11,12-Epoxyecosatrienoic acids mitigate endothelial dysfunction associated with estrogen loss and aging: Role of membrane depolarization.

OBJECTIVE: Endothelial dysfunction, including upregulation of inflammatory adhesion molecules and impaired vasodilatation, is a key element in cardiovascular disease. Aging and estrogen withdrawal in women are associated with endothelial inflammation, vascular stiffness and increased cardiovascular disease. Epoxyecosatrienoic acids (EETs), the products of arachidonic acid metabolism mediated by cytochrome P450 (CYP) 2J, 2C and other isoforms, are regulated by soluble epoxide hydrolase (sEH)-catalyzed conversion into less active diols. We hypothesized that 11,12-EETs would reduce the endothelial dysfunction associated with aging and estrogen loss. APPROACH/RESULTS: When stabilized by an sEH inhibitor (seHi), 11,12-EET at a physiologically low dose (0.1nM) reduced cytokine-stimulated upregulation of adhesion molecules on human aorta endothelial cells (HAEC) and monocyte adhesion under shear flow through marked depolarization of the HAEC when combined with TNFα. Mechanistically, neither 11,12-EETs nor 17ß-estradiol (E2) at physiologic concentrations prevented activation of NFκB by TNFα. E2 at physiological concentrations reduced sEH expression in HAEC, but did not alter CYP expression, and when combined with TNFα depolarized the cell. We also examined vascular dysfunction in adult and aged ovariectomized Norway brown rats (with and without E2 replacement) using an ex-vivo model to analyze endothelial function in an intact segment of artery. sEHi and 11,12-EET with or without E2 attenuated phenylephrine induced constriction and increased endothelial-dependent dilation of aortic rings from ovariectomized rats. CONCLUSIONS: Increasing 11,12-EETs through sEH inhibition effectively attenuates inflammation and may provide an effective strategy to preserve endothelial function and prevent atherosclerotic heart disease in postmenopausal women.

Depth asymmetries of the pore-lining segments of the Na+ channel revealed by cysteine mutagenesis.

We used serial cysteine mutagenesis to study the structure of the outer vestibule and selectivity region of the voltage-gated Na channel. The voltage dependence of Cd(2+) block enabled us to determine the locations within the electrical field of cysteine-substituted mutants in the P segments of all four domains. The fractional electrical distances of the substituted cysteines were compared with the differential sensitivity to modification by sulfhydryl-specific modifying reagents. These experiments indicate that the P segment of domain II is external, while the domain IV P segment is displaced internally, compared with the first and third domain P segments. Sulfhydryls with a steep voltage dependence for Cd(2+) block produced changes in monovalent cation selectivity; these included substitutions at the presumed selectivity filter, as well as residues in the domain IV P segment not previously recognized as determinants of selectivity. A new structural model is presented in which each of the P segments contribute unique loops that penetrate the membrane to varying depths to form the channel pore.

Adenovirus-mediated expression of a voltage-gated potassium channel in vitro (rat cardiac myocytes) and in vivo (rat liver). A novel strategy for modifying excitability.

Excitability is governed primarily by the complement of ion channels in the cell membrane that shape the contour of the action potential. To modify excitability by gene transfer, we created a recombinant adenovirus designed to overexpress a Drosophila Shaker potassium channel (AdShK). In vitro, a variety of mammalian cell types infected with AdShK demonstrated robust expression of the exogenous channel. Spontaneous action potentials recorded from cardiac myocytes in primary culture were abbreviated compared with noninfected myocytes. Intravascular infusion of AdShK in neonatal rats induced Shaker potassium channel mRNA expression in the liver, and large potassium currents could be recorded from explanted hepatocytes. Thus, recombinant adenovirus technology has been used for in vitro and in vivo gene transfer of ion channel genes designed to modify cellular action potentials. With appropriate targeting, such a strategy may be useful in gene therapy of arrhythmias, seizure disorders, and myotonic muscle diseases.

Nitric oxide modulates cardiac Na(+) channel via protein kinase A and protein kinase G.

We directly tested the effects of nitric oxide (NO) on Na(+) channels in guinea pig and mouse ventricular myocytes using patch-clamp recordings. We have previously shown that NO donors have no observed effects on expressed Na(+) channels. In contrast, NO (half-blocking concentration of 523 nmol/L) significantly reduces peak whole-cell Na(+) current (I(Na)) in isolated ventricular myocytes. The inhibitory effect of NO on I(Na) was not associated with changes in activation, inactivation, or reactivation kinetics. At the single-channel level, the reduction in macroscopic current was mediated by a decrease in open probability and/or a decrease in the number of functional channels with no change in single-channel conductance. Application of cell permeable analogs of cGMP or cAMP mimics the inhibitory effects of NO. Furthermore, the effects of NO on I(Na) can only be blocked by inhibition of both cGMP and cAMP pathways. Sulfhydryl-reducing agent does not reverse the effect of NO. In summary, although NO exerts its action via the known guanylyl cyclase (GC)/cGMP pathway, our findings provide evidence that NO can mediate its function via a GC/cGMP-independent mechanism involving the activation of adynylyl cyclase (AC) and cAMP-dependent protein kinase.

Charged residues between the selectivity filter and S6 segments contribute to the permeation phenotype of the sodium channel.

The deep regions of the Na(+) channel pore around the selectivity filter have been studied extensively; however, little is known about the adjacent linkers between the P loops and S6. The presence of conserved charged residues, including five in a row in domain III (D-III), hints that these linkers may play a role in permeation. To characterize the structural topology and function of these linkers, we neutralized the charged residues (from position 411 in D-I and its homologues in D-II, -III, and -IV to the putative start sites of S6) individually by cysteine substitution. Several cysteine mutants displayed enhanced sensitivities to Cd(2+) block relative to wild-type and/or were modifiable by external sulfhydryl-specific methanethiosulfonate reagents when expressed in TSA-201 cells, indicating that these amino acids reside in the permeation pathway. While neutralization of positive charges did not alter single-channel conductance, negative charge neutralizations generally reduced conductance, suggesting that such charges facilitate ion permeation. The electrical distances for Cd(2+) binding to these residues reveal a secondary "dip" into the membrane field of the linkers in domains II and IV. Our findings demonstrate significant functional roles and surprising structural features of these previously unexplored external charged residues.

Functional association of the beta 1 subunit with human cardiac (hH1) and rat skeletal muscle (mu 1) sodium channel alpha subunits expressed in Xenopus oocytes.

Native cardiac and skeletal muscle Na channels are complexes of alpha and beta 1 subunits. While structural correlates for activation, inactivation, and permeation have been identified in the alpha subunit and the expression of alpha alone produces functional channels, beta 1-deficient rat skeletal muscle (mu 1) and brain Na channels expressed in Xenopus oocytes do not gate normally. In contrast, the requirement of a beta 1 subunit for normal function of Na channels cloned from rat heart or human heart (hH1) has been disputed. Coinjection of rat brain beta 1 subunit cRNA with hH1 (or mu 1) alpha subunit cRNA into oocytes increased peak Na currents recorded 2 d after injection by 240% (225%) without altering the voltage dependence of activation. In mu 1 channels, steady state inactivation was shifted to more negative potentials (by 6 mV, p < 0.01), but the shift of 2 mV was not significant for hH1 channels. Nevertheless, coexpression with beta 1 subunit speeded the decay of macroscopic current of both isoforms. Ensemble average hH1 currents from cell-attached patches revealed that coexpression of beta 1 increases the rate of inactivation (quantified by time to 75% decay of current; p < 0.01 at -30, -40, and -50 mV). Use-dependent decay of hH1 Na current during repeated pulsing to -20 mV (1 s, 0.5 Hz) after a long rest was reduced to 16 +/- 2% of the first pulse current in oocytes coexpressing alpha and beta 1 subunits compared to 35 +/- 8% use-dependent decay for oocytes expressing the alpha subunit alone. Recovery from inactivation of mu 1 and hH1 Na currents after 1-s pulses to -20 mV is multiexponential with three time constants; coexpression of beta 1 subunit decreased all three recovery time constants. We conclude that the beta 1 subunit importantly influences the function of Na channels produced by coexpression with either the hH1 or mu 1 alpha subunits.

Amiloride-quinidine interaction: adverse outcomes.

OBJECTIVES: Previous studies have reported beneficial antiarrhythmic effects when selected drugs were combined. The purpose of this study was to assess whether a favorable interaction would occur with amiloride and quinidine. DESIGN: The antiarrhythmic and electrophysiologic effects of quinidine alone and in combination with amiloride were assessed in 10 patients with inducible sustained ventricular tachycardia. Parallel electrophysiologic studies assessed this drug combination in guinea pig papillary muscle. RESULTS: None of the patients had adverse effects during quinidine monotherapy. However, seven of 10 patients had adverse responses to the combination treatment: three patients had suppression of inducible ventricular tachycardia during quinidine monotherapy but had sustained ventricular tachycardia induced during combination treatment; three other patients had somatic side effects that resulted in discontinuation of the combination therapy but were absent during quinidine monotherapy; and one patient had 12 episodes of sustained ventricular tachycardia during this combination therapy. The patient had no such response during monotherapy. Surface QRS duration was significantly more prolonged during combination therapy than during monotherapy. Parallel electrophysiologic effects assessed this drug combination in guinea pig papillary muscle. The combination of amiloride (1 mumol/L) and quinidine (10 mumol/L) synergistically decreased the maximum rate of rise of phase 0 of the action potential (Vmax) (43 +/- 12 V/sec) compared with quinidine alone (24 +/- 9 V/sec) because of a greater degree of tonic block of Vmax (14% +/- 6%) as compared to quinidine alone (3% +/- 3%) with no significant change in action potential duration. CONCLUSIONS: Amiloride exaggerates the effects of quinidine on QRS duration in patients and on Vmax during in vitro study, which implies that the proarrhythmic effect of the combination of amiloride and quinidine may be associated with synergistic increase in sodium channel blockade.

Use-dependent electrophysiologic effects of amiodarone in coronary artery disease and inducible ventricular tachycardia.

Amiodarone produces use-dependent block of cardiac sodium channels in vitro. This study assessed whether similar use-dependent block occurred in 19 patients with coronary artery disease and inducible, sustained, monomorphic ventricular tachycardia treated with amiodarone. Beat-to-beat measurements of ventricular paced QRS durations during 12-beat trains at cycle lengths of 700, 600, 400 and 300 ms were analyzed at a baseline antiarrhythmic drug-free study and after 2 and 10 weeks of amiodarone therapy. At the drug-free study, there were no significant changes in paced QRS durations within the 12-beat trains at any pacing cycle lengths. After 2 and 10 weeks of amiodarone therapy, progressive prolongation of paced QRS durations occurred over the 12-beat trains at pacing cycle lengths of 600, 400 and 300 ms (p less than 0.05). Significant changes in QRS duration were not observed at a pacing cycle length of 700 ms. This progressive prolongation in QRS duration can be fitted as a function of beat number to a monoexponential equation and occurred with an onset time constant of 1.02 +/- 0.41 beats (306 +/- 122 ms) at a pacing cycle length of 300 ms. The magnitude of QRS prolongation increased as the pacing cycle length was shortened. The magnitudes of QRS prolongation were similar after 2 and 10 weeks of amiodarone therapy. In conclusion, use-dependent prolongation in QRS duration occurs at rapid pacing cycle lengths in humans receiving amiodarone.

Prospects for genetic manipulation of cardiac excitability.

Despite impressive advances in the therapy of a number of types of heart disease in the last two decades, sudden cardiac death remains a public health problem of staggering dimensions. Current treatment options include antiarrhythmic drugs that have higher than desired failure rates and implantable defibrillators that incur significant costs to the patient and society. The development of therapies that better suppress the cardiac arrhythmias responsible for sudden cardiac death requires a broad and comprehensive understanding of the basic mechanisms underlying electrical instability in the heart. This study explores the scientific basis for a molecular genetic approach to modify cardiac excitability and thereby to create animal models of sudden cardiac death. The availability of such models will open up new avenues of research in arrhythmogenesis and facilitate the development of novel antiarrhythmic agents.

Interaction between selected sodium and potassium channel blockers in guinea pig papillary muscle.

Previous studies have reported that enhanced antiarrhythmic effects occur when agents that prolong repolarization are combined with agents that block the sodium channels. The mechanism(s) of this interaction have not been elucidated. In this study, the interactions between the prolongation of action potential duration (APD) by a potassium channel blocker and the reduction in the maximal upstroke velocity of phase 0 of action potential (Vmax) by sodium channel blockers were investigated in guinea pig papillary muscle using conventional microelectrode techniques. Agents that produce selective electrophysiologic effects were chosen, including low concentrations of barium chloride (BaCl2), which selectively blocks the inwardly rectifying potassium current without effects on other repolarizing or depolarizing currents, O-demethyl-encainide (ODME), which blocks the activated sodium channel with slow onset/offset kinetics, and mexiletine, which preferentially blocks the inactivated sodium channel with rapid onset/offset kinetics. Mexiletine (4 x 10(-6) M) decreased Vmax from 195 +/- 29 V/sec at baseline to 180 +/- 26 V/sec (P < .05). Whereas BaCl2 (10(-5) M) prolonged action potential duration, it had no effect on Vmax. However, the addition of BaCl2 to mexiletine synergistically decreased Vmax from 180 +/- 26 V/sec with mexiletine to 166 +/- 18 V/sec (P < .05). ODME (3 x 10(-7) M) decreased Vmax from 179 +/- 17 V/sec at baseline to 133 +/- 15 V/sec (P < .01). However, the addition of BaCl2 to ODME did not produce a further decrease in Vmax as compared with ODME alone. In summary, a synergistic effect on Vmax was observed when BaCl2 and mexiletine were combined.(ABSTRACT TRUNCATED AT 250 WORDS)

Direct inhibition of expressed cardiac L-type Ca2+ channels by S-nitrosothiol nitric oxide donors.

NO donors have complex effects on Ca2+ currents in native cardiac cells, with reports of direct stimulation and indirect cGMP-mediated inhibition or stimulation. To investigate the molecular basis of these effects, we tested the effects of one class of NO donors, S-nitrosothiols (RSNOs), on expressed cardiovascular L-type Ca2+ channels (alpha 1C +/- beta 1a +/- alpha 2 or alpha 1C +/- beta 2a +/- alpha 2) in human embryonic kidney (HEK293) cells. The RSNO compounds we used were S-nitroso-N-acetylpenicillamine (SNAP, 5 to 10 nmol/L or 100 to 800 mumol/L), S-nitrosocysteine (SNC, 100 mumol/L or 1 mmol/L), and S-nitrosoglutathione (GSNO, 1 mmol/L). Currents were measured using whole-cell patch recordings with 2 to 10 mmol/L Ba2+ as the charge carrier. SNAP reduced the amplitude of barium currents (IBa) through all the subunit combinations, with and EC50 of 360 mumol/L for alpha 1C + beta 1a channels. SNC or GSNO also inhibited IBa, albeit less potently. The inhibitory effect of SNAP was not affected by methylene blue (10 to 30 mumol/L) or 8-bromo-cGMP (200 to 400 mumol/L). The effects are relatively specific for Ca2+ channels, as expressed cardiac or skeletal muscle Na+ channels, which have a similar overall architecture, were barely affected by SNAP at concentrations as high as 1 mmol/L. We conclude that in the HEK293 expression system, the S-nitrosothiol NO donors inhibit L-type Ca2+ channels by a mechanism independent of cGMP.

Structure of the sodium channel pore revealed by serial cysteine mutagenesis.

The pores of voltage-gated cation channels are formed by four intramembrane segments that impart selectivity and conductance. Remarkably little is known about the higher order structure of these critical pore-lining or P segments. Serial cysteine mutagenesis reveals a pattern of side-chain accessibility that contradicts currently favored structural models based on alpha-helices or beta-strands. Like the active sites of many enzymes of known structure, the sodium channel pore consists of irregular loop regions.

Control of ion flux and selectivity by negatively charged residues in the outer mouth of rat sodium channels.

1. The sodium channel has a ring of negatively charged amino acids on its external face. This common structural feature of cation-selective channels has been proposed to optimize conduction by electrostatic attraction of permeant cations into the channel mouth. We tested this idea by mutagenesis of mu1 rat skeletal sodium channels expressed in Xenopus oocytes. 2. Replacement of the external glutamate residue in domain II by cysteine reduces sodium current by decreasing single-channel conductance. While this effect can be reversed by the negatively charged sulfhydryl modifying reagent methanethiosulphonate ethylsulphonate (MTSES), the flux saturation behaviour cannot be rationalized simply by changes in the surface charge. 3. The analogous mutations in domains I, III and IV affect not only conductance but also selectivity. These changes in selectivity are only partially reversed by exposure to MTSES. 4. Our findings necessitate revision of prevailing concepts regarding the role of superficial negatively charged residues in the process of ion permeation. These residues do not act solely by electrostatic attraction of permeant ions, but instead may help to form ion-specific binding sites within the pore.

Mechanism of anode break stimulation in the heart.

Anodal stimulation is routinely observed in cardiac tissue, but only recently has a mechanism been proposed. The bidomain cardiac tissue model proposes that virtual cathodes induced at sites distant from the electrode initiate the depolarization. In contrast, none of the existing cardiac action potential models (Luo-Rudy phase I and II, or Oxsoft) predict anodal stimulation at the single-cell level. To determine whether anodal stimulation has a cellular basis, we measured membrane potential and membrane current in mammalian ventricular myocytes by using whole-cell patch clamp. Anode break responses can be readily elicited in single ventricular cells. The basis of this anodal stimulation in single cells is recruitment of the hyperpolarization-activated inward current I(f). The threshold of activation for I(f) is -80 mV in rat cells and -120 mV in guinea pig or canine cells. Persistent I(f) "tail" current upon release of the hyperpolarization drives the transmembrane potential toward the threshold of sodium channels, initiating an action potential. Time-dependent block of the inward rectifier, I(K1), at hyperpolarized potentials decreases membrane conductance and thereby potentiates the ability of I(f) to depolarize the cell on the break of an anodal pulse. Inclusion of I(f), as well as the block and unblock kinetics of I(K1), in the existing Luo-Rudy action potential model faithfully reproduces anode break stimulation. Thus active cellular properties suffice to explain anode break stimulation in cardiac tissue.

Effects of intracellular calcium on sodium current density in cultured neonatal rat cardiac myocytes.

1. Na+ channel mRNA levels in the heart can be modulated by changes in intracellular Ca2+ ([Ca2+]i). We have investigated whether this regulation of Na+ channel biosynthesis by cytosolic Ca2+ translates into functional Na+ channels that can be detected electrophysiologically. 2. Whole-cell Na+ currents (INa) were recorded using patch-clamp techniques from single ventricular myocytes isolated from neonatal rats and maintained in tissue culture for 24 h. Na+ current density, measured at a membrane potential of -10 mV, was significantly decreased in the cells which were exposed for 24 h to culture medium containing 10 mM of both external Ca2+ and K+ in order to raise [Ca2+]i compared with control cells which were maintained in culture medium containing 2 and 5 mM of Ca2+ and K+, respectively. In contrast, Na+ current density (at -10 mV) was significantly increased in cells exposed for 24 h to 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetra-acetic acid tetraacetoxymethyl ester (BAPTA AM; a cell membrane-permeable Ca2+ chelator) which lowered the average [Ca2+]i compared with control. 3. Changes in current density were not associated with changes in the voltage dependence of activation and inactivation of INa. There were no changes in single-channel conductances. 4. It is concluded that Na+ current density in neonatal rat cardiac myocytes is modulated by [Ca2+]i. The findings suggest that the differences in current density are attributable to a change in Na+ channel numbers rather than to changes in single-channel conductance or gating. These changes are consistent with the previously documented modulation of Na+ channel biosynthesis by cytosolic Ca2+.

Changes in Ca(2+) cycling proteins underlie cardiac action potential prolongation in a pressure-overloaded guinea pig model with cardiac hypertrophy and failure.

Ventricular arrhythmias are common in both cardiac hypertrophy and failure; cardiac failure in particular is associated with a significant increase in the risk of sudden cardiac death. We studied the electrophysiologic changes in a guinea pig model with aortic banding resulting in cardiac hypertrophy at 4 weeks and progressing to cardiac failure at 8 weeks using whole-cell patch-clamp and biochemical techniques. Action potential durations (APDs) were significantly prolonged in banded animals at 4 and 8 weeks compared with age-matched sham-operated animals. APDs at 50% and 90% repolarization (APD(50) and APD(90) in ms) were the following: 4 week, banded, 208+/-51 and 248+/-49 (n = 15); 4 week, sham, 189+/-68 and 213+/-69 (n = 16); 8 week, banded, 197+/-40 and 226+/-40 (n = 21); and 8 week, sham, 156+/-42 and 189+/-45 (n = 22), respectively; P<0.05 comparing banded versus sham-operated animals. We observed no significant differences in the K(+) currents between the 2 groups of animals at 4 and 8 weeks. However, banded animals exhibited a significant increase in Na(+) and Na(+)-Ca(2+) exchange current densities compared with controls. Furthermore, we have found a significant attenuation in the Ca(2+)-dependent inactivation of the L-type Ca(2+) current in the banded compared with sham-operated animals, likely as a result of the significant downregulation of the sarcoplasmic reticulum Ca(2+) ATPase, which has been documented previously in the heart failure animals. Our data provide an alternate mechanism for APD prolongation in cardiac hypertrophy and failure and support the notion that there is close interaction between Ca(2+) handling and action potential profile.

Mechanisms of sodium/calcium selectivity in sodium channels probed by cysteine mutagenesis and sulfhydryl modification.

A conserved lysine residue in the "P loop" of domain III renders sodium channels highly selective. Conversion of this residue to glutamate, to mimic the homologous position in calcium channels, enables Ca2+ to permeate sodium channels. Because the lysine-to-glutamate mutation converts a positively charged side chain to a negative one, it has been proposed that a positive charge at this position suffices for Na+ selectivity. We tested this idea by converting the critical lysine to cysteine (K1237C) in mu 1 rat skeletal sodium channels expressed in Xenopus oocytes. Selectivity of the mutant channels was then characterized before and after chemical modification to alter side-chain charge. Wild-type channels are highly selective for Na+ over Ca2+ (PCa/PNa < 0.01). The K1237C mutation significantly increases permeability to Ca2+ (PCa/PNa = 0.6) and Sr2+. Analogous mutations in domains I (D400C), II (E755C), and IV (A1529C) did not alter the selectivity for Na+ over Ca2+, nor did any of the domain IV mutations (G1530C, W1531C, and D1532C) that are known to affect monovalent selectivity. Interestingly, the increase in permeability to Ca2+ in K1237C cannot be reversed by simply restoring the positive charge to the side chain by using the sulfhydryl modifying reagent methanethiosulfonate ethylammonium. Single-channel studies confirmed that modified K1237C channels, which exhibit a reduced unitary conductance, remain permeable to Ca2+, with a PCa/PNa of 0.6. We conclude that the chemical identity of the residue at position 1237 is crucial for channel selectivity. Simply rendering the 1237 side chain positive does not suffice to restore selectivity to the channel.

External pore residue mediates slow inactivation in mu 1 rat skeletal muscle sodium channels.

1. Upon depolarization, voltage-gated sodium channels assume non-conducting inactivated states which may be characterized as "fast' or "slow' depending on the length of the repolarization period needed for recovery. Skeletal muscle Na+ channel alpha-subunits expressed in Xenopus laevis oocytes display anomalous gating behaviour, with substantial slow inactivation after brief depolarizations. We exploited this kinetic behaviour to examine the structural basis for slow inactivation. 2. While fast inactivation in Na+ channels is mediated by cytoplasmic occlusion of the pore by III-IV linker residues, the structural features of slow inactivation are unknown. Since external pore-lining residues modulate C-type inactivation in potassium channels, we performed serial cysteine mutagenesis in the permeation loop (P-loop) of the rat skeletal muscle Na+ channel (mu 1) to determine whether similarly placed residues are involved in Na+ channel slow inactivation. 3. Wild-type and mutant alpha-subunits were heterologously expressed in Xenopus oocytes, and Na+ currents were recorded using a two-electrode voltage clamp. Slow inactivation after brief depolarizations was eliminated by the W402C mutation in domain I. Cysteine substitution of the homologous tryptophan residues in domains II, III and IV did not alter slow inactivation. 4. Analogous to the W402C mutation, coexpression of the wild-type alpha-subunit with rat brain Na+ channel beta 1-subunit attenuated slow inactivation. However, the W402C mutation imposed a delay on recovery from fast inactivation, while beta 1-subunit coexpression did not. We propose that the W402C mutation and the beta 1-subunit modulate gating through distinct mechanisms. 5. Removal of fast inactivation in wild-type alpha-subunits with the III-IV linker mutation I1303Q; F1304Q; M1305Q markedly slowed the development of slow inactivation. We propose that slow inactivation in Na+ channels involves conformational changes in the external pore. Mutations that affect fast and slow inactivation appear to interact despite their remote positions in the channel.

Functional consequences of sulfhydryl modification in the pore-forming subunits of cardiovascular Ca2+ and Na+ channels.

The structure and function of many cysteine-containing proteins critically depend on the oxidation state of the sulfhydryl groups. In such proteins, selective modification of sulfhydryl groups can be used to probe the relation between structure and function. We examined the effects of sulfhydryloxidizing and -reducing agents on the function of the heterologously expressed pore-forming subunits of the cloned rabbit smooth muscle L-type Ca2+ channel and the human cardiac tetrodotoxin-insensitive Na+ channel. The known sequences of the channels suggest the presence of three or four cysteine residues within the putative pores of Ca2+ or Na+ channels, respectively, as well as multiple other cysteines in regions of unknown function. We determined the effects of sulfhydryl modification on Ca2+ and Na+ channel gating and permeation by using the whole-cell and single-channel variants of the patch-clamp technique. Within 10 minutes of exposure to 2,2'-dithiodipyridine (DTDP, a specific lipophilic oxidizer of sulfhydryl groups), Ca2+ current was reduced compared with the control value, with no significant change in the kinetics and no shift in the current-voltage relations. The effect could be readily reversed by 1,4-dithiothreitol (an agent that reduces disulfide bonds). Similar results were obtained by using the hydrophilic sulfhydryl-oxidizing agent thimerosal. The effects were Ca(2+)-channel specific: DTDP induced no changes in expressed human cardiac Na+ current. Single-channel Ba2+ current recordings revealed a reduction in open probability and mean open time by DTDP but no change in single-channel conductance, implying that the reduction of macroscopic Ca2+ current reflects changes in gating and not permeation. In summary, the pore-forming (alpha 1) subunit of the L-type Ca2+ channel contains functionally important free sulfhydryl groups that modulate gating. These free sulfhydryl groups are accessible from the extracellular side by an aqueous pathway.

Ionic mechanism of action potential prolongation in ventricular myocytes from dogs with pacing-induced heart failure.

Membrane current abnormalities have been described in human heart failure. To determine whether similar current changes are observed in a large animal model of heart failure, we studied dogs with pacing-induced cardiomyopathy. Myocytes isolated from the midmyocardium of 13 dogs with heart failure induced by 3 to 4 weeks of rapid ventricular pacing and from 16 nonpaced control dogs did not differ in cell surface area or resting membrane potential. Nevertheless, action potential duration (APD) was significantly prolonged in myocytes isolated from failing ventricles (APD at 90% repolarization, 1097 +/- 73 milliseconds [failing hearts, n = 30] versus 842 +/- 56 milliseconds [control hearts, n = 25]; P < .05), and the prominent repolarizing notch in phase 1 was dramatically attenuated. Basal L-type Ca2+ current and whole-cell Na+ current did not differ in cells from failing and from control hearts, but significant differences in K+ currents were observed. The density of the inward rectifier K+ current (IKl) was reduced in cells from failing hearts at test potentials below -90 mV (at -150 mV, -19.1 +/- 2.2 pA/pF [failing hearts, n = 18] versus -32.2 +/- 5.1 pA/pF [control hearts, n = 15]; P < .05). The small outward current component of IKl was also reduced in cells from failing hearts (at -60 mV, 1.7 +/- 0.2 pA/pF [failing hearts] versus 2.5 +/- 0.2 pA/pF [control hearts]; P < .05). The peak of the Ca(2+)-independent transient outward current (Ito) was dramatically reduced in myocytes isolated from failing hearts compared with nonfailing control hearts (at +80 mV, 7.0 +/- 0.9 pA/pF [failing hearts, n = 20] versus 20.4 +/- 3.2 pA/pF [control hearts, n = 15]; P < .001), while the steady state component was unchanged. There were no significant differences in Ito kinetics or single-channel conductance. A reduction in the number of functional Ito channels was demonstrated by nonstationary fluctuation analysis (0.4 +/- 0.03 channels per square micrometer [failing hearts, n = 5] versus 1.2 +/- 0.1 channels per square micrometer [control hearts, n = 3]; P < .001). Pharmacological reduction of Ito by 4-aminopyridine in control myocytes decreased the notch amplitude and prolonged the APD. Current clamp-release experiments in which current was injected for 8 milliseconds to reproduce the notch sufficed to shorten the APD significantly in cells from failing hearts. These data support the hypothesis that downregulation of Ito in pacing-induced heart failure is at least partially responsible for the action potential prolongation. Because the repolarization abnormalities mimic those in cells isolated from failing human ventricular myocardium, canine pacing-induced cardiomyopathy may provide insights into the development of repolarization abnormalities and the mechanisms of sudden death in patients with heart failure.