Contribution of potassium accumulation in narrow extracellular spaces to the genesis of nicorandil-induced large inward tail current in guinea-pig ventricular cells

Resolution of hyposmotic stress in isolated mouse ventricular myocytes causes sealing of t-tubules

It has recently been shown that various stress-inducing manipulations in isolated ventricular myocytes may lead to significant remodelling of t-tubules. Osmotic stress is one of the most common complications in various experimental and clinical settings. This study was therefore designed to determine the effects of a physiologically relevant type of osmotic stress, hyposmotic challenge, to the integrity of the t-tubular system in mouse ventricular myocytes using the following two approaches: (1) electrophysiological measurements of membrane capacitance and inward rectifier (IK1) tail currents originating from K(+) accumulation in t-tubules; and (2) confocal microscopy of fluorescent dextrans trapped in sealed t-tubules. Importantly, we found that removal of '0.6 Na' (60% NaCl) hyposmotic solution, but not its application to myocytes, led to a ∼27% reduction in membrane capacitance, a ∼2.5-fold reduction in the amplitude of the IK1 tail current and a ∼2-fold reduction in the so-called IK1 'inactivation' (due to depletion of t-tubular K(+)) at negative membrane potentials; all these data were consistent with significant detubulation. Confocal imaging experiments also demonstrated that extracellularly applied dextrans become trapped in sealed t-tubules only upon removal of hyposmotic solutions, i.e. during the shrinking phase, but not during the initial swelling period. In light of these data, relevant previous studies, including those on excitation-contraction coupling phenomena during hyposmotic stress, may need to be reinterpreted, and the experimental design of future experiments should take into account the novel findings.

Modelling the cardiac transverse-axial tubular system

The transverse-axial tubular system (TATS) of cardiac ventricular myocytes is a complex network of tubules that arises as invaginations of the surface membrane; it appears to form a specialised region of cell membrane that is particularly important for excitation-contraction coupling. However, much remains unknown about the structure and role of the TATS. In this brief review we use experimental data and computer modelling to address the following key questions: (i) What fraction of the cell membrane is within the TATS? (ii) Is the composition of the TATS membrane the same as the surface membrane? (iii) How good is electrical coupling between the surface and TATS membranes? (iv) What fraction of each current is within the TATS? (v) How important is the complex structure of the TATS network? (vi) What is the effect of current inhomogeneity on lumenal ion concentrations? (vii) Does the TATS contribute to the functional changes observed in heart failure? Although there are many areas in which experimental evidence is lacking, computer models provide a method to assess and predict the possible function of the TATS; such models suggest that although the surface and TATS membranes are electrically well coupled, concentration of ion flux pathways within the TATS, coupled to restricted diffusion, may result in the ionic composition in the TATS lumen being different from that in the bulk extracellular space, and varying with activity and in pathological conditions.

A model of the guinea-pig ventricular cardiac myocyte incorporating a transverse-axial tubular system

A model of the guinea-pig cardiac ventricular myocyte has been developed that includes a representation of the transverse-axial tubular system (TATS), including heterogeneous distribution of ion flux pathways between the surface and tubular membranes. The model reproduces frequency-dependent changes of action potential shape and intracellular ion concentrations and can replicate experimental data showing ion diffusion between the tubular lumen and external solution in guinea-pig myocytes. The model is stable at rest and during activity and returns to rested state after perturbation. Theoretical analysis and model simulations show that, due to tight electrical coupling, tubular and surface membranes behave as a homogeneous whole during voltage and current clamp (maximum difference 0.9 mV at peak tubular INa of -38 nA). However, during action potentials, restricted diffusion and ionic currents in TATS cause depletion of tubular Ca2+ and accumulation of tubular K+ (up to -19.8% and +3.4%, respectively, of bulk extracellular values, at 6 Hz). These changes, in turn, decrease ion fluxes across the TATS membrane and decrease sarcoplasmic reticulum (SR) Ca2+ load. Thus, the TATS plays a potentially important role in modulating the function of guinea-pig ventricular myocyte in physiological conditions.

Galphai3 primes the G protein-activated K+ channels for activation by coexpressed Gbetagamma in intact Xenopus oocytes

G protein-activated K+ channels (GIRK) mediate postsynaptic inhibitory effects of neurotransmitters in the atrium and in the brain by coupling to G protein-coupled receptors (GPCRs). In neurotransmitter-dependent GIRK signalling, Gbetagamma is released from the heterotrimeric Galphabetagamma complex upon GPCR activation, activating the channel and attenuating its rectification. Now it becomes clear that Galpha is more than a mere Gbetagamma donor. We have proposed that Galphai3-GDP regulates GIRK gating, keeping its basal activity low but priming (predisposing) the channel for activation by agonist in intact cells, and by Gbetagamma in excised patches. Here we have further investigated GIRK priming by Galphai3 using a model in which the channel was activated by coexpression of Gbetagamma, and the currents were measured in intact Xenopus oocytes using the two-electrode voltage clamp technique. This method enables the bypass of GPCR activation during examination of the regulation of the channel in intact cells. Using this method, we further characterize the priming phenomenon. We tested and excluded the possibility that our estimates of priming are affected by artifacts caused by series resistance or large K+ fluxes. We demonstrate that both Galphai3 and membrane-attached Gbetagamma scavenger protein, m-phosducin, reduce the basal channel activity. However, Galphai3 allows robust channel activation by coexpressed Gbetagamma, in sharp contrast to m-phosducin, which causes a substantial reduction in the total Gbetagamma-induced current. Furthermore, Galphai3 also does not impair the Gbetagamma-dependent attenuation of the channel rectification, in contrast to m-phosducin, which prevents this Gbetagamma-induced modulation. The Galphai3-induced enhancement of direct activation of GIRK by Gbetagamma, demonstrated here for the first time in intact cells, strongly supports the hypothesis that Galphai regulates GIRK gating under physiological conditions.

K+ currents activated by depolarization in cardiac fibroblasts

K(+) currents expressed in freshly dispersed rat ventricular fibroblasts have been studied using whole-cell patch-clamp recordings. Depolarizing voltage steps from a holding potential of -90 mV activated time- and voltage-dependent outward currents at membrane potentials positive to approximately -30 mV. The relatively slow activation kinetics exhibited strong dependence on the membrane potential. Selected changes in extracellular K(+) concentration ([K(+)](o)) revealed that the reversal potentials of the tail currents changed as expected for a K(+) equilibrium potential. The activation and inactivation kinetics of this K(+) current, as well as its recovery from inactivation, were well-fitted by single exponential functions. The steady-state inactivation was well described by a Boltzmann function with a half-maximal inactivation potential (V(0.5)) of -24 mV. Increasing [K(+)](o) (from 5 to 100 mM) shifted this V(0.5) in the hyperpolarizing direction by -11 mV. Inactivation was slowed by increasing [K(+)](o) to 100 mM, and the rate of recovery from inactivation was decreased after increasing [K(+)](o). Block of this K(+) current by extracellular tetraethylammonium also slowed inactivation. These [K(+)](o)-induced changes and tetraethylammonium effects suggest an important role for a C-type inactivation mechanism. This K(+) current was sensitive to dendrotoxin-I (100 nM) and rTityustoxin Kalpha (50 nM).

Analysis of K+ accumulation reveals privileged extracellular region in the vicinity of glial cells in situ

Astrocytes and oligodendrocytes in rat and mouse spinal cord slices, characterized by passive membrane currents during de- and hyperpolarizing stimulation pulses, express a high resting K+ conductance. In contrast to the case for astrocytes, a depolarizing prepulse in oligodendrocytes produces a significant shift of reversal potential (Vrev) to positive values, arising from the larger accumulation of K+ in the vicinity of the oligodendrocyte membrane. As a result, oligodendrocytes express large tail currents (Itail) after a depolarizing prepulse due to the shift of K+ into the cell. In the present study, we used a mathematical model to calculate the volume of the extracellular space (ECS) in the vicinity of astrocytes and oligodendrocytes (ESVv), defined as the volume available for K+ accumulation during membrane depolarization. A mathematical analysis of membrane currents revealed no differences between glial cells from mouse (n = 59) or rat (n = 60) spinal cord slices. We found that the Vrev of a cell after a depolarizing pulse increases with increasing Itail, expressed as the ratio of the integral inward current (Qin) after the depolarizing pulse to the total integral outward current (Qout) during the pulse. In astrocytes with small Itail and Vrev ranging from -50 to -70 mV, the Qin was only 3-19% of Qout, whereas, in oligodendrocytes with large Itail and Vrev between -20 and 0 mV, Qin/Qout was 30-75%. On the other hand, ESVv decreased with increasing values of Vrev. In astrocytes, ESVv ranged from 2 to 50 microm3, and, in oligodendrocytes, it ranged from 0.1 to 2.0 microm3. Cell swelling evoked by the application of hypotonic solution shifted Vrev to more positive values by 17.2 +/- 1.8 mV and was accompanied by a decrease in ESVv of 3.6 +/- 1.3 microm3. Our mathematical analysis reveals a 10-100 times smaller region of the extracellular space available for K+ accumulation during cell depolarization in the vicinity of oligodendrocytes than in the vicinity of astrocytes. The presence of such privileged regions around cells in the CNS may affect the accumulation and diffusion of other neuroactive substances and alter communication between cells in the CNS.

Voltage dependence of ATP-dependent K+ current in rat cardiac myocytes is affected by IK1 and IK(ACh)

In this study we have investigated the voltage dependence of ATP-dependent K+ current (I(K(ATP))) in atrial and ventricular myocytes from hearts of adult rats and in CHO cells expressing Kir6.2 and SUR2A. The current-voltage relation of 2,4-dinitrophenole (DNP) -induced I(K(ATP)) in atrial myocytes and expressed current in CHO cells was linear in a voltage range between 0 and -100 mV. In ventricular myocytes, the background current-voltage relation of which is dominated by a large constitutive inward rectifier (I(K1)), the slope conductance of I(K(ATP)) was reduced at membrane potentials negative to E(K) (around -50 mV), resulting in an outwardly rectifying I-V relation. Overexpression of Kir2.1 by adenoviral gene transfer, a subunit contributing to I(K1) channels, in atrial myocytes resulted in a large I(K1)-like background current. The I-V relation of I(K(ATP)) in these cells showed a reduced slope conductance negative to E(K) similar to ventricular myocytes. In atrial myocytes with an increased background inward-rectifier current through Kir3.1/Kir3.4 channels (I(K(ACh))), irreversibly activated by intracellular loading with GTP-gamma-S, the I-V relation of I(K(ATP)) showed a reduced slope negative to E(K), as in ventricular myocytes and atrial myocytes overexpressing Kir2.1. It is concluded that the voltage dependencies of membrane currents are not only dependent on the molecular composition of the charge-carrying channel complexes but can be affected by the activity of other ion channel species. We suggest that the interference between inward I(K(ATP)) and other inward rectifier currents in cardiac myocytes reflects steady-state changes in K+ driving force due to inward K+ current.

T-tubule localization of the inward-rectifier K(+) channel in mouse ventricular myocytes: a role in K(+) accumulation

1. The properties of the slow inward 'tail currents' (I(tail)) that followed depolarizing steps in voltage-clamped, isolated mouse ventricular myocytes were examined. Depolarizing steps that produced large outward K(+) currents in these myocytes were followed by a slowly decaying inward I(tail) on repolarization to the holding potential. These currents were produced only by depolarizations: inwardly rectifying K(+) currents, I(K1), produced by steps to potentials negative to the holding potential, were not followed by I(tail). 2. For depolarizations of equal duration, the magnitude of I(tail) increased as the magnitude of outward current at the end of the depolarizing step increased. The apparent reversal potential of I(tail) was dependent upon the duration of the depolarizing step, and the reversal potential shifted to more depolarized potentials as the duration of the depolarization was increased. 3. Removal of external Na(+) and Ca(2+) had no significant effect on the magnitude or time course of I(tail). BaCl(2) (0.25 mM), which had no effect on the magnitude of outward currents, abolished I(tail) and I(K1) simultaneously. 4. Accordingly, I(tail) in mouse ventricular myocytes probably results from K(+) accumulation in a restricted extracellular space such as the transverse tubule system (t-tubules). The efflux of K(+) into the t-tubules during outward currents produced by depolarization shifts the K(+) Nernst potential (E(K)) from its 'resting' value (close to -80 mV) to more depolarized potentials. This suggests that I(tail) is produced by I(K1) in the t-tubules and is inward because of the transiently elevated K(+) concentration and depolarized value of E(K) in the t-tubules. 5. Additional evidence for the localization of I(K1) channels in the t-tubules was provided by confocal microscopy using a specific antibody against Kir2.1 in mouse ventricular myocytes.

Localization of K(+) channels in the tubules of cardiomyocytes as suggested by the parallel decay of membrane capacitance, IK(1) and IK(ATP) during culture and by delayed IK(1) response to barium

Adult ventricular myocytes lose T-tubules over few days in culture, which causes the loss of about 60% of the cell membrane capacitance (Cm) (Mitcheson et al., 1996). In this study, we have measured, in whole-cell voltage-clamped rabbit right ventricular myocytes at 0, 1, 2 and 3-5 days of culture (nine to 20 myocytes at each age) in a defined Dulbecco's modified Eagle's medium, the value of Cm and the magnitudes of the background inward rectifier current (IK(1)) and of the 2,4-dinitrophenol-induced ATP-sensitive potassium current (IK(ATP)). Cm, IK(1) and IK(ATP) all had decreased significantly by 51, 83 and 88%, respectively after 4 days of culture. Analysis using a single exponential decay function of time gave time constants of, 2.6+/-0.2, 2.2+/-0.5 and 2.4+/-0.4 days, respectively. Linear regressions of IK(1) and IK(ATP) versus Cm had regression coefficients of 0.93 and 0. 98, respectively. These observations are consistent with a strong link of the decay of IK(1) and IK(ATP) currents to that of Cm. Furthermore, the time course of changes in IK(1) when an external blocker (100 microm BaCl(2)) was applied and washed by local perfusion (95% change in 50 ms) agrees with a model including a diffusion time constant of 300 ms. This value is consistent with the known kinetics of diffusion of divalent cations in the T-tubules. Taken together, these results could be explained by the localization of a major part of the IK(1) and IK(ATP) currents of ventricular cardiomyocytes in the T-tubules. As a consequence, transient accumulation of K(+) ions in cardiac T-tubules may take place and modulate excitation-contraction coupling.

Propafenone blocks ATP-sensitive K+ channels in rabbit atrial and ventricular cardiomyocytes

Propafenone, a class I antiarrhythmic agent, inhibits several membrane currents (I(Na), I(Ca), I(K), Ito), however, its effects on ATP-sensitive potassium current (I(K)ATP) of cardiac cells have not been tested. We evaluated the blocking effects of 0.1 to 100 microM propafenone applications at 35 degrees C on the whole-cell I(K)ATP as triggered by dinitrophenol (75 microM) in adult rabbit dissociated atrial and ventricular cardiomyocytes in comparison. The block of I(K)ATP by propafenone was dose-dependent, fully reversible and voltage-independent. The dose-response relation, as evaluated at 0 mV for atrial myocytes (ED50 = 1.26+/-0.17 microM, Hill number = 1.25+/-0.22) was significantly shifted to the left vs. that in ventricular myocytes (ED50 = 4.94+/-0.59 microM, Hill number = 1.22+/-0.14). It is concluded that propafenone blocks cardiac I(K)ATP at a single site with 4 times higher affinity for the drug in atrial myocytes. This block of cardiac I(K)ATP might play a role in the beneficial and adverse effects of the drug.

Examination of the transverse tubular system in living cardiac rat myocytes by 2-photon microscopy and digital image-processing techniques

The transverse tubular system (t-system) of cardiac muscle is a structure that allows rapid propagation of excitation into the cell interior. Using 2-photon molecular excitation microscopy and digital image-processing methods, we have obtained a comprehensive overview of the t-system of rat ventricular myocytes in living cells. We show that it is possible to quantify the morphology of the t-system in terms of average local tubule diameter, branching pattern, and local abundance of the t-system by immersing living myocytes in a dextran-linked fluorescein solution. Our data suggest that previous electron microscopic examinations of t-system structure have underestimated both the geometric complexity of the t-system morphology and the fraction of cell volume occupied by the t-system (3.6% in this species). About 40% of tubules occur between Z-lines, and the t-tubule diameter is 255+/-0.85 nm (mean+/-SEM). The t-tubules leave the outer surface of the cell in an approximately rectangular array; however, at some points junctions between the t-tubules and the surface membrane are missing. In view of the complexity of the t-system apparent from our images, we propose that the t-system be renamed the "sarcolemmal Z rete." The methods presented here are generally applicable to the quantification of the sarcolemmal Z rete and other structures within cells by fluorescence microscopy in a variety of cell types.

Effects of the ATP-sensitive K channel opener nicorandil on the QT interval and the effective refractory period in patients with congenital long QT syndrome. Investigator Group for K-Channel Openers and Arrhythmias

Congenital or idiopathic long QT syndrome is characterized by a frequently lethal ventricular arrhythmia called torsades de pointes (TdP) as well as a prolonged QT interval. The long QT interval related to an abnormal gene of the Na+ channel has been shown to be shortened by mexiletine. However, the action of K+ channel openers on the QT interval associated with abnormal genes of the K channel has yet to be studied. Seven patients of five families with long QT syndrome were included in this study, of whom six had syncope and six had documented TdP. Either long QT interval or sudden cardiac death had been observed in family members of all seven patients. At 1 to 3 weeks after admission, when TdP or frequent ventricular arrhythmia had subsided, nicorandil, an ATP-sensitive K channel opener, was administered orally at a dose of 15 mg/day in five patients and at 30 mg/day in the remaining two patients, and the effects were assessed on the third day after drug administration. In four patients, the effective refractory period was measured in the right ventricle before and after administration of K channel opener administration. The QT interval (QTc) prior to administration of the K channel opener was 0.60 +/- 0.09 ms (mean +/- SD) (0.61 +/- 0.10 second(1/2)), which was shortened to 0.54 +/- 0.05 ms (0.55 +/- 0.06 second(1/2)) on the third day of drug administration (P < .05 for both): 10.4 +/- 8.0% (8.6 +/- 5.5%). The QT interval at varying preceding R-R intervals on Holter electrocardiograms showed a shift toward the right as a result of the drug administration. The effective refractory period showed a significant prolongation, 256 +/- 26 ms versus 280 +/- 22 ms before and after drug administration, respectively (P <.05). Intravenous administration of nicorandil resulted in no significant change in heart rate or blood pressure, while QTc showed a tendency to shorten, but nonsignificantly (P = .08). However, a hump on the monophasic action potential was abolished, especially at the long preceding R-R interval induced by premature stimulation of the ventricle. It is concluded that nicorandil shortens the QT interval slightly when administered orally, whereas the effective refractory period shows a slight prolongation. The physiologic and clinical significance of these effects needs to be studied further.