Re-Evaluation of the Action Potential Upstroke Velocity as a Measure of the Na+ Current in Cardiac Myocytes at Physiological Conditions

Background: The SCN5A encoded sodium current (INa) generates the action potential (AP) upstroke and is a major determinant of AP characteristics and AP propagation in cardiac myocytes. Unfortunately, in cardiac myocytes, investigation of kinetic properties of INa with near-physiological ion concentrations and temperature is technically challenging due to the large amplitude and rapidly activating nature of INa, which may seriously hamper the quality of voltage control over the membrane. We hypothesized that the alternating voltage clamp-current clamp (VC/CC) technique might provide an alternative to traditional voltage clamp (VC) technique for the determination of INa properties under physiological conditions. Principal Findings: We studied INa under close-to-physiological conditions by VC technique in SCN5A cDNA-transfected HEK cells or by alternating VC/CC technique in both SCN5A cDNA-transfected HEK cells and rabbit left ventricular myocytes. In these experiments, peak INa during a depolarizing VC step or maximal upstroke velocity, dV/dtmax, during VC/CC served as an indicator of available INa. In HEK cells, biophysical properties of INa, including current density, voltage dependent (in)activation, development of inactivation, and recovery from inactivation, were highly similar in VC and VC/CC experiments. As an application of the VC/CC technique we studied INa in left ventricular myocytes isolated from control or failing rabbit hearts

Sodium current inhibition by nanosecond pulsed electric field (nsPEF)--fact or artifact?

In two recent publications in Bioelectromagnetics it has been demonstrated that the voltage-gated sodium current (I(Na)) is inhibited in response to a nanosecond pulsed electric field (nsPEF). At the same time, there was an increase in a non-inactivating "leak" current (I(leak)), which was attributed to the formation of nanoelectropores or larger pores in the plasma membrane. We demonstrate that the increase in I(leak), in combination with a residual series resistance, leads to an error in the holding potential in the patch clamp experiments and an unanticipated inactivation of the sodium channels. We conclude that the observed inhibition of I(Na) may be largely, if not fully, artifactua

Pacemaker activity of the human sinoatrial node: Role of the hyperpolarization-activated current, I-f

The mechanism of primary, spontaneous cardiac pacemaker activity of the sinoatrial node (SAN) has extensively been studied in several animal species, but is virtually unexplored in man. Understanding the mechanisms of human SAN pacemaker activity is important for developing new therapeutic approaches for controlling the heart rate in the sick sinus syndrome and in diseased myocardium. Here we review the functional role of the hyperpolarization-activated 'funny' current, I-f, in human SAN pacemaker activity. Despite the many animal studies performed over the years, the contribution of I-f to pacemaker activity is still controversial and not fully established. However, recent clinical data on mutations in the I-f encoding HCN4 gene, which is thought to be the most abundant isoform of the HCN gene family in SAN, suggest a functional role of I-f in human pacemaker activity. These clinical findings are supported by recent experimental data from single isolated human SAN cells that provide direct evidence that I-f contributes to human SAN pacemaker activity. Therefore, controlling heart rate in clinical practice via I-f blockers offers a valuable approach to lowering heart rate and provides an attractive alternative to conventional treatment for a wide range of patients with confirmed stable angina, while upregulation or artificial expression of I-f may relieve disease-causing bradycardias. (C) 2008 Elsevier Ireland Ltd. All rights reserve

Beta-Adrenergic Modulation of Heart Rate:Contribution of the Slow Delayed Rectifier K+ Current (IKs)

To assess the role of the slow delayed rectifier potassium current (IKs) in the β-adrenergic modulation of heart rate, we experimentally determined the effect of β adrenergic stimulation on IKs and used the thus obtained data in computer simulations of SA nodal pacemaker activity, employing the mathematical model of a primary rabbit SA node pacemaker cell by Kurata and coworkers. Incorporation of our experimental findings into the SA nodal cell model resulted in a 12 ms decrease in cycle length. This decrease in cycle length is similar to the 13 ms decrease observed upon incorporation of our experimental data on the effect of β-adrenergic stimulation on the hyperpolarization-activated funny current' (If), also known as 'pacemaker current'. We conclude that IKs is an important contributor to the β-adrenergic modulation of heart rat

Effects of heart failure on brain-type Na+ channels in rabbit ventricular myocytes: Reply

La conférence annuelle de la British Sociological Association (BSA) aura lieu du 7 au 9 avril 2010 à Glasgow Caledonian University. Le thème de la conférence est inégalités et justice sociale (Inequalities & Social Justice). L'appel à contribution est ouvert

Effects of heart failure on brain-type Na+ channels in rabbit ventricular myocytes: Reply

AIMS: Brain-type alpha-subunit isoforms of the Na(+) channel are present in various cardiac tissue types and may control pacemaker activity and excitation-contraction coupling. Heart failure (HF) alters pacemaker activity and excitation-contraction coupling. Here, we studied whether HF alters brain-type Na(+) channel properties. METHODS AND RESULTS: HF was induced in rabbits by volume/pressure overload. Na(+) currents of ventricular myocytes were recorded in the cell-attached mode of the patch-clamp technique using macropatches. Macropatch recordings were conducted from the middle portions of myocytes or from intercalated disc regions between cell pairs. Both areas exhibited a fast activating and inactivating current, 8.5 times larger in intercalated disc regions. Tetrodotoxin (TTX) (50 nM) did not block currents in the intercalated disc regions, but did block in the middle portions, indicating that the latter currents were TTX-sensitive brain-type Na(+) currents. Macropatch recordings from these regions were used to study the effects of HF on brain-type Na(+) current. Neither current density nor gating properties (activation, inactivation, recovery from inactivation, slow inactivation) differed between CTR and HF. CONCLUSION: The density and gating properties of brain-type Na(+) current are not altered in our HF model. In the volume/pressure-overload rabbit model of HF, the role of brain-type Na(+) current in HF-induced changes in excitation-contraction coupling is limite

Dietary fish oil reduces the occurrence of early afterdepolarizations in pig ventricular myocytes

Fish oil reduces sudden cardiac death in post myocardial infarction patients. Life-threatening arrhythmias in heart failure are associated with repolarization abnormalities leading to EAD(1) formation. We examined the effects of incorporated fish oil omega 3-PUFAs2 on EAD formation in pig myocytes. Pigs were fed a diet rich in fish oil or sunflower oil (control) for 8 weeks. Myocytes were isolated by enzymatic dissociation and patch-clamped. Susceptibility to EAD formation was tested using E4031 (5 mu M), a blocker of I-Kr. The fish oil diet in pigs resulted in increased incorporation of omega 3-PUFAs in the sarcolemma of the myocytes compared to the control diet and caused a reduced occurrence of E4031-induced EADs in pig myocytes. A shorter action potential, a reduced action potential prolongation in response to E-4031 and a reduced reactivation of I-Ca,I-L by omega 3-PUFAs may explain the observed reduction in EADs. A diet rich in fish oil protects against EAD formation. (c) 2006 Elsevier Inc. All rights reserved