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Cardiac t tubules undergo significant remodeling in various pathological and experimental conditions, which can be associated with mechanical or osmotic stress. In particular, it has been shown that removal of hyposmotic stress can lead to sealing of t tubules. However, the mechanisms underlying the sealing process remain essentially unknown. In this study we used dextran trapping assay to demonstrate that in adult mouse cardiomyocytes, t-tubular sealing can also be induced by hyperosmotic challenge and that both hypo- and hyperosmotic sealing display a clear threshold behavior requiring ≈100 mosmol/L minimal stress. Importantly, during both hypo- and hyperosmotic challenges, the sealing of t tubules occurs only during the shrinking phase. Analysis of the time course of t-tubular remodeling following removal of hyposmotic stress shows that t tubules become sealed essentially instantly, well before any significant reduction in cell size can be observed. Overall, the data support the hypothesis that the critical event in the process of t-tubular sealing during osmotic challenges is detachment (peeling) of the membrane from the underlying cytoskeleton due to suprathreshold stress. NEW & NOTEWORTHY This study provides new insights into how t-tubular membranes respond to osmotic forces. In particular, the data show that osmotically induced sealing of cardiac t tubules is a threshold phenomenon initiated by detachment of t-tubular membrane from the underlying cytoskeleton. The findings are consistent with the hypothesis that final sealing of t tubules is driven by negative hydrostatic intracellular pressure coincident with cell shrinking.

Loss-of-function (LOF) variants in SCN1B, encoding voltage-gated sodium channel β1 subunits, are linked to human diseases with high risk of sudden death, including developmental and epileptic encephalopathy and cardiac arrhythmia. β1 Subunits modulate the cell-surface localization, gating, and kinetics of sodium channel pore-forming α subunits. They also participate in cell-cell and cell-matrix adhesion, resulting in intracellular signal transduction, promotion of cell migration, calcium handling, and regulation of cell morphology. Here, we investigated regulated intramembrane proteolysis (RIP) of β1 by BACE1 and γ-secretase and show that β1 subunits are substrates for sequential RIP by BACE1 and γ-secretase, resulting in the generation of a soluble intracellular domain (ICD) that is translocated to the nucleus. Using RNA sequencing, we identified a subset of genes that are downregulated by β1-ICD overexpression in heterologous cells but upregulated in Scn1b-null cardiac tissue, which lacks β1-ICD signaling, suggesting that the β1-ICD may normally function as a molecular brake on gene transcription in vivo. We propose that human disease variants resulting in SCN1B LOF cause transcriptional dysregulation that contributes to altered excitability. Moreover, these results provide important insights into the mechanism of SCN1B-linked channelopathies, adding RIP-excitation coupling to the multifunctionality of sodium channel β1 subunits.

Cardiac t-tubules (TTs) form a network of complex surface membrane invaginations that is essential for proper excitation-contraction coupling. Although electron and optical microscopy studies provided a wealth of important information about the structure of TTs, assessing their functional properties remains a challenge. In this study, we investigated the diffusional accessibility of TTs in intact isolated adult mouse ventricular myocytes using, to our knowledge, a novel fluorescence-based assay. In this approach, a small part of TTs is first locally filled with fluorescent dextran and then its diffusion out of TTs is monitored after rapid removal of extracellular dextran. In normal cells, diffusion of 3 kDa dextran is characterized by an average time constant of 3.9 ± 1.2 s with the data ranging from 1.8 to 10.5 s. The data are consistent with essentially free diffusion of dextran in TTs although measurable contribution of binding is also evident. TT fluorescence is abolished in cells treated with high concentration of formamide or after hyposmotic stress. Importantly, the assay we use allows for quantitative, repetitive measurements of subtle dynamic changes in TT structure of the same cell that are not possible to observe with other approaches. In particular, dextran diffusion rate decreases two-to-threefold during cell swelling, suggesting significant structural remodeling of TTs. Computer modeling shows that diffusional accessibility and electrical properties of TTs are primarily determined by the constrictions and dilations of individual TTs and that, from a functional perspective, TTs cannot be considered as a network of cylinders of the same average diameter. Constriction/dilation model of cardiac TTs is in a quantitative agreement with previous high-resolution microscopy studies of TT structure and alternative measurements of diffusional and electrical time constants of TTs. The data also show that the apparent electrical length constant of cardiac TTs is likely several-fold smaller than that estimated in earlier studies.

Cardiac t-tubules are critical for efficient excitation-contraction coupling but become significantly remodeled during various stress conditions. However, the mechanisms by which t-tubule remodeling occur are poorly understood. Recently, we demonstrated that recovery of mouse ventricular myocytes after hyposmotic shock is associated with t-tubule sealing. In this study, we found that the application of Small Membrane Permeable Molecules (SMPM) such as DMSO, formamide and acetamide upon washout of hyposmotic solution significantly reduced the amount of extracellular dextran trapped within sealed t-tubules. The SMPM protection displayed sharp biphasic concentration dependence that peaks at ∼140 mM leading to >3- to 4-fold reduction in dextran trapping. Consistent with these data, detailed analysis of the effects of DMSO showed that the magnitude of normalized inward rectifier tail current ( IK1,tail), an electrophysiological marker of t-tubular integrity, was increased ∼2-fold when hyposmotic stress was removed in the presence of 1% DMSO (∼140 mM). Analysis of dynamics of cardiomyocytes shrinking during resolution of hyposmotic stress revealed only minor increase in shrinking rate in the presence of 1% DMSO, and cell dimensions returned fully to prestress values in both control and DMSO groups. Application and withdrawal of 10% DMSO in the absence of preceding hyposmotic shock induced classical t-tubule sealing. This suggests that the biphasic concentration dependence originated from an increase in secondary t-tubule sealing when high SMPM concentrations are removed. Overall, the data suggest that SMPM protect against sealing of t-tubules following hyposmotic stress, likely through membrane modification and essentially independent of their osmotic effects.

Efficient excitation-contraction coupling in ventricular myocytes depends critically on the presence of the t-tubular network. It has been recently demonstrated that cholesterol, a major component of the lipid bilayer, plays an important role in long-term maintenance of the integrity of t-tubular system although mechanistic understanding of underlying processes is essentially lacking. Accordingly, in this study we investigated the contribution of membrane cholesterol to t tubule remodeling in response to acute hyposmotic stress. Experiments were performed using isolated left ventricular cardiomyocytes from adult mice. Depletion and restoration of membrane cholesterol was achieved by applying methyl-β-cyclodextrin (MβCD) and water soluble cholesterol (WSC), respectively, and t-tubule remodeling in response to acute hyposmotic stress was assessed using fluorescent dextran trapping assay and by measuring t-tubule dependent IK1 tail current (IK1, tail). The amount of dextran trapped in t-tubules sealed in response to stress was significantly increased when compared to control cells, and reintroduction of cholesterol to cells treated with MβCD restored the amount of trapped dextran to control values. Alternatively, application of WSC to normal cells significantly reduced the amount of trapped dextran further suggesting the protective effect of cholesterol. Importantly, modulation of membrane cholesterol (without osmotic stress) led to significant changes in various parameters of IK1, tail strongly suggesting significant but essentially hidden remodeling of t-tubules prior to osmotic stress. Results of this study demonstrate that modulation of the level of membrane cholesterol has significant effects on the susceptibility of cardiac t-tubules to acute hyposmotic stress.

We have recently shown that in mouse ventricular myocytes, t-tubules can be quickly and tightly sealed during the resolution of hyposmotic shock of physiologically relevant magnitude. Sealing of t-tubules is associated with trapping extracellular solution inside the myocytes but the ionic homeostasis of sealed t-tubules and the consequences of potential transtubular ion fluxes remain unknown. In this study we investigated the dynamics of Ca(2+) movements associated with sealing of t-tubules. The data show that under normal conditions sealed t-tubules contain Ca(2+) at concentrations below 100μM. However, blockade of voltage-dependent Ca(2+) channels with 10μM nicardipine, or increasing extracellular concentration of K(+) from 5.4mM to 20mM led to several fold increase in concentration of t-tubular Ca(2+). Alternatively, the release of Ca(2+) from sarcoplasmic reticulum using 10mM caffeine led to the restoration of t-tubular Ca(2+) towards extracellular levels within few seconds. Sealing of t-tubules in the presence of extracellular 1.5mM Ca(2+) and 5.4mM extracellular K(+) led to occasional and sporadic intracellular Ca(2+) transients. In contrast, sealing of t-tubules in the presence of 10mM caffeine was characterized by a significant long lasting increase in intracellular Ca(2+). The effect was completely abolished in the absence of extracellular Ca(2+) and significantly reduced in pre-detubulated myocytes but was essentially preserved in the presence of mitochondrial decoupler dinitrophenol. This study shows that sealed t-tubules are capable of highly regulated transport of Ca(2+) and present a major route for Ca(2+) influx into the cytosol during sealing process.

It has been recently shown that various stress-inducing manipulations in isolated ventricular myocytes may lead to significant remodeling of t-tubules. Osmotic stress is one of the most common complications in various experimental and clinical settings. Therefore, this study was designed to determine the effects of a physiologically relevant type of osmotic stress, hypo-osmotic challenge, to the integrity of t-tubular system in mouse ventricular myocytes using 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) hypo-osmotic solution, but not its application to myocytes, led to ~27% reduction in membrane capacitance, ~2.5-fold reduction in the amplitude of IK1 tail current and ~2-fold reduction in so-called IK1 ‘inactivation’ (due to depletion of t-tubular K+) at negative membrane potentials – all data being consistent with significant detubulation. Confocal imaging experiments also demonstrated that extracellularly applied dextrans become trapped in sealed t-tubules only upon removal of hypo-osmotic solutions, i.e. during shrinking phase, but not during initial swelling period. In light of these data, relevant previous studies, including those on EC coupling phenomena during hypo-osmotic stress, may need to be reinterpreted and the experimental design of future experiments should take into account the novel findings.

Cardiac ventricular myocytes possess an extensive t-tubular system that facilitates the propagation of membrane potential across the cell body. It is well established that ionic currents at the restricted t-tubular space may lead to significant changes in ion concentrations, which, in turn, may affect t-tubular membrane potential. In this study, we used the whole cell patch-clamp technique to study accumulation and depletion of t-tubular potassium by measuring inward rectifier potassium tail currents ( IK1,tail), and inward rectifier potassium current ( IK1) “inactivation”. At room temperatures and in the absence of Mg2+ ions in pipette solution, the amplitude of IK1,tail measured ∼10 min after the establishment of whole cell configuration was reduced by ∼18%, but declined nearly twofold in the presence of 1 mM cyanide. At ∼35°C IK1,tail was essentially preserved in intact cells, but its amplitude declined by ∼85% within 5 min of cell dialysis, even in the absence of cyanide. Intracellular Mg2+ ions played protective role at all temperatures. Decline of IK1,tail was accompanied by characteristic changes in its kinetics, as well as by changes in the kinetics of IK1 inactivation, a marker of depletion of t-tubular K+. The data point to remodeling of t tubules as the primary reason for the observed effects. Consistent with this, detubulation of myocytes using formamide-induced osmotic stress significantly reduced IK1,tail, as well as the inactivation of inward IK1. Overall, the data provide strong evidence that changes in t tubule volume/structure may occur on a short time scale in response to various types of stress.

It has long been recognized that cationic nanoparticles induce cell membrane permeability. Recently, it has been found that cationic nanoparticles induce the formation and/or growth of nanoscale holes in supported lipid bilayers. In this paper, we show that noncytotoxic concentrations of cationic nanoparticles induce 30-2000 pA currents in 293A (human embryonic kidney) and KB (human epidermoid carcinoma) cells, consistent with a nanoscale defect such as a single hole or group of holes in the cell membrane ranging from 1 to 350 nm(2) in total area. Other forms of nanoscale defects, including the nanoparticle porating agents adsorbing onto or intercalating into the lipid bilayer, are also consistent; although the size of the defect must increase to account for any reduction in ion conduction, as compared to a water channel. An individual defect forming event takes 1-100 ms, while membrane resealing may occur over tens of seconds. Patch-clamp data provide direct evidence for the formation of nanoscale defects in living cell membranes. The cationic polymer data are compared and contrasted with patch-clamp data obtained for an amphiphilic phenylene ethynylene antimicrobial oligomer (AMO-3), a small molecule that is proposed to make well-defined 3.4 nm holes in lipid bilayers. Here, we observe data that are consistent with AMO-3 making approximately 3 nm holes in living cell membranes.

In neurons, voltage-gated sodium channel beta subunits regulate the expression levels, subcellular localization, and electrophysiological properties of sodium channel alpha subunits. However, the contribution of beta subunits to sodium channel function in heart is poorly understood. We examined the role of beta1 in cardiac excitability using Scn1b null mice. Compared to wildtype mice, electrocardiograms recorded from Scn1b null mice displayed longer RR intervals and extended QT(c) intervals, both before and after autonomic block. In acutely dissociated ventricular myocytes, loss of beta1 expression resulted in a approximately 1.6-fold increase in both peak and persistent sodium current while channel gating and kinetics were unaffected. Na(v)1.5 expression increased in null myocytes approximately 1.3-fold. Action potential recordings in acutely dissociated ventricular myocytes showed slowed repolarization, supporting the extended QT(c) interval. Immunostaining of individual myocytes or ventricular sections revealed no discernable alterations in the localization of sodium channel alpha or beta subunits, ankyrin(B), ankyrin(G), N-cadherin, or connexin-43. Together, these results suggest that beta1 is critical for normal cardiac excitability and loss of beta1 may be associated with a long QT phenotype.

Sarcolemmal ATP-sensitive potassium (KATP) channels are activated after pathological depletion of intracellular ATP, unlike their pancreatic β-cell counterparts, which dynamically regulate membrane excitability in response to changes in blood glucose. We recently engineered a series of transgenic (TG) mice overexpressing an ATP-insensitive inward rectifying K+ channel protein (Kir)6.2 mutant (Kir6.2[ΔN30,K185Q]) or the accessory sulfonylurea receptor (SUR)2A (FLAG-SUR2A) or SUR1 (FLAG-SUR1) subunits of the KATP channel, under transcriptional control of the α-myosin heavy chain promoter. In the present study, we generated double transgenic (DTG) animals overexpressing both Kir6.2[ΔN30,K185Q] and FLAG-SUR1 or FLAG-SUR2A and examined the effects on cardiac excitability in vivo. No animals expressing both FLAG-SUR1 and Kir6.2[ΔN30,K185Q] transgenes at a high level were obtained. DTG mice expressing one transgene at a high level and the other at a lower level are born, but they die prematurely. Electrocardiographic analysis of both anesthetized and conscious animals revealed a constellation of arrhythmias in DTG animals, but not in wild-type or single TG littermates. The proarrhythmic effect of the transgene combination is intrinsic to the myocardium, since it persists in isolated hearts. Importantly, this effect is specific for SUR1-expressing DTG animals: DTG animals expressing both Kir6.2[ΔN30,K185Q] and FLAG-SUR2A at high levels exhibit neither impaired survival nor increased arrhythmia frequency, even with both subunits expressed at high levels. In demonstrating the profound arrhythmic consequences of KATP channels comprised of SUR1 and Kir6.2[ΔN30,K185Q] in the myocardium specifically, the results highlight the critical differential activation of SUR1 versus SUR2A, and indicate that expression of hyperactive KATP in the heart is likely to be proarrhythmic.

G protein-coupled receptors play a pivotal role in regulating cardiac automaticity. Their function is controlled by regulator of G protein signaling (RGS) proteins acting as GTPase-activating proteins for Gα subunits to suppress Gαi and Gαq signaling. Using knock-in mice in which Gαi2-RGS binding and negative regulation are disrupted by a genomic Gαi2G184S (GS) point mutation, we recently (Fu Y, Huang X, Zhong H, Mortensen RM, D'Alecy LG, Neubig RR. Circ Res 98: 659–666, 2006) showed that endogenous RGS proteins suppress muscarinic receptor-mediated bradycardia. To determine whether this was due to direct regulation of cardiac pacemakers or to alterations in the central nervous system or vascular responses, we examined isolated, perfused hearts. Isoproterenol-stimulated beating rates of heterozygote (+/GS) and homozygote (GS/GS) hearts were significantly more sensitive to inhibition by carbachol than were those of wild type (+/+). Even greater effects were seen in the absence of isoproterenol; the potency of muscarinic-mediated bradycardia was enhanced fivefold in GS/GS and twofold in +/GS hearts compared with +/+. A1-adenosine receptor-mediated bradycardia was unaffected. In addition to effects on the sinoatrial node, +/GS and GS/GS hearts show significantly increased carbachol-induced third-degree atrioventricular (AV) block. Atrial pacing studies demonstrated an increased PR interval and AV effective refractory period in GS/GS hearts compared with +/+. Thus loss of the inhibitory action of endogenous RGS proteins on Gαi2 potentiates muscarinic inhibition of cardiac automaticity and conduction. The severe carbachol-induced sinus bradycardia in Gαi2G184S mice suggests a possible role for alterations of Gαi2 or RGS proteins in sick sinus syndrome and pathological AV block.

Previous studies have suggested an important role for the inward rectifier K+ current (IK1) in stabilizing rotors responsible for ventricular tachycardia (VT) and fibrillation (VF). To test this hypothesis, we used a line of transgenic mice (TG) overexpressing Kir 2.1–green fluorescent protein (GFP) fusion protein in a cardiac-specific manner. Optical mapping of the epicardial surface in ventricles showed that the Langendorff-perfused TG hearts were able to sustain stable VT/VF for 350 ± 1181 s at a very high dominant frequency (DF) of 44.6 ± 4.3 Hz. In contrast, tachyarrhythmias in wild-type hearts (WT) were short-lived (3 ± 9 s), and the DF was 26.3 ± 5.2 Hz. The stable, high frequency, reentrant activity in TG hearts slowed down, and eventually terminated in the presence of 10 μm Ba2+, suggesting an important role for IK1. Moreover, by increasing IK1 density in a two-dimensional computer model having realistic mouse ionic and action potential properties, a highly stable, fast rotor (≈45 Hz) could be induced. Simulations suggested that the TG hearts allowed such a fast and stable rotor because of both greater outward conductance at the core and shortened action potential duration in the core vicinity, as well as increased excitability, in part due to faster recovery of Na+ current. The latter resulted in a larger rate of increase in the local conduction velocity as a function of the distance from the core in TG compared to WT hearts, in both simulations and experiments. Finally, simulations showed that rotor frequencies were more sensitive to changes (doubling) in IK1, compared to other K+ currents. In combination, these results provide the first direct evidence that IK1 up-regulation in the mouse heart is a substrate for stable and very fast rotors.

Recent studies have shown that Kir2 channels display differential sensitivity to intracellular polyamines, and have raised a number of questions about several properties of inward rectification important to the understanding of their physiological roles. In this study, we have carried out a detailed characterization of steady-state and kinetic properties of block of Kir2.1–3 channels by spermine. High-resolution recordings from outside-out patches showed that in all Kir2 channels current–voltage relationships display a ‘crossover’ effect upon change in extracellular K+. Experiments at different concentrations of spermine allowed for the characterization of two distinct shallow components of rectification, with the voltages for half-block negative (V11/2) and positive (V21/2) to the voltage of half-block for the major steep component of rectification (V01/2). While V11/2 and V21/2 voltages differ significantly between Kir2 channels, they were coupled to each other according to the equation V11/2−V21/2= constant, strongly suggesting that similar structures may underlie both components. In Kir2.3 channels, the V21/2 was ∼50 mV positive to V01/2, leading to a pattern of outward currents distinct from that of Kir2.1 and Kir2.2 channels. The effective valency of spermine block (Z0) was highest in Kir2.2 channels while the valencies in Kir2.1 and Kir2.3 channels were not significantly different. The voltage dependence of spermine unblock was similar in all Kir2 channels, but the rates of unblock were ∼7-fold and ∼16-fold slower in Kir2.3 channels than those in Kir2.1 and Kir2.2 when measured at high and physiological extracellular K+, respectively. In all Kir2 channels, the instantaneous phase of activation was present. The instantaneous phase was difficult to resolve at high extracellular K+ but it became evident and accounted for nearly 30–50% of the total current when recorded at physiological extracellular K+. In conclusion, the data are consistent with the universal mechanism of rectification in Kir2 channels, but also point to significant, and physiologically important, quantitative differences between Kir2 isoforms.

T-tubules are membrane invaginations of cardiac myocytes critical for effective excitation-contraction coupling and are significantly altered in various diseases. However, the mechanisms underlying t-tubule remodeling remain largely unknown, particularly due to a number of limitations of currently available technologies. Therefore, we have developed an assay that employs the diffusion of fluorescent 3 KD dextran out of the t-tubule network of live intact isolated ventricular cardiomyocytes. To significantly improve the quality of measurements, dextran is applied locally using a patch-clamp pipette (∼2-3 μm diameter) under constant flow of washing solution through 0.5 mm ID capillary glass, which limits dextran filling of t-tubules to a small area of the cardiomyocyte. With 40X objective and small pinholes in the light paths, excitation is limited to ∼9-10 μm diameter area and fluorescence emission is measured from ∼5-6 μm diameter region using a photomultiplier. Upon cessation of dye application, fluorescence decline displays 2 major components: a fast component (τ ∼5-50 ms) corresponding to dye washout from the solution surrounding the cell and a slow component corresponding to dye diffusion out of the t-tubule system. In healthy mouse ventricular myocytes the slow component had a time constant of 1304 ± 98 ms (n=24). In contrast, “detubulation” by acute hyposmotic stress significantly decreased the time constant to 848 ± 99 ms (n=27, p

The molecular basis of G-protein inhibition of inward rectifier K+ currents was examined by co-expression of G-proteins and cloned Kir2 channel subunits in Xenopus oocytes. Channels encoded by Kir2.3 (HRK1/HIR/BIRK2/BIR11) were completely suppressed by co-expression with G-protein betagamma subunits, whereas channels encoded by Kir2. 1 (IRK1), which shares 60% amino acid identity with Kir2.3, were unaffected. Co-expression of Galphai1 and Galphaq subunits also partially suppressed Kir2.3 currents, but Galphat, Galphas, and a constitutively active mutant of Galphail (Q204L) were ineffective. Gbetagamma and Kir2.3 subunits were co-immunoprecipitated using an anti-Kir2.3 antibody. Direct binding of G-protein betagamma subunits to fusion proteins containing Kir2.3 N terminus, but not to fusion proteins containing Kir2.1 N terminus, was also demonstrated. The results are consistent with suppression of Kir2.3 currents resulting from a direct protein-protein interaction between the channel and G-protein betagamma subunits. When Kir2.1 and Kir2.3 subunits were coexpressed, the G-protein inhibitory phenotype of Kir2.3 was dominant, suggesting that co-expression of Kir2.3 with other Kir subunits might give rise to novel G-protein-inhibitable inward rectifier currents.

Two different approaches were used to examine the in vivo role of polyamines in causing inward rectification of potassium channels. In two-microelectrode voltage-clamp experiments, 24-hr incubation of Xenopus oocytes injected with 50 nl of difluoromethylornithine (5 mM) and methylglyoxal bis(guanylhydrazone) (1 mM) caused an approximate doubling of expressed Kir2.1 currents and relieved rectification by causing an approximately +10-mV shift of the voltage at which currents are half-maximally inhibited. Second, a putrescine auxotrophic, ornithine decarboxylase-deficient Chinese hamster ovary (O-CHO) cell line was stably transfected with the cDNA encoding Kir2.3. Withdrawal of putrescine from the medium led to rapid (1-day) loss of the instantaneous phase of Kir2.3 channel activation, consistent with a decline of intracellular putrescine levels. Four days after putrescine withdrawal, macroscopic conductance, assessed using an 86Rb+ flux assay, was approximately doubled, and this corresponded to a +30-mV shift of V1/2 of rectification. With increasing time after putrescine withdrawal, there was an increase in the slowest phase of current activation, corresponding to an increase in the spermine-to-spermidine ratio over time. These results provide direct evidence for a role of each polyamine in induction of rectification, and they further demonstrate that in vivo modulation of rectification is possible by manipulation of polyamine levels using genetic and pharmacological approaches.

The effects of permeant (K+) ions on polyamine (PA)-induced rectification of cloned strong inwardly rectifying channels (IRK1, Kir2.1) expressed in Xenopus oocytes were examined using patch-clamp techniques. The kinetics of PA-induced rectification depend strongly on external, but not internal, K+ concentration. Increasing external [K+] speeds up "activation" kinetics and shifts rectification to more positive membrane potentials. The shift of rectification is directly proportional to the shift in the K+ reversal potential (EK) with slope factors +0.62, +0.81, and +0.91 for 1 mM putrescine (Put), 100 microM spermidine and 20 microM spermine (Spm), respectively. The time constant of current activation, resulting from unblock of Spm, also shifts directly in proportion to EK with slope factor +1.1. Increasing internal [K+] slows down activation kinetics and has a much weaker relieving effect on block by PA: Spm-induced rectification and time constant of activation (Spm unblock) shift directly in proportion to the corresponding change in EK with slope factors -0.15 and +0.31, respectively, for 20 microM Spm. The speed up of activation kinetics caused by increase of external [K+] cannot be reversed by equal increase of internal [K+]. The data are consistent with the hypothesis that the conduction pathway of strong inward rectifiers is a long and narrow pore with multiple binding sites for PA and K+.

The mechanism of spermidine release from Xenopus oocytes was examined by measuring release of radioactive [3H]spermidine under different ionic conditions, and under voltage-clamp. In normal solution (2 mM K), the efflux rate is less than 1% per hour, and is stimulated 2-fold by inclusion of Ca (1 mM) in the incubation medium. Spermidine efflux is stimulated 10-fold in high [K] (KD98) solution. In KD98 solution, efflux is strongly inhibited by divalent cations (K for Ba block of spermidine efflux is 0.1 mM), but not by tetraethylammonium ions or verapamil. Spermidine efflux rates were not different between control oocytes and those expressing HRK1 inward rectifier K (Kir) channels. When the membrane potential was clamped, either by changing external [K] in oocytes expressing HRK1, or by 2-microelectrode voltage-clamp, spermidine efflux was shown to be strongly dependent on voltage, as expected for a simple electrodiffusive process, where spermidine is the effluxing species. This result argues against spermidine diffusing out as an uncharged species, or in exchange for similarly charged counterions. These results are the first conclusive demonstration of a simple electrodiffusive pathway for spermidine efflux from cells.

It has been recently shown that various stress-inducing manipulations in isolated ventricular cardiac myocytes may lead to significant remodeling of t-tubules. Osmotic stress is one of the most common complications in various experimental and clinical settings, and therefore, this study was designed to test a hypothesis that osmotic challenge may affect the integrity of t-tubules. T-tubular remodeling in mouse ventricular myocytes in response to various osmotic challenges was studied using two approaches: (1) electrophysiologically, by measuring membrane capacitance and IK1 tail currents originating from K+ accumulation in ttubules, and (2) using confocal microscopy of fluorescent dextrans trapped in vesiculated t-tubules. In particular, application and removal of 0.6 T (60% of NaCl) hypo-osmotic solution to myocytes led to ∼30% reduction in membrane capacitance, ∼3-fold reduction in the amplitude of IK1 tail current and ∼2-fold reduction in so-called IK1 ‘inactivation' (due to depletion of t-tubular K+) at negative membrane potentials – all being consistent with strong detubulation. Importantly, confocal imaging experiments showed that extracellularly applied dextrans become trapped inside the myocytes only upon removal of hypo-osmotic solutions (i.e. during shrinking phase) but not during initial swelling period. In light of these data some relevant previous studies, including those on EC coupling phenomena during hypo-osmotic stress, may need to be reinterpreted and the experimental design of future experiments should take into account the novel findings.

INWARDLY rectifying potassium channels conduct ions more readily in the inward than the outward direction, an essential property for normal electrical activity1,2. Although voltage-dependent block by internal magnesium ions may underlie inward rectification in some channels3–5, an intrinsic voltage-dependent closure of the channel plays a contributory, or even exclusive, role in others4,6–9. Here we report that, rather than being intrinsic to the channel protein, so-called intrinsic rectification of strong inward rectifiers requires soluble factors that are not Mg2+ and can be released from Xenopus oocytes and other cells. Biochemical and biophysical characterization identifies these factors as polyamines (spermine, spermidine, putrescine and cadaverine). The results suggest that intrinsic rectification results from voltage-dependent block of the channel pore by polyamines, not from a voltage sensor intrinsic to the channel protein10.

A complementary DNA encoding an inward rectifier K+ channel (HRK1) was isolated from human hippocampus using a 392-base pair cDNA (HHCMD37) as a probe. HRK1 shows sequence similarity to three recently cloned inwardly rectifying potassium channels (IRK1, GIRK1, and ROMK1, 60, 42, and 37%, respectively) and has a similar proposed topology of two membrane spanning domains that correspond to the inner core structure of voltage gated K+ channels. When HRK1 was expressed in Xenopus oocytes, large inward K+ currents were observed below the K+ reversal potential but very little outward K+ current was observed. In on-cell membrane patches, single channel conductance (g) was estimated to be 10 picosiemens by both direct measurement and noise analysis, in 102 mM external [K+]. HRK1 currents were blocked by external Ba2+ and Cs+ (K(0) = 183 microM, and K(-130) = 30 microM, respectively), and internal tetraethylammonium ion (K(0) = 62 microM), but were insensitive to external tetraethylammonium ion. The functional properties of HRK1 are very similar to those of glial cell inward rectifier K+ channels and HRK1 may represent a glial cell inward rectifier.

Cardiac I(K1) and I(KACh) are the major potassium currents displaying classical strong inward rectification, a unique property that is critical for their roles in cardiac excitability. In the last 15 years, research on I(K1) and I(KACh) has been propelled by the cloning of the underlying inwardly rectifying potassium (Kir) channels, the discovery of the molecular mechanism of strong rectification and the linking of a number of disorders of cardiac excitability to defects in genes encoding Kir channels. Disease-causing mutations in Kir genes have been shown experimentally to affect one or more of the following channel properties: structure, assembly, trafficking, and regulation, with the ultimate effect of a gain- or a loss-of-function of the channel. It is now established that I(K1) and I(KACh) channels are heterotetramers of Kir2 and Kir3 subunits, respectively. Each homomeric Kir channel has distinct biophysical and regulatory properties, and individual Kir subunits often display different patterns of regional, cellular, and membrane distribution. These differences are thought to underlie important variations in the physiological properties of I(K1) and I(KACh). It has become increasingly clear that the contribution of I(K1) and I(KACh) channels to cardiac electrical activity goes beyond their long recognized role in the stabilization of resting membrane potential and shaping the late phase of action potential repolarization in individual myocytes but extends to being critical elements determining the overall electrical stability of the heart.

Relaxation abnormalities are prevalent in heart failure and contribute to clinical outcomes. Disruption of Ca2+homeostasis in heart failure delays relaxation by prolonging the intracellular Ca2+transient. We sought to speed cardiac relaxation in vivo by cardiac-directed transgene expression of parvalbumin (Parv), a cytosolic Ca2+buffer normally expressed in fast-twitch skeletal muscle. A key feature of Parv's function resides in its Ca2+/Mg2+binding affinities that account for delayed Ca2+buffering in response to the intracellular Ca2+transient. Cardiac Parv expression decreased sarcoplasmic reticulum Ca2+content without otherwise altering intracellular Ca2+homeostasis. At high physiological mouse heart rates in vivo, Parv modestly accelerated relaxation without affecting cardiac morphology or systolic function. Ex vivo pacing of the isolated heart revealed a marked heart rate dependence of Parv's delayed Ca2+buffering effects on myocardial performance. As the pacing frequency was lowered (7 to 2.5 Hz), the relaxation rates increased in Parv hearts. However, as pacing rates approached the dynamic range in humans, Parv hearts demonstrated decreased contractility, consistent with Parv buffering systolic Ca2+. Mathematical modeling and in vitro studies provide the underlying mechanism responsible for the frequency-dependent fractional Ca2+buffering action of Parv. Future studies directed toward refining the dose and frequency-response relationships of Parv in the heart or engineering novel Parv-based Ca2+buffers with modified Mg2+and Ca2+affinities to limit systolic Ca2+buffering may hold promise for the development of new therapies to remediate relaxation abnormalities in heart failure.

It is established that prolonged hypoxia leads to activation of KATP channels and action potential (AP) shortening, but the mechanisms behind the early phase of metabolic stress remain controversial. Under normal conditions IK1 channels are constitutively active while KATP channels are closed. Therefore, early changes in IK1 may underlie early AP shortening. This hypothesis was tested using transgenic mice with suppressed IK1 (AAA-TG). In isolated AAA-TG hearts AP shortening was delayed by ∼ 24 s compared to WT hearts. In WT ventricular myocytes, blocking oxidative phosphorylation with 1 mM cyanide (CN; 28 °C) led to a 29% decrease in APD90 within ∼ 3–5 min. The effect of CN was reversed by application of 100 μM Ba2+, a selective blocker of IK1, but not by 10 μM glybenclamide, a selective blocker of KATP channels. Accordingly, voltage-clamp experiments revealed that both CN and true hypoxia lead to early activation of IK1. In AAA-TG myocytes, neither CN nor glybenclamide or Ba2+ had any effect on AP. Further experiments showed that buffering of intracellular Ca2+ with 20 mM BAPTA prevented IK1 activation by CN, although CN still caused a 54% increase in IK1 in a Ca2+-free bath solution. Importantly, both (i) 20 μM ruthenium red, a selective inhibitor of SR Ca2+-release, and (ii) depleting SR by application of 10 μM ryanodine + 1 mM caffeine, abolished the activation of IK1 by CN. The above data strongly argue that in the mouse heart IK1, not KATP, channels are responsible for the early AP shortening during hypoxia.

Previous studies have suggested an important role for the inward rectifier K+ current (I K1) in stabilizing rotors responsible for ventricular tachycardia (VT) and fibrillation (VF). To test this hypothesis, we used a line of transgenic mice (TG) overexpressing Kir 2.1-green fluorescent protein (GFP) fusion protein in a cardiac-specific manner. Optical mapping of the epicardial surface in ventricles showed that the Langendorff-perfused TG hearts were able to sustain stable VT/VF for 350 +/- 1181 s at a very high dominant frequency (DF) of 44.6 +/- 4.3 Hz. In contrast, tachyarrhythmias in wild-type hearts (WT) were short-lived (3 +/- 9 s), and the DF was 26.3 +/- 5.2 Hz. The stable, high frequency, reentrant activity in TG hearts slowed down, and eventually terminated in the presence of 10 mum Ba2+, suggesting an important role for I K1. Moreover, by increasing I K1 density in a two-dimensional computer model having realistic mouse ionic and action potential properties, a highly stable, fast rotor (approximately 45 Hz) could be induced. Simulations suggested that the TG hearts allowed such a fast and stable rotor because of both greater outward conductance at the core and shortened action potential duration in the core vicinity, as well as increased excitability, in part due to faster recovery of Na+ current. The latter resulted in a larger rate of increase in the local conduction velocity as a function of the distance from the core in TG compared to WT hearts, in both simulations and experiments. Finally, simulations showed that rotor frequencies were more sensitive to changes (doubling) in I K1, compared to other K+ currents. In combination, these results provide the first direct evidence that I K1 up-regulation in the mouse heart is a substrate for stable and very fast rotors.

The lack of pathological consequences of cardiac ATP-sensitive potassium channel (K(ATP)) channel gene manipulation is in stark contrast to the effect of similar perturbations in the pancreatic beta-cell. Because the pancreatic and cardiac channel share the same pore-forming subunit (Kir6.2), the different effects of genetic manipulation likely reflect, at least in part, the tissue-specific expression of the regulatory subunit (SUR1 in pancreas vs. SUR2A in heart) of the bipartite channel complex. To examine this, we have generated transgenic (TG) mice that overexpress epitope-tagged SUR1 or SUR2A under the transcriptional control of the alpha-myosin heavy chain promoter. Western blot and real time RT-PCR analysis confirm transgene expression in the heart, and variable levels of SUR1 RNA and protein, in 16 viable founder lines. Surprisingly, activation of channels by either pharmacological agents (diazoxide and pinacidil) or metabolic inhibitors (oligomycin and 2-deoxyglucose) reveals a suppression of total K(ATP) conductance in high expressing TG mice. Moreover, K(ATP) channel activity was significantly reduced in excised cardiac patches from TG myocytes that overexpress either SUR1 or SUR2A. Using a recombinant cell system, we show that overexpression of either SUR1 or Kir6.2 suppresses the functional expression of K(ATP) from optimized dimeric SUR1-Kir6.2. Thus, the graded effect of SUR1 expression in the intact heart appears to demonstrate an in vivo requirement for 1:1 expression ratio of Kir6.2 and SURx.

To assess the functional significance of upregulation of the cardiac current ( IK1), we have produced and characterized the first transgenic (TG) mouse model of IK1upregulation. To increase IK1density, a pore-forming subunit of the Kir2.1 (green fluorescent protein-tagged) channel was expressed in the heart under control of the α-myosin heavy chain promoter. Two lines of TG animals were established with a high level of TG expression in all major parts of the heart: line 1 mice were characterized by 14% heart hypertrophy and a normal life span; line 2 mice displayed an increased mortality rate, and in mice ≤1 mo old, heart weight-to-body weight ratio was increased by >100%. In adult ventricular myocytes expressing the Kir2.1-GFP subunit, IK1conductance at the reversal potential was increased ∼9- and ∼10-fold in lines 1 and 2, respectively. Expression of the Kir2.1 transgene in line 2 ventricular myocytes was heterogeneous when assayed by single-cell analysis of GFP fluorescence. Surface ECG recordings in line 2 mice revealed numerous abnormalities of excitability, including slowed heart rate, premature ventricular contractions, atrioventricular block, and atrial fibrillation. Line 1 mice displayed a less severe phenotype. In both TG lines, action potential duration at 90% repolarization and monophasic action potential at 75–90% repolarization were significantly reduced, leading to neuronlike action potentials, and the slow phase of the T wave was abolished, leading to a short Q-T interval. This study provides a new TG model of IK1upregulation, confirms the significant role of IK1in cardiac excitability, and is consistent with adverse effects of IK1upregulation on cardiac electrical activity.

Reducing the ATP sensitivity of the sarcolemmal ATP-sensitive K+ (KATP) channel is predicted to lead to active channels in normal metabolic conditions and hence cause shortened ventricular action potentials and reduced myocardial inotropy. We generated transgenic (TG) mice that express an ATP-insensitive KATP channel mutant [Kir6.2(ΔN2–30,K185Q)] under transcriptional control of the α-myosin heavy chain promoter. Strikingly, myocyte contraction amplitude was increased in TG myocytes (15.68 ± 1.15% vs. 10.96 ± 1.49%), even though KATP channels in TG myocytes are very insensitive to inhibitory ATP. Under normal metabolic conditions, steady-state outward K+ currents measured under whole cell voltage clamp were elevated in TG myocytes, consistent with threshold KATP activation, but neither the monophasic action potential measured in isolated hearts nor transmembrane action potential measured in right ventricular muscle preparations were shortened at physiological pacing cycles. Taken together, these results suggest that there is a compensatory remodeling of excitation-contraction coupling in TG myocytes. Whereas there were no obvious differences in other K+ conductances, peak L-type Ca2+ current ( ICa) density (–16.42 ± 2.37 pA/pF) in the TG was increased compared with the wild type (–8.43 ± 1.01 pA/pF). Isoproterenol approximately doubled both ICa and contraction amplitude in wild-type myocytes but failed to induce a significant increase in TG myocytes. Baseline and isoproterenol-stimulated cAMP concentrations were not different in wild-type and TG hearts, suggesting that the enhancement of ICa in the latter does not result from elevated cAMP. Collectively, the data demonstrate that a compensatory increase in ICa counteracts a mild activation of ATP-insensitive KATP channels to maintain the action potential duration and elevate the inotropic state of TG hearts.

When membrane patches are isolated from cells, one gains the ability to regulate precisely the composition of the solution bathing both surfaces of the membrane and to change the composition rapidly. However, in detaching the membrane from the underlying cytoskeleton, one irreversibly changes the integrated system of which the channel protein is a part, and begins an inexorable loss of channel function. In this chapter we will deal primarily with the techniques available for measuring the response of patch currents to changes in bathing solution and the practical problems to be minimized, or overcome, in implementing such techniques and analyzing the results obtained. Other chapters will deal with patch-clamping in general. Chapter 6 deals specifically with methods of changing the concentration of the solution bathing the intrapipet face of the membrane, and we will concentrate on consideration of the extrapipet solution changes, i.e., changes at the intracellular membrane surface in inside-out patches, or the external face in outside-out membrane patches. The purpose of this chapter is to describe practical approaches to methods and problems encountered in performing and analyzing experiments with inside-out membrane patches. Four sections follow, dealing with

The inward rectifier potassium current in the heart, I(K1), has been suggested to play a significant role in cardiac excitability by contributing to the late phase of action potential (AP) repolarization and the stabilization of resting potential. To further assess the role of I(K1) in cardiac excitability we have produced transgenic mice expressing a dominant-negative subunit of the Kir2.1 channel, a major molecular determinant of I(K1) in the heart, and studied the effects of I(K1) suppression on major potassium currents, APs and the overall electrical activity of the heart. Kir2.1 channel subunits with a mutated signature sequence (AAA for GYG substitution) were expressed in the heart under control of the alpha-myosin heavy chain promoter. Two lines of transgenic mice were established, both expressing high levels of Kir2.1-AAA-GFP (GFP, green fluorescent protein) subunits in all major parts of the heart. In ventricular myocytes isolated from transgenic mice, I(K1) was reduced by 95% in both lines, leading to a significant prolongation of APs. Surface ECG recordings from anesthetized transgenic mice revealed significant changes in key parameters of excitability, including prolongation of QRS complexes and QT intervals. This study confirms the significant role of I(K1) in control of AP repolarization and major ECG intervals in the intact heart.

Cardiac t-tubules are critical for efficient excitation-contraction coupling and undergo significant remodeling during various experimental and clinical stress conditions including heart failure. Recently, we have shown that recovery of ventricular myocytes after a brief hyposmotic shock is associated with significant sealing of t-tubules. Dimethyl Sulfoxide (DMSO) is a commonly used solvent and has been reported to have cardioprotective properties although the mechanisms underlying these protective effects remain largely unknown. Therefore, we tested whether the cardioprotective effects of DMSO are mediated by its action on t-tubular remodeling. We found that application of DMSO at the time of resolution of hyposmotic stress, when t-tubule sealing occurs, reduced the amount of fluorescent dextran trapped within sealed t-tubules. The effect of DMSO displayed sharp biphasic concentration dependence with 1% being the most effective dose (∼4-fold reduction of dextran trapping; p

The cardiac inward rectifier potassium current (I(K1)), present in all ventricular and atrial myocytes, has been suggested to play a major role in repolarization of the action potential and stabilization of the resting potential. The molecular basis is now ascribed to members of the Kir2 sub-family of inward rectifier K channel genes, and the availability of recombinant expression systems has led to elucidation of the mechanism of inward rectification, as well as additional regulatory mechanisms involving intracellular pH and phosphorylation. In vivo manipulation of the genes encoding I(K1)and regulatory proteins now promise to provide new insights to the role of this conductance in the heart. This review details recent advances and considers the prospects for further elucidation of the role of this conductance in cardiac electrical activity.

Many different types of K+ channels are involved in cardiac electrical activity (Noble, 1965). Various depolarization-activated K+ channels (Kv channels) determine the shape of the cardiac action potential. Cardiac cells also contain Kir channels (Fig. 1) that are open at very negative potentials but show a reduced conductance at positive membrane potentials, a phenomenon termed inward, or anomalous, rectification (Fig. 2A; Katz, 1949). Work over the past 30 years has described three main types of Kir channel in cardiac tissue. (1) The classical inward rectifier, I K1, present in atrial and ventricular myocytes, shows “strong” inward rectification. Essentially no current flows through these channels at potentials positive to —40 mV (Noble, 1965; Vandenberg, 1994). (2) The KATP channel is found in ventricular, atrial, and nodal cells. KATP channels display “weak” rectification, allowing substantial outward current to flow at positive potentials (Noma, 1983; Nichols and Lederer, 1991). (3) The muscarinicreceptor-activated K + channel, K(ACh), is a strong inward rectifier found predominantly in atrial tissue (Bond et al., 1994). K(ACh) channels open in response to acetylcholine released from the vagus nerve and underlie the resultant slowing of the heart rate.

Cardiac ventricular myocytes possess an extensive t-tubule system which plays a number of important roles including facilitation of membrane potential propagation across the cell body. It has been shown that ion fluxes at a highly restricted t-tubular space may lead to significant accumulation/depletion of specific ions which in turn may affect t-tubular, as well as whole-cell, membrane potential. The extent of ion accumulation/depletion depends on the current densities and the volume/structure of t-tubules. In this study we used the whole-cell patch-clamp technique to monitor t-tubular accumulation of potassium (caused by outward potassium currents in response to 400 ms voltage step to +50 mV) by measuring inward rectifier IK1 tail currents (Itail) at −75 mV. At room temperatures of ∼21-23 °C the amplitude of Itail current measured 9-10 minutes after the establishment of whole-cell configuration was essentially unchanged (93.6% of initial value), but declined to 53.5% upon application (within 2-5 minutes after the beginning of cell dialysis ) of 1 mM cyanide, a blocker of oxidative phosphorylation (n=9 and 6, respectively; p

Inward rectification of cardiac I(K1)channels was modulated by genetic manipulation of the naturally occurring polyamines. Ornithine decarboxylase (ODC) was overexpressed in mouse heart under control of the cardiac alpha -myosin heavy chain promoter (alpha MHC). In ODC transgenic hearts, putrescine and cadaverine levels were highly elevated ( identical with 35-fold for putrescine), spermidine was increased 3.6-fold, but spermine was essentially unchanged. I(K1)density was reduced by identical with 38%, although the voltage-dependence of rectification was essentially unchanged. Interestingly, the fast component of transient outward (I(to,f)) current was increased, but the total outward current amplitude was unchanged. I(K1)and I(to)currents were also studied in myocytes from mutant Gyro (Gy) mice in which the spermine synthase gene is disrupted, leading to a complete loss of spermine. I(K1)current densities were not altered in Gy myocytes, but the steepness of rectification was reduced indicating a role for spermine in controlling rectification. Intracellular dialysis of myocytes with putrescine, spermidine and spermine caused reduction, no change and increase of the steepness of rectification, respectively. Taken together with kinetic analysis of I(K1)activation these results are consistent with spermine being a major rectifying factor at potentials positive to E(K), spermidine dominating at potentials around and negative to E(K), and putrescine playing no significant role in rectification in the mouse heart.

This is an exciting time in the development of techniques to examine ion channel distributions in cells and tissues. As outlined above, novel fluorescent, functional and structural tools are being developed for microscopic and even nanoscopic localization. Following recent discoveries of channel structural components and biophysical mechanisms of permeation and gating, such techniques will be crucial for a new wave of ion channel studies. These will seek to determine exact locations of ion channels and specific patterns of their membrane localization and co-localization with underlying structures as well as the temporal sequence of events of channel biogenesis.

▪ Abstract The past three years have seen remarkable progress in research on the molecular basis of inward rectification, with significant implications for basic understanding and pharmacological manipulation of cellular excitability. Expression cloning of the first inward rectifier K channel (Kir) genes provided the necessary breakthrough that has led to isolation of a family of related clones encoding channels with the essential functional properties of classical inward rectifiers, ATP-sensitive K channels, and muscarinic receptor-activated K channels. High-level expression of cloned channels led to the discovery that classical inward so-called anomalous rectification is caused by voltage-dependent block of the channel by polyamines and Mg2+ ions, and it is now clear that a similar mechanism results in inward rectification of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)-kainate receptor channels. Knowledge of the primary structures of Kir channels and the ability to mutate them also has led to the determination of many of the structural requirements of inward rectification.

Abstract Since the cloning of the first inwardly rectifying K + channel in 1993, a family of related clones has been isolated, with many members being expressed in the heart. Exogenous expression of different clones has demonstrated that between them they encode channels with the essential functional properties of classic inward rectifier channels, ATP-sensitive K + channels, and muscarinic receptor–activated inward rectifier channels. High-level expression of cloned channels has led to the discovery that classic strong inward, or anomalous, rectification is caused by very steeply voltage-dependent block of the channel by polyamines, with an additional contribution by Mg 2+ ions. Knowledge of the primary structures of inward rectifying channels and the ability to mutate them have led to the determination of many of the structural requirements of inward rectification. The implications of these advances for basic understanding and pharmacological manipulation of cardiac excitability may be significant. For example, cellular concentrations of polyamines are altered under different conditions and can be manipulated pharmacologically. Simulations predict that changes in polyamine concentrations or changes in the relative proportions of each polyamine species could have profound effects on cardiac excitability.