Beta-blocker/ACE inhibitor therapy differentially impacts the steady state signaling landscape of failing and non-failing hearts

Background

The molecular underpinnings of heart failure with reduced ejection fraction (HFrEF) involves a complex remodeling of the contractile, metabolic and electrical functions. Current pharmacotherapy for patients presenting HFrEF includes combination of angiotensin-converting enzyme inhibitors (ACEi) and β-adrenergic receptor blockers (β-AR blockers). Yet, a knowledge gap exists regarding the molecular changes accompanying such treatment.

Purpose

The present study takes an omics approach to study protein and phosphorylation signaling derangement in HFrEF and to define the global changes resulting from treatment with β-AR blocker (metoprolol) and ACE inhibitor (enalapril) in control- and HFrEF hearts.

Methods and results

For induction of HFrEF, a tight and permanent ligature was applied to constrict the transverse aorta in male C57BL6 mice. Eight weeks post-surgery, an osmotic pump was implanted delivering either vehicle or treatment (enalapril, ACE inhibitor, and metoprolol, β-AR blocker) for a two week period. The proteome- and phosphoproteome of left ventricular tissue was resolved using high-resolution liquid chromatography-mass spectrometry. The resulting dataset covered 6,004 proteins and 14,967 phosphorylation events. HFrEF was characterized by profound downregulation of mitochondrial proteins coupled with derangement of β-adrenergic and pyruvate dehydrogenase signaling. Upon treatment, phosphorylation changes consequent to HFrEF were reversed, including a reversal of Pdhk4 activity. In controls, treatment mainly led to downregulation of canonical PKA signaling. Overall, the signaling response elicited by treatment was profoundly different for failing than for control hearts.

Conclusions

We used state-of-the-art proteomics and phosphoproteomics approaches to analyze changes in protein abundance and phosphorylation state of the left ventricle resulting from combination therapy with a β-AR blocker and an ACE inhibitor in failing and non-failing hearts. Our observation of divergent signaling outcomes depending on the disease state of the heart underscores the importance of evaluating drug effects within the context of the specific conditions present in the recipient heart.

Funding Acknowledgement

Type of funding sources: Foundation. Main funding source(s): The Danish Council for independent ResearchLundbeck FoundationFondation Leducq Transatlantic Network of Excellence

4965Non-transcriptional disruption of Ca2+i homeostasis and Cx43 function in the right ventricle precedes overt arrhythmogenic cardiomyopathy in PKP2-deficient mice

Background

Plakophilin-2 (PKP2) is classically defined as a protein of the desmosome, an intercellular adhesion structure that also acts as a signaling hub to maintain structural and electrical homeostasis. Mutations in PKP2 associate with most cases of gene-positive arrhythmogenic right ventricular cardiomyopathy (ARVC). A better understanding of PKP2 cardiac biology can help elucidate the mechanisms underlying arrhythmic and cardiomyopathic events that occur consequent to its mutation. Here we sought to captureearly molecular/cellular events that can act as nascent substrates for subsequent arrhythmic/cardiomyopathic phenotypes.

Methods

We used multiple quantitative imaging modalities, as well as biochemical and high-resolution mass spectrometry methods to study the functional/structural properties of cells/tissues derived from cardiomyocyte-specific, tamoxifen-activated, PKP2 knockout mice (“PKP2cKO”). Studies were carried out 14 days post-tamoxifen injection, a time point preceding an overt electrical or structural phenotype.Myocytes from right or left ventricular free wall were studied separately, to detect functional/structural asymmetries.

Results

Most properties of PKP2cKO left ventricular (LV) myocytes were not different from control; in contrast, PKP2cKO right ventricular (RV) myocytes showed increased amplitude and duration of Ca2+transients, increased frequency of spontaneous Ca2+release events, increased [Ca2+] in the cytoplasm and sarcoplasmic reticulum compartments, and dynamic Ca2+accumulation in mitochondria. In addition, RyR2 in RV presented enhanced sensitivity to Ca2+and preferential phosphorylation in a domain known to modulate Ca2+gating. RNAseq at 14 days post-TAM showed no relevant difference in transcript abundance between RV and LV, neither in control nor in PKP2cKO cells, suggesting that in the earliest stage, [Ca2+]i dysfunction is not transcriptional. Rather, we found an RV-predominant increase in membrane permeability that can permit Ca2+entry into the cell. Cx43 ablation mitigated the increase in membrane permeability, the accumulation of cytoplasmic Ca2+and the early stages of RV dysfunction.

Conclusions

Loss of PKP2 creates an RV-predominant arrhythmogenic substrate (Ca2+ dysregulation) that precedes the cardiomyopathy and that is, at least in part, mediated by a Cx43-dependent membrane conduit. Given that asymmetric Ca2+ dysregulation precedes the cardiomyopathic stage, we speculate that abnormal Ca2+ handling in RV myocytes can be a trigger for gross structural changes observed at a later stage.

Potential new mechanisms of pro-arrhythmia in arrhythmogenic cardiomyopathy: focus on calcium sensitive pathways

Arrhythmogenic cardiomyopathy, or its most well-known subform arrhythmogenic right ventricular cardiomyopathy (ARVC), is a cardiac disease mainly characterised by a gradual replacement of the myocardial mass by fibrous and fatty tissue, leading to dilatation of the ventricular wall, arrhythmias and progression towards heart failure. ARVC is commonly regarded as a disease of the intercalated disk in which mutations in desmosomal proteins are an important causative factor. Interestingly, the Dutch founder mutation PLN R14Del has been identified to play an additional, and major, role in ARVC patients within the Netherlands. This is remarkable since the phospholamban (PLN) protein plays a leading role in regulation of the sarcoplasmic reticulum calcium load rather than in the establishment of intercellular integrity. In this review we outline the intracellular cardiac calcium dynamics and relate pathophysiological signalling, induced by disturbed calcium handling, with activation of calmodulin dependent kinase II (CaMKII) and calcineurin A (CnA). We postulate a thus far unrecognised role for Ca2+ sensitive signalling proteins in maladaptive remodelling of the macromolecular protein complex that forms the intercalated disk, during pro-arrhythmic remodelling of the heart.

Minimum Information about a Cardiac Electrophysiology Experiment (MICEE): standardised reporting for model reproducibility, interoperability, and data sharing

Cardiac experimental electrophysiology is in need of a well-defined Minimum Information Standard for recording, annotating, and reporting experimental data. As a step towards establishing this, we present a draft standard, called Minimum Information about a Cardiac Electrophysiology Experiment (MICEE). The ultimate goal is to develop a useful tool for cardiac electrophysiologists which facilitates and improves dissemination of the minimum information necessary for reproduction of cardiac electrophysiology research, allowing for easier comparison and utilisation of findings by others. It is hoped that this will enhance the integration of individual results into experimental, computational, and conceptual models. In its present form, this draft is intended for assessment and development by the research community. We invite the reader to join this effort, and, if deemed productive, implement the Minimum Information about a Cardiac Electrophysiology Experiment standard in their own work.

Functional demonstration of connexin-protein binding using surface plasmon resonance

Surface plasmon resonance (SPR) allows examination of protein-protein interactions in real time, from which both binding affinities and kinetics can be directly determined. We have used the SPR technique to search for proteins in heart tissue that would be candidate binding partners for the cardiac gap junction protein, connexin43 (Cx43). Heart lysate showed a strong, pH-dependent binding to the carboxyl terminus (CT) of Cx43 (amino acids 254-382) covalently linked to an SPR cuvette. Binding was inhibited by the presence of v-src transfected 3T3 cell lysate, suggesting that binding partners in these two lysates may compete for overlapping epitopes on Cx43CT. The combined application of proteomic and functional studies is expected to identify which proteins within heart tissue interact with Cx43 and what roles they may play in gap junction function.

Heterotypic docking of Cx43 and Cx45 connexons blocks fast voltage gating of Cx43

Immunohistochemical co-localization of distinct connexins (Cxs) in junctional areas suggests the formation of heteromultimeric channels. To determine the docking effects of the heterotypic combination of Cx43 and Cx45 on the voltage-gating properties of their channels, we transfected DNA encoding Cx43 or Cx45 into N2A neuroblastoma or HeLa cells. Using a double whole-cell voltage-clamp technique, we determined macroscopic and single-channel gating properties of the intercellular channels formed. Cx43-Cx45 heterotypic channels had rectifying properties where Cx45 connexons inactivated rapidly upon hyperpolarizing voltage pulses applied to the Cx45-expressing cell. During depolarizing pulses to the Cx45-expressing cell, Cx43 connexons inactivated with substantially reduced kinetics as compared with homotypic Cx43 channels. Similar slow kinetics was observed for homotypic Cx43M257 (truncation mutant). Heterotypic channels had a main conductance whose value was predicted by the sum of corresponding homomeric connexon conductances; it was not voltage dependent and had no detectable residual conductance. The voltage-gating kinetics of heterotypic channels and their single-channel behavior implicate a role for the Cx43 carboxyl-terminal domain in the fast gating mechanism and in the establishment of residual conductance. Our results also suggest that heterotypic docking may lead to conformational changes that inhibit this action of the Cx43 carboxyl-terminal domain.

Null mutation of connexin43 causes slow propagation of ventricular activation in the late stages of mouse embryonic development

Connexin43 (Cx43) is the principal connexin isoform in the mouse ventricle, where it is thought to provide electrical coupling between cells. Knocking out this gene results in anatomic malformations that nevertheless allow for survival through early neonatal life. We examined electrical wave propagation in the left (LV) and right (RV) ventricles of isolated Cx43 null mutated (Cx43(-/-)), heterozygous (Cx43(+/)(-)), and wild-type (WT) embryos using high-resolution mapping of voltage-sensitive dye fluorescence. Consistent with the compensating presence of the other connexins, no reduction in propagation velocity was seen in Cx43(-/-) ventricles at postcoital day (dpc) 12.5 compared with WT or Cx43(+/)(-) ventricles. A gross reduction in conduction velocity was seen in the RV at 15.5 dpc (in cm/second, mean [1 SE confidence interval], WT 9.9 [8.7 to 11.2], Cx43(+/)(-) 9.9 [9.0 to 10.9], and Cx43(-/-) 2.2 [1.8 to 2.7; P<0.005]) and in both ventricles at 17.5 dpc (in RV, WT 8.4 [7.6 to 9.3], Cx43(+/)(-) 8.7 [8.1 to 9.3], and Cx43(-/-) 1.1 [0.1 to 1.3; P<0.005]; in LV, WT 10.1 [9.4 to 10.7], Cx43(+/)(-) 8.3 [7.8 to 8.9], and Cx43(-/-) 1.7 [1.3 to 2.1; P<0.005]) corresponding with the downregulation of Cx40. Cx40 and Cx45 mRNAs were detectable in ventricular homogenates even at 17.5 dpc, probably accounting for the residual conduction function. Neonatal knockout hearts were arrhythmic in vivo as well as ex vivo. This study demonstrates the contribution of Cx43 to the electrical function of the developing mouse heart and the essential role of this gene in maintaining heart rhythm in postnatal life.

The carboxyl terminal domain regulates the unitary conductance and voltage dependence of connexin40 gap junction channels

Chemical regulation of connexin (Cx) 40 and Cx43 follows a ball-and-chain model, in which the carboxyl terminal (CT) domain acts as a gating particle that binds to a receptor affiliated with the pore. Moreover, Cx40 channels can be closed by a heterodomain interaction with the CT domain of Cx43 and vice versa. Here, we report similar interactions in the establishment of the unitary conductance and voltage-dependent profile of Cx40 in N2A cells. Two mean unitary conductance values ("lower conductance" and "main") were detected in wild-type Cx40. Truncation of the CT domain at amino acid 248 (Cx40tr248) caused the disappearance of the lower-conductance state. Coexpression of Cx40tr248 with the CT fragment of either Cx40 (homodomain interactions) or Cx43 (heterodomain interactions) rescued the unitary conductance profile of Cx40. In the N2A cells, the time course of macroscopic junctional current relaxation was best described by a biexponential function in the wild-type Cx40 channels, but it was reduced to a single-exponential function after truncation. However, macroscopic junctional currents recorded in the oocyte expression system were not significantly different between the wild-type and mutant channels. Concatenation of the CT domain of Cx43 to amino acids 1 to 248 of Cx40 yielded a chimeric channel with unitary conductance and voltage-gating profile indistinguishable from that of wild-type Cx40. We conclude that residence of Cx40 channels in the lower-conductance state involves a ball-and-chain type of interaction between the CT domain and the pore-forming region. This interaction can be either homologous (Cx40 truncation with Cx40CT) or heterologous (with the Cx43CT).

Coexpression of connexins 40 and 43 enhances the pH sensitivity of gap junctions: a model for synergistic interactions among connexins

Gap junctions are formed by oligomerization of a protein called connexin. Most cells express more than one connexin isotype. Atrial myocytes, for example, coexpress connexin (Cx) 40 and Cx43. The consequence of connexin coexpression on the regulation of gap junctions is not well understood. In the present study, we show that cells coexpressing Cx40 and Cx43 are more susceptible to acidification-induced uncoupling than those cells expressing only one connexin isotype. Xenopus oocytes were injected with mRNA for Cx40, Cx43, or a combination of both. Intracellular pH and junctional conductance were simultaneously measured while cells were progressively acidified by superfusion with a bicarbonate-buffered solution gassed with increasing concentrations of carbon dioxide. The data show that the pKa (ie, the pH at which junctional conductance decreased to 50% from maximum) shifted from approximately 6.7 when cells expressed only Cx40 or only Cx43 to approximately 7.0 when one of the oocytes was coexpressing both connexins. Truncation of the carboxyl terminal domains of the connexins caused the loss of pH sensitivity even after coexpression. The data are interpreted on the basis of previous studies from our laboratory that demonstrated heterodomain interactions in the regulation of Cx40 and Cx43 gap junctions. The possible implications of these findings on the regulation of native gap junctions that express both connexins remain to be determined.

Identification of a protein kinase activity that phosphorylates connexin43 in a pH-dependent manner

The carboxyl-terminal (CT) domain of connexin43 (Cx43) has been implicated in both hormonal and pH-dependent gating of the gap junction channel. An in vitro assay was utilized to determine whether the acidification of cell extracts results in the activation of a protein kinase that can phosphorylate the CT domain. A glutathione S-transferase (GST)-fusion protein was bound to Sephadex beads and used as a target for protein kinase phosphorylation. A protein extract produced from sheep heart was allowed to bind to the fusion protein-coated beads. The bound proteins were washed and then incubated with 32P-ATP. Phosphorylation was assessed after the proteins were resolved by SDS-PAGE. Incubation at pH 7.5 resulted in a minimal amount of phosphorylation while incubation at pH 6.5 resulted in significant phosphorylation reaction. Maximal activity was achieved when both the binding and kinase reactions were performed at pH 6.5. The protein kinase activity was stronger when the incubations were performed with manganese rather than magnesium. Mutants of Cx43 which lack the serines between amino acids 364-374 could not be phosphorylated in the in vitro kinase reaction, indicating that this is a likely target of this reaction. These results indicate that there is a protein kinase activity in cells that becomes more active at lower pH and can phosphorylate Cx43.

Characterization of conduction in the ventricles of normal and heterozygous Cx43 knockout mice using optical mapping

INTRODUCTION

Gap junction channels are important determinants of conduction in the heart and may play a central role in the development of lethal cardiac arrhythmias. The recent development of a Cx43-deficient mouse has raised fundamental questions about the role of specific connexin isoforms in intercellular communication in the heart. Although a homozygous null mutation of the Cx43 gene (Cx43-/-) is lethal, the heterozygous (Cx43+/-) animals survive to adulthood. Reports on the cardiac electrophysiologic phenotype of the Cx43+/- mice are contradictory. Thus, the effects of a null mutation of a single Cx43 allele require reevaluation.

METHODS AND RESULTS

High-resolution video mapping techniques were used to study propagation in hearts from Cx43+/- and littermate control (Cx43+/+) mice. Local conduction velocities (CVs) and conduction patterns were quantitatively measured by determining conduction vectors. We undertook the characterization of ECG parameters and epicardial CVs of normal and Cx43+/- mouse hearts. ECG measurements obtained from 12 Cx43+/+ and 6 Cx43+/- age matched mice did not show differences in any parameter, including QRS duration (14.5 +/- 0.9 and 15.7 +/- 2.3 msec for Cx43+/+ and Cx43+/-, respectively). In addition, using a sensitive method of detecting changes in local CV, video images of epicardial wave propagation revealed similar activation patterns and velocities in both groups of mice.

CONCLUSION

A sensitive method that accurately measures local CVs throughout the ventricles revealed no changes in Cx43+/- mice, which is consistent with the demonstration that ECG parameter values in the heterozygous mice are the same as those in wild-type mice.

Proton and zinc effects on HERG currents

The proton and Zn2+ effects on the human ether-a-go-go related gene (HERG) channels were studied after expression in Xenopus oocytes and stable transfection in the mammalian L929 cell line. Experiments were carried out using the two-electrode voltage clamp at room temperature (oocytes) or the whole-cell patch clamp technique at 35 degrees C (L929 cells). In oocytes, during moderate extracellular acidification (pHo = 6.4), current activation was not shifted on the voltage axis, the time course of current activation was unchanged, but tail current deactivation was dramatically accelerated. At pHo < 6.4, in addition to accelerating deactivation, the time course of activation was slower and the midpoint voltage of current activation was shifted to more positive values. Protons and Zn2+ accelerated the kinetics of deactivation with apparent Kd values about one order of magnitude lower than for tail current inhibition. For protons, the Kd values for the effect on tail current amplitude versus kinetics were, respectively, 1.8 microM (pKa = 5.8) and 0.1 microM (pKa = 7.0). In the presence of Zn2+, the corresponding Kd values were, respectively, 1.2 mM and 169 microM. In L929 cells, acidification to pHo = 6.4 did not shift the midpoint voltage of current activation and had no effect on the time course of current activation. Furthermore, the onset and recovery of inactivation were not affected. However, the acidification significantly accelerated tail current deactivation. We conclude that protons and Zn2+ directly interact with HERG channels and that the interaction results, preferentially, in the regulation of channel deactivation mechanism.

Hetero-domain interactions as a mechanism for the regulation of connexin channels

Previous studies have shown that chemical regulation of connexin43 (Cx43) depends on the presence of the carboxyl terminal (CT) domain. A particle-receptor (or "ball-and-chain") model has been proposed to explain the mechanism of gating. We tested whether the CT region behaved as a functional domain for other members of the connexin family. The pH sensitivity of wild-type and Ct-truncated connexins was quantified by use of electrophysiological and optical techniques and the Xenopus oocyte system. The CT domain of Cx45 had no role in pH regulation, although a partial role was shown for Cx37 and Cx50. A prominent effect was observed for Cx40 and Cx43. In addition, we found that the CT domain of Cx40 that was expressed as a separate fragment rescued the pH sensitivity of the truncated Cx40 (Cx40tr), which was in agreement with a particle-receptor model. Because Cx40 and Cx43 often colocalize and possibly heteromerize, we tested the pH sensitivity of Cx40tr when coexpressed with the CT domain of Cx43 (hetero-domain interactions). We found that the CT domain of Cx43 enhanced the pH sensitivity of Cx40tr; similarly, the CT domain of Cx40 restored the pH sensitivity of the truncated Cx43. In addition, the CT domain of Cx43 granted insulin sensitivity to the otherwise insulin-insensitive Cx26 or Cx32 channels. These data show that the particle-receptor model is preserved in Cx40 and the regulatory domain of one connexin can specifically interact with a channel formed by another connexin. Hetero-domain interactions could be critical for the regulation of heteromeric channels.

Connexin diversity and gap junction regulation by pHi

The molecular mechanisms controlling pH-sensitivity of gap junctions formed of two different connexins are yet to be determined. We used a proton-sensitive fluorophore and electrophysiological techniques to correlate changes in intracellular pH (pHi) with electrical coupling between connexin-expressing Xenopus oocytes. The pH sensitivities of alpha 3 (connexin46), alpha 2 (connexin38), and alpha 1 (connexin43) were studied when these proteins were expressed as: 1) nonjunctional hemichannels (for alpha 3 and alpha 2), 2) homotypic gap junctions, and 3) heterotypic gap junctions. We found that alpha 3 hemichannels are sensitive to changes in pHi within a physiological range (pKa = 7.13 +/- 0.03; Hill coefficient = 3.25 +/- 1.73; n = 8; mean +/- SEM); an even more alkaline pKa was obtained for alpha 2 hemichannels (pKa = 7.50 +/- 0.03; Hill coefficient = 3.22 +/- 0.66; n = 13). The pH sensitivity curves of alpha 2 and alpha 3 homotypic junctions were indistinguishable from those recorded from hemichannels of the same connexin. Based on a comparison of pKa values, both alpha 3 and alpha 2 gap junctions were more pHi-dependent than alpha 1. The pH sensitivity of alpha 2-containing heterotypic junctions could not be predicted from the behavior of the two connexons in the pair. When alpha 2 was paired with alpha 3, the pH sensitivity curve was similar to that obtained from alpha 2 homotypic pairs. Yet, pairing alpha 2 with alpha 1 shifted the curve similar to homotypic alpha 1 channels. Pairing alpha 2 with a less pH sensitive mutant of alpha 1 (M257) yielded the same curve as when alpha 1 was used. However, the pH sensitivity curve of alpha 3/alpha 1 channels was similar to alpha 3/alpha 3, while alpha 3/M257 was indistinguishable from alpha 3/alpha 1. Our results could not be consistently predicted by a probabilistic model of two independent gates in series. The data show that dissimilarities in the pH regulation of gap junctions are due to differences in the primary sequence of connexins. Moreover, we found that pH regulation is an intrinsic property of the hemichannels, but pH sensitivity is modified by the interactions between connexons. These interactions should provide a higher level of functional diversity to gap junctions that are formed by more than one connexin.

A particle-receptor model for the insulin-induced closure of connexin43 channels

Connexin43(Cx43) channels can be regulated by a variety of factors, including low pHi. Structure/function studies from this laboratory have demonstrated that pH gating follows a particle-receptor mechanism, similar to the "ball-and-chain" model of voltage-dependent inactivation of ion channels. The question whether the particle-receptor model is applicable only to pH gating or to other forms of Cx43 regulation as well remains. To address this question, we looked at the uncoupling effects of insulin and of insulin-like growth factor-1 (IGF) on Cx43 channels expressed in Xenopus oocytes. These agonists do not induce changes in pHi. Junctional conductance (Gj) was measured by the dual 2-electrode voltage-clamp technique. Control studies showed that relative Gj did not change spontaneously as a function of time. Continuous exposure of Cx43-expressing oocytes to insulin (10 micro/L) led to a decrease in Gj. After 80 minutes, Gj was 54+/-5% from control (n= 12). Exposure of oocytes to IGF (10 nmol/L) caused an even more pronounced change in Gj (37+/-4% of control, n=6). The time course of the IGF-induced uncoupling was similar to that observed after insulin exposure. The effect of insulin was abolished by truncation of the carboxyl-terminal domain of Cx43 at amino acid 257 (M257). Interestingly, as in the case of pH gating, coexpression of the carboxyl-terminal domain (amino acids 258 to 282) together with M257 rescued the ability of insulin to reduce coupling (Gj, 39+/-12% from control; n=6). Structure/function experiments using various deletion mutants of the carboxyl-terminal domain showed that insulin treatment does not modify Gj if amino acids 261 to 280 are missing from the Cx43 sequence. Our results suggest that a particle-receptor (or ball-and-chain) mechanism, similar to that described for pH gating, also applies to chemical regulation of Cx43 by other factors.

A 17mer peptide interferes with acidification-induced uncoupling of connexin43

Structure/function analysis shows that the carboxyl terminal (CT) domain of connexin43 (Cx43) is essential for the chemical regulation of cell-cell communication. Of particular interest is the region between amino acids 260 and 300. Structural preservation of this region is essential for acidification-induced uncoupling (ie, pH gating). In this study, we report data showing that a 17mer peptide of the same sequence as amino acids 271 to 287 of Cx43 (CSSPTAPLSPMSPPGYK) can prevent pH gating of Cx43-expressing oocytes. Experiments were carried out in pairs of Xenopus oocytes previously injected with connexin38 antisense and expressing wild-type Cx43. Junctional conductance was measured electrophysiologically. pHi was determined from the light emission of the proton-sensitive dye dextran-seminaphthorhodafluor. Intracellular acidification was induced by superfusion with a bicarbonate-buffered solution gassed with a progressively increasing concentration of CO2. Injection of water alone into both oocytes of a Cx43-expressing pair or injection of a peptide from region 321 to 337 of Cx43 did not modify pH sensitivity. However, injection of a polypeptide corresponding to amino acids 241 to 382 of Cx43 interfered with the ability of gap junctions to close on acidification. Similar results were obtained when a 17mer peptide (region 271 to 287) was injected into both oocytes of the pair. Normal Cx43 pH gating was observed if (1) the amino acid sequence of the 17mer peptide was scrambled or (2) the N and the C ends of the 17mer peptide were not included in the sequence. This is the first demonstration of a molecule that can interfere with the chemical regulation of connexin channels in a cell pair. The data may lead to the development of small molecules that can be used in Cx43-expressing multicellular preparations to study the role of gap junction regulation in normal as well as diseased states.

Structure of connexin43 and its regulation by pHi

Electrical coupling in the heart provides an effective mechanism for propagating the cardiac action potential efficiently throughout the entire heart. Cells within the heart are electrically coupled through specialized membrane channels called gap junctions. Studies have shown that gap junctions are dynamic, carefully regulated channels that are important for normal cardiogenesis. We have recently been interested in the molecular mechanisms by which intracellular acidification leads to gap junction channel closure. Previous results in this lab have shown that truncation of the carboxyl terminal (CT) of connexin43 (Cx43) does not interfere with functional channel expression. Further, the pH-dependent closure of Cx43 channels is significantly impaired by removal of this region of the protein. Other studies have shown that the CT is capable of interacting with its receptor even when not covalently attached to the channel protein. From these data we have proposed a particle-receptor model to explain the pH-dependent closure of Cx43 gap junction channels. Detailed analysis of the CT has revealed interesting new information regarding its possible structure. Here we review the most recent studies that have contributed to our understanding of the molecular mechanisms of regulation of the cardiac gap protein Cx43.

PH regulation of connexin43: molecular analysis of the gating particle

Gap junction channels allow for the passage of ions and small molecules between neighboring cells. These channels are formed by multimers of an integral membrane protein named connexin. In the heart and other tissues, the most abundant connexin is a 43-kDa, 382-amino acid protein termed connexin43 (Cx43). A characteristic property of connexin channels is that they close upon acidification of the intracellular space. Previous studies have shown that truncation of the carboxyl terminal of Cx43 impairs pH sensitivity. In the present study, we have used a combination of optical, electrophysiological, and molecular biological techniques and the oocyte expression system to further localize the regions of the carboxyl terminal that are involved in pH regulation of Cx43 channels. Our results show that regions 261-300 and 374-382 are essential components of a pH-dependent "gating particle," which is responsible for acidification-induced uncoupling of Cx43-expressing cells. Regions 261-300 and 374-382 seem to be interdependent. The function of region 261-300 may be related to the presence of a poly-proline repeat between amino acids 274 and 285. Furthermore, site-directed mutagenesis studies show that the function of region 374-382 is not directly related to its net balance of charges, although mutation of only one amino acid (aspartate 379) for asparagine impairs pH sensitivity to the same extent as truncation of the carboxyl terminal domain (from amino acid 257). The mutation in which serine 364 is substituted for proline, which has been associated with some cases of cardiac congenital malformations in humans, also disrupts the pH gating of Cx43, although deletion of amino acids 364-373 has no effect on acidification-induced uncoupling. These results provide new insight into the molecular mechanisms responsible for acidification-induced uncoupling of gap junction channels in the heart and in other Cx43-expressing structures.

Intramolecular interactions mediate pH regulation of connexin43 channels

We have previously proposed that acidification-induced regulation of the cardiac gap junction protein connexin43 (Cx43) may be modeled as a particle-receptor interaction between two separate domains of Cx43: the carboxyl terminal (acting as a particle), and a region including histidine 95 (acting as a receptor). Accordingly, intracellular acidification would lead to particle-receptor binding, thus closing the channel. A premise of the model is that the particle can bind its receptor, even if the particle is not covalently bound to the rest of the protein. The latter hypothesis was tested in antisense-injected Xenopus oocyte pairs coexpressing mRNA for a pH-insensitive Cx43 mutant truncated at amino acid 257 (i.e., M257) and mRNA coding for the carboxyl terminal region (residues 259-382). Intracellular pH (pHo) was recorded using the dextran form of the proton-sensitive dye seminaphthorhodafluor (SNARF). Junctional conductance (Gj) was measured with the dual voltage clamp technique. Wild-type Cx43 channels showed their characteristic pH sensitivity. M257 channels were not pH sensitive (pHo tested: 7.2 to 6.4). However, pH sensitivity was restored when the pH-insensitive channel (M257) was coexpressed with mRNA coding for the carboxyl terminal. Furthermore, coexpression of the carboxyl terminal of Cx43 enhanced the pH sensitivity of an otherwise less pH-sensitive connexin (Cx32). These data are consistent with a model of intramolecular interactions in which the carboxyl terminal acts as an independent domain that, under the appropriate conditions, binds to a separate region of the protein and closes the channel. These interactions may be direct (as in the ball-and-chain mechanism of voltage-dependent gating of potassium channels) or mediated through an intermediary molecule. The data further suggest that the region of Cx43 that acts as a receptor for the particle is conserved among connexins. A similar molecular mechanism may mediate chemical regulation of other channel proteins.

A model study of changes in excitability of ventricular muscle cells: inhibition, facilitation, and hysteresis

A model study was carried out to investigate the mechanism of changes in excitability at long cycle lengths (i.e., > 1,000 ms), which are responsible for various phenomena, including electrotonic inhibition, active facilitation, and hysteresis of excitability in ventricular muscle at slow frequencies of stimulation. Experimental studies suggested that with repetitive activity the inward rectifier potassium current (IK1) is not a passive component of membrane response and that the dynamics of IK1 are responsible for the changes in excitability at long cycle lengths. In the present study, we have used new experimental data as the basis to modify the equations for IK1 in the ionic model for ventricular muscle of the Luo and Rudy (LR) model. The modified equations for IK1 incorporate an additional slow gate (s-gate), which governs the transition from a high steady-state conductance at rest to a lower conductance with repetitive stimulation. In simulation studies, electronic inhibition was seen in the original and the modified LR model and was shown to depend on changes in the delayed rectifier current (IK). However, addition of the s-gate to IK1 of the LR model extended the frequency dependence of excitability to longer cycle lengths and allowed for the demonstration of active facilitation and hysteresis. These results support the hypothesis that the inward rectifier is involved in the dynamic control of membrane excitability. The overall results provide mechanistic explanations for heart rate-dependent excitation abnormalities that may be involved in the genesis of cardiac arrhythmias.

Characterization of an E4031-sensitive potassium current in quiescent AT-1 cells

INTRODUCTION

A cardiac culture cell line (AT-1) recently has been generated from transgenic mice. Initial studies have yielded opposing results as to the nature of the major repolarizing current(s) in these cells. The present study describes the ion selectivity, voltage dependence, and E4031 sensitivity of the major time-dependent outward current present in AT-1 cells. In addition, we have determined whether an outward current with the characteristics we observed could be capable of modulating action potential duration in a frequency-dependent manner (for stimulation cycle lengths between 250 and 1000 msec).

METHODS AND RESULTS

Action potentials and membrane currents were recorded from nonconfluent cultures of quiescent AT-1 cells using the "perforated patch" technique. AT-1 cells showed a round appearance 1 or 2 days after plating. An E4031-insensitive transient outward current seemed to be absent in these cells. The main time-dependent outward current was a rapidly activating and rectifying potassium current with properties similar to those of IKr. Most of the potassium current was sensitive to the benzenesulfonamide E4031 (5 microM). The same concentration of E4031 led to a 38% increase in action potential duration. Action potential parameters were independent of the stimulation cycle length within the range of 250 to 1000 msec, thus suggesting that the membrane currents involved in the action potential of AT-1 cells are completely reset within a diastolic interval of approximately 150 msec.

CONCLUSION

AT-1 cells present a unique electrophysiologic phenotype, which is clearly different from that reported for freshly dissociated adult atrial or ventricular myocytes from other species. AT-1 cells may be a good model to study IKr, since there seems to be minimal contamination by other outward conductances (such as IKs). In addition, the feasibility of culturing AT-1 cells provides us with a system where electrophysiologic experiments on IKr currents could be combined with biochemical or molecular biological studies requiring significant periods of incubation in a cell culture system.

Electrotonic inhibition and active facilitation of excitability in ventricular muscle

INTRODUCTION

The effects of subthreshold electrical pulses on the response to subsequent stimulation have been described previously in experimental animal studies as well as in the human heart. In addition, previous studies in cardiac Purkinje fibers have shown that diastolic excitability may decrease after activity (active inhibition) and, to a lesser extent, following subthreshold responses (electrotonic inhibition). However, such dynamic changes in excitability have not been explored in isolated ventricular muscle, and it is uncertain whether similar phenomena may play any role in the activation patterns associated with propagation abnormalities in the myocardium.

METHODS AND RESULTS

Experiments were performed in isolated sheep Purkinje fibers and papillary muscles, and in enzymatically dissociated guinea pig ventricular myocytes. In all types of preparations introduction of a conditioning subthreshold pulse between two suprathreshold pulses was followed by a transient decay in excitability (electrotonic inhibition). The degree of inhibition was directly related to the amplitude and duration of the conditioning pulse and inversely related to the postconditioning interval. Yet, inhibition could be demonstrated long after (> 1 sec) the end of the conditioning pulse. Electronic inhibition was found at all diastolic intervals and did not depend on the presence of a previous action potential. In Purkinje fibers, conditioning action potentials led to active inhibition of subsequent responses. In contrast, in muscle cells, such action potentials had a facilitating effect (active facilitation). Electrotonic inhibition and active facilitation were observed in both sheep ventricular muscle and guinea pig ventricular myocytes. Accordingly, during repetitive stimulation with pulses of barely threshold intensity, we observed: (1) bistability (i.e., with the same stimulating parameters, stimulus:response patterns were either 1:1 or 1:0, depending on previous history), and (2) abrupt transitions between 1:1 and 1:0 (absence of intermediate Wenckebach-like patterns). Simulations utilizing an ionic model of cardiac myocytes support the hypothesis that electrotonic inhibition in well-polarized ventricular muscle is the result of partial activation of IK following subthreshold pulses. On the other hand, active facilitation may be the result of an activity-induced decrease in the conductance of IK1.

CONCLUSION

Diastolic excitability of well-polarized ventricular myocardium may be transiently depressed following local responses and transiently enhanced following action potentials. On the other hand, diastolic excitability decreases during quiescence. Active facilitation and electrotonic inhibition may have an important role in determining the dynamics of excitation of the myocardium in the presence of propagation abnormalities.

Role of histidine 95 on pH gating of the cardiac gap junction protein connexin43

We have studied the role of histidine 95 (H95) on the pH gating of the cardiac gap junction protein connexin43 (Cx43). Wild-type and mutant rat cardiac Cx43 channels were expressed in antisense-injected Xenopus oocytes. Junctional conductance was measured using the dual voltage-clamp technique, and intracellular acidification was induced by superfusion with a sodium acetate-containing solution balanced at a pH of 6.2. H95 was substituted by other amino acids by use of oligonucleotide-directed site-specific mutagenesis. Replacing H95 for the hydrophobic residues methionine or phenylalanine, for the charged basic residue arginine, or for the noncharged residue glutamine (H95Q) yielded nonfunctional channels. Functional expression of H95Q was rescued by placing a histidine residue in position 93 (H95Q-L93H), 94 (H95Q-A94H), or 97 (H95Q-F97H) but not in position 96. Further experiments showed that replacing H95 with either aspartate (an acidic residue) or tyrosine (a polar uncharged residue) led to the expression of functional channels with a reduced susceptibility to acidification-induced uncoupling, whereas lysine (a basic residue) was more susceptible to uncoupling than the wild-type protein. The susceptibility to acidification-induced uncoupling was enhanced for the H95Q-A94H mutant when compared with the wild-type mutant, but it was significantly reduced when histidine was placed at position 93 (H95Q-L93H). Our data indicate that a properly placed histidine residue is an important structural element for functional expression as well as for pH regulation of Cx43. The results suggest that the importance of H95 on pH gating may be associated with a possible protonation of this residue on acidification of the intracellular environment.(ABSTRACT TRUNCATED AT 250 WORDS)

Immunohistochemical localization of gap junction protein channels in hamster sinoatrial node in correlation with electrophysiologic mapping of the pacemaker region

INTRODUCTION

Gap junction proteins are thought to form the low resistance pathways that connect neighboring cells within the sinoatrial node, and to mediate pacemaker synchronization.

METHODS AND RESULTS

We have carried out microelectrode mapping experiments of the hamster sinoatrial region to localize the primary pacemaker area for subsequent light, electron, and immunofluorescence microscopic studies aimed at testing the hypothesis that the major cardiac gap junction protein (connexin43) is present in such an area. The site of earliest activation is unifocal and the pattern of activation, obtained by multiple sequential microelectrode recordings of the sinoatrial region, is qualitatively similar to that previously described for other species. However, quantitatively, the impulse transmission time from the primary pacemaker area to the crista (sulcus) terminalis in the hamster sinoatrial node is about 50% briefer than that of the guinea pig and five times faster than that of the rabbit. Immunolocalization studies in the hamster sinoatrial node using anti-connexin43 antisera demonstrated specific staining at the areas of cell-to-cell apposition and suggested that the apparently high degree of electrical coupling in this tissue is the result of abundant connexin43 expression. The immunofluorescence data were supported by light microscopic studies, which demonstrated the typical morphologic characteristics of sinus nodal cells in the pacemaker area. In addition, an electron microscopic study of the sinoatrial region revealed the presence of gap junctions in the junctional complex at areas of cell-to-cell contact.

CONCLUSION

Our results demonstrate that cells in the sinoatrial region of the hamster heart are electrically well coupled and strongly suggest that such coupling is mediated by gap junctional channels formed by connexin43.

Effects of diacetyl monoxime on the electrical properties of sheep and guinea pig ventricular muscle

OBJECTIVE

Diacetyl monoxime (DAM), a nucleophilic agent with "phosphatase-like" activity, has been found to effectively and reversibly block cardiac muscle contraction, while the cells remain capable of generating transmembrane action potentials. The aim of this study was to characterise the effects of DAM on the electrical properties of cardiac muscle.

METHODS

Sheep epicardial muscle, guinea pig papillary muscle, and guinea pig ventricular myocytes were studied using conventional microelectrode techniques as well as single electrode current and voltage clamp techniques.

RESULTS

DAM (5-20 mM) decreased action potential duration at 50% and 90% repolarisation levels (APD50, APD90) and refractory period in a dose dependent manner without causing significant changes in action potential amplitude, maximum upstroke velocity, or resting membrane potential. DAM induced a slight decrease in action potential conduction velocity in both the longitudinal and transverse directions, but on average the conduction velocity recorded in the presence of the drug was not significantly different from control. The time course of the APD restitution curve was not significantly changed but the frequency dependent APD variations were reduced. The ionic bases for these changes were studied in guinea pig ventricular myocytes. As with the results obtained in tissue preparations, DAM 15 mM decreased APD50 and APD90 by 35% and 29%, respectively. Under voltage clamp conditions, DAM led to a 35% reduction of ICa. The delayed rectifier IK current and the inward rectifier background current were also partially depressed by DAM but to a lesser extent. All of these effects were reversible upon washout.

CONCLUSIONS

Aside from its well known effect as an electromechanical uncoupler, DAM causes a small, reversible, and non-selective reduction of several membrane conductances. Provided such effects are taken into consideration, DAM is a valuable tool in electrophysiological studies.

Ionic mechanisms of electronic inhibition and concealed conduction in rabbit atrioventricular nodal myocytes

BACKGROUND

The term "concealed conduction" is used in electrocardiography when a proximal (atrial or ventricular) impulse penetrates the atrioventricular (AV) node but fails to traverse it completely. Its penetration into the node is inferred by its after effects on the propagation of succeeding impulses. Concealed AV nodal conduction is a well-established phenomenon, but its precise cellular and subcellular mechanisms are unknown. It has been suggested that concealed conduction results from a transient impairment of excitability caused by the subthreshold depolarization (ie, electrotonic inhibition) that is elicited downstream of the site of block.

METHODS AND RESULTS

We studied the ionic mechanism of electrotonic inhibition and concealed conduction in single myocytes isolated from the rabbit AV node. Cells were paced using just-threshold current pulses delivered at a constant (basic) cycle length of 1 second. Appropriately timed interpolation of a conditioning pulse of depolarizing but subthreshold current led to failure of the subsequent, previously successful activation. The ability of the subthreshold response to inhibit subsequent excitation was increased when the interval between the conditioning and succeeding pulses was shortened, when the amplitude of the conditioning pulse was increased, or when the inward sodium current was blocked by superfusion with tetrodotoxin (30 microM). Voltage clamp analysis demonstrated that electrotonic inhibition results from partial inactivation of the transient calcium current (ICa.T). Similar results were obtained using a mathematical model (Hodgkin-Huxley type) of the AV nodal myocyte. Additional simulations in a linear array of AV nodal cells showed that when a premature impulse fails to traverse the AV node, the subthreshold depolarization elicited downstream of the site of block may lead to a transient reduction of excitability with consequent delay or block of the succeeding impulse.

CONCLUSIONS

The overall data strongly suggest that some of the electrocardiographic manifestations of concealed AV conduction are the result of electrotonic inhibition of excitability secondary to a transient decrease in ICa.T.

A structural basis for the unequal sensitivity of the major cardiac and liver gap junctions to intracellular acidification: the carboxyl tail length

The regulation of junctional conductance (Gi) of the major cardiac (connexin43; Cx43) and liver (connexin32; Cx32) gap junction proteins by intracellular hydrogen ion concentration (pH; pHi), as well as well as that of a truncation mutant of Cx43 (M257) with 125 amino acids deleted from the COOH terminus, was characterized in pairs of Xenopus laevis oocytes expressing homologous channels. Oocytes were injected with 40 nl mRNAs (2 micrograms/microliters) encoding the respective proteins; subsequently, cells were stripped, paired, and incubated for 20-24 h. Gj was measured in oocyte pairs using the dual electrode voltage-clamp technique, while pHi was recorded simultaneously in the unstimulated cell by means of a proton-selective microelectrode. Because initial experiments showed that the pH-sensitive microelectrode responded more appropriately to acetate than to CO2 acidification, oocytes expressing Cx32 and wild type and mutant Cx43 were exposed to a sodium acetate saline, which was balanced to various levels of pH using NaOH and HCl. pH was changed in a stepwise manner, and quasi-steady-state Gj -pHi relationships were constructed from data collected at each step after both Gj and pHi had reached their respective asymptotic values. A moderate but significant increase of Gj was observed in Cx43 pairs as pHi decreased from 7.2 to 6.8. In both Cx32 and M257 pairs, Gj increased significantly over a wider pH range (i.e., between 7.2 and 6.3). Further acidification reversibly reduced Gj to zero in all oocyte pairs. Pooled data for the individual connexins obtained during uncoupling were fitted by the Hill equation; apparent 50%-maximum (pK;pKa) values were 6.6 and 6.1 for Cx43 and Cx32, respectively, and Hill coefficients were 4.2 for Cx43 and 6.2 for Cx32. Like Cx32, M257 had a more acidic pKa (6.1) and steeper Hill coefficient (6.0) than wild type Cx43. The pKa and Hill coefficient of M257 were very similar to those of Cx32. These experiments provide the first direct comparison of the effects of acidification on Gj in oocyte pairs expressing Cx43 or Cx32. The results indicate that structural differences in the connexins are the basis for their unequal sensitivity to intracellular acidification in vivo. The data further suggest that a common pH gating mechanism may exist between amino acid residues 1 and 256 in both Cx32 and Cx43. However, the longer carboxyl tail of Cx43 relative to Cx32 or M257 provides additional means to facilitate acidification-induced gating; its presence shifts the pKa from 6.1 (Cx32 and M257) to 6.6 (Cx43) in the conductance of these channels.

Effects of 2,4-dinitrophenol or low [ATP]i on cell excitability and action potential propagation in guinea pig ventricular myocytes

Inhibition of aerobic metabolism leads to a major disruption of cardiac cell homeostasis. The purpose of the present study was twofold: 1) We determined the relative importance of junctional and nonjunctional membrane resistance (Rj and Rm, respectively) in the development of propagation failure during inhibition of aerobic metabolism in guinea pig ventricular cell pairs. 2) We used the patch-action potential clamp technique in single ventricular myocytes to study some of the properties of the membrane channels that are responsible for shortening of action potential duration and eventual failure of cell excitation after metabolic blockade. In most experiments, whole-cell patch pipettes were filled with a solution containing 1 mM EGTA, 5 mM HEPES, and 5 mM ATP. Our results in cell pairs showed that pharmacological inhibition of aerobic metabolism with the mitochondrial uncoupler 2,4-dinitrophenol (DNP) led to a drop in Rm followed by an increase in Rj. The increase in Rj was not sufficient to cause a measurable delay in cell-to-cell propagation, whereas the drop in Rm consistently led to failure of cell excitation. Similar results were obtained in additional experiments in which the EGTA concentration in the pipette was reduced to 50 microM. Similar results were also obtained by loading the recording patch pipettes with a solution containing only 0.1 mM ATP. Our patch-action potential clamp experiments, on the other hand, revealed that DNP induced the opening of time- and voltage-independent membrane channels, with a unitary conductance of 23 pS. The channels allowed for the passage of outward current in the voltage range of the action potential, and the increase in membrane patch conductance correlated with the observed shortening of action potential duration during DNP superfusion. Our experiments provide the first simultaneous recordings of action potentials and DNP-induced channel currents in guinea pig ventricular myocytes. Overall, the data provide new evidence for the understanding of the cellular and subcellular mechanisms involved in the development of slow conduction velocity and propagation block after metabolic blockade.

Analytical modeling of the hysteresis phenomenon in guinea pig ventricular myocytes

In the present study, we have demonstrated hysteresis phenomena in the excitability of single, enzymatically dissociated guinea pig ventricular myocytes. Membrane potentials were recorded with patch pipettes in the whole-cell current clamp configuration. Repetitive stimulation with depolarizing current pulses of constant cycle length and duration but varying strength led to predictable excitation (1:1) and non-excitation (1:0) patterns depending on current strength. In addition, transition between patterns depended on the direction of current intensity change and stable hysteresis loops were obtained in stimulus:response pattern vs. current intensity plots in 14 cells. Increase of pulse duration and decrease of stimulation rate contributed to a reduction in hysteresis loop areas. Changes in amplitude and shape of the subthreshold responses during the transitions from one stable pattern to the other, suggested that activity led to an increase in membrane resistance, particularly in the voltage domain between resting potential, and threshold. Therefore, we modelled the dynamic behaviour of the single cells as a function of diastolic membrane resistance, using previously published analytical solutions. Numerical iteration of the analytical model equations closely reproduced the experimental hysteresis loops in both qualitative and quantitative ways. In particular, the effect of stimulation frequency on the model was similar to the experimental findings. The overall study suggests that the excitability pattern of guinea pig ventricular myocytes accounts for hysteresis and bistabilities when current intensity is allowed to fluctuate around threshold levels.

Gap junctional channels in adult mammalian sinus nodal cells. Immunolocalization and electrophysiology

The subcellular mechanism of cell-to-cell communication in the natural pacemaker region of the mammalian heart was studied using electrophysiological and immunofluorescence techniques in isolated pairs of rabbit sinus nodal cells. By measuring whole-cell currents using a double patch-clamp approach, it was demonstrated that communication in the sinus node is mediated through gap junctional channels similar to those in other types of adult cardiac cell pairs. Macroscopic sinus nodal junctional resistance had a mean value of 387.9 +/- 97.1 M omega (mean +/- SEM, n = 10) and was greatly increased by superfusion with alkanols. Single-channel junctional conductance could be resolved in three cell pairs. Given their high membrane resistance (1.16 +/- 0.32 G omega, n = 12), the electrical coupling provided by as few as three gap junctional channels between nodal cells will allow for pacemaker synchronization. Further evidence for the presence of the channels was obtained from immunofluorescent double-labeling of desmin and the gap junction protein (connexin43) in sinus nodal tissue as well as in cultured sinus nodal cells. Using antisera against residues 243-257 of the connexin43 protein, a specific staining at the site of cell-to-cell apposition was demonstrated. These data provide direct evidence in favor of electronic coupling as the means for achieving pacemaker synchronization in the rabbit sinus node.

Immunolocalization and expression of functional and nonfunctional cell-to-cell channels from wild-type and mutant rat heart connexin43 cDNA

The carboxyl terminal cytoplasmic domain of distinct gap junction proteins may play an important role in assembly of functional channels as well as differential responsiveness to pH, voltage, and intracellular second messengers. Oligonucleotide-directed site-specific mutagenesis in a paired Xenopus laevis oocyte expression system was used to examine the expression of mRNAs encoding wild-type and carboxyl terminal mutant connexin43 (Cx43) proteins. Oocytes were stripped, injected with mRNA or distilled water (dH2O), preincubated for 16-20 hours, and then paired for 5-10 hours; this process was followed by electrophysiological recording using the dual voltage-clamp technique. Initial experiments compared the relative junctional conductances (Gjs) in oocyte pairs expressing Cx43 (382 amino acid residues) and two truncated mutants lacking most or a portion of the cytoplasmic carboxyl terminal. The shortest mutant (M241) contained 240 amino acid residues and was devoid of all phosphorylatable serine residues in the cytoplasmic tail; its length approximated the length of liver connexin26. The longest mutant (M257) tested contained 256 amino acid residues, including two serine residues. Oocyte pairs expressing M241 yielded a Gj similar to that of oocytes injected with dH2O, whereas M257 yielded a Gj similar to that of oocytes injected with Cx43. Immunoprecipitation studies showed that Cx43, M257, and M241 proteins were readily detectable in oocytes injected with their respective mRNAs, indicating that the lack of Gj observed with the M241 mRNA was not due to reduced translation. Immunocytochemical studies revealed that wild-type and both truncated mutants were localized to the area of cell-to-cell contact between the paired oocytes, indicating that protein targeting to the membrane was not inhibited in oocytes injected with M241 mRNA. Oocyte pairs expressing mutants in which serine residues were replaced with nonphosphorylatable amino acids (serine codon No. 255 AGC was converted to GCC, alanine, designated as M255S----A, and serine codon No. 244 AGC was converted to GGC, glycine, designated as M244S----G) showed Gjs similar to M257, indicating that these serine residues and, by inference, their phosphorylation state are not critical for expression of functional channels. The importance of the length of the carboxyl terminus was assessed by comparing the Gjs in a series of mutants that were intermediate in length between M257 and M241. Gradual shortening of the carboxyl terminus produced a gradual reduction of Gj relative to M257. However, simple deletion of amino acid residues 241-257 from the wild-type Cx43 did not affect Gj relative to M257.(ABSTRACT TRUNCATED AT 400 WORDS)

Dynamics of the inward rectifier K+ current during the action potential of guinea pig ventricular myocytes

The potassium selective, inward rectifier current (IK1) is known to be responsible for maintaining the resting membrane potential of quiescent ventricular myocytes. However, the contribution of this current to the different phases of the cardiac action potential has not been adequately established. In the present study, we have used the action potential clamp (APC) technique to characterize the dynamic changes of a cesium-sensitive (i.e., Ik1) current which occur during the action potential. Our results show that (a) Ik1 is present during depolarization, as well as in the final phase of repolarization of the cardiac action potential. (b) The current reaches the zone of inward-going rectification before the regenerative action potential ensues. (c) The maximal outward current amplitude during repolarization is significantly lower than during depolarization, which supports the hypothesis that in adult guinea pig ventricular myocytes, Ik1 rectification is accentuated during the action potential plateau. Our results stress the importance of Ik1 in the modulation of cell excitability in the ventricular myocyte.

Dynamics of the background outward current of single guinea pig ventricular myocytes. Ionic mechanisms of hysteresis in cardiac cells

Subthreshold potentials are thought to be mediated by time-independent, "passive" background currents. In this study, we show that the background current-voltage (I-V) relation of guinea pig ventricular myocytes is changed significantly by repetitive stimulation, in such a way that cell excitability becomes enhanced. Myocytes were used for whole-cell voltage-clamp experiments. A voltage-clamp ramp (100 mV/sec) to -50 mV was applied from a holding potential of -100 mV. Subsequently, a train of square voltage-clamp pulses to +10 mV (duration, 300 msec; interpulse interval, 300 msec) was delivered from a holding potential of -85 mV. A new ramp was applied again immediately after the train, and the resulting I-V curve was compared with that obtained before the train. Pulsing displaced the I-V relation to the right, the zero-current point becoming 1-2 mV less negative, and increased the degree of inward-going rectification. These changes were insensitive to tetrodotoxin (30 microM); disappeared during superfusion with cobalt (2 mM), verapamil (22 microM), or ryanodine (5 microM); and could not be mimicked by agonists of the protein kinase C system. In the presence of cesium (8 mM), pulsing still displaced the I-V curve to the right. However, the linear portion of the curve became steeper after the train. Subtraction of the cesium-sensitive current from control revealed that, although the zero-current point remained constant, the I-V relation showed a stronger inward-going rectification after pulsing. In accordance with these results, we have demonstrated hysteresis of excitability in ventricular myocytes. We conclude that the observed changes are mediated by an increase in intracellular calcium, which leads to an increase in rectification of IK1, as well as to activation of another membrane-conductance system, perhaps the Na-Ca exchange or the Ca(2+)-activated, nonselective current.

Hysteresis in the excitability of isolated guinea pig ventricular myocytes

Hysteresis phenomena were demonstrated in the excitability of single, enzymatically dissociated guinea pig ventricular myocytes. Membrane potentials were recorded with patch pipettes in the whole-cell current-clamp configuration. Repetitive stimulation with depolarizing current pulses of constant cycle length and duration but varying strength led to predictable excitation (1:1) and nonexcitation (1:0) patterns depending on current strength. However, transition between patterns depended on the direction of current strength change, and stable hysteresis loops were obtained in stimulus-response pattern versus current strength plots in 31 cells. Increase of pulse duration and decrease of stimulation rate contributed to a reduction in hysteresis loop areas. In addition, at the abrupt transitions from 1:0 to 1:1 patterns, a latency adaptation phenomenon was consistently observed. Bath application of tetrodotoxin (30 microM) produced no change of hysteresis, whereas hysteresis was substantially decreased in cobalt (2 mM) superfusion experiments. Analysis of the changes in amplitude and shape of the subthreshold responses during the transitions from one stable pattern to the other suggested that activity led to an increase in membrane resistance, particularly in the voltage domain between resting and threshold potentials. We therefore modeled the dynamic behavior of the single cells, using an analytical solution aimed at calculating the recovery of activation latency as a function of diastolic membrane resistance. Numerical iteration of the analytical model equations closely reproduced the experimental hysteresis loops in both qualitative and quantitative ways. The effect of stimulation frequency on the model was similar to the experimental findings. The overall study suggests that the excitability pattern of guinea pig ventricular myocytes is responsible for hysteresis and bistabilities when current intensity is allowed to fluctuate around threshold levels.

Phase resetting and entrainment of pacemaker activity in single sinus nodal cells

The phase-resetting and entrainment properties of single pacemaker cells were studied using computer simulations in a model of the rabbit sinus nodal cell, as well as using the whole-cell patch-clamp (current-clamp) technique in isolated rabbit sinus nodal cells. Spontaneous electrical activity in the cell model was reconstructed using Hodgkin-Huxley-type equations describing time- and voltage-dependent membrane currents. In both simulations and experiments, single subthreshold current pulses (depolarizing or hyperpolarizing) were used to scan the spontaneous cycle of the cells. Such pulses perturbed the subsequent discharge, producing temporary phasic changes in pacemaker period, and enabled the construction of phase response curves. On the basis of these results, we studied entrainment characteristics of the cells. For example, application of repetitive pulses allowed for phasic changes in the spontaneous cycle and resulted in stable 1:1 entrainment at a range of basic cycle length around the spontaneous cycle, or a 2:1 pattern at basic cycle length values about half the spontaneous cycle length. Between the two entrainment zones, complex Wenckebach-like patterns (e.g., 5:4, 4:3, and 3:2) were observed. The experimental data from the isolated cell were further analyzed from a theoretical perspective, and the results showed that the topological characteristics of the phase-resetting behavior accounts for the experimentally observed patterns during repetitive stimulation of the cell. This first demonstration of phase resetting in single cells provides the basis for phenomena such as mutual entrainment between electrically coupled pacemaker cells, apparent intranodal conduction, and reflex vagal control of heart rate.

Wenckebach periodicity in single atrioventricular nodal cells from the rabbit heart

Previous studies have suggested that Wenckebach periodicity in cardiac tissues may occur because of discontinuous propagation across junctional areas in which there is high intercellular resistivity or different cell types. Under these conditions, the impulse may stop altogether at a given junction, or may renew its propagation but only after a step delay imposed by the diastolic time-dependent recovery in the excitability of cells distal to that junction. Accordingly, Wenckebach periodicity in the atrioventricular node may be explained in terms of electrotonically mediated delay in the activation of the nodal cells. To test this hypothesis, we have studied recovery of excitability, and susceptibility to rate-dependent activation failure in single myocytes isolated from the adult rabbit atrioventricular node. Recordings were obtained by using the patch technique in the whole-cell, current clamp configuration. Repetitive stimulation of single atrioventricular nodal myocytes with depolarizing current pulses of critical amplitude yielded frequency-dependent stimulus response patterns that ranged from 1:1, through various Wenckebachlike periodicities (e.g., 5:4 and 4:3) to 2:1 and 3:1. Both typical and atypical Wenckebach structures were demonstrated, as well as "complex" patterns (e.g., reverse Wenckebach or alternating Wenckebach) previously ascribed to multiple levels of block. The diastolic recovery of excitability curve, determined by application of repetitive stimuli at cycle lengths that were longer than the action potential duration, showed a monotonic function with a refractory period outlasting the action potential duration (i.e., postrepolarization refractoriness). Abbreviation of the stimulation cycle length to values below those of the action potential duration revealed the existence of a period of supernormal excitability during the repolarizing phase of the action potential. In either case, the stimulus response patterns obtained were a direct consequence of the shape of the recovery of excitability curve. The monotonic portion of the recovery curve was fitted to an empirical equation that when iterated reproduced the stimulus response patterns observed in the atrioventricular nodal cell. Our data demonstrate that recovery of excitability after an action potential is indeed a function of the diastolic interval, and that this slow process sets the conditions for the development of Wenckebach periodicity in the atrioventricular node.

Directional differences in excitability and margin of safety for propagation in sheep ventricular epicardial muscle

Computer simulations and isolated tissue experiments were used to characterize the relation between excitability and margin of safety for propagation in anisotropic ventricular myocardium. Longitudinal, uniform transverse, and nonuniform transverse tissue directions were modeled in a one-dimensional Beeler-Reuter based cable. Stimulation threshold was smallest in the nonuniform transverse direction. The safety factor for propagation was determined in the model as the total axial charge that was available for depolarizing downstream tissue divided by the threshold charge that was just sufficient for continued propagation and was largest in the longitudinal direction. The strength-interval plot for the junction between simulated longitudinal and nonuniform transverse directions identified a range of stimulus strengths and intervals that resulted in nonuniform transverse but not longitudinal propagation. When high values of transverse resistance were used, higher stimulus strengths during premature stimulation resulted in longitudinal but not nonuniform transverse propagation. The experimental strength interval plots from 17 L-shaped preparations of isolated sheep epicardial muscles had similar characteristics. In nine additional L-shaped tissue experiments, changing extracellular K+ concentrations from 4 to 20 mM resulted in progressive membrane depolarization and conduction impairment in both directions. However, in eight of nine experiments, complete block occurred first in the transverse direction. In one experiment, block was simultaneous in both directions. We conclude that, under normal conditions, threshold requirements for active propagation are lower for transverse than for longitudinal propagation. In addition, when active membrane properties are impaired, the safety factor for propagation is larger in the direction along the longitudinal axis of the cells.

Electrophysiology of single heart cells from the rabbit tricuspid valve

1. The electrophysiology of single myocytes isolated from the rabbit tricuspid valve was studied using the patch-clamp method (whole-cell configuration). Cell dispersion was achieved by collagenase treatment, using the Langendorff retrograde perfusion procedure. 2. After isolation, and while incubating in the recovery (Kraftbrühe) solution, cells had clear striations and were mostly spindle-shaped, or rod-like (less than 10%), with length varying from 35 microns to over 150 microns, and diameter from 3 to 10 microns. 3. Upon exposure to Tyrode solution, the calcium-tolerant cells were mostly rounded with smooth surfaces and well-defined borders. The mean diameter of these cells was 15 +/- 5 microns (S.D., n = 9). A smaller percentage (about 30%) retained the original elongated shape. 4. Patch pipette recordings showed the presence of spontaneous activity in about 30% of round cells, and less frequently in elongated cells. Maximum diastolic potentials (MDPs) in the round cells averaged -82 +/- 6 mV, with a take-off potential of -56 +/- 3 mV (n = 9), and an average maximum upstroke velocity (Vmax) value of 6.3 +/- 0.6 V/s (n = 4). In quiescent cells, the mean resting potential was 69 +/- 12 mV (n = 43). 5. Voltage clamp ramps revealed a steady-state I-V relation with a negative slope region. The mean input resistance value was 25 +/- 9 M omega (n = 16) for the elongated, and 883 +/- 481 M omega (n = 8) for the round cells. 6. Hyperpolarizing 5 s pulses (holding potential = -50 mV) occasionally revealed a slow, time-dependent inward current whose peak increased progressively as a function of clamp potential. The slowly activating current was sensitive to caesium 2 mM), indicating its similarity to the so-called 'pacemaker current' (iF). In alternate voltage- and current-clamp experiments, blocking of iF did not stop pacemaker activity, but there was up to a fourfold increase in pacemaker cycle length. 7. In some cells, 5 s hyperpolarizing steps from a holding potential of -40 or -50 mV produced large, inwardly directed and voltage-dependent current surges that decayed rapidly with time, similar to the inactivation described for the inward rectifier current, iK1. The current was very prominent at voltages more negative than -100 mV, and its decay process was best fitted by two time constants, one fast and one slow. For example, at -150 mV the time constants were 61 and 634 ms. The inward current was blocked by barium (1 mM).(ABSTRACT TRUNCATED AT 400 WORDS)

Ionic basis and analytical solution of the wenckebach phenomenon in guinea pig ventricular myocytes

The ionic mechanisms of slow recovery of cardiac excitability and rate-dependent activation failure were studied in single, enzymatically dissociated guinea pig ventricular myocytes and in computer simulations using a modified version of the Beeler and Reuter model for the ventricular cell. On the basis of our results, we developed a simplified analytical model for recovery of cell excitability during diastole. This model was based on the equations for current distribution in a resistive-capacitive circuit. A critical assumption in the model is that, in the voltage domain of the subthreshold responses, the sodium and calcium inward currents do not play a significant role, and only the two potassium outward currents, the delayed rectifier (IK) and the inward rectifier, are operative. The appropriate parameters needed to numerically solve the analytical model were measured in the guinea pig ventricular myocyte, as well as in the Beeler and Reuter cell. The curves of recovery of excitability and the rate-dependent activation patterns generated by numerical iteration of the analytical model equations closely reproduced the experimental results. Our analysis demonstrates that slow deactivation of the delayed rectifier current determines the observed variations in excitability during diastole, whereas the inward rectifier current determines the amplitude and shape of the subthreshold response. Both currents combined are responsible for the development of Wenckebach periodicities in the ventricular cell. The overall study provides new insight into the ionic mechanisms of rate-dependent conduction block processes and may have important clinical implications as well.

Slow recovery of excitability and the Wenckebach phenomenon in the single guinea pig ventricular myocyte

The cellular mechanisms of Wenckebach periodicity were investigated in single, enzymatically dissociated guinea pig ventricular myocytes, as well as in computer reconstructions of transmembrane potential of the ventricular cell. When depolarizing current pulses of the appropriate magnitude were delivered repetitively to a well-polarized myocyte, rate-dependent activation failure was observed. Such behavior accurately mimicked the Wenckebach phenomenon in cardiac activation and was the consequence of variations in cell excitability during the diastolic phase of the cardiac cycle. The recovery of cell excitability during diastole was studied through the application of single test pulses of fixed amplitude and duration at variable delays with respect to a basic train of normal action potentials. The results show that recovery of excitability is a slow process that can greatly outlast action potential duration (i.e., postrepolarization refractoriness). Two distinct types of subthreshold responses were recorded when activation failure occurred: one was tetrodotoxin- and cobalt-insensitive (type 1) and the other was sensitive to sodium-channel blockade (type 2). Type 1 responses, which were commonly associated with the typical structure of the Wenckebach phenomenon (Mobitz type 1 block), were found to be the result of the nonlinear conductance properties of the inward rectifier current, IK1. Type 2 sodium-channel-mediated responses were associated with the so-called "millisecond Wenckebach." These responses may be implicated in the mechanism of Mobitz type 2 rate-dependent block. Single-cell voltage-clamp experiments suggest that variations in excitability during diastole are a consequence of the slow deactivation kinetics of the delayed rectifier, IK. Computer simulations of the ventricular cell response to depolarizing current pulses reproduced very closely all the response patterns obtained in the experimental preparation. It is concluded that postrepolarization refractoriness and Wenckebach periodicity are properties of normal cardiac excitable cells and can be explained in terms of the voltage dependence and slow kinetics of potassium outward currents. The conditions for the occurrence of intermittent activation failure during diastole will depend on the frequency and magnitude of the driving stimulus.