Conditioning prepulses and kinetics of potassium conductance in the frog node

Voltage-dependent action of valproate on potassium channels in frog node of Ranvier

The influence of the anti-epileptic drug, valproate, on K conductance (gK) was investigated in voltage-clamped Ranvier nodes of Xenopus laevis. A double pulse method was used in order to eliminate the effect of accumulation of potassium ions in the perinodal space, thus enabling the determination of the 'true' magnitude of gK. Valproate (2.4 mM) had a voltage-dependent action on the magnitude of gK. With small step depolarizations more negative than about -50 mV, valproate increased gK (20 ms after the step) to approximately 12% of the maximal gK, an increase which disappeared due to a relatively rapid (less than 200 ms) inactivation process. However, with step depolarizations more positive than about -50 mV, valproate markedly reduced gK (20 ms after the step) at greater depolarizations, with a maximum of about 40% of the maximal gK. Moreover, at these voltages gK was inactivated completely (less than or equal to 10 s), whereas under control conditions the inactivation was only partial. Both the temporary increase and the steady state decrease of gK could contribute to an anti-epileptic effect by increasing the action potential threshold and by preventing excessive depolarizations of the nerve during epileptic seizures, respectively.

The action of pyrethroids on sodium channels in myelinated nerve fibres and spinal ganglion cells of the frog

The interaction of pyrethroids with the voltage-dependent sodium channel was studied in voltage-clamped nodes of Ranvier and isolated spinal ganglion neurons of the clawed frog, Xenopus laevis. In the node, pyrethroids prolonged the sodium tail current associated with a step repolarization of the membrane. It was found that the amplitude of the slow, pyrethroid-induced, sodium tail current (PIT) first increased and then decreased as a function of the duration of membrane depolarization (to -5 mV). This decrease of the PIT amplitude was absent when depolarizations to the sodium equilibrium potential (+40 mV) were used. Measurements of changes in sodium reversal potential indicated that sodium ion depletion in the perinodal space is largely responsible for the inactivation of the pyrethroid-modified sodium current. Inactivation is not completely abolished by pyrethroid treatment since the probability of channel opening, measured in membrane patches excised from spinal ganglion cells, decreased slowly during prolonged depolarization. Analysis of unitary currents indicated that both activation and inactivation are retarded by pyrethroids. The arrival of sodium channels in the pyrethroid-modified open state followed a time course that was slower than both activation and inactivation of unmodified sodium channels. Our findings indicate that sodium channels are modified when in the closed resting state and that both opening and closing kinetics are delayed by pyrethroids.

Sodium depletion in the periaxonal space of the squid axon treated with pyrethroids

Pyrethroids are known to increase the steady-state sodium current during a step depolarization and to increase and prolong the tail sodium current associated with a step repolarization of the membrane. The pyrethroid-induced tail sodium current of squid axons developed as a function of the duration of the conditioning depolarizing pulse. However, with further lengthening the conditioning pulse duration, it decreased, further increased, or remained constant depending on the direction of sodium current during the depolarization, irrespective of the membrane potential per se. The depletion or accumulation of sodium in the periaxonal space during a 200-ms conditioning depolarizing pulse in the axon internally treated with pronase, pyrethroids, or both, was demonstrated by measurements of the changes in sodium reversal potential. Thus the observed changes in tail current amplitude as a function of the conditioning pulse duration are explicable in terms of changes in sodium concentration in the periaxonal space without assuming inactivation of the pyrethroid-modified channel.

Potassium ion accumulation slows the closing rate of potassium channels in squid axons

Potassium ion accumulation in the periaxonal space between squid axonal membrane and the Schwann cell surrounding the axon slows the rate of potassium channel closing to a degree that is consistent with the effect on channel closing of an equivalent change in the bulk external potassium concentration. The alteration of channel gating is independent of membrane potential, V, for V less than or equal to -60 mV, which suggests that the effect is mediated at a site on the outer surface of the membrane, rather than a site within the channel.

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)

Current-dependent inactivation induced by sodium depletion in normal and batrachotoxin-treated frog node of Ranvier

In batrachotoxin (BTX)-treated frog node of Ranvier, in spite of a marked reduction in Na inactivation, the Na current still presents a time- and voltage-dependent inactivation that could induce a 50-60% decrease in the current. The inactivation was found to be modified by changing the amplitude of a conditioning pulse, adding tetrodotoxin in the external solution, or replacing NaCl with KCl in the external solution. Conditioning pulses were able to alter the reversal potential of the BTX-modified Na current (Vrev). Vrev was shifted toward negative values for inward conditioning currents and was shifted toward positive values for outward conditioning currents. The change in Vrev was proportional to the conditioning current amplitude. Large inward currents induced 15-25 mV shifts of Vrev. During a 10-20-ms depolarizing pulse, the inactivation and change in Vrev were proportional to the time integral of the current. For longer depolarizations, Vrev reached a steady state level proportional to the current amplitude. The conductance, as calculated from the current and the actual Vrev, showed an inactivation proportional to exp(Vrev F/RT). These observations suggest that the BTX-modified Na current induces a decrease in local Na concentrations, which results in an alteration of the driving force and the conductance. During a pulse that induced a large inward current, the Na space concentration [( Na]s) changed from 114 to 50-60 mM. In normal fibers, the reversal potential of Na current was also shifted toward negative values by a prepulse that induced a large inward current. The change in Vrev reached 5-15 mV, which corresponded to a decrease in [Na]s of 20-50 mM. This change in Vrev slightly altered the time course of Na current. On the basis of a three-compartment model (axoplasm-perinodal space-bulk solution), a Na permeability of the barrier between the space and the bulk solution (PNa,s) and a mean thickness of the space (theta) were calculated. The mean value of PNa,s was 0.0051 cm X s-1 in both normal and BTX-treated fibers, whereas the value of theta was 0.29 micron in BTX-treated fibers and 0.05 micron in normal fibers. When compared with the values calculated during K accumulation, PNa,s was 10 times smaller than PK,s and theta Na-BTX was equal to theta K.(ABSTRACT TRUNCATED AT 400 WORDS)

Single potassium channel conductance in the frog node of Ranvier

The single K+-channel conductance was calculated from the variance of the spontaneous potassium noise currents in voltage clamped frog node. Essential for this calculation is the mean potassium conductance during the noise measurement. So far this quantity has been underestimated, apparently due to K+-ion accumulation. With the proper values, the single K+-channel conductance is an increasing function of membrane voltage.