Mechanisms of cation permeation in cardiac sodium channel: description by dynamic pore model

Na/K pump inactivation, subsarcolemmal Na measurements, and cytoplasmic ion turnover kinetics contradict restricted Na spaces in murine cardiac myocytes

The Na/K pump exports cytoplasmic Na ions while importing K ions, and its activity is thought to be affected by restricted intracellular Na diffusion in cardiac myocytes. Lu and Hilgemann find instead that the pump can enter an inactivated state and that inactivation can be relieved by cytoplasmic Na.

Decades ago, it was proposed that Na transport in cardiac myocytes is modulated by large changes in cytoplasmic Na concentration within restricted subsarcolemmal spaces. Here, we probe this hypothesis for Na/K pumps by generating constitutive transsarcolemmal Na flux with the Na channel opener veratridine in whole-cell patch-clamp recordings. Using 25 mM Na in the patch pipette, pump currents decay strongly during continuous activation by extracellular K (τ, ∼2 s). In contradiction to depletion hypotheses, the decay becomes stronger when pump currents are decreased by hyperpolarization. Na channel currents are nearly unchanged by pump activity in these conditions, and conversely, continuous Na currents up to 0.5 nA in magnitude have negligible effects on pump currents. These outcomes are even more pronounced using 50 mM Li as a cytoplasmic Na congener. Thus, the Na/K pump current decay reflects mostly an inactivation mechanism that immobilizes Na/K pump charge movements, not cytoplasmic Na depletion. When channel currents are increased beyond 1 nA, models with unrestricted subsarcolemmal diffusion accurately predict current decay (τ ∼15 s) and reversal potential shifts observed for Na, Li, and K currents through Na channels opened by veratridine, as well as for Na, K, Cs, Li, and Cl currents recorded in nystatin-permeabilized myocytes. Ion concentrations in the pipette tip (i.e., access conductance) track without appreciable delay the current changes caused by sarcolemmal ion flux. Importantly, cytoplasmic mixing volumes, calculated from current decay kinetics, increase and decrease as expected with osmolarity changes (τ >30 s). Na/K pump current run-down over 20 min reflects a failure of pumps to recover from inactivation. Simulations reveal that pump inactivation coupled with Na-activated recovery enhances the rapidity and effectivity of Na homeostasis in cardiac myocytes. In conclusion, an autoregulatory mechanism enhances cardiac Na/K pump activity when cytoplasmic Na rises and suppresses pump activity when cytoplasmic Na declines.

Sodium channel diversity in the vestibular ganglion: NaV1.5, NaV1.8, and tetrodotoxin-sensitive currents

Firing patterns differ between subpopulations of vestibular primary afferent neurons. The role of sodium (NaV) channels in this diversity has not been investigated because NaV currents in rodent vestibular ganglion neurons (VGNs) were reported to be homogeneous, with the voltage dependence and tetrodotoxin (TTX) sensitivity of most neuronal NaV channels. RT-PCR experiments, however, indicated expression of diverse NaV channel subunits in the vestibular ganglion, motivating a closer look. Whole cell recordings from acutely dissociated postnatal VGNs confirmed that nearly all neurons expressed NaV currents that are TTX-sensitive and have activation midpoints between -30 and -40 mV. In addition, however, many VGNs expressed one of two other NaV currents. Some VGNs had a small current with properties consistent with NaV1.5 channels: low TTX sensitivity, sensitivity to divalent cation block, and a relatively negative voltage range, and some VGNs showed NaV1.5-like immunoreactivity. Other VGNs had a current with the properties of NaV1.8 channels: high TTX resistance, slow time course, and a relatively depolarized voltage range. In two NaV1.8 reporter lines, subsets of VGNs were labeled. VGNs with NaV1.8-like TTX-resistant current also differed from other VGNs in the voltage dependence of their TTX-sensitive currents and in the voltage threshold for spiking and action potential shape. Regulated expression of NaV channels in primary afferent neurons is likely to selectively affect firing properties that contribute to the encoding of vestibular stimuli.

Kinetic models for stochastically modified ionic channels

Ionic channels form pores in biomembranes. These pores are large macromolecular structures. Due to thermal fluctuations of countless degrees-of-freedom of the biomembrane material, the actual form of the pores is permanently subject to modification. Furthermore, the arrival of an ion at the binding site can change this form by repolarizing the surrounding aminoacids. In any case the variations of the pore structure are stochastic. In this paper, we discuss the effect of such modifications on the channel conductivity. Applying a simple kinetic description, we show that stochastic variations in channel properties can significantly alter the ionic current, even leading to its substantial increase or decrease for the specific matching of some time-scales of the system.

Molecular determinant for specific Ca/Ba selectivity profiles of low and high threshold Ca2+ channels

Voltage-gated Ca²⁺ channels (VGCC) play a key role in many physiological functions by their high selectivity for Ca²⁺ over other divalent and monovalent cations in physiological situations. Divalent/monovalent selection is shared by all VGCC and is satisfactorily explained by the existence, within the pore, of a set of four conserved glutamate/aspartate residues (EEEE locus) coordinating Ca²⁺ ions. This locus however does not explain either the choice of Ca²⁺ among other divalent cations or the specific conductances encountered in the different VGCC. Our systematic analysis of high- and low-threshold VGCC currents in the presence of Ca²⁺ and Ba²⁺ reveals highly specific selectivity profiles. Sequence analysis, molecular modeling, and mutational studies identify a set of nonconserved charged residues responsible for these profiles. In HVA (high voltage activated) channels, mutations of this set modify divalent cation selectivity and channel conductance without change in divalent/monovalent selection, activation, inactivation, and kinetics properties. The Ca_V2.1 selectivity profile is transferred to Ca_V2.3 when exchanging their residues at this location. Numerical simulations suggest modification in an external Ca²⁺ binding site in the channel pore directly involved in the choice of Ca²⁺, among other divalent physiological cations, as the main permeant cation for VGCC. In LVA (low voltage activated) channels, this locus (called DCS for divalent cation selectivity) also influences divalent cation selection, but our results suggest the existence of additional determinants to fully recapitulate all the differences encountered among LVA channels. These data therefore attribute to the DCS a unique role in the specific shaping of the Ca²⁺ influx between the different HVA channels.

Selectivity filter residues contribute unequally to pore stabilization in voltage-gated sodium channels

Mutations in the putative selectivity filter region of the voltage-gated Na+ channel, the so-called DEKA-motif, not only affect selectivity but also alter the channel's gating properties, suggesting functional coupling between permeation and gating. We have previously reported that charge-altering mutations at position 1237 in the P-loop of domain III (position K of the DEKA-motif in the adult rat skeletal muscle Na+ channel, rNa(v)1.4) dramatically enhanced entry to an inactivated state from which the channels recovered with a very slow time constant on the order of approximately 100 s (Todt, H., Dudley, S. C. J., Kyle, J. W., French, R. J., and Fozzard, H. A. (1999) Biophys. J. 76, 1335-1345). This state, termed "ultra-slow inactivation", may reflect a complex molecular rearrangement of the channel's pore region that involves both the extracellular and the cytoplasmic pore. Here, we tested whether charged DEKA-motif residues other than K1237 were also important determinants of a channel's gating properties. Therefore, we constructed the charge-neutralizing mutations D400A, E755A, and K1237A and studied the effects of these mutations on I(US). We found that, compared to wild-type rNa(v)1.4 channels, mutant D400A and K1237A but not E755A channels exhibited enhanced entry into ultra-slow inactivation. Selectivity for Na+ over K+, as judged from shifts in reversal potentials, was preserved in D400A, reduced in E755A, and completely lost in K1237A. These data suggest that an electrostatic interaction between the positively charged residue K1237 and the negatively charged residue D400 stabilizes the structure of the pore and thereby prevents I(US). Moreover, the interaction between K1237 and E755 appears to provide the basis for selective permeation of Na+ over K+.

I(Ca(TTX)) channels are distinct from those generating the classical cardiac Na(+) current

The Na(+) current component I(Ca(TTX)) is functionally distinct from the main body of Na(+) current, I(Na). It was proposed that I(Ca(TTX)) channels are I(Na) channels that were altered by bathing media containing Ca(2+), but no, or very little, Na(+). It is known that Na(+)-free conditions are not required to demonstrate I(Ca(TTX).) We show here that Ca(2+) is also not required. Whole-cell, tetrodotoxin-blockable currents from fresh adult rat ventricular cells in 65 mm Cs(+) and no Ca(2+) were compared to those in 3 mM Ca(2+) and no Cs(+) (i.e., I(Ca(TTX))). I(Ca(TTX)) parameters were shifted to more positive voltages than those for Cs(+). The Cs(+) conductance-voltage curve slope factor (mean, -4.68 mV; range, -3.63 to -5.72 mV, eight cells) is indistinguishable from that reported for I(Ca(TTX)) (mean, -4.49 mV; range, -3.95 to -5.49 mV). Cs(+) current and I(Ca(TTX)) time courses were superimposable after accounting for the voltage shift. Inactivation time constants as functions of potential for the Cs(+) current and I(Ca(TTX)) also superimposed after voltage shifting, as did the inactivation curves. Neither of the proposed conditions for conversion of I(Na) into I(Ca(TTX)) channels is required to demonstrate I(Ca(TTX)). Moreover, we find that cardiac Na(+) (H1) channels expressed heterologously in HEK 293 cells are not converted to I(Ca(TTX)) channels by Na(+)-free, Ca(2+)-containing bathing media. The gating properties of the Na(+) current through H1 and those of Ca(2+) current through H1 are identical. All observations are consistent with two non-interconvertable Na(+) channel populations: a larger that expresses little Ca(2+) permeability and a smaller that is appreciably Ca(2+)-permeable.

Molecular identification of a TTX-sensitive Ca(2+) current

The TTX-sensitive Ca(2+) current [I(Ca(TTX))] observed in cardiac myocytes under Na(+)-free conditions was investigated using patch-clamp and Ca(2+)-imaging methods. Cs(+) and Ca(2+) were found to contribute to I(Ca(TTX)), but TEA(+) and N-methyl-D-glucamine (NMDG(+)) did not. HEK-293 cells transfected with cardiac Na(+) channels exhibited a current that resembled I(Ca(TTX)) in cardiac myocytes with regard to voltage dependence, inactivation kinetics, and ion selectivity, suggesting that the cardiac Na(+) channel itself gives rise to I(Ca(TTX)). Furthermore, repeated activation of I(Ca(TTX)) led to a 60% increase in intracellular Ca(2+) concentration, confirming Ca(2+) entry through this current. Ba(2+) permeation of I(Ca(TTX)), reported by others, did not occur in rat myocytes or in HEK-293 cells expressing cardiac Na(+) channels under our experimental conditions. The report of block of I(Ca(TTX)) in guinea pig heart by mibefradil (10 microM) was supported in transfected HEK-293 cells, but Na(+) current was also blocked (half-block at 0.45 microM). We conclude that I(Ca(TTX)) reflects current through cardiac Na(+) channels in Na(+)-free (or "null") conditions. We suggest that the current be renamed I(Na(null)) to more accurately reflect the molecular identity of the channel and the conditions needed for its activation. The relationship between I(Na(null)) and Ca(2+) flux through slip-mode conductance of cardiac Na(+) channels is discussed in the context of ion channel biophysics and "permeation plasticity."

Tityustoxin effect on nerve compound action potentials requires extracellular sodium

Previous studies have demonstrated that Li(+) ions can substitute for Na(+) in a variety of functional systems. Using the single sucrose-gap recording technique, we measured the nerve compound action potential to study the effects of tityustoxin (an alpha-scorpion toxin that selectively inhibits fast Na(+) channel inactivation) upon removal of extracellular Na(+). Our results suggest that tityustoxin requires the presence of extracellular Na(+) to produce its typical pharmacological effect on Na(+) channel inactivation kinetics, but not to bind to its site.