sweep timing, and display. However, the digital oscilloscope not only acquires and displays analog signals, it also stores them in a solid state memory, and thereby offers many advantages to the user. The resolution and accuracy of an analog oscilloscope are restricted by screen size, phosphor characteristics, and the limitations of the human eye to a typical range of 2 or 3 percent. In contrast, many modern digital oscilloscopes offer a voltage resolution of one part in 4096, or the ability to detect a 0.25 mV change on a one-volt signal. Similarly, horizontal timing is significantly improved in a digital oscilloscope since the incoming signal is sampled at a rate determined by a crystal-controlled clock. Each sample is converted to a digital value and stored in memory, producing a digital image of the original waveform. This digital representation of the analog signal is extremely accurate, with the voltage and time coordinates of each point precisely known. Thus, although the image is reconverted to analog form for display, the reliance on CRT characteristics for measurement accuracy is completely avoided.

The effect of internal barium on the K current of the node of Ranvier.

Voltage clamp experiments were performed on single myelinated nerve fibres of the frog Rana esculenta to study the influence of internally applied Ba on the potassium outward current IK. Potassium currents of Ba-treated fibres decay rapidly during 30 ms depolarizations of more than 60 mV. The time constant tau of decay and the steady state potassium current IKss are strongly voltage dependent. The equilibrium dissociation constant K of the Ba-receptor complex decreases e-fold for a 39 mV change of potential. The analysis suggests that the binding site is located at 30% of the membrane field seen from the inside of the axon.

Properties of miniature excitatory junctional currents at the locust nerve-muscle junction.

1. Miniature excitatory junctional currents (m.e.j.c.s) were examined in conditions where inward current was carried mainly by Na(+) (i.e. in normal medium, Ca(2+)-free medium and Cl(-)-free medium). M.e.j.c.s were also examined in isotonic Ca(2+) where the inward post-synaptic current was carried mainly by Ca(2+).2. In normal medium, mean m.e.j.c. amplitude = 2.34+/-0.05 nA. The decay time constant of m.e.j.c.s (excluding a small percentage with abnormal shapes) was tau(m.e.j.c.) = 2.62+/-0.11 msec (V(m) = -80 mV, T = 22 degrees C). Decay-time was not markedly changed in Ca(2+)-free or Cl(-)-free medium. tau(m.e.j.c.) approaches the life-time of glutamate activated junctional channels.3. Excitatory junctional currents, evoked by nerve impulses, decayed slightly faster than m.e.j.c.s obtained in the same fibres. Extracellularly recorded m.e.j.c.s and voltage-clamped m.e.j.c.s were similar in time course.4. tau(m.e.j.c.) decreased exponentially with membrane hyperpolarization. An e-fold change was produced by 182.+/-24.8 mV change in V(m).5. The dependence of mean m.e.j.c. amplitude on clamp potential showed a slight non-linearity at hyperpolarized levels. The equilibrium potential for transmitter action was close to 0 mV in normal solution as well as in Ca(2+)-free and Cl(-)-free solutions.6. The kinetics of junctional channels are altered in isotonic Ca(2+). M.e.j.c. amplitude was reduced to about one-third normal size; mean m.e.j.c. = 0.74+/-0.03 nA. The decay time becomes markedly briefer, tau(m.e.j.c.) = 1.01+/-0.08 msec, indicating a reduction in mean channel life-time (V(m) = -80 mV, T = 22 degrees C).7. A population of slow time course and composite m.e.j.c.s appear when muscle fibres are hyperpolarized in isotonic Ca(2+), thus producing a prolongation in mean tau(m.e.j.c.). This results from an influence of post-synaptic membrane potential on presynaptic transmitter release. If such m.e.j.c.s are ignored the voltage dependence of tau(m.e.j.c.) of the remaining events is abolished or even reversed indicating that voltage sensitivity of channel life-time is altered in isotonic Ca(2+). The equilibrium potential for transmitter action may be slightly more positive than normal.8. We estimate that a single packet of neurally released transmitter normally opens, on average, 250 ion channels at these junctions.

Role of lipids in the Neurospora crassa membrane: IV. Biochemical and electrophysiological changes caused by growth on phytanic acid.

Neurospora crassa strain cel, which is deficient in fatty acid synthesis, was grown with phytanic acid supplementation. The temperature dependence of membrane potential is increased by growth on phytanic acid. A temperature change of 40 degrees C produces a change of 184 mV in phytanic acid-grown cells as compared to a 50 mV change for cel grown on palmitic acid or wild-type. Membrane resistance (measured as DC input resistance) of phytanic acid-grown cells did not differ from cel grown on palmitic acid or wild-type. Lipid analysis of cel grown on phytanic acid revealed approximately 7 mole percent phytanic acid incorporation into phospholipids, no change in phospholipid base composition, a reduction of ergosterol content from 80 to 30 percent, and the induction of beta sitosterol, a sterol not usually present in Neurospora. beta sitosterol accounted for approximately 60 percent of the sterol present. Incorporation of 7 mole percent phytanic acid into phospholipids lowers the phase transition temperature by approximately 5 degrees C, and decreases the heat content of the phase transition (delta H) slightly. Results are discussed in relation to Refsum's disease, a human neurological disorder associated with high plasma levels of phytanic acid. It is proposed that high intracellular phytanic acid concentration induces novel sterol synthesis and that the incorporation of the novel sterol into the membrane is responsible for the increased temperature sensitivity of membrane potential. The excitable membrane deficits observed in patients with Refsum's disease may be explained by such a mechanism.

Surface potentials and sodium entry in frog skin epithelium.

1. The effects which alterations in the surface potential of the apical membrane of isolated Rana catesbeiana skin have on Na entry were examined. 2. Changes in the external ionic strength have little effect upon the rate of Na transport across the frog skin epithelium. 3. Uranyl ion (UO2(2+), 2.5 mM) induces a +145 mV change in the surface potential of phosphatidylserine monolayers, and a +60mV change in the surface potential of monolayers made from phosphatidylcholine. 4. UO2(2+) inhibits the short-circuit current (Isc) by a maximum of 20% in R. catesbeiana skin, while stimulating Isc by 40% in R. temporaria skin. Neither Isc stimulation nor inhibition by UO2(2+) can be seen in the presence of 10(-4) M-amiloride. 5. From points 1 and 2 above, we conclude that the surface charge density in the neighbourhood of the Na-selective entry site located in the apical membrane is small (greater than 1e-/600 A2). The results obtained using UO2(2+) suggest that Na entry is not affected by changes in the membrane surface potential.

Inward rectification in frog skeletal muscle fibres and its dependence on membrane potential and external potassium.

1. Experiments were carried out using a voltage-clamp technique to investigate the dependence of inward rectification on membrane potential and on the equilibrium potential for K+, changed either by changing [K]o or changing [K]i. 2. The relationship between gK, the potassium chord conductance, and membrane potential depended on membrane potential and [K]o, but not on [K]i. 3. Under hyperpolarization, K currents increased with time, but instantaneous current-voltage relations also showed inward rectification. The time constants for activation fell with hyperpolarization, e -fold for an 18 mV change in membrane potential. 4. The time constants for activation depended on [K]o but not on [K]i. 5. Under depolarization, the activation of K currents was partly reversed, but between activation and membrane potential, determined from two-pulse experiments, also appeared to depend on [K]o but not on [K]i. 5. Under depolarization, the activation of K currents was partly reversed, but between activation and membrane potential, determined from two-pulse experiments, also appeared to depend on [K]o but not on [K]i. 6. The rate of activation of K currents under hyperpolarization had a Q10 of 2.64 +/- 0.08 (n = 5). Currents, measured per unit length, increased with temperature, with a Q10 of 1.66 +/- 0.11 (n = 5).

The rising phase of the miniature endplate current at the frog neuromuscular junction

(1) The rising phase of minature endplate currets was recorded at the frog's neuromuscular junction using both the two electrode voltage clamp and a single external electrode, or Strickholm, voltage clamp. (2) The Q(10) of the miniature endplate current rising phase was 2.3 in a variety of solutions selected to alter presynaptic behavior. (3) Increasing the solution's viscosity by an amount sufficient to slow the diffusion coefficient of acetylcholine by a third has no effect on the duration of the rising or the decay phase. This solution does seem to further slow the miniature endplate current decay phase, but not the rising phase, after inhibition of the acetylcholinesterase. (4) As the membrane potential is made more positive, the miniature endplate current rising phase is prolonged, with an e- fold slowing per 170 mV change. (5) It is concluded that neither presynaptic nor subsynaptic events determine the rising phase of miniature endplate currents at the frog neuromuscular junction. Rather, the limiting step occurs within the membrane and is most likely a change in the binding constant of the receptor for the acetylcholine molecule.

The kinetics of monazomycin-induced voltage-dependent conductance. I. Proof of the validity of an empirical rate equation.

Monazomycin (a positively-charged, polyene-like antibiotic) induces a strongly voltage-dependent conductance in thin lipid membranes when added to one of the bathing solutions. We show here that the kinetics of conductance changes after a step of membrane potential are only superficially similar to the kinetics of the potassium gating system of squid giant axons, in that the beginning of conductance increases are growth functions of the time, as opposed to power functions of the time. We find that the rate constant (reciprocal of the time constant) of the growth varies with the approximately 2.6 power of the monazomycin concentration. The rate constant also varies exponentially with membrane potential such that an e-fold change is associated with a 10-11 mV change of membrane potential. We show that solutions of a simple differential equation are able to reproduce the actual conductance changes almost exactly. In the accompanying paper (Muller and Peskin. 1981. J. Gen. Physiol. 78:201-229), we derive the differential equation from a molecular model and use the theoretical equation so obtained to investigate the gating current of this system and to predict an interesting form of memory.

The membrane potentials of Tetrahymena vorax

1. 1. Tetrahymena in a solution containing 1 mM CaCl 2, 1 mM KCl, 1 mM TrisHCl pH 6.8, and 0.1 mM EDTAKOH had a resting potential of −28.7 ± 2.1 mV and a membrane resistance of 44.1 ± 2.6 MΩ . 2. 2. A 10-fold increase in external K + led to a 28.3 mV change, and a 10-fold change in Ca 2+ led to a 9.7 mV change in resting potential. 3. 3. Spontaneous hyperpolarizations were observed. These were of 150 msec duration, 12 mV magnitude and had a rate of change of potential of 220 mV sec −1. 4. 4. Spontaneous depolarizations were observed. There were of two main types: (a) the fast type had a maximum rate of rise of potential of 500 mV sec −1 with the spike peaking at + 10 mV. The depolarization phase lasted 100 msec and the repolarizing phase 400 msec. The fast spikes had an all-or-none characteristic and a threshold between 3 and 7 mV; and (b) the slow depolarizations did not overshoot 0 mV, were 15 mV in magnitude, with a rate of rise of potential of 50 mV sec −1. The time course was variable. Both fast and slow depolarizations could occur in the same cell. 5. 5. Following spikes, sustained depolarizations (plateaus) were observed in sodium free solutions. They had a variable duration and were usually at 0 mV membrane potential. 6. 6. Electrical stimulation elicited the fast type of depolarization in 80% of Tetrahymena. The response had an all-or-none characteristic and the threshold was 0.25 nA. 7. 7. The electrophysiological properties of Tetrahymena appear similar to but not identical with those of Paramecium.

Automating a class a evaporation pan for semi-continuous recording

Continuos monitoring of water loss from an evaporation pan was automated by using a system of 16 general-purpose Constantan wire-resistance strain gages mounted on four supporting arms. A change in pan weight from water loss proportionally changed the electrical resistance of the wire gages. The system was assembled with a Class A evaporation pan and tested under field conditions as part of a larger evapotranspiration study. A dual-channel carrier—amplifier recorder in line with a digital data acquisition system was used to record the output of the system once every 2 min. The system assembly had a sensitivity of 0.034 mm of water for 0.1 mV change in bridge output. Total evaporation measured with this system was within 5% of manual measurements.

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.

Effect of membrane polarization and synaptic activity on the timing of antidromic invasion

The latency from the stimulus (S) to the IS and SD components of the antidromic spike was measured in motoneurones and spinocerebellar tract cells following displacement of the membrane potential either by current pulses or by synaptic potentials. Changes in the latency to the SD spike (S-SD delay) were mainly caused by changes in the IS-SD delay and varied from 10 to 100 musec per mV change in membrane potential, depending on the initial value of the IS-SD delay. Changes in the S-IS delay were also observed and these changes could, especially in spinocerebellar cells, give a significant contribution to the change in the total delay. EPSPs shortened the S-SD delay as efficiently as current-evoked depolarizations of similar magnitude while IPSPs were often more effective in prolonging the delay than current-evoked hyperpolarizations. This difference was related to the larger conductance increase during IPSPs than during IPSPs and to the longer IS-SD delays at hyperpolarized potentials. The presented data contribute to the understanding of the method which uses extracellular recording of antidromic latency changes as an indirect measure of intracellular membrane potential changes. Our results show that the recording of antidromic latency changes is a particularly sensitive method for detecting inhibition of neurones.

Sites including those of origin and termination of replication are not freely available to single-cut restriction endonucleases in the supercompact form of simian virus 40 minichromosome

Simian virus 40 minichromosomes, isolated from both virions (MV) and infected cells (MI), have highly compact structures in buffer containing 0.15 M NaCl and sediment with S values of about 90–100 and 115–130, respectively. Under the electron microscope, also, MI appear the more compact of the two. Only 30–35% of the sites of origin and termination of replication in MV are freely available to the restriction endonucleases Bgl 1 and Bam H1. MV are similarly resistant to Eco R1 and Hpa II. In contrast, almost no sites in MI are available to any of the above single-cut endonucleases. In 0.6 M NaCl, MV and MI change to relaxed structures of 45–55 S and 50–60 S, respectively, containing 20–24 nucleosomes per genome, and become more sensitive to Bgl 1, Bam H1, Eco R1, and Hpa II.

Intracellular citrate or externaly applied tetraethylammonium ions produce calcium-dependent action potentials in an insect motoneurone cell body.

1. Electrophysiological observations have been made upon the cell body of an identified motoneurone of the cockroach, Periplaneta americana. Normal responses were compared with those observed after intracellular injection of citrate anions or when the preparation was bathed in solutions containing tetraethylammonium ions (TEA+).2. Normally when depolarized, the motoneurone soma gave a series of damped oscillations; the amplitude of these responses increased with increase in the applied current.3. After citrate ions had been injected into the neurone soma, all-or-none action potentials were evoked by depolarization; such responses appeared about 5-10 min after the onset of citrate injection. Injection of EGTA produced similar effects. Citrate and EGTA probably produce their effect through a reduction in the intracellular free calcium concentration.4. When preparations were bathed in saline solution containing 50 mM-TEA+, soma depolarization produced prolonged all-or-none action potentials (up to approximately 100 msec duration).5. The action potentials produced by citrate injection or externally applied TEA+ appeared to have a similar ionic mechanism; they were not depressed by sodium-free solutions or by tetrodotoxin (4 x 10(-6)M) but were reversibly blocked in saline solution containing 40 mM-manganous chloride.6. The overshoot amplitude of action potentials recorded after injection of citrate anions or in solutions containing TEA+ showed a 22.5 mV change for a ten-fold change in the external calcium concentration.7. Both intracellular citrate and external TEA+ caused a significant increase in the input resistance and membrane time constant of the motoneurone.8. It is concluded that action potentials generated under various experimental conditions in the soma of this insect motoneurone map have differing ionic mechanisms.

Characteristics of sodium and calcium conductance changes produced by membrane depolarization in an Aplysia neurone.

1. The time course and voltage dependence of Na and Ca conductance changes produced by depolarization of the soma of the neurone R15 in the abdominal ganglion of Aplysia juliana were examined at temperatures of 10--14 degrees C. 2. During a maintained depolarization, Na currents turned on then decayed (inactivated). Inactivation was exponential with time constant tauh. Activation (after correction for inactivation) was reasonably well described by the expression G'Na(t) = G'Na (infinity) (1 - exp [-t/taum])3 over a wide range of potentials. 3. taum and tauh were both voltage dependent. In the range -20 to +40 mV, taum varied from 5 to 0.5 msec and tauh from 25 to 8 msec (13.5 degrees C). Steady-state Na conductance (corrected for inactivation) was voltage dependent also, increasing sigmoidally with depolarization to a maximum of 25--30 muS at +10 to +20 mV. Half-maximal Na conductance occurred at a membrane potential of -8 mV and from -15 to -5 mV, a 5 mV change in membrane potential produced an e-fold change in steady-state Na conductance. 4. Steady-state inactivation of Na conductance (hNa(infinity)) was voltage dependent with half-inactivation occurring at a membrane potential of -32 mV. Recovery from Na inactivation followed an exponential time course with a voltage-dependent time constant. 5. During a maintained depolarization Ca currents activated then decayed (inactivated) more slowly than Na currents. The decay was exponential with time constant tauH. The decay of Ca current was not an artifact porduced by an outward current. The amplitude of calcium tail currents, produced by voltage steps back to epsilonK at different times during the decay of ICa, decayed also with a time constant close to tauH. 6. Ca conductance (after correction for inactivation) could be described approximately by the expression G'Ca(t) = G'Ca(infinity) (1 - exp [-t/tauM])p but it was necessary to vary p from 1 to 2 at different potentials. No value of p gave as good a fit to this model as that obtained for Na currents. 7. taum and tauH were voltage dependent. In the range of potentials from 0 to +60 mV, tauM varied from 9 to 5 msec and tauH from 300 to 50 msec (13.5 degrees C). Steady-state Ca conductance (corrected for inactivation) was voltage dependent also, increasing sigmoidally with depolarization to a maximum of 10--15 muS at +30 to +40 mV. Half-maximal Ca conductance occurred at a membrane potential of +12 mV, and from +10 to +20 mV a 6 mV change in membrane potential produced an e-fold change in Ca conductance. 8. Steady-state inactivation of Ca conductance (hCa(infinity)) varied with holding potential (VH). Half-inactivation occurred with depolarization to -20 mV. At potentials more negative than -40 mV, hCa(infinity) was less than at -40 mV, i.e. hyperpolarization produced Ca 'inactivation'. 9...

Potassium effect on the electrophysiological properties of the giant nerve cell bodies of Arion subfuscus (gastropoda:pulmonata)

1. 1. The effect of the extracellular potassium concentration [K 0 +] on the membrane and action potentials of Arion subfuscus subesophageal ganglion has been investigated. 2. 2. An increase in [K 0 +] depolarizes cellular membrane as in many other preparations. 3. 3. A ten-fold change in [K 0 +] results in a 29.4 mV change in the resting potential. 4. 4. A modified constant field equation account for the behaviour of the resting potentials that differ considerably from the Nernst equation. 5. 5. Mean values of internal potassium concentration [K i +] and P Na/P K determined. 6. 6. Changes in frequency and shape of action potentials of spontaneous discharging neurons were observed when exposed to a high [K 0 +].

A potential‐ and time‐dependent blockade of inward rectification in frog skeletal muscle fibres by barium and strontium ions.

1. A three-electrode voltage clamp method was used to investigate the effects of Ba and Sr ions on the inwardly rectifying K conductance of resting frog sartorius muscle fibres. 2. When Ba2+ (0.01-5 mM) was added to the control (115 mM-K+) solution the inward currents recorded during hyperpolarizing voltage steps turned off exponentially with time as the blockade by Ba2+ developed. Outward currents showed no time-dependence. 3. Ba2+ ions reduced both the instantaneous and the steady-state values of currents recorded on hyperpolarization. The blockade was potential-dependent, steady-state currents being increasingly reduced with increasing hyperpolarization. 4. The concentration-effect relation for the blockade of instantaneous currents by Ba2+ could be fitted assuming 1:1 binding of Ba2+ to a receptor, with the block being proportional to the number of Ba2+-filled receptors. The apparent dissociation constant at the holding potential (-5 mV) was 0.65 mM. Concentration-effect relations were shifted along the concentration axis to lower concentrations by hyperpolarization. The apparent dissociation constant was reduced e-fold for a 16.8 mV change in potential. 5. Increasing the [Ba]o increased the rate of onset of the blockade at a given potential. 6. The rate of onset of the blockade had a high temperature dependence (Q10 = 3.15 +/- 0.08). 7. When [K]o was doubled to 230 mM, under conditions where [K]i was also doubled, [Ba]o had to be raised approximately fourfold to produce the same degree and rate of onset of blockade. Similarly, when [K]o was decreased, the degree and rate of onset of blockade were increased for a given [Ba]o. 8. The blockade could be readily removed by removal of Ba2+ from the bathing solution. In addition the blockade which develops on hyperpolarization is removed exponentially on return to the holding potential. 9. The blockade which exists at the holding potential may be removed by a depolarizing prepulse. 10. Sr causes a similar potential-dependent blockade to that by Ba2+, but is around 400 times less effective. 11. The results have been fitted with a model assuming that the permeability mechanism is an aqueous pore with a site which binds one Ba2+ ion or two K+ ions. The site must have affinity for Ba2+ and a low affinity for K+.