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J R Balser

Publications and source records attributed to J R Balser.

At least 37 records · Page 2Linked to original sources
PubMed1999

Molecular dynamics of the sodium channel pore vary with gating: interactions between P-segment motions and inactivation.

Disulfide trapping studies have revealed that the pore-lining (P) segments of voltage-dependent sodium channels undergo sizable motions on a subsecond time scale. Such motions of the pore may be necessary for selective ion translocation. Although traditionally viewed as separable properties, gating and permeation are now known to interact extensively in various classes of channels. We have investigated the interaction of pore motions and voltage-dependent gating in micro1 sodium channels engineered to contain two cysteines within the P segments. Rates of catalyzed internal disulfide formation (kSS) were measured in K1237C+W1531C mutant channels expressed in oocytes. During repetitive voltage-clamp depolarizations, increasing the pulse duration had biphasic effects on the kSS, which first increased to a maximum at 200 msec and then decreased with longer depolarizations. This result suggested that occupancy of an intermediate inactivation state (IM) facilitates pore motions. Consistent with the known antagonism between alkali metals and a component of slow inactivation, kSS varied inversely with external [Na+]o. We examined the converse relationship, namely the effect of pore flexibility on gating, by measuring recovery from inactivation in Y401C+E758C (YC/EC) channels. Under oxidative conditions, recovery from inactivation was slower than in a reduced environment in which the spontaneous YC/EC cross-link is disrupted. The most prominent effects were slowing of a component with intermediate recovery kinetics, with diminution of its relative amplitude. We conclude that occupancy of an intermediate inactivation state facilitates motions of the P segments; conversely, flexibility of the P segments alters an intermediate component of inactivation.

Animals↗
PubMed1999

Enhancement of HERG K+ currents by Cd2+ destabilization of the inactivated state.

We have studied the functional effects of extracellular Cd(2+) on human ether-a-go-go-related gene (HERG) encoded K(+) channels. Low concentrations (10-200 microM) of extracellular Cd(2+) increased outward currents through HERG channels; 200 microM Cd(2+) more than doubled HERG currents and altered current kinetics. Cd(2+) concentrations up to 200 microM did not change the voltage dependence of channel activation, but shifted the voltage dependence of inactivation to more depolarized membrane potentials. Cd(2+) concentrations >or=500 microM shifted the voltage dependence of channel activation to more positive potentials. These results are consistent with a somewhat specific ability of Cd(2+) to destabilize the inactivated state. We tested the hypothesis that channel inactivation is essential for Cd(2+)-induced increases in HERG K(+) currents, using a double point mutation (G628C/S631C) that diminishes HERG inactivation (Smith, P. L., T. Baukrowitz, and G. Yellen. 1996. Nature (Lond.). 379:833-836). This inactivation-removed mutant is insensitive to low concentrations of Cd(2+). Thus, Cd(2+) had two distinct effects on HERG K(+) channels. Low concentrations of Cd(2+) caused relatively selective effects on inactivation, resulting in a reduction of the apparent rectification of the channel and thereby increasing HERG K(+) currents. Higher Cd(2+) concentrations affected activation gating as well, possibly by a surface charge screening mechanism or by association with a lower affinity site.

Animals↗
PubMed1999

Structure and function of the cardiac sodium channels.

The coincident cloning of the voltage-gated Na channel from the electric eel electroplax and development of patch-clamp methodology has allowed an explosive phase of investigation into the structural basis of cardiac Na channel function. Recognizing the importance of structural motifs that underlie gating (charged S4 segments, III-IV linker) and permeation (P-loops) have complemented new molecular information surrounding inherited cardiac arrhythmias, such as the chromosome 3-linked form of the long QT syndrome. Although the proarrhythmic potential recognized in the CAST trial [1] slowed the development of class I antiarrhythmic agents, our emerging understanding of the molecular pharmacology of Na channels may motivate strategies for Na-channel drug discovery that involve targeting particular structural domains.

Anesthetics, Local↗
PubMed1999

Pro: All patients should receive pharmacologic prophylaxis for atrial fibrillation after cardiac surgery.

In addition to the physiologic manifestations of atrial fibrillation (stroke, ventricular arrhythmia), patients with postoperative arrhythmias have subjective concerns because of symptoms (palpitations), and prolonged length of stay and hospital cost are significant sources of visceral morbidity to both patients and their physicians. Efforts to terminate this arrhythmia after its initiation are fraught with problems ranging from ineffectiveness to toxicity. Fortunately, a variety of pharmacologic strategies are now available to prevent atrial fibrillation after cardiac surgery. At a minimum, low-dose postoperative beta-adrenergic blockade is valuable for patients who receive these medications preoperatively and may be beneficial in all patients. Moreover, emerging data suggest that prophylaxis with antiarrhythmic compounds can significantly decrease the incidence of atrial fibrillation, length of hospital stay, and cost. Future trials will be focused on evaluating the risks and benefits of the newer prophylactic therapies and defining which subpopulations benefit most from such therapy.

Anti-Arrhythmia Agents↗
PubMed1998

Mechanistic link between lidocaine block and inactivation probed by outer pore mutations in the rat micro1 skeletal muscle sodium channel.

1. Mutations that disrupt Na+ channel fast inactivation attenuate lidocaine (lignocaine)-induced use dependence; however, the pharmacological role of slower inactivation processes remains unclear. In Xenopus oocytes, tryptophan substitution in the outer pore of the rat skeletal muscle channel (micro1-W402) alters partitioning among fast- and slow-inactivated states. We therefore examined the effects of W402 mutations on lidocaine block. 2. Recovery from inactivation exhibited three kinetic components (IF, fast; IM, intermediate; IS, slow). The effects of W402A and W402S on IF and IS differed, but both mutants (with or without beta1 subunit coexpression) decreased the amplitude of IM. In wild-type channels, lidocaine imposed a delayed recovery component with intermediate kinetics, and use-dependent block was attenuated in both W402A and W402S. 3. To examine the pharmacological role of IS relative to IM, drug-exposed beta1-coexpressed channels were subjected to 2 min depolarizations. Lidocaine had no effect on sodium current (INa) after a 1 s hyperpolarization interval that allowed recovery from IM but not IS, suggesting that lidocaine affinity for IS is low. 4. Both W402 mutations reduced occupancy of IM in drug-free conditions, and also induced resistance to use-dependent block. We propose that lidocaine-induced use dependence may involve an allosteric conformational change in the outer pore.

Animals↗
PubMed1998

Lidocaine action on Na+ currents in ventricular myocytes from the epicardial border zone of the infarcted heart.

Myocytes overlying a zone of infarction form the primary substrate for serious reentrant ventricular arrhythmias. In vitro and in vivo studies suggest that antiarrhythmic agents affect Na+ channels of cells from the epicardial border zone (EBZ) of the 5-day infarcted heart differently than they affect those of normal muscle. However, the mechanisms responsible for this difference remain unclear. Previous studies have revealed differences in Na+ current (INa) density and inactivation gating kinetics in myocytes dispersed from the EBZ (IZs). Since changes in inactivation gating could influence lidocaine action, we examined the effects of lidocaine on INa of IZs (n=38) and epicardial myocytes from the noninfarcted heart (NZs) (n=50) using the whole-cell variation of the patch-clamp technique. In drug-free conditions, the voltage dependence of steady-state inactivation of IZs was shifted negative to that of NZs, causing greater inactivation of IZ channels at depolarized (> or = -100-mV) holding potentials. Consistent with a high affinity for the inactivated channel conformation, lidocaine produced more tonic block in IZs than NZs at depolarized holding potentials. Additionally, in drug-free conditions, IZ INa exhibited an enhanced rate of inactivation from closed states, a delay in recovery from inactivation, and increased use-dependent reduction in amplitude during rapid (1- to 3-Hz) pulse trains. In both IZs and NZs, lidocaine (20 to 120 micromol/L) accelerated the rate of time-dependent loss of availability and markedly delayed recovery from availability, inducing significant use-dependent reduction of INa. However, at drug concentrations > or =60 micromol/L, the difference in use-dependent current reduction between IZs and NZs was minimized. The action of lidocaine to render Na+ channel inactivation in NZs more similar to that of IZs may be central to its (pro)antiarrhythmic effects.

Animals↗
PubMed1998

Phenotypic characterization of a novel long-QT syndrome mutation (R1623Q) in the cardiac sodium channel.

BACKGROUND: A heritable form of the long-QT syndrome (LQT3) has been linked to mutations in the cardiac sodium channel gene (SCN5A). Recently, a sporadic SCN5A mutation was identified in a Japanese girl afflicted with the long-QT syndrome. In contrast to the heritable mutations, this externally positioned domain IV, S4 mutation (R1623Q) neutralized a charged residue that is critically involved in activation-inactivation coupling. METHODS AND RESULTS: We have characterized the R1623Q mutation in the human cardiac sodium channel (hH1) using both whole-cell and single-channel recordings. In contrast to the autosomal dominant LQT3 mutations, R1623Q increased the probability of long openings and caused early reopenings, producing a threefold prolongation of sodium current decay. Lidocaine restored rapid decay of the R1623Q macroscopic current. CONCLUSIONS: The R1623Q mutation produces inactivation gating defects that differ mechanistically from those caused by LQT3 mutations. These findings provide a biophysical explanation for this severe long-QT phenotype and extend our understanding of the mechanistic role of the S4 segment in cardiac sodium channel inactivation gating and class I antiarrhythmic drug action.

Animals↗
PubMed1998

Beta-adrenergic blockade accelerates conversion of postoperative supraventricular tachyarrhythmias.

BACKGROUND: Postoperative supraventricular tachyarrhythmia is a common complication of surgery. Because chemical cardioversion is often ineffective, ventricular rate control remains a principal goal of therapy. The authors hypothesized that patients with supraventricular tachyarrhythmia after major noncardiac surgery who receive intravenous beta-adrenergic blockade for ventricular rate control would experience conversion to sinus rhythm at a rate that differs from those receiving intravenous calcium channel blockade. METHODS: The rate of conversion to sinus rhythm at 2 and 12 h after treatment was examined in 64 cases of postoperative supraventricular tachyarrhythmia. After adenosine administration, patients who remained in supraventricular tachyarrhythmia were prospectively randomized to receive either intravenous diltiazem or intravenous esmolol for ventricular rate control (unblinded). Loading and infusion rates were adjusted to achieve equivalent degrees of ventricular rate control. RESULTS: Patients were similar with regard to age and Apache III score. Most patients in both groups had atrial fibrillation (esmolol, 79%; diltiazem, 81%), and none experienced stable conversion with adenosine. Patients randomized to receive esmolol experienced a 59% rate of conversion to sinus rhythm within 2 h of treatment, compared with only 33% for patients randomized to receive diltiazem (intention to treat, P = 0.049; odds ratio, 2.9; 95% confidence interval, 1.046 to 7.8). After 12 h of therapy, the number of patients converting to sinus rhythm increased in both groups (esmolol, 85%; diltiazem, 62%), and the rates of conversion no longer differed significantly. Ventricular rates when supraventricular tachyarrhythmia began and after 2 and 12 h of rate control therapy were similar in the two treatment groups. The in-hospital mortality rate and length of stay in the intensive care unit were not significantly influenced by treatment group. CONCLUSIONS: Among adenosine-resistant patients in the intensive care unit with atrial fibrillation after noncardiac surgery, intravenous esmolol produced a more rapid (2-h) conversion to sinus rhythm than did intravenous diltiazem.

Adrenergic beta-Antagonists↗
PubMed1998

"Renal dose" dopamine in surgical patients: dogma or science?

OBJECTIVE: "Renal dose" dopamine is widely used in the perioperative period to provide renal protection. A comprehensive review of the literature was performed to determine whether dopamine does in fact confer protection on the kidneys of surgical patients. SUMMARY BACKGROUND DATA: Studies in healthy animals and human volunteers reveal that dopamine causes diuresis and natriuresis, as well as some degree of renal vasodilatation. RESULTS: Studies of the perioperative use of dopamine fail to demonstrate any benefit of dopamine in preventing renal failure. Studies in congestive heart failure, critical illness, and sepsis also fail to show any benefit of dopamine other than diuresis. Further, dopamine administration is not completely without risk, because of dopamine's catecholamine and neuroendocrine functions. CONCLUSIONS: Routine use of prophylactic "renal dose" dopamine in surgical patients is not recommended.

Animals↗
PubMed1998

Radial artery pressure monitoring underestimates central arterial pressure during vasopressor therapy in critically ill surgical patients.

OBJECTIVES: Radial artery pressure is known to differ from central arterial pressure in normal patients (distal pulse amplification) and in the early postcardiopulmonary bypass period. The adequacy of the radial artery as a site for blood pressure monitoring in critically ill patients receiving high-dose vasopressors has not been carefully examined. DESIGN: Prospective observational study comparing simultaneous intra-arterial measurements of radial (peripheral) and femoral artery (central) pressures. SETTING: Clinical investigation in a university-based surgical intensive care unit. PATIENTS: Fourteen critically ill patients with presumed sepsis who received norepinephrine infusions at a rate of > or =5 microg/min. INTERVENTIONS: All patients were managed in accordance with our standard practice for presumed sepsis, which consisted of intravascular volume repletion followed by vasopressor administration titrated to a mean arterial pressure of > or =60 mm Hg. MEASUREMENTS AND MAIN RESULTS: Systolic and mean arterial pressures were significantly higher when measured from the femoral vs. radial site (p < .005). The higher mean arterial pressures enabled an immediate reduction in norepinephrine infusions in 11 of the 14 patients. No change in cardiac output or pulmonary artery occlusion pressure was noted after dose reduction. In the two patients in whom simultaneous recordings were made after discontinuation of norepinephrine infusions, equalization of mean arterial pressures was observed. CONCLUSIONS: Radial artery pressure underestimates central pressure in hypotensive septic patients receiving high-dose vasopressor therapy. Clinical management, based on radial pressures, may lead to excessive vasopressor administration. Awareness of this phenomena may help minimize adverse effects of these potent agents by enabling dosage reduction.

Bias↗
PubMed1997

Proton inhibition of sodium channels: mechanism of gating shifts and reduced conductance.

Extracellular acidosis affects both permeation and gating of the expressed rat skeletal muscle Na+ channel (micro1). Reduction of the extracellular pH produced a progressive decrease in the maximal whole-cell conductance and a depolarizing shift in the whole-cell current-voltage relationship. A smaller depolarizing shift in the steady-state inactivation curve was observed. The pK of the reduction of maximal conductance was 6.1 over the pH range studied. An upper limit estimate of the pK of the shift of the half-activation voltage was 6.1. The relative reduction in the maximal whole-cell conductance did not change with higher [Na+]o. The conductance of single fenvalerate-modified Na+ channels was reduced by extracellular protons. Although the single-channel conductance increased with higher [Na+]o, the maximal conductances at pH 7.6, 7.0 and 6.0 did not converge at [Na+]o up to 280 mm, inconsistent with a simple electrostatic effect. A model incorporating both Na+ and H+ binding in the pore and cation binding to a Gouy-Chapman surface charge provided a robust fit to the single-channel conductance data with an estimated surface charge density of 1e-/439A2. Neither surface charge nor proton block alone suffices to explain the effects of extracellular acidosis on Na+ channel permeation; both effects play major roles in mediating the response to extracellular pH.

Animals↗
PubMed1997

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.

Amino Acid Sequence↗
PubMed1997

The rational use of intravenous amiodarone in the perioperative period.

Intravenous amiodarone is a potentially valuable therapy for perioperative patients experiencing life-threatening ventricular arrhythmias refractory to conventional antiarrhythmic agents perioperatively. Its potential for hemodynamic and pulmonary toxicity perioperatively suggests that it should remain an alternative therapy rather than a first-line option. However, because of its impressive efficacy in nonsurgical trials, its role in perioperative arrhythmia management will dramatically expand if clinical studies become available that clarify its safety in surgical populations.

Administration, Oral↗
PubMed1997

High dose neostigmine treatment of malignant sinus tachycardia.

Sinus tachycardia caused by circulating catecholamines in the setting of congestive heart failure may impair systemic perfusion because of decreased diastolic filling time. We report the case of a patient with Wolff-Parkinson-White syndrome with angina and cardiogenic shock who improved dramatically following administration of neostigmine. Cardiac output, blood pressure, and stroke volume increased as heart rate was reduced. A previous attempt at heart rate control, in the same patient, using a low dose beta-antagonist, precipitated hemodynamic collapse. The remarkable recovery of our patient suggests that acetylcholinesterase inhibitors may warrant further investigation in patients with severe sinus tachycardia.

Adult↗
PubMed1996

Local anesthetics as effectors of allosteric gating. Lidocaine effects on inactivation-deficient rat skeletal muscle Na channels.

Time- and voltage-dependent local anesthetic effects on sodium (Na) currents are generally interpreted using modulated receptor models that require formation of drug-associated nonconducting states with high affinity for the inactivated channel. The availability of inactivation-deficient Na channels has enabled us to test this traditional view of the drug-channel interaction. Rat skeletal muscle Na channels were mutated in the III-IV linker to disable fast inactivation (F1304Q: FQ). Lidocaine accelerated the decay of whole-cell FQ currents in Xenopus oocytes, reestablishing the wild-type phenotype; peak inward current at -20 mV was blocked with an IC50 of 513 microM, while plateau current was blocked with an IC50 of only 74 microM (P < 0.005 vs. peak). In single-channel experiments, mean open time was unaltered and unitary current was only reduced at higher drug concentrations, suggesting that open-channel block does not explain the effect of lidocaine on FQ plateau current. We considered a simple model in which lidocaine reduced the free energy for inactivation, causing altered coupling between activation and inactivation. This model readily simulated macroscopic Na current kinetics over a range of lidocaine concentrations. Traditional modulated receptor models which did not modify coupling between gating processes could not reproduce the effects of lidocaine with rate constants constrained by single-channel data. Our results support a reinterpretation of local anesthetic action whereby lidocaine functions as an allosteric effector to enhance Na channel inactivation.

Allosteric Regulation↗
PubMed1996

Coupling between fast and slow inactivation revealed by analysis of a point mutation (F1304Q) in mu 1 rat skeletal muscle sodium channels.

1. We sought to elucidate the mechanism of the defective inactivation that characterizes sodium channels containing mutations in the cytoplasmic loop between the third and fourth domains (the III-IV linker). Specifically, we measured whole-cell and single-channel currents through wild-type and F1304Q mutant mu 1 rat skeletal muscle Na+ channels expressed in Xenopus laevis oocytes. 2. In wild-type channels, inactivation is complete and the faster of two decay components predominates. In F1304Q, inactivation is incomplete; the slow decay component is larger in amplitude and slower than in wild-type. The fraction of non-inactivating current is substantial (37 +/- 2% of peak current at -20 mV) in F1304Q. 3. Cell-attached patch recordings confirmed the profound kinetic differences and indicated that permeation was not altered by the F1304Q mutation. The F1304Q phenotype must be conferred entirely by changes in gating properties and is not remedied by coexpression with the beta 1-subunit. 4. Recovery from inactivation of F1304Q channels is faster than for wild-type channels and three exponentials are required to describe recovery adequately following long (5 s) depolarizations. Thus, there are three inactivated states even in 'inactivation-deficient' F1304Q channels. 5. The steady-state voltage dependence of F1304Q inactivation is right-shifted by 26 +/- 2 mV. 6. A gating model incorporating three inactivated states, all directly accessible from multiple closed states or the open state, was constrained to fit wild-type and F1304Q inactivation (h infinitive) data and repriming data simultaneously. While it was necessary to alter the rate constants entering and exiting all three inactivated states, the model accounted for the F1304Q-induced rightward shift in steady-state inactivation without imposing voltage dependence on the inactivation rate constants. 7. We conclude that the F1304Q mutation in mu 1 sodium channels modifies several inactivation processes simultaneously. The fact that a single amino acid substitution profoundly alters both fast and slow inactivation indicates that these processes share physical determinants in Na+ channels.

Animals↗
PubMed1996

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.

Amino Acid Sequence↗