Behavior of delayed current under voltage clamp in the supramedullary neurons of puffer

Structural correlates of selectivity and inactivation in potassium channels

Potassium channels are involved in a tremendously diverse range of physiological applications requiring distinctly different functional properties. Not surprisingly, the amino acid sequences for these proteins are diverse as well, except for the region that has been ordained the "selectivity filter". The goal of this review is to examine our current understanding of the role of the selectivity filter and regions adjacent to it in specifying selectivity as well as its role in gating/inactivation and possible mechanisms by which these processes are coupled. Our working hypothesis is that an amino acid network behind the filter modulates selectivity in channels with the same signature sequence while at the same time affecting channel inactivation properties. This article is part of a Special Issue entitled: Membrane protein structure and function.

State-dependent inactivation of K+ currents in rat type II alveolar epithelial cells

Inactivation of K+ channels responsible for delayed rectification in rat type II alveolar epithelial cells was studied in Ringer, 160 mM K-Ringer, and 20 mM Ca-Ringer. Inactivation is slower and less complete when the extracellular K+ concentration is increased from 4.5 to 160 mM. Inactivation is faster and more complete when the extracellular Ca2+ concentration is increased from 2 to 20 mM. Several observations suggest that inactivation is state-dependent. In each of these solutions depolarization to potentials near threshold results in slow and partial inactivation, whereas depolarization to potentials at which the K+ conductance, gK, is fully activated results in maximal inactivation, suggesting that open channels inactivate more readily than closed channels. The time constant of current inactivation during depolarizing pulses is clearly voltage-dependent only at potentials where activation is incomplete, a result consistent with coupling of inactivation to activation. Additional evidence for state-dependent inactivation includes cumulative inactivation and nonmonotonic from inactivation. A model like that proposed by C.M. Armstrong (1969. J. Gen. Physiol. 54: 553-575) for K+ channel block by internal quaternary ammonium ions accounts for most of these properties. The fundamental assumptions are: (a) inactivation is strictly coupled to activation (channels must open before inactivating, and recovery from inactivation requires passage through the open state); (b) the rate of inactivation is voltage-independent. Experimental data support this coupled model over models in which inactivation of closed channels is more rapid than that of open channels (e.g., Aldrich, R.W. 1981. Biophys. J. 36:519-532). No inactivation results from repeated depolarizing pulses that are too brief to open K+ channels. Inactivation is proportional to the total time that channels are open during both a depolarizing pulse and the tail current upon repolarization; repolarizing to more negative potentials at which the tail current decays faster results in less inactivation. Implications of the coupled model are discussed, as well as additional states needed to explain some details of inactivation kinetics.

Potassium currents in dissociated cells of the rat pineal gland

The properties of K currents of pineal cells were studied using the whole-cell variant of the patch-clamp technique. The total K current could be separated in two distinct components: a fast, transient current (It) and a slow current (Is). The activation threshold of It was at -35 to -30 mV. On depolarization to +50 mV it reaches a peak in 2-3 ms and inactivates almost completely in 50 ms. Half steady state inactivation occurs at -45 mV. Inactivation of It is voltage-dependent and is well fitted by single exponentials with time constants between 17.2 ms at +50 mV and 27.2 ms at -10 mV. Inactivation is removed with time and the recovery period shortened by membrane hyperpolarization. The slow K current has a threshold at -20 to -15 mV. It reaches a maximum in about 30-40 ms and inactivates slightly, to about 80% of the peak value at the end of pulses lasting 200 ms. With 80 mM external K, tail currents recorded after short (1-2 ms) depolarizations were about 2.5 times faster than the tails recorded at the end of 50 ms pulses. The fast tails were removed by depolarizing prepulses but the slow tails remained unaltered. Thus, the fast and slow tails are probably a reflection of the closing of the transient and slow K channels. The transient K current of pineal cells has general characteristics similar to transient currents recorded in non-secretory cells, but also has particular kinetic properties.

Slow inactivation and reactivation of the K+ channel in squid axons. A tail current analysis

Potassium current inactivation and reactivation in squid axons were measured from tail current amplitudes after voltage clamp prepulses to the potassium equilibrium potential, EK, in seawater containing elevated levels of potassium ion concentration, Ko. Little or no inactivation resulted with prepulses lasting less than 100 ms. Longer pulses caused the current to inactivate in two phases, one between 0.1 and 1 s, and a second phase between 5 and 100 s. Inactivation was incomplete. The time constant of the tail current after a prepulse to EK was independent of pulse duration (0.1-120 s). Inactivation was independent of Ko (10 less than or equal to Ko less than or equal to 300 mM), and it was independent of membrane potential, V, for -40 less than or equal to V less than or equal to 0 mV. Reactivation was measured with a three-pulse protocol. The reactivation time course was sigmoidal with a delay of approximately 100 ms before significant reactivation occurred. These results were described by a model consisting of three inactivated states arranged in a linear sequence. The rate constants of the model are of the form (A + B exp (CV), or 1/(A + B exp (CV], which are required to describe the non-inactivating conductance component.

Ionic conductances of squid giant fiber lobe neurons

The cell bodies of the neurons in the giant fiber lobe (GFL) of the squid stellate ganglion give rise to axons that fuse and thereby form the third-order giant axon, whose initial portion functions as the postsynaptic element of the squid giant synapse. We have developed a preparation of dissociated, cultured cells from this lobe and have studied the voltage-dependent conductances using patch-clamp techniques. This system offers a unique opportunity for comparing the properties and regional differentiation of ionic channels in somatic and axonal membranes within the same cell. Some of these cells contain a small inward Na current which resembles that found in axon with respect to tetrodotoxin sensitivity, voltage dependence, and inactivation. More prominent is a macroscopic inward current, carried by Ca2+, which is likely to be the result of at least two kinetically distinct types of channels. These Ca channels differ in their closing kinetics, voltage range and time course of activation, and the extent to which their conductance inactivates. The dominant current in these GFL neurons is outward and is carried by K+. It can be accounted for by a single type of voltage-dependent channel. This conductance resembles the K conductance of the axon, except that it partially inactivates during relatively short depolarizations. Ensemble fluctuation analysis of K currents obtained from excised outside-out patches is consistent with a single type of K channel and yields estimates for the single channel conductance of approximately 13 pS, independently of membrane potential. A preliminary analysis of single channel data supports the conclusion that there is a single type of voltage-dependent, inactivating K channel in the GFL neurons.

Acetylcholine raises excitability by inhibiting the fast transient potassium current in cultured hippocampal neurons

The effects of acetylcholine on cultured hippocampal neurons were investigated by using the whole-cell version of the patch-clamp technique. The CA1 region of the hippocampus was excised from brain slices of young rats (12-19 day old), incubated in a papain solution, and dissociated. Neurons were plated on a glial feeder layer. The experiments were conducted mostly on neurons cultured for 2-6 days. Upon depolarization under voltage clamp, these cells exhibited a fast transient outward current (A-current), which was inhibited by 4-aminopyridine (2.5 mM). Acetylcholine (0.1 microM) also inhibited this A-current, as did the muscarinic agonists bethanechol and muscarine. As expected from their inhibition of the A-current, acetylcholine and 4-aminopyridine both increased the amplitude of the action potential and prolonged its duration. We conclude that the inhibition of the A-current constitutes a mechanism by which acetylcholine exerts its excitatory influence on hippocampal neurons.

The kinetics of recovery and development of potassium channel inactivation in perfused squid (Loligo pealei) giant axons

K+ currents were studied at a normal (-69 mV) and at a depolarized (-49 mV) membrane potential in voltage-clamped squid giant axons perfused with 350 mM-K+ and bathed in K+-free artificial sea water containing tetrodotoxin to block the Na+ channels. Steady-state and instantaneous K+ currents were reduced by over 50% at corresponding voltages at the depolarized membrane potential. Instantaneous chord conductance-voltage curves showed that the depolarized membrane potential caused a uniform reduction of K+ conductance across the voltage range under study. The driving force for K+ ions was comparable at both membrane potentials when a short (2 ms) pre-pulse was used to open the K+ channels. When a longer (7.5 ms) pre-pulse was used, the driving force was actually larger at the depolarized membrane potential. The depolarized membrane potential did drive some K+ ions into the periaxonal space. The amount of K+ ions driven into the periaxonal space was estimated by two independent methods, with similar results. The resulting increase of K+ ions in the periaxonal space (10 mM) was about 40 times too small to account for the large reduction in currents in terms of a reduced driving force for K+ ions. The kinetics of recovery and development of inactivation were monitored by repeatedly applying a 7.5 ms test pulse followed by a long conditioning potential. Both recovery and development of inactivation, from the depolarized membrane potential, were described by the sum of two exponential terms plus a constant. The time constant-voltage curves for both phases of inactivation peaked at about -54 mV at 10 degrees C. The time constant of the slow phase of inactivation at -54 mV was about 12.4 s, while the corresponding time constant for the fast phase was about 2.3 s. The slow relaxation had an apparent plateau of about 11 s at more depolarized membrane potentials. Recovery from inactivation was rapid at hyperpolarized membrane potentials. The steady-state inactivation curve of the K+ channel was incomplete in the depolarizing region; and apparent plateau was reached with about 75% of the K+ current inactivated. The temperature sensitivity of both phases of inactivation corresponded to a Q10 of about 3. Elevated external concentrations of K+ ions did not block either phase of the inactivation process, although the kinetics of recovery from inactivation were slightly faster under these conditions.(ABSTRACT TRUNCATED AT 400 WORDS)

Periodic responses in squid axon membrane exposed intracellularly and extracellularly to solutions containing a single species of salt

A periodic membrane potential change was found to occur in squid giant axons which were internally and externally perfused with solutions of an identical composition and were hyperpolarized by passing a sustained inward current. The solution contained Co2+ or Mn2+ as the sole cation species at a concentration of 1-10 mM. The amplitude of the response was roughly 100 mV. The current intensity and the ion concentration had large effects on the response. The voltage-clamp technique revealed an N-shaped I-V characteristic of the membrane system. The membrane emf of the resting and excited states was almost the same but the membrane conductance was increased in the excited states. The response was suppressed with 4-aminopyridine reversibly but unchanged with tetrodotoxin or D-600. Those unusual ionic conditions did not deprive axons of their ability to produce ordinary action potentials in physiological solutions. The experimental conditions employed and the results obtained were very close to those for some of the artificial membrane models. Applicability of the physico-chemical theories developed for these models is discussed.

Fast inward and outward current channels in a non-spiking neurone

Although the crustacean coxal receptors and no-spiking, indirect pharmacological and electrophysiological evidence suggests that fast sodium channels may be present in their membrane. The properties of these channels are not known, but it has been suggested that they might be "incompletely differentiated", perhaps lacking "appropriate gating mechanisms", and/or "too sparsely distributed". The former hypothesis is not supported by the results of voltage-clamping experiments done on dendritic segments isolated from these mechanoreceptors. Instead, the results reported here provide direct evidence for a voltage-dependent fast inward current, sensitive to tetrodotoxin (TTX) and requiring external sodium (but not calcium). This current is shunted by a transient fast outward current, also voltage dependent, and it is suggested that this shunting may account, at least in part, for the non-spiking behaviour of the coxal receptors.

Facilitation of membrane electrical excitability in Drosophila

Prior electrical activity in the indirect flight muscles of Drosophila facilitates membrane excitability. The mechanism of facilitation involves the inactivation of an early, fast, transient outward current by prior membrane depolarizaton. In the facilitated state the calcium-dependent spike-like response has a decreased current and voltage threshold. The facilitated state persists for 1.5 sec after a membrane active response. A single nerve-driven spike is sufficient to facilitate membrane excitability.

Inactivation of the slow potassium outward current in crab muscle fibre

The slow outward current (IK2) recorded in crab muscle fibre using the double sucrose gap method decreases when high and maintained depolarizations are applied. This decrease corresponds to a true inactivation of the potassium conductance rather than to a shift in the reversal potential of the charge carrying ion following local accumulation.

Inactivation of delayed outward current in molluscan neurone somata

1. Inactivation of delayed outward current was studied by voltage clamp of isolated neurone somata of the molluscs Archidoris and Anisodoris. During prolonged voltage clamp steps in normal artificial sea water delayed outward current rises to a peak and then declines to a non-zero steady-state. During repetitive clamp pulses at repetition rates slower than 2/sec, the amplitude of peak outward current in the second pulse is commonly less than the amplitude at the end of the preceding pulse, giving the impression of continued inactivation during the repolarized interval. We have termed this property cumulative inactivation. 2. Two components of delayed outward current were separated using tetraethyl ammonium ions (TEA) and cobalt ions (Co). External TEA blocks 90% of a voltage and time dependent outward current termed K current (IK). External Co blocks 85% of a Ca activated delayed outward current termed Ca current (ICa does not inactivate during prolonged or repetitive voltage clamp pulses. IK, however, inactivates during prolonged voltage clamp steps and shows cumulative inactivation during repetitive voltage clamp pulses. 3. Inactivation of IK is voltage and time dependent and does not require influx of Ca ions. 4. As measured by a prepulse method, the onset of inactivation is characterized by a two time constant process. Fast inactivation occurs with a time course comparable to the rate of rise of outward current and can account for 90% of total inactivation. 5. Recovery from inactivation is slow with a time constant approximately an order of magnitude slower than the onset of inactivation. 6. The current-voltage (I-V) curve for peak IK can be N-shaped, with a region of negative slope resistance in the range of +30 to +80 mV. The I-V curve for steady-state IK, however, shows little or no tendency to form a local maximum. 7. The pattern of delayed outward current varies considerably between cells. A major contributing factor to this variability appears to be the relative contributions of ICa and IK to delayed outward current.

A mechanism for minimizing temperature effects on repetitive firing frequency

The magnitudes and time courses of conductance changes in molluscan neurons show marked temperature dependence. Interestingly, though, the relationship between repetitive firing frequency and stimulus current is not greatly affected by moderated temperature changes (6-8 degrees C). The transient potassium current, IA, is largely reponsible for the interspike voltage trajectory, hence, repetitive interval, in molluscan neurons. It is shown in this study that temperature dependencies of the rate constants and magnitude of the IA system are balanced in such a way that the observed temperature insensitivity of the repetitive response is predicted.

Calcium-activated conductance in skate electroreceptors: voltage clamp experiments

Voltage clamp experiments allow further characterization of the calcium-dependent repolarizing process in skate electroreceptor epithelium. Four current components are described: a prolonged capacity current, a leakage current, an early active current which flows inward across the lumenal membranes of the receptor cells, and a late current which flows outward. The leakage and capacity currents are linear and may be substracted from the total current, giving net active currents. The early active current is carried by calcium and does not undergo inactivation for at least several seconds. When large stimuli exceed the reversal potential for the early calcium current, the late current is suppressed. Reduction of the ionized calcium concentration in the lumen lowers the reversal potential for the early current and the suppression potential for the late current by the same amount. We conclude that the late current is initiated by a calcium influx into the cytoplasm. During pulses of moderate duration, activation of the late current does not begin until a fixed amount of calcium has entered the receptor cells. The required amount of calcium is reduced if a recent calcium influx has occurred. We suggest that the calcium-activated outward current is mediated by a distinct macromolecule that is insensitive to voltage. Such macromolecules are likely to have an important role in the regulation of electrical activity in excitable cells.

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.

Potassium activation in Helix aspersa neurones under voltage clamp: a component mediated by calcium influx

1. Helix aspersa neurones under voltage clamp generate prolonged outward currents (potassium currents) in response to depolarizing command pulses. 2. The potassium currents recorded from cell A were reversibly reduced 25-50% by 10 mM cobalt ions in the bathing medium; 1 mM lanthanum, 10(-6) g/ml. D-600 and 10(-6) g/ml. iproveratril had similar effects but were only partially reversible. 3. The relationship between the potassium currents and the membrane potential had an "n" shape in normal saline. In calcium-free saline (containing 25 mM magnesium) the potassium currents were reduced and the "n" shape was abolished. The effect of calcium-free saline was readily reversible. 4. The voltage-dependence of the calcium-sensitive potassium currents was similar to that of the "late" calcium channel in squid axons (Baker, Hodgkin & Ridgway, 1971). 5. When cell A was depolarents were made up of two exponentially declining components. The slower of the two components was reduced in calcium-free saline. 6. When cell A was depolarized by 150 mV for 10 msec and then repolarized the "tail" currents were made up of a single rapidly declining component. The reversal potential of this component changed by 58 mV for a tenfold change in the external potassium concentration as predicted by the Nernst equation. 7. The reversal potential of "tail" currents having both components was less sensitive to changes in the external potassium concentration. 8. Tetraethylammonium (TEA) ions blocked both calcium dependent and voltage sensitive potassium currents. Each receptor was found to bind a single molecule of TEA. The dissociaton constant was about 10 mM in each case. 9. The intracellular concentration of ionized calcium was estimated from the potential at which there was no apparent calcium influx (the null point). It was between 3 x 10(-8) M and 8 x 10(-8) M with 10(-2) M calcium in the bathing medium. 10. The null point changed 30 mV for a tenfold change in the external calcium concentration as predicted by the Nernst equation. 11. It is concluded that depolarization of Helix neurones activates two typesof potassium channel. One channel is voltage dependent and highly selective for potassium. Activation of the other channel is dependent on the influx (or injection, see Meech, 1972, 1974a) of calcium. This calcium mediated potassium activation system saturates at high external calcium concentrations and is inhibited by external magnesium ions.

Potassium rectifications of the starfish oocyte membrane and their changes during oocyte maturation

1. The current-voltage relations of the oocyte membrane of the starfish, Asterina pectinifera, and their changes during maturation were investigated using current-clamp techniques. 2. The resting potential of the oocyte membrane in sea water was found to be determined by the diffusion potential of K ions. 3. In the absence of Na and Ca inward currents the steady-state current-voltage relation of the oocyte membrane had inward-going K rectification at membrane potentials more negative than -65 mV and outward-going K rectification at potentials more positive than -30 mV, forming a S-shaped I-V curve. 4. A negative resistance region of the steady-state I-V curve was revealed with voltage-clamp technique in the potential range between -65 and -30 mV. 5. Transient K activation occurred when the membrane was brought from a resting potential of about -75 mV to potentials more positive than -20 mV, and this was immediately followed by K inactivation. Accordingly, the steady-state I-V relation showed only slight outward-going rectification. 6. At the beginning of meiosis, which is signalled by break-down of the nucleus, the limiting slope conductance in the inward rectifying region of the I-V curve decreased sevenfold. The cell membrane lost its selective permeability to K ions and was depolarized from -70 to between -20 and OmV in standard artificial sea-water. The depolarized resting potential was partly due to the relative increase in Na permeability. K conductance began to increase again within 30 min after breakdown of the nucleus. The resting potential became gradually larger and eventually attained -70 mV in the mature egg. 7. In the mature egg, K activation upon depolarization was no longer followed by inactivation. Accordingly, the slope conductance in the outward rectifying region of the I-V curve increased. 8. The action potential was augmented at the stage of nuclear breakdown. Thereafter the maximum rate of rise decreased and the duration of the action potential shortened. These changes were caused primarily by changes in K conductance during maturation. 9. Fertilization of the egg during the maturation process did not affect the changes in the I-V relation described above, except for a transient change of the membrane permeability upon fertilization.

Analysis of non-linearity observed in the current-voltage relation of the tunicate embryo

1. In the gastrula of the tunicate (Halocynthia aurantium) the resting potential of the embryonic membrane was about -70 mV in both std ASW and Na-free ASW.2. The potential responses to depolarizing current in the early gastrula embryo showed distinct potential jumps from about -50 to about +40 mV during the application of current and from about 0 to resting level after its cessation thus forming a plateau at about 0 mV. Those jumps were not altered significantly by the removal of both Na and Ca ions from ASW except for the duration of the plateau after the cessation of current.3. Blastomeres in an embryo were tightly coupled with each other electrotonically at the blastula and gastrula stages. The fact that the coupling ratio was nearly 1.0 made it reasonable to treat an embryo as one cell as far as electrical properties are concerned.4. The I-V relation of the embryonic membrane consisted of an inward rectifying region, outward rectifying region and transitional zone between the two, resulting in a S-shaped curve as in the egg cell membrane. But in the gastrula embryo, the transitional zone actually included the discontinuous part of the I-V curve (from about -50 to about +40 mV) due to the potential jump.5. The ionic current (I(i)) during the potential jump in Na-free ASW was estimated from the slope of the potential response based on the assumption of constant capacitance during the membrane potential changes. The calculated I(i)-V curve complemented the interrupted I-V curve smoothly and showed the existence of a negative resistance region from about -50 to about 0 mV in the steady-state I-V relation.6. The steady-state I-V relation was altered significantly by changing the K concentration in ASW, and the existence of the negative resistance can be reasonably explained by the marked inward-going rectification or anomalous rectification of K conductance.7. The replacement of K with Rb in ASW produced the marked suppression of the inward-going rectification of the embryonic membrane.

Aplysia giant cell: soma-axon voltage clamp current differences

Under voltage clamp, local membrane currents have been measured in several regions of the soma. The early inward current appears to pass largely through the membrane of the axon and of the soma near the axon in normal media. After 10(-5) molar tetrodotoxin (TTX) is added to the bathing medium the larger inward current is found in the somatic membrane away from the axon. The late currents are larger at the soma in both normal and TTX media.

Potassium ion accumulation near a pace-making cell of Aplysia

1. A delayed current decrease associated with prolonged depolarization was studied in R(15) (the parabolic burster) of Aplysia by using intracellular recording and voltage clamp techniques.2. For long duration command pulses (3 sec), the outward current shows a delayed decrease. The current goes from a maximum near 100 msec and falls until a steady-state outward current is reached between 1.5 and 2.5 sec after the beginning of the command step. This final steady-state current is usually only about 20-30% of the peak outward current.3. Double step voltage clamps show that this current decrease is associated with a large shift of e.m.f. Measurements of conductance, on the other hand, fail to show any significant difference in conductance associated with peak and steady-state currents.4. From the results of application of high K(+) ringer, the conclusion is reached that this shift in e.m.f. is due to an accumulation of K(+) near the exterior cell membrane. Several other experiments exclude the possibility of either metabolic events or compensating conductance changes producing the phenomenon.5. The location of the accumulation is considered on the basis of anatomical studies. It is concluded that the accumulation takes place in the extensive infoldings found in cells like R(15). An explanation of the difference in delayed current decrease between pace-makers and non-pace-makers is suggested, since the pace-makers apparently have more extensive invaginations than the non-pace-makers. This suggestion is lent support by measurements of capitance and current density.

Post-stimulus hyperpolarization and slow potassium conductance increase in Aplysia giant neurone

1. Intracellular records from Aplysia giant (R2) cell somata showed long lasting 4-10 mV hyperpolarizations after passage of outward current through a second intracellular electrode.2. An increase in membrane slope conductance occurred simultaneously with the post-stimulus hyperpolarization (PSH).3. Both the PSH and conductance-increase varied strongly with stimulus amplitude and duration.4. Both the PSH and the conductance increase occurred in Ca-free medium containing tetrodotoxin, when action-potential production was completely blocked.5. The PSH persisted in the presence of ouabain or DNP, with cooling, with removal of external K(+), and in media where all the Na(+) was replaced with Li(+), suggesting that it was not due to the activity of an electrogenic pump.6. A reversal potential for the PSH was demonstrated by application of maintained inward current following the end of an outward-directed stimulus.7. The PSH reversal potential varied with [K](o), but not with [Cl](o) or [Na](o), suggesting that the PSH was mainly due to an increase in K conductance.8. The PSH and the conductance increase were reduced strongly when all the Na(+) was replaced with Tris, and only slightly when Na(+) was replaced with sucrose.

Inward and delayed outward membrane currents in isolated neural somata under voltage clamp

1. Membrane current-voltage relationships were investigated under voltage clamp conditions in isolated neural somata of marine gastropods.2. Step depolarizations from the resting potential produce an initial inward current followed by a delayed outward current.3. Inward current appears to be carried by both sodium and calcium ions and displays time and voltage dependent properties similar to those of other excitable membranes.4. Activation and deactivation of the delayed outward current follow a more complicated time course than that of a single exponential raised to a power but can be fitted by the product of two exponential functions.5. Delayed outward current inactivation proceeds with a time constant which decreases as membrane voltage is made more positive. The steady-state levels of inactivation as a function of membrane voltage are related by an S-shaped curve similar to that for K inactivation in squid giant axon.

Voltage clamp studies of a transient outward membrane current in gastropod neural somata

1. Outward directed membrane currents have been studied in voltage clamp experiments on isolated neural somata of the marine gastropod Anisodoris.2. Stepping the membrane potential from a hyperpolarized level to a value in the neighbourhood of resting potential (-35 to -50 mV at 5 degrees C) results in an outward current transient, I(A), which is apparently carried by potassium ions.3. The peak amplitude of I(A) is dependent upon both the holding voltage level and the test step voltage while the time courses of development and decay are independent of, or only slightly dependent on, these parameters.4. The developing and decaying phases of I(A) are approximated by exponentials, leading to time constants for development of 10-25 msec and for decay of 220-600 msec over the aggregate of cells studied (data at 5 degrees C). Q(10) for the processes is approximately 3.5. It is concluded that the transport mechanism for I(A) is at least operationally distinct from the mechanism underlying delayed outward current, I(K).

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.

Inactivation of the potassium conductance and related phenomena caused by quaternary ammonium ion injection in squid axons

Several analogues of the tetraethylammonium (TEA(+)) ion were injected into the giant axon of the squid, and the resultant changes in time course and magnitude of the potassium current (I(K)) were studied. For all the analogues used, three of the ethyl side chains of TEA(+) were left unchanged, while the fourth chain was either lengthened or shortened. Increasing the length of this chain increased binding to the blocking site in the channel by a factor of roughly two for each added CH(2) group. The effect on the rate of entry into the blocking site was relatively slight. Thus the concentration for half-suppression of g(K) decreased by about the same factor of two for each added CH(2). All the analogues caused anomalous or ingoing rectification. The longest chain analogue used, pentyltriethylammonium ion, caused rapid inactivation of g(K), and this inactivation had properties quite similar to g(Na) inactivation. The anomalous rectification and the g(K) inactivation caused by these compounds have the same basic mechanism.

Membrane properties of the stretch receptor neurones of crayfish with particular reference to mechanisms of sensory adaptation

1. Membrane properties of neurones of the two morphologically different types of stretch receptor of crayfish, the slowly adapting (RM(1)) and the rapidly adapting (RM(2)) receptors, were investigated with two microelectrodes inserted into the same neurone.2. The action potential was usually larger in the slowly adapting than in the rapidly adapting neurone. But the distributions of the height were not sharply delimited, and there was an overlap from the two groups of neurone.3. There were no marked differences in the current-voltage relationship between the two types.4. Under voltage clamp, depolarizations evoked a large delayed outward current, which slowly diminished during maintained depolarization (K-inactivation). Under a moderate depolarization, development of the K-permeability increase was very slow.5. When stimulated by intracellularly applied constant currents, the slowly adapting neurone always adapted slowly, and gave rise to long-lasting trains of spikes, whereas the rapidly adapting neurone never produced maintained repetitive discharges.6. The same marked differences in the adaptation behaviour of spike discharge between the two types were also observed when the neurones were stimulated by constant currents applied through external electrodes.7. When the stimulating point was shifted along the axon of the slowly adapting neurones, the ability to produce long-lasting repetitive discharges was found to be confined to the axonal region near the soma, where the diameter was very small, and where impulses were first initiated.8. Possible ionic mechanisms of the adaptation of the spike generating membrane were discussed. The importance of slowly occurring changes in the Na- and K-permeabilities and changes in the electromotive force of the membrane due to electrogenic pump was emphasized.

Analysis of spike electrogenesis of Eel electroplaques with phase plane and impedance measurements

Eel electroplaques provide experimental conditions in which registration of phase plane trajectories (dV/dt vs. V) and impedance measurements with an AC Wheatstone bridge, in conjunction with spike electrogenesis describe quantitatively the ionic processes of the electrogenesis. Thus, these data employing as they do measurements of transients, permit an independent test of the validity of the assumptions which underlie the Hodgkin-Huxley equivalent circuit: independent ionic channels with fixed ionic batteries and exhibiting time-variant conductance changes with different kinetics for the different channels. The analysis accords with earlier findings on voltage-clamped electroplaques and this agreement confirms the validity of the equivalent circuit despite the fact that the current-voltage characteristics of the axons and electroplaques differ profoundly. As for squid axons, the equivalent circuit of the electroplaques has four branches: a capacity and three ionic channels. One of the latter is an invariant leak channel (G(L)) of high conductance. A K channel (G(K)) is fully open at rest, but rapidly undergoes inactivation when the cell is depolarized by more than 40 mv. G(L) and G(K) have a common inside negative emf (E(K)). A Na channel (G(Na)) with an inside positive emf (E(Na)) is closed at rest, but opens transiently upon depolarization.

Post-tetanic hyperpolarization and electrogenic Na pump in stretch receptor neurone of crayfish

1. Two types of after-potentials in the stretch receptor neurone of crayfish are described.2. A short-duration after-hyperpolarization associated with a single spike or a few spikes is diminished and reversed on applying hyperpolarizing currents. However, a much longer-lasting post-tetanic hyperpolarization (PTH) is enhanced by conditioning hyperpolarization; thus, no reversal potential can be obtained.3. No changes in membrane conductance occur during PTH.4. Reducing K concentration in the bathing fluid diminishes PTH, while it shifts the reversal potential of the short after-potential toward greater negativity.5. Replacement of Na with Li, or addition of 2,4-dinitrophenol in the bathing fluid suppresses PTH in a reversible manner.6. Electrophoretic injection of Na into the cell induces a long-lasting hyperpolarization.7. No change in K-equilibrium potential, as indicated by the reversal point of the short after-potential, is detected during PTH.8. It is concluded that the short after-potential is caused by a permeability increase for potassium ion, whereas PTH is produced by an electrogenic Na-pump.

Analysis of depolarizing and hyperpolarizing inactivation responses in gymnotid electroplaques

In electroplaques of several gymnotid fishes hyperpolarizing or depolarizing currents can evoke all-or-none responses that are due to increase in membrane resistance as much as 10- to 12-fold. During a response the emf of the membrane shifts little, if at all, when the cell either is at its normal resting potential, or is depolarized by increasing external K, and in the case of depolarizing responses when either Cl or an impermeant anion is present. Thus, the increase in resistance is due mainly, or perhaps entirely, to decrease in K permeability, termed depolarizing or hyperpolarizing K inactivation, respectively. In voltage clamp measurements the current-voltage relation shows a negative resistance region. This characteristic accounts for the all-or-none initiation and termination of the responses demonstrable in current clamp experiments. Depolarizing inactivation is initiated and reversed too rapidly to measure with present techniques in cells in high K. Both time courses are slowed in cells studied in normal Ringer's. Once established, the high resistance state is maintained as long as an outward current is applied. Hyperpolarizing inactivation occurs in normal Ringer's or with moderate excess K. Its onset is more rapid with stronger stimuli. During prolonged currents it is not maintained; i.e., there is a secondary increase in conductance. Hyperpolarizing inactivation responses exhibit a long refractory period, presumably because of persistence of this secondary increase in conductance.

Analysis of K inactivation and TEA action in the supramedullary cells of puffer

Under the voltage clamp condition, the K inactivation was analyzed in cells bathed in the isosmotic KCl Lophius-Ringer solution. After conditioning hyperpolarization, the cells respond to depolarizations with increased K permeability, which in turn is decreased during maintained depolarizations. The steady-state levels of the K inactivation as a function of the membrane potential are related by an S-shaped curve similar to that which describes the steady-state Na inactivation in the squid giant axon. TEA reduced the K conductance by a factor which is independent of the potential, and without a shift of the inactivation curve along the voltage axis. The rapid phase of the K activation is less susceptible to TEA than the slow phase of the K activation. Hyperpolarizing steps remove the K inactivation, the rate of the removal being faster the larger the hyperpolarization from the standard potential of about -60 mv.