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Biomedical subjects

D M Roden

Publications and source records attributed to D M Roden.

At least 73 records · Page 4Linked to original sources
PubMed1997

Rapid inactivation determines the rectification and [K+]o dependence of the rapid component of the delayed rectifier K+ current in cardiac cells.

Two characteristic features of the rapid component of the cardiac delayed rectifier current (IKr) are prominent inward rectification and an unexpected reduction in activating current with decreased [K+]o. Similar features are observed with heterologous expression of HERG, the gene thought to encode the channel carrying IKr, moreover, recent studies indicate that the mechanism underlying rectification of HERG current is the inactivation that channels rapidly undergo during depolarizing pulses. The present studies were designed to determine the mechanism of IKr rectification and [K+]o sensitivity in the mouse atrial myocyte cell line, AT-1 cells. Reducing [Mg2+]i to 0, which reverses inward rectification of some K+ channels, did not alter IKr current-voltage relationships, although it did decrease sensitivity to the IKr blockers dofetilide and quinidine 2- to 5-fold. To determine the presence and extent of fast inactivation of IKr in AT-1 cells, a brief hyperpolarizing pulse (20 ms to -120 mV) was applied during long depolarizations. Immediately after this pulse, a very large outward current that decayed rapidly to the previous activating current baseline was observed. This outward current component was blocked by the IKr-specific inhibitor dofetilide, indicating that it represented recovery from fast inactivation during the hyperpolarizing step, with fast reinactivation during the return to depolarized potential. With removal of inactivation using this approach, current-voltage relationships for IKr ([K+]o, 1 to 20 mmol/L) were linar and reversed close to the predicted Nernst potential for K+. In addition, decreased [K+]o decreased the time constants for open-->inactivated and inactivated-->open transitions. Thus, in these cardiac myocytes, as with heterologously expressed HERG, IKr undergoes fast inactivation that determines its characteristic inward rectification. These studies demonstrate that the mechanism underlying decreased activating current observed at low [K+]o is more extensive fast inactivation.

Animals↗
PubMed1997

Inhibition of cardiac potassium currents by the vesnarinone analog OPC-18790: comparison with quinidine and dofetilide.

OPC-18790 is a vesnarinone analog currently in clinical trials for treatment of heart failure. In vitro studies have shown that, in addition to its positive inotropic actions, OPC-18790 prolongs cardiac action potentials. Therefore, in this study, the effects of OPC-18790 on cardiac potassium currents were compared with those we previously observed for the blockers quinidine and dofetilide in two test systems, i.e., L-cells stably transfected with mammalian cardiac potassium channel clones (Kv1.4, Kv1.5 and Kv2.1) and mouse AT-1 cells, in which the rapidly inactivating component of the cardiac delayed rectifier (I(Kr)) is the major repolarizing current. In L-cells, 10 to 100 microM OPC-18790 reduced Kv1.4, Kv1.5 and Kv2.1 currents by <30%, whereas quinidine was a more potent blocker (EC50 < 10 microM) and the I(Kr)-specific blocker dofetilide was without effect. In contrast, in AT-1 cells, OPC-18790 blocked I(Kr) with an EC50 (0.96 +/- 0.12 microM, n = 10) similar to that of quinidine (0.9 +/- 0.2 microM). For both drugs, block was voltage dependent, increasing at positive potentials. OPC-18790 and quinidine showed no frequency dependence, implying block of resting channels and/or very rapid block of open channels; this is in contrast to dofetilide, which displayed slow onset kinetics of block. Thus, we conclude that, 1) unlike quinidine, OPC-18790 does not significantly inhibit currents obtained by expression of the cardiac potassium channel clones Kv1.4, Kv1.5 and Kv2.1; 2) like quinidine and dofetilide, OPC-18790 blocks I(Kr) in AT-1 cells, but the kinetics of block onset more closely resemble those of quinidine than dofetilide; and 3) block of I(Kr) appears to be an important mechanism underlying the action potential-prolonging properties of OPC-18790.

Animals↗
PubMed1996

Multiple mechanisms in the long-QT syndrome. Current knowledge, gaps, and future directions. The SADS Foundation Task Force on LQTS.

The congenital long-QT syndrome (LQTS) is characterized by prolonged QT intervals, QT interval lability, and polymorphic ventricular tachycardia. The manifestations of the disease vary, with a high incidence of sudden death in some affected families but not in others. Mutations causing LQTS have been identified in three genes, each encoding a cardiac ion channel. In families linked to chromosome 3, mutations in SCN5A, the gene encoding the human cardiac sodium channel, cause the disease, Mutations in the human ether-à-go-go-related gene (HERG), which encodes a delayed-rectifier potassium channel, cause the disease in families linked to chromosome 7. Among affected individuals in families linked to chromosome 11, mutations have been identified in KVLQT1, a newly cloned gene that appears to encode a potassium channel. The SCN5A mutations result in defective sodium channel inactivation, whereas HERG mutations result in decreased outward potassium current. Either mutation would decrease net outward current during repolarization and would thereby account for prolonged QT intervals on the surface ECG. Preliminary data suggest that the clinical presentation in LQTS may be determined in part by the gene affected and possibly even by the specific mutation. The identification of disease genes in LQTS not only represents a major milestone in understanding the mechanisms underlying this disease but also presents new opportunities for combined research at the molecular, cellular, and clinical levels to understand issues such as adrenergic regulation of cardiac electrophysiology and mechanisms of susceptibility to arrhythmias in LQTS and other settings.

Cardiology↗
PubMed1996

Ionic mechanisms for prolongation of refractoriness and their proarrhythmic and antiarrhythmic correlates.

Drugs that prolong cardiac refractoriness exert antiarrhythmic effects, probably by reducing dispersion of refractoriness and thereby reducing the likelihood of reentrant excitation. This electrophysiologic effect can be achieved in fast-response tissues by sodium channel block or by action potential prolongation; drugs with either attribute can exert antiarrhythmic effects. However, both types of drugs can also cause proarrhythmic effects. For sodium channel blockers, proarrhythmic actions can be attributed to conduction slowing and include increased frequency of episodes of ventricular tachycardia as well as slowing of atrial flutter with 1:1 atrioventricular conduction and increases in ventricular rate. In addition, sodium channel block has been implicated as the mechanism underlying increased mortality with sodium channel blockers in the Cardiac Arrhythmia Suppression Trial (CAST); some data suggest that intercurrent ischemia increases this risk. For drugs that prolong action potentials, torsades de pointes is the most common proarrhythmic syndrome, occurring most often with underlying bradyarrhythmias and hypokalemia. The mechanism(s) underlying normal refractoriness and its modulation by antiarrhythmic drugs, as well as the mechanism(s) underlying these proarrhythmic syndromes, are discussed.

Action Potentials↗
PubMed1996

Epinephrine-induced changes in serum potassium and cardiac repolarization and effects of pretreatment with propranolol and diltiazem.

Although increases in serum epinephrine are known to cause hypokalemia, the epinephrine dosages and concentrations at which this effect occurs, and the electrocardiographic consequences, have not been evaluated. Because epinephrine infusion is now being used to provoke arrhythmias in some patients, we have determined the physiologic effects of a range of dosages of epinephrine. The effects of pretreatment with propranolol and diltiazem on these indexes of epinephrine effect were also evaluated. Epinephrine dose ranging started at 10 ng/kg/min, with doubling of the dose every 10 minutes until a predetermined end point was reached. At the end of each dosage level, serum electrolytes, catecholamines, and an electrocardiogram were recorded. Whereas even the lowest dosage of epinephrine significantly increased heart rate, serum glucose levels increased and serum potassium decreased only when dosages of 160 to 320 ng/kg/min were administered. Plasma concentrations of epinephrine at these dosages were mean +/- SD 1,328 +/- 902 pg/ml, comparable to those observed in these subjects during maximal exercise (1,003 +/- 527 pg/ml). The major electrocardiographic effect of epinephrine infusion was a dose-related increase in QTc, but pretreatment with propranolol blunted this effect and tended to shorten QTc. At an epinephrine dose of 40 ng/kg/min, QTc prolongation persisted and was inhibited by diltiazem. These data suggest that the major electrocardiographic effect of epinephrine infusion is mediated by increased calcium current. At dosages > 80 ng/kg/min, plasma epinephrine concentrations are comparable to those observed with severe stress, and hypokalemia is common. The use of epinephrine as an electrophysiologic provoker at dosages > 80 ng/kg/min results in both a direct effect, as well as an indirect effect due to hypokalemia.

Adrenergic beta-Antagonists↗
PubMed1996

Loss of quinidine gluconate injection in a polyvinyl chloride infusion system.

The effect of a polyvinyl chloride (PVC) i.v. administration system on the availability of quinidine gluconate was studied. Quinidine gluconate diluted in 5% dextrose injection was administered intravenously to five healthy volunteers via conventional PVC infusion sets, and the subjects received oral quinidine sulfate two days later. The mean +/- S.D. oral bioavailability of quinidine was, unexpectedly, greater than 100% (147 +/- 44%). To test the possibility that this occurred because of reduced delivery of i.v. quinidine, the percentage of drug delivered via two systems was evaluated in simulation studies, one involving a conventional PVC administration set and the other a glass syringe attached to shorter PVC tubing and a winged i.v. catheter. Spectrophotometric analysis revealed a 5-7% reduction in absorbance associated with loss of quinidine in the PVC infusion bag and a further 34-38% reduction in absorbance attributable to quinidine loss in the PVC tubing. However, with the winged i.v. catheter system the loss was reduced to less than 3%. More than 40% of a dose of quinidine gluconate was lost when the drug was administered with a conventional PVC i.v. administration set. Drug loss was reduced by using a winged i.v. catheter and shorter tubing.

Administration, Oral↗
PubMed1996

Extracellular potassium modulation of drug block of IKr. Implications for torsade de pointes and reverse use-dependence.

BACKGROUND: Torsade de pointes often occurs with underlying hypokalemia and bradycardia. A common effect of many drugs producing torsade de pointes is block of the rapidly activating component of the cardiac delayed rectifier (IKr). In this study, we evaluated the effect of changing extracellular potassium ([K+]o) on IKr block by the nonspecific agent quinidine and by the specific IKr blocker dofetilide. METHODS AND RESULTS: IKr was measured in AT-1 cells, where contaminating outward currents are absent. The drug concentration producing 50% inhibition of IKr tails (IC50) was strikingly [K+]o-dependent. Elevating [K+]o from 1 to 8 mmol/L increased the IC50 for dofetilide block from 2.7 +/- 0.9 to 79 +/- 32 nmol/L and for quinidine block from 0.4 +/- 0.1 to 3.8 +/- 1.2 mumol/L. CONCLUSIONS: (1) The increase in drug block with low [K+]o provides a mechanism to explain the link between hypokalemia and torsade de pointes. (2) Elevations in [K+]o occur with myocardial ischemia and with rapid pacing. Possible consequences of blunted drug block with high [K+]o include loss of drug efficacy with ischemia and with rapid pacing; the latter may contribute to "reverse use-dependent" action potential prolongation. Extracellular potassium is a critical determinant of drug block of IKr, with substantial clinical implications.

Animals↗
PubMed1996

Suprachoroidal effusion following argon laser trabeculoplasty.

PURPOSE: This is first report of suprachoroidal effusion occurring subsequent to argon laser trabeculoplasty (ALT). METHODS: Review of the records of the patients in question. RESULTS: A 77-year-old woman with bilateral pseudophakia and primary open-angle glaucoma was treated with ALT when her visual fields deteriorated despite topical timolol therapy. Although ALT was initially performed without complication in one eye, treatment of the other eye led to a choroidal detachment. This was associated with temporary reduction in visual acuity, shallowing of the anterior chamber and hypotony. CONCLUSION: Suprachoroidal effusion appears to be another complication of ALT. In the reported case, this application and its effects were temporary and resolved with conservative management.

Aged↗
PubMed1996

The cardiac ion channels: relevance to management of arrhythmias.

The electrical activity of cardiac tissue is determined by the highly regulated flow of ions across the cell membrane during the cardiac action potential. Ion channels are pore-forming proteins through which these electric currents flow. In this review, the ion currents that underlie the action potential are first described. Then, the way in which expression of individual ion-channel genes results in such ion currents is discussed. Finally, the concept that arrhythmias may be due to abnormalities of structure, function, or number of ion channels, or the way in which they respond to abnormalities in their environment (such as acute ischemia), is reviewed. Further understanding of the molecular mechanisms underlying normal and abnormal cardiac electrophysiologic behavior should allow the development of safer and more effective antiarrhythmic interventions.

Animals↗
PubMed1996

Regulation of sodium current development in cultured atrial tumor myocytes (AT-1 cells).

AT-1 cells, derived from atrial tumors in transgenic mice, have many features similar to cardiac myocytes. However, their sodium current (INa) has not been evaluated on detail. In this study, two INa phenotypes were identified in AT-1 cells: one at 3 days in culture and the other at 14 days. INa was smaller at 3 days than at 14 days (12 +/- 2 vs. 37 +/- 5 pA/pF) and activated more slowly (time to peak INa at -30 mV: 9.8 +/- 0.4 vs. 1.4 +/- 0.1 ms). Inactivation at 14 days was faster and shifted 16 mV negative compared with that at 3 days. Acute protein kinase A or C stimulation in 3-day cells did not alter INa gating. However, the 14-day phenotype was observed in 3-day cells when the adenosine 3',5'-cyclic monophosphate analogue 8-(4-chlorophenylthio)-adenosine 3',5'-cyclic monophosphate, the phorbol ester phorbol 12-myristate 13-acetate, or okadaic acid was added to the culture medium from days 0 to 3. Conversely, adenosine 3',5'-cyclic monophosphothioate triethylamine, the protein kinase A inhibitor, prevented the normal development of the 14-day phenotype if the exposure was early and reverted the phenotype to that at 3 days if the exposure was later. Thus, in AT-1 cells, as in other mammalian cardiac myocytes, INa undergoes a maturation process that is dependent on intracellular phosphorylation processes. The data raise the possibility that an important consequence of altered intracellular signaling in disease is lability in INa amplitude or gating.

Animals↗
PubMed1996

Is there a need for new antiarrhythmic drugs?

Currently available antiarrhythmic drugs were developed at a time when the basic mechanisms underlying most arrhythmias were conjectural at best. Indeed, the molecular targets upon which drugs act to prevent (or to exacerbate) arrhythmias are only now being defined. Thus, recent advances in the treatment of patients with arrhythmias have emphasized non-pharmacologic approaches, which use new information on the pathophysiology of specific arrhythmias to deliver targeted therapies. For some arrhythmias, such as atrial fibrillation, it seems likely that drug therapy will remain an important part of treatment. With increasing cellular and molecular understanding of the determinants of normal and abnormal cardiac electrogenesis, it should be possible to develop effective and safer drugs for the treatment of cardiac arrhythmias.

Amiodarone↗
PubMed1995

Ibutilide, a methanesulfonanilide antiarrhythmic, is a potent blocker of the rapidly activating delayed rectifier K+ current (IKr) in AT-1 cells. Concentration-, time-, voltage-, and use-dependent effects.

BACKGROUND: Ibutilide is an action potential-prolonging antiarrhythmic currently in clinical trials. The drug shares structural similarities with E-4031 and dofetilide, specific blockers of the rapidly activating delayed rectifier K+ current (IKr). However, previous in vitro studies in guinea pig myocytes have indicated that ibutilide does not block IKr but rather increases a slow inward sodium current. METHODS AND RESULTS: In this study, we compared the effects of ibutilide with those of dofetilide on outward current in mouse atrial tumor myocytes (AT-1 cells), a preparation in which, unlike guinea pig, a typical IKr is the major delayed rectifier and can be readily recorded in isolation from other currents. In AT-1 cells, ibutilide and dofetilide were both potent IKr blockers, with EC50 values of 20 (n = 12) and 12 (n = 8) nmol/L, respectively, at +20 mV. The time and voltage dependence of IKr inhibition by the two compounds were virtually identical. The following characteristics were most consistent with open channel block: (1) block increased with depolarizing pulses; (2) block increased with longer pulses; (3) currents deactivated more slowly in the presence of drug, resulting in a "crossover" typical of open channel block; and (4) with repetitive pulsing after drug wash-in, use-dependent block was observed. CONCLUSIONS: These data suggest that the clinical actions of ibutilide are mediated at least in part by block of IKr; an effect on inward currents is not excluded. AT-1 cells are a useful model system for the study of drug block of this important repolarizing current.

Animals↗
PubMed1995

Mechanism of block of a human cardiac potassium channel by terfenadine racemate and enantiomers.

1. The cardiac toxicity of racemic terfenadine (marked QT prolongation and polymorphic ventricular arrhythmias) is probably due to potassium channel blockade. To test whether one of its enantiomers would be a less efficient potassium channel blocker, we compared the mechanism of action of the racemate with that of the individual enantiomers. 2. We synthesized the individual enantiomers of terfenadine and examined under whole cell voltage-clamp conditions the mechanism of action of the racemate, both enantiomers and a major metabolite on a cloned human cardiac potassium channel, hKv1.5. This delayed rectifier is sensitive to quinidine, clofilium and other 'class III' antiarrhythmic drugs at clinically relevant concentrations. 3. Upon depolarization, racemic terfenadine and its enantiomers induced a fast decline of hKv1.5 current towards a reduced steady state current level. During subsequent repolarization the tail currents deactivated more slowly than the control, resulting in a 'crossover' phenomenon. 4. The voltage-dependence of block was biphasic with a steep increase in block over the voltage range of channel opening (-30 to 0 mV), and a more shallow phase positive to 0 mV (where the channel is fully open). The latter was consistent with a binding reaction sensing 21% of the transmembrane electrical field (with reference to the cell interior). 5. The EC50 for hKv1.5 block by racemic terfenadine was 0.88 microM, while the values for R- and S-terfenadine were 1.19 microM and 1.16 microM, respectively. In contrast, the acid metabolite reduced hKv1.5 current by only 5% at a concentration of 50 microM. 6. These findings suggest that terfenadine blocks the hKvl.5 channel after it opens by entering into the internal mouth of the channel. We have previously shown that quinidine blocks hKvl.5 in a similar manner but with an apparent affinity of ~6 micro M. Thus, terfenadine and its enantiomers are approximately equipotent open state blockers of this human K+ channel and about 6 times more potent than quinidine. The similar state-, time-, and voltage-dependence of hKvl.5 block by both enantiomers also indicates that the chiral centre does not significantly constrain the orientation of critical binding determinants of terfenadine with respect to the receptor site.

Animals↗
PubMed1995

Recent advances in understanding the molecular mechanisms of the long QT syndrome.

Competing theories to explain the congenital long QT syndrome have included an imbalance in sympathetic innervation of the heart or a defect in repolarizing ion currents. Recent studies have identified at least four chromosomal loci at which mutations cause the congenital long QT syndrome in different families. The specific genes mutated in affected individuals have been identified at two of these loci, and both encode cardiac ion channels. The affected genes are SCN5A, the cardiac sodium channel gene, and HERG, whose protein product likely underlies IKr, the rapidly activating delayed rectifier. Thus, currently available evidence indicates that the congenital long QT syndrome is a primary disease of cardiac ion channels. Abnormalities in either inward or outward currents can cause the disease. Ongoing studies are evaluating the function of the mutant ion channels and the relationship between individual mutations and the clinical manifestations of the syndrome.

Animals↗
PubMed1995

A randomized, double-blind, placebo-controlled, dose-ranging study of dofetilide in patients with inducible sustained ventricular tachyarrhythmias.

INTRODUCTION: Dofetilide is a new antiarrhythmic agent with potent IK blocking properties in vitro. We developed a dose-ranging, placebo-controlled study design to define the range of effective doses and to evaluate the clinical electrophysiology of intravenous dofetilide in patients in whom sustained ventricular tachycardia or fibrillation was reproducibly inducible at baseline electrophysiologic testing. METHODS AND RESULTS: The initial four patients received low doses that were increased in subsequent groups of four if adverse effects were absent. In each group of four patients, one patient was randomly assigned to placebo (double blind). Twenty-four patients were studied at six incremental loading and maintenance infusion regimens. Dofetilide (0.1 to 8.0 ng/mL) produced concentration-related increases in the % delta of QT (r = 0.79, P < 0.001), QTc (r = 0.60, P = 0.02), RR (r = 0.62, P < 0.02), and right ventricular effective refractory period (cycle length 600 msec; r = 0.68, P = 0.04). Placebo produced no changes in any of these measurements. Sustained ventricular tachycardia or ventricular fibrillation was no longer inducible in 1 of 6 patients receiving placebo and 8 of 18 receiving dofetilide (4 to 13 sec nonsustained ventricular tachycardia was induced in 4 of these 8). One patient developed torsades de pointes at a high concentration (5.3 ng/mL). CONCLUSIONS: We conclude that: (1) dofetilide produces concentration-related IK blocking effects in patients; (2) an incremental dose-ranging study design aids in identifying the range of doses demonstrating electrophysiologic effects and efficacy; (3) a concomitant placebo group provides important data to assess reproducibility of results over time; and (4) further studies of dofetilide's efficacy and toxicity should be conducted.

Adolescent↗
PubMed1995

Anti-minK antisense decreases the amplitude of the rapidly activating cardiac delayed rectifier K+ current.

The rapidly and slowly activating delayed rectifier K+ currents (IKr and IKs, respectively), which have different physiological properties have been identified in cardiac cells from several species, including humans. Although expression of the minimal K+ channel protein (minK) cDNA in some systems results in a current resembling IKs, the role of this gene product in channel function remains controversial. In atrial tumor myocytes (AT-1 cells), no IKs is recorded, but minK mRNA is detected, raising the possibility that expression of the minK gene serves an as-yet-unidentified function. In these experiments, AT-1 cells were exposed to antisense oligonucleotides targeting the 5' translation start site of the minK cDNA cloned from an AT-1 library. Cell size, IKr, and L-type and T-type Ca2+ currents were measured 24 to 48 hours after exposure and compared with data in cells exposed to the corresponding sense oligonucleotide or grown in medium only. Antisense oligonucleotide significantly reduced IKr compared with sense and medium-only control cells in 0 of 2 experiments (n = 3 to 6 cells per treatment in each experiment) at 50 nmol/L, 1 of 2 at 250 nmol/L, 6 of 6 at 1000 nmol/L, and 2 of 2 at 10,000 nmol/L. At 1000 nmol/L, maximum tail current in antisense-exposed cells was 2.5 +/- 0.1 pA/pF (mean +/- SEM, n = 28, 6 separate experiment), 6.6 +/- 0.4 pA/pF in sense-exposed cells (n = 27), 5.4 +/- 0.6 pA/pF in medium-only cells (n = 21), and 5.8 +/- 0.7 pA/pF in cells exposed to a random oligonucleotide (n = 9).(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗