L-cysteine prevents oxidation-induced block of the cardiac Na+ channel via interaction with heart-specific cysteinyl residues in the P-loop region

The Plant V-ATPase

V-ATPase is the dominant proton pump in plant cells. It contributes to cytosolic pH homeostasis and energizes transport processes across endomembranes of the secretory pathway. Its localization in the trans Golgi network/early endosomes is essential for vesicle transport, for instance for the delivery of cell wall components. Furthermore, it is crucial for response to abiotic and biotic stresses. The V-ATPase's rather complex structure and multiple subunit isoforms enable high structural flexibility with respect to requirements for different organs, developmental stages, and organelles. This complexity further demands a sophisticated assembly machinery and transport routes in cells, a process that is still not fully understood. Regulation of V-ATPase is a target of phosphorylation and redox-modifications but also involves interactions with regulatory proteins like 14-3-3 proteins and the lipid environment. Regulation by reversible assembly, as reported for yeast and the mammalian enzyme, has not be proven in plants but seems to be absent in autotrophic cells. Addressing the regulation of V-ATPase is a promising approach to adjust its activity for improved stress resistance or higher crop yield.

Inhibition of the cardiac Na⁺ channel Nav1.5 by carbon monoxide

Sublethal carbon monoxide (CO) exposure is frequently associated with myocardial arrhythmias, and our recent studies have demonstrated that these may be attributable to modulation of cardiac Na(+) channels, causing an increase in the late current and an inhibition of the peak current. Using a recombinant expression system, we demonstrate that CO inhibits peak human Nav1.5 current amplitude without activation of the late Na(+) current observed in native tissue. Inhibition was associated with a hyperpolarizing shift in the steady-state inactivation properties of the channels and was unaffected by modification of channel gating induced by anemone toxin (rATX-II). Systematic pharmacological assessment indicated that no recognized CO-sensitive intracellular signaling pathways appeared to mediate CO inhibition of Nav1.5. Inhibition was, however, markedly suppressed by inhibition of NO formation, but NO donors did not mimic or occlude channel inhibition by CO, indicating that NO alone did not account for the actions of CO. Exposure of cells to DTT immediately before CO exposure also dramatically reduced the magnitude of current inhibition. Similarly, l-cysteine and N-ethylmaleimide significantly attenuated the inhibition caused by CO. In the presence of DTT and the NO inhibitor N(ω)-nitro-L-arginine methyl ester hydrochloride, the ability of CO to inhibit Nav1.5 was almost fully prevented. Our data indicate that inhibition of peak Na(+) current (which can lead to Brugada syndrome-like arrhythmias) occurs via a mechanism distinct from induction of the late current, requires NO formation, and is dependent on channel redox state.

Enhancing effects of salicylate on quinidine-induced block of human wild type and LQT3 related mutant cardiac Na+ channels

It is unknown whether salicylate enhances the action of antiarrhythmic agents on human Na+ channels with state dependency and tissue specificity. We therefore investigated effects of salicylate on quinidine-induced block of human cardiac and skeletal muscle Na+ channels. Human cardiac wild-type (hH1), LQT3-related mutant (ΔKPQ), and skeletal muscle (hSkM1) Na+ channel α subunits were expressed in COS7 cells. Effects of salicylate on quinidine-induced tonic and use-dependent block of Na+ channel currents were examined by the whole-cell patch-clamp technique. Salicylate enhanced the quinidine-induced tonic and use-dependent block of both hH1 and hSkM1 currents at a holding potential (HP) of -100 mV but not at -140 mV. Salicylate decreased the IC50 value for the quinidine-induced tonic block of hH1 at an HP of -100 mV, and produced a negative shift in the steady-state inactivation curve of hH1 in the presence of quinidine. According to the modulated receptor theory, it is probable that salicylate decreases the dissociation constant for quinidine binding to inactivated-state channels. Furthermore, salicylate significantly enhanced the quinidine-induced tonic and use-dependent block of the peak and steady-state ΔKPQ channel currents. The results suggest that salicylate enhances quinidine-induced block of Na+ channels via increasing the affinity of quinidine to inactivated state channels.

Hydrogen sulfide is a partially redox-independent activator of the human jejunum Na+ channel, Nav1.5

Hydrogen sulfide (H(2)S) is produced endogenously by L-cysteine metabolism. H(2)S modulates several ion channels with an unclear mechanism of action. A possible mechanism is through reduction-oxidation reactions attributable to the redox potential of the sulfur moiety. The aims of this study were to determine the effects of the H(2)S donor NaHS on Na(V)1.5, a voltage-dependent sodium channel expressed in the gastrointestinal tract in human jejunum smooth muscle cells and interstitial cells of Cajal, and to elucidate whether H(2)S acts on Na(V)1.5 by redox reactions. Whole cell Na(+) currents were recorded in freshly dissociated human jejunum circular myocytes and Na(V)1.5-transfected human embryonic kidney-293 cells. RT-PCR amplified mRNA for H(2)S enzymes cystathionine β-synthase and cystathionine γ-lyase from the human jejunum. NaHS increased native Na(+) peak currents and shifted the half-point (V(1/2)) of steady-state activation and inactivation by +21 ± 2 mV and +15 ± 3 mV, respectively. Similar effects were seen on the heterologously expressed Na(V)1.5 α subunit with EC(50)s in the 10(-4) to 10(-3) M range. The reducing agent dithiothreitol (DTT) mimicked in part the effects of NaHS by increasing peak current and positively shifting steady-state activation. DTT together with NaHS had an additive effect on steady-state activation but not on peak current, suggesting that the latter may be altered via reduction. Pretreatment with the Hg(2+)-conjugated oxidizer thimerosal or the alkylating agent N-ethylmaleimide inhibited or decreased NaHS induction of Na(V)1.5 peak current. These studies show that H(2)S activates the gastrointestinal Na(+) channel, and the mechanism of action of H(2)S is partially redox independent.

State-Dependent Blocking Actions of Azimilide Dihydrochlo-ride (NE-10064) on Human Cardiac Na+ Channels

BACKGROUND

Azimilide reportedly blocks Na(+) channels, although its mechanism remains unclear.

METHODS AND RESULTS

The kinetic properties of the azimilide block of the wild-type human Na(+) channels (WT: hH1) and mutant DeltaKPQ Na(+) channels (DeltaKPQ) expressed in COS7 cells were investigated using the whole-cell patch clamp technique and a Markovian state model. Azimilide induced tonic block of WT currents by shifting the h infinity curve in the hyperpolarizing direction and caused phasic block of WT currents with intermediate recovery time constant. The peak and steady-state DeltaKPQ currents were blocked by azimilide, although with only a slight shift in the h infinity curve. The phasic block of peak and steady-state DeltaKPQ currents by azimilide was significantly larger than the blocking of the peak WT current. The affinity of azimilide predicted by a Markovian state model was higher for both the activated state (Kd(A) =1.4 micromol/L), and the inactivated state (Kd(I) =1.4 micromol/L), of WT Na(+) channels than that for the resting state (Kd(R) =102.6 micromol/L).

CONCLUSIONS

These experimental and simulation studies suggest that azimilide blocks the human cardiac Na(+) channel in both the activated and inactivated states.

Developmental Changes of Ni2+ Sensitivity and Automaticity in Nkx2.5-Positive Cardiac Precursor Cells From Murine Embryonic Stem Cell

BACKGROUND

It is controversial which subtypes of T type Ca(2+) channels are implicated in automaticity of cardiac cells during the embryonic period.

METHOD AND RESULTS

The effect of Ni(2+) on the automaticity of Nkx2.5-positive cardiac precursor cells sorted from embryonic stem cells during their differentiation was examined using patch clamp techniques. Although 40 micromol/L Ni(2+), which is enough to block Ni(2+)sensitive T type-Ca(2+) channels, decreased the spontaneous beating rate in all cells in the early and intermediate stage, Ni(2+) did not show any effects on the automaticity of 50% of the cells in the late stage.

CONCLUSION

These results indicate that Ni(2+)-sensitive T-type Ca(2+) channels expressed in the Nkx2.5-positive cardiac precursor cells are developmentally regulated.