Existence of two fast inactivation states in cardiac Na channels confirmed by two-stage action of proteolytic enzymes

Effect of phenytoin on sodium conductances in rat hippocampal CA1 pyramidal neurons

The antiepileptic drug phenytoin (PHT) is thought to reduce the excitability of neural tissue by stabilizing sodium channels (Na_V) in inactivated states. It has been suggested the fast-inactivated state (I_F) is the main target, although slow inactivation (I_S) has also been implicated. Other studies on local anesthetics with similar effects on sodium channels have implicated the Na_V voltage sensor interactions. In this study, we reexamined the effect of PHT in both equilibrium and dynamic transitions between fast and slower forms of inactivation in rat hippocampal CA1 pyramidal neurons. The effects of PHT were observed on fast and slow inactivation processes, as well as on another identified "intermediate" inactivation process. The effect of enzymatic removal of I_F was also studied, as well as effects on the residual persistent sodium current (I_NaP). A computational model based on a gating charge interaction was derived that reproduced a range of PHT effects on Na_V equilibrium and state transitions. No effect of PHT on I_F was observed; rather, PHT appeared to facilitate the occupancy of other closed states, either through enhancement of slow inactivation or through formation of analogous drug-bound states. The overall significance of these observations is that our data are inconsistent with the commonly held view that the archetypal Na_V channel inhibitor PHT stabilizes fast inactivation states, and we demonstrate that conventional slow activation "I_S" and the more recently identified intermediate-duration inactivation process "I_I" are the primary functional targets of PHT. In addition, we show that the traditional explanatory frameworks based on the "modulated receptor hypothesis" can be substituted by simple, physiologically plausible interactions with voltage sensors. Additionally, I_NaP was not preferentially inhibited compared with peak I_Na at short latencies (50 ms) by PHT.

Properties of an intermediate-duration inactivation process of the voltage-gated sodium conductance in rat hippocampal CA1 neurons

Rapid transmembrane flow of sodium ions produces the depolarizing phase of action potentials (APs) in most excitable tissue through voltage-gated sodium channels (NaV). Macroscopic currents display rapid activation followed by fast inactivation (IF) within milliseconds. Slow inactivation (IS) has been subsequently observed in several preparations including neuronal tissues. IS serves important physiological functions, but the kinetic properties are incompletely characterized, especially the operative timescales. Here we present evidence for an “intermediate inactivation” (II) process in rat hippocampal CA1 neurons with time constants of the order of 100 ms. The half-inactivation potentials ( V0.5) of steady-state inactivation curves were hyperpolarized by increasing conditioning pulse duration from 50 to 500 ms and could be described by a sum of Boltzmann relations. II state transitions were observed after opening as well as subthreshold potentials. Entry into II after opening was relatively insensitive to membrane potential, and recovery of II became more rapid at hyperpolarized potentials. Removal of fast inactivation with cytoplasmic papaine revealed time constants of INa decay corresponding to II and IS with long depolarizations. Dynamic clamp revealed attenuation of trains of APs over the 102-ms timescale, suggesting a functional role of II in repetitive firing accommodation. These experimental findings could be reproduced with a five-state Markov model. It is likely that II affects important aspects of hippocampal neuron response and may provide a drug target for sodium channel modulation.

Modulation of human Nav1.7 channel gating by synthetic α-scorpion toxin OD1 and its analogs

Nine different voltage-gated sodium channel isoforms are responsible for inducing and propagating action potentials in the mammalian nervous system. The Nav1.7 channel isoform plays an important role in conducting nociceptive signals. Specific mutations of this isoform may impair gating behavior of the channel resulting in several pain syndromes. In addition to channel mutations, similar or opposite changes in gating may be produced by spider and scorpion toxins binding to different parts of the voltage-gated sodium channel. In the present study, we analyzed the effects of the α-scorpion toxin OD1 and 2 synthetic toxin analogs on the gating properties of the Nav1.7 sodium channel. All toxins potently inhibited channel inactivation, however, both toxin analogs showed substantially increased potency by more than one order of magnitude when compared with that of wild-type OD1. The decay phase of the whole-cell Na(+) current was substantially slower in the presence of toxins than in their absence. Single-channel recordings in the presence of the toxins revealed that Na(+) current inactivation slowed due to prolonged flickering of the channel between open and closed states. Our findings support the voltage-sensor trapping model of α-scorpion toxin action, in which the toxin prevents a conformational change in the domain IV voltage sensor that normally leads to fast channel inactivation.

Slow sodium channel inactivation and use-dependent block modulated by the same domain IV S6 residue

Voltage- and/or conformation-dependent association and dissociation of local anesthetic-class drugs from a putative receptor site in domain IV S6 of the sodium channel and slow conformation transitions of the drug-associated channel have been proposed as mechanisms of use- and frequency-dependent reduction in sodium current. To distinguish these possibilities, we have explored the reactivity to covalent modification by thiols and block of the mutations F1760C and F1760A at the putative receptor site of the cardiac sodium channel expressed as stable cell lines in HEK-293 cells. Both mutations decreased steady-state fast inactivation, shifting V1/2h from -86 +/- 1.3 mV (WT) to -72.3 +/- 1.4 mV (F1760C) and -67.7 +/- 1 mV (F1760A). In the absence of drug, the F1760C mutant channel displayed use-dependent current reduction during pulse-train stimulation, and faster onset of slow inactivation. This mutant also retained some sensitivity to lidocaine. In contrast, the F1760A mutant showed no use-dependent current reduction or sensitivity to lidocaine. The covalent-modifying agent MTS-ET enhanced use-dependent current reduction of the F1760C mutant channel only. The use-dependent reduction in current of the covalently modified channel completely recovered with rest. Lidocaine produced no additional block during exposure to MTS-ET-treated cells (MTS-ET 43 +/- 2.7%: MTS-ET lidocaine 47 +/- 4.5%), implying interaction at a common binding site. The data suggest that use-dependent binding at the F1760 site results in enhanced slow inactivation rather than alteration of drug association and dissociation from that site and may be a general mechanism of action of sodium-channel blocking agents.

Inactivation of single cardiac Na+ channels in three different gating modes

In small cell-attached patches containing one and only one Na+ channel, inactivation was studied in three different gating modes, namely, the fast-inactivating F mode and the more slowly inactivating S mode and P mode with similar inactivation kinetics. In each of these modes, ensemble-averaged currents could be fitted with a Hodgkin-Huxley-type model with a single exponential for inactivation (tauh). tauh declined from 1.0 ms at -60 mV to 0.1 ms at 0 mV in the F mode, from 4.6 ms at -40 mV to 1.1 ms at 0 mV in the S mode, and from 4.5 ms at -40 mV to 0.8 ms at +20 mV in the P mode, respectively. The probability of non-empty traces (net), the mean number of openings per non-empty trace (op/tr), and the mean open probability per trace (popen) were evaluated at 4-ms test pulses. net inclined from 30% at -60 mV to 63% at 0 mV in the F mode, from 4% at -90 mV to 90% at 0 mV in the S mode, and from 2% at -60 mV to 79% at +20 mV in the P mode. op/tr declined from 1.4 at -60 mV to 1.1 at 0 mV in the F mode, from 4.0 at -60 mV to 1.2 at 0 mV in the S mode, and from 2.9 at -40 mV to 1.6 at +20 mV in the P mode. popen was bell-shaped with a maximum of 5% at -30 mV in the F mode, 48% at -50 mV in the S mode, and 16% at 0 mV in the P mode. It is concluded that 1) a switch between F and S modes may reflect a functional change of inactivation, 2) a switch between S and P modes may reflect a functional change of activation, 3) tauh is mainly determined by the latency until the first channel opening in the F mode and by the number of reopenings in the S and P modes, 4) at least in the S and P modes, inactivation is independent of pore opening, and 5) in the S mode, mainly open channels inactivate, and in the P mode, mainly closed channels inactivate.

A direct effect of forskolin on sodium channel bursting

A long-lasting component of current through voltage-dependent Na channels is believed to contribute to the plateau phase of the cardiac action potential. Here we report that in cardiac ventricular myocytes forskolin increases the contribution of a very slow component of decay (tau = 36 +/- 16 ms, n = 13) in ensemble currents in response to step depolarizations to 0 mV. Long-lasting bursts of openings (mean duration of 27 +/- 14 ms, n = 10) accounted for this behavior. The slow time constant of decay was not altered by forskolin (5-50 microM). Rather, an increase in the probability of bursting behavior produced a forskolin concentration-dependent increase in the amplitude of this very slow component. This action of forskolin was not the result of stimulation of adenylyl cyclase because it was not affected when cAMP-dependent phosphorylation was inhibited by the protein kinase inhibitor H-89, and it could not be mimicked by addition of isoproterenol, membrane-permeant cAMP [8-(4-chlorophenylthio)-cAMP], or the phosphatase inhibitor okadaic acid. In addition, bursting was not augmented by guanosine 5'-O-(3-thiotriphosphate) (GTP [gamma S]) either applied to the bath or directly to the intracellular face of the channel in inside-out macropatches. Furthermore, 1,9-dideoxy-forskolin, which does not stimulate adenylyl cyclase and 6-(3-dimethylaminopropionyl)-forskolin, a hydrophilic derivative of forskolin, also augmented late channel activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Gating of cardiac Na+ channels in excised membrane patches after modification by alpha-chymotrypsin

Single cardiac Na+ channels were investigated after intracellular proteolysis to remove the fast inactivation process in an attempt to elucidate the mechanisms of channel gating and the role of slow inactivation. Na+ channels were studied in inside-out patches excised from guinea-pig ventricular myocytes both before and after very brief exposure (2-4 min) to the endopeptidase, alpha-chymotrypsin. Enzyme exposure times were chosen to maximize removal of fast inactivation and to minimize potential nonspecific damage to the channel. After proteolysis, the single channel current-voltage relationship was approximately linear with a slope conductance of 18 +/- 2.5 pS. Na+ channel reversal potentials measured before and after proteolysis by alpha-chymotrypsin were not changed. The unitary current amplitude was not altered after channel modification suggesting little or no effect on channel conductance. Channel open times were increased after removal of fast inactivation and were voltage-dependent, ranging between 0.7 (-70 mV) and 3.2 (-10 mV) ms. Open times increased with membrane potential reaching a maximum at -10 mV; at more positive membrane potentials, open times decreased again. Fast inactivation appeared to be completely removed by alpha-chymotrypsin and slow inactivation became more apparent suggesting that fast and slow inactivation normally compete, and that fast inactivation dominates in unmodified channels. This finding is not consistent with a slow inactivated state that can only be entered through the fast inactivated state, since removal of fast inactivation does not eliminate slow inactivation. The data indicate that cardiac Na+ channels can enter the slow inactivated state by a pathway that bypasses the fast inactivated state and that the likelihood of entering the slow inactivated state increases after removal of fast inactivation.

Chemically modified cardiac Na+ channels and their sensitivity to antiarrhythmics: is there a hidden drug receptor?

Elementary Na+ currents were recorded at 19 degrees C in inside-out patches from cultured neonatal rat cardiocytes. In analyzing the sensitivity of chemically modified Na+ channels to several class 1 antiarrhythmic drugs, the hypothesis was tested that removal of Na+ inactivation may be accompanied by a distinct responsiveness to these drugs, open channel blockade. Iodate-modified and trypsin-modified cardiac Na+ channels are noninactivating but strikingly differ from each other by their open state kinetics, a O1-O2 reaction (tau open(1) 1.4 +/- 0.3 msec; tau open(2) 5.4 +/- 1.1 msec; at -40 mV) in the former and a single open state (tau open 3.0 +/- 0.5 msec; at -40 mV) in the latter. Lidocaine (150 mumol/liter) like propafenone (10 mumol/liter), diprafenone (10 mumol/liter) and quinidine (20 mumol/liter) in cytoplasmic concentrations effective to depress NPo significantly can interact with both types of noninactivating Na+ channels to reduce the dwell time in the conducting configuration. Iodate-modified Na+ channels became drug sensitive during the O2 state. At -40 mV, for example, lidocaine reduced tau open(2) to 62 +/- 5% of the control without detectable changes in tau open(1). No evidence could be obtained that these inhibitory molecules would flicker-block the open Na+ pore. Drug-induced shortening of the open state, thus, is indicative for a distinct mode of drug action, namely interference with the gating process. Lidocaine proved less effective to reduce tau open(2) when compared with the action of diprafenone. Both drugs apparently interacted with individual association rate constants, a(lidocaine) was 0.64 x 10(6) mol-1 sec-1 and a(diprafenone) 13.6 x 10(6) mol-1 sec-1. Trypsin-modified Na+ channels also appear capable of discriminating among these antiarrhythmics, the ratio a(diprafenone)/a(lidocaine) even exceeded the value in iodate-modified Na+ channels. Obviously, this antiarrhythmic drug interaction with chemically modified Na+ channels is receptor mediated: drug occupation of such a hypothetical hidden receptor that is not available in normal Na+ channels may facilitate the exit from the open state.

Responsiveness of cardiac Na+ channels to a site-directed antiserum against the cytosolic linker between domains III and IV and their sensitivity to other modifying agents

Elementary Na+ currents were recorded in inside-out patches from neonatal rat heart cardiocytes to analyze the influence of a site-directed polyclonal anti-serum against the linker region between the domains III and IV (amino acids 1489-1507 of the cardiac Na+ channel protein) on Na+ channel gating and to test whether this part of the alpha-subunit may be considered as a target for modifying agents such as the (-)-enantiomer of DPI 201-106. Anti-SLP 1 serum (directed against amino acids 1490-1507) evoked, usually within 10-15 min after cytosolic administration, modified Na+ channel activity. Antiserum-modified Na+ channels retain a single open state but leave, at -60 mV for example, their conducting configuration consistently with an about threefold lower rate than normal Na+ channels. Another outstanding property of noninactivating Na+ channels, enhanced burst activity, may be quite individually pronounced, a surprising result which is difficult to interpret in terms of structure-function relations. Removal of inactivation led to an increase of reconstructed peak INa (indicating a rise in NPo) and changed INa decay to obey second-order kinetics, i.e., open probability declined slowly but progressively during membrane depolarization. The underlying deactivation process is voltage dependent and responds to a positive voltage shift with a deceleration but may operate even at the same membrane potential with different rates. Iodate-modified Na+ channels exhibit very similar properties including a conserved conductance. They are likewise controlled by an efficient, voltage-dependent deactivation process.(ABSTRACT TRUNCATED AT 250 WORDS)