Molecular Pathophysiology of Congenital Long QT Syndrome

Ion channels represent the molecular entities that give rise to the cardiac action potential, the fundamental cellular electrical event in the heart. The concerted function of these channels leads to normal cyclical excitation and resultant contraction of cardiac muscle. Research into cardiac ion channel regulation and mutations that underlie disease pathogenesis has greatly enhanced our knowledge of the causes and clinical management of cardiac arrhythmia. Here we review the molecular determinants, pathogenesis, and pharmacology of congenital Long QT Syndrome. We examine mechanisms of dysfunction associated with three critical cardiac currents that comprise the majority of congenital Long QT Syndrome cases: 1) IKs, the slow delayed rectifier current; 2) IKr, the rapid delayed rectifier current; and 3) INa, the voltage-dependent sodium current. Less common subtypes of congenital Long QT Syndrome affect other cardiac ionic currents that contribute to the dynamic nature of cardiac electrophysiology. Through the study of mutations that cause congenital Long QT Syndrome, the scientific community has advanced understanding of ion channel structure-function relationships, physiology, and pharmacological response to clinically employed and experimental pharmacological agents. Our understanding of congenital Long QT Syndrome continues to evolve rapidly and with great benefits: genotype-driven clinical management of the disease has improved patient care as precision medicine becomes even more a reality.

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.

A computational model of Purkinje fibre single cell electrophysiology: implications for the long QT syndrome

Computer modelling has emerged as a particularly useful tool in understanding the physiology and pathophysiology of cardiac tissues. Models of ventricular, atrial and nodal tissue have evolved and include detailed ion channel kinetics and intercellular Ca(2+) handling. Purkinje fibre cells play a central role in the electrophysiology of the heart and in the genesis of cardiac arrhythmias. In this study, a new computational model has been constructed that incorporates the major membrane currents that have been isolated in recent experiments using Purkinje fibre cells. The model, which integrates mathematical models of human ion channels based on detailed biophysical studies of their kinetic and voltage-dependent properties, recapitulates distinct electrophysiological characteristics unique to Purkinje fibre cells compared to neighbouring ventricular myocytes. These characteristics include automaticity, hyperpolarized voltage range of the action potential plateau potential, and prolonged action potential duration. Simulations of selective ion channel blockade reproduce responses to pharmacological challenges characteristic of isolated Purkinje fibres in vitro, and importantly, the model predicts that Purkinje fibre cells are prone to severe arrhythmogenic activity in patients harbouring long QT syndrome 3 but much less so for other common forms of long QT. This new Purkinje cellular model can be a useful tool to study tissue-specific drug interactions and the effects of disease-related ion channel dysfunction on the cardiac conduction system.

Mutation-specific pharmacology of the long QT syndrome

The congenital long QT syndrome is a rare disease in which inherited mutations of genes coding for ion channel subunits, or channel interacting proteins, delay repolarization of the human ventricle and predispose mutation carriers to the risk of serious or fatal arrhythmias. Though a rare disorder, the long QT syndrome has provided invaluable insight from studies that have bridged clinical and pre-clinical (basic science) medicine. In this brief review, we summarize some of the key clinical and genetic characteristics of this disease and highlight novel findings about ion channel structure, function, and the causal relationship between channel dysfunction and human disease, that have come from investigations of this disorder.

Structural effects of an LQT-3 mutation on heart Na+ channel gating

Computational methods that predict three-dimensional structures from amino acid sequences have become increasingly accurate and have provided insights into structure-function relationships for proteins in the absence of structural data. However, the accuracy of computational structural models requires experimental approaches for validation. Here we report direct testing of the predictions of a previously reported structural model of the C-terminus of the human heart Na(+) channel. We focused on understanding the structural basis for the unique effects of an inherited C-terminal mutation (Y1795C), associated with long QT syndrome variant 3 (LQT-3), that has pronounced effects on Na(+) channel inactivation. Here we provide evidence that this mutation, in which a cysteine replaces a tyrosine at position 1795 (Y1795C), enables the formation of disulfide bonds with a partner cysteine in the channel. Using the predictions of the model, we identify the cysteine and show that three-dimensional information contained in the sequence for the channel protein is necessary to understand the structural basis for some of the effects of the mutation. The experimental evidence supports the accuracy of the predicted structural model of the human heart Na(+) channel C-terminal domain and provides insight into a structural basis for some of the mutation-induced altered channel function underlying the disease phenotype.

Stimulation of protein kinase C inhibits bursting in disease-linked mutant human cardiac sodium channels

BACKGROUND

Mutations in SCN5A, the gene coding for the human cardiac Na+ channel alpha-subunit, are associated with variant 3 of the long-QT syndrome (LQT-3). Several LQT-3 mutations promote a mode of Na+ channel gating in which a fraction of channels fail to inactivate, contributing sustained Na+ channel current (Isus), which can delay repolarization and prolong the QT interval. Here, we investigate the possibility that stimulation of protein kinase C (PKC) may modulate Isus, which is prominent in disease-related Na+ channel mutations.

METHODS AND RESULTS

We measured the effects of PKC stimulation on Na+ currents in human embryonic kidney (HEK 293) cells expressing 3 previously reported disease-associated Na+ channel mutations (Y1795C, Y1795H, and DeltaKPQ). We find that the PKC activator 1-oleoyl-2-acetyl-sn-glycerol (OAG) significantly reduced Isus in the mutant but not wild-type channels. The effect of OAG on Isus was reduced by the PKC inhibitor staurosporine (2.5 micromol/L), ablated by the mutation S1503A, and mimicked by the mutation S1503D. Isus recorded in myocytes isolated from mice expressing DeltaKPQ channels was similarly inhibited by OAG exposure or stimulation of alpha1-adrenergic receptors by phenylephrine. The actions of phenylephrine on Isus were blocked by the PKC inhibitor chelerythrine.

CONCLUSIONS

We conclude that stimulation of PKC inhibits channel bursting in disease-linked mutations via phosphorylation-induced alteration of the charge at residue 1503 of the Na+ channel alpha-subunit. Sympathetic nerve activity may contribute directly to suppression of mutant channel bursting via alpha-adrenergic receptor-mediated stimulation of PKC.

Leucine/isoleucine zipper coordination of ion channel macromolecular signaling complexes in the heart. Roles in inherited arrhythmias

The sympathetic nervous system controls the force and rate of contraction of the heart. The rapid response to stress and exercise mediated by increased sympathetic nervous system (SNS) activity requires the coordinated regulation of several ion channels in response to activation of beta-adrenergic receptors. The microenvironment of target channels is mediated by the assembly of macromolecular signaling complexes in which targeting proteins recruit phosphatases and kinases and in turn bind directly to the channel protein via highly conserved leucine/isoleucine zippers (LIZs). Disruption of local signaling by disease-associated LIZ mutations unbalances the physiologic response to SNS stimulation and increases the risk of arrhythmia in mutation carriers.

Inherited Brugada and long QT-3 syndrome mutations of a single residue of the cardiac sodium channel confer distinct channel and clinical phenotypes

Defects of the SCN5A gene encoding the cardiac sodium channel alpha-subunit are associated with both the long QT-3 (LQT-3) subtype of long-QT syndrome and Brugada syndrome (BrS). One previously described SCN5A mutation (1795insD) in the C terminus results in a clinical phenotype combining QT prolongation and ST segment elevation, indicating a close interrelationship between the two disorders. Here we provide additional evidence that these two disorders are closely related. We report the analysis of two novel mutations on the same codon, Y1795C (LQT-3) and Y1795H (BrS), expressed in HEK 293 cells and characterized using whole-cell patch clamp procedures. We find marked and opposing effects on channel gating consistent with activity associated with the cellular basis of each clinical disorder. Y1795H speeds and Y1795C slows the onset of inactivation. The Y1795H, but not the Y1795C, mutation causes a marked negative shift in the voltage dependence of inactivation, and neither mutation affects the kinetics of the recovery from inactivation. Interestingly, both mutations increase the expression of sustained Na+ channel activity compared with wild type (WT) channels, although this effect is most pronounced for the Y1795C mutation, and both mutations promote entrance into an intermediate or a slowly developing inactivated state. These data confirm the key role of the C-terminal tail of the cardiac Na+ channel in the control of channel gating, illustrate how subtle changes in channel biophysics can have significant and distinct effects in human disease, and, additionally, provide further evidence of the close interrelationship between BrS and LQT-3 at the molecular level.

Molecular basis of the delayed rectifier current I(ks)in heart

J. Kurokawa, H. Abriel and R. S. Kass. Molecular Basis of the Delayed Rectifier Current I(Ks)in Heart. Journal of Molecular and Cellular Cardiology (2001) 33, 873-882. Electrical activity underlies the control of the frequency, strength, and duration of contraction of the heart. During the cardiac cycle, a regular rhythmic pattern must be established in time-dependent changes in ionic conductances in order to ensure events that underlie normal cardiac function. This pattern must be tightly regulated by sympathetic nervous activity to ensure a physiologically relevant relationship between diastolic filling and ejection times with variable heart rate. The duration of the ventricular action potential is controlled in part by a slowly activated potassium channel current, I(Ks). The molecular identity of the subunits that comprise the channels conducting this current is important, not only for understanding the fundamental mechanisms that control electrical activity in healthy individuals, but also for understanding the molecular basis of at least one inherited human disease, LQTS-1. This brief review summarizes key points of information regarding the molecular determinants of the activity of these channels, their relationship to human disease, and what is known, and yet to be discovered, about the molecular determinants of the regulation of this channel by sympathetic nervous activity.

Novel arrhythmogenic mechanism revealed by a long-QT syndrome mutation in the cardiac Na(+) channel

Variant 3 of the congenital long-QT syndrome (LQTS-3) is caused by mutations in the gene encoding the alpha subunit of the cardiac Na(+) channel. In the present study, we report a novel LQTS-3 mutation, E1295K (EK), and describe its functional consequences when expressed in HEK293 cells. The clinical phenotype of the proband indicated QT interval prolongation in the absence of T-wave morphological abnormalities and a steep QT/R-R relationship, consistent with an LQTS-3 lesion. However, biophysical analysis of mutant channels indicates that the EK mutation changes channel activity in a manner that is distinct from previously investigated LQTS-3 mutations. The EK mutation causes significant positive shifts in the half-maximal voltage (V(1/2)) of steady-state inactivation and activation (+5.2 and +3.4 mV, respectively). These gating changes shift the window of voltages over which Na(+) channels do not completely inactivate without altering the magnitude of these currents. The change in voltage dependence of window currents suggests that this alteration in the voltage dependence of Na(+) channel gating may cause marked changes in action potential duration because of the unique voltage-dependent rectifying properties of cardiac K(+) channels that underlie the plateau and terminal repolarization phases of the action potential. Na(+) channel window current is likely to have a greater effect on net membrane current at more positive potentials (EK channels) where total K(+) channel conductance is low than at more negative potentials (wild-type channels), where total K(+) channel conductance is high. These findings suggest a fundamentally distinct mechanism of arrhythmogenesis for congenital LQTS-3.

Characterization of sodium channel alpha- and beta-subunits in rat and mouse cardiac myocytes

BACKGROUND

Sodium channels isolated from mammalian brain are composed of alpha-, beta(1)-, and beta(2)-subunits. The composition of sodium channels in cardiac muscle, however, has not been defined, and disagreement exists over which beta-subunits are expressed in the myocytes. Some investigators have demonstrated beta(1) expression in heart. Others have not detected any auxiliary subunits. On the basis of Northern blot analysis of total RNA, beta(2) expression has been thought to be exclusive to neurons and absent from cardiac muscle.

METHODS AND RESULTS

The goal of this study was to define the subunit composition of cardiac sodium channels in myocytes. We show that cardiac sodium channels are composed of alpha-, beta(1)-, and beta(2)-subunits. Nav1.5 and Nav1.1 are expressed in myocytes and are associated with beta(1)- and beta(2)-subunits. Immunocytochemical localization of Nav1.1, beta(1), and beta(2) in adult heart sections showed that these subunits are expressed at the Z lines, as shown previously for Nav1.5. Coexpression of Nav1.5 with beta(2) in transfected cells resulted in no detectable changes in sodium current.

CONCLUSIONS

Cardiac sodium channels are composed of alpha- (Nav1.1 or Nav1.5), beta(1)-, and beta(2)-subunits. Although beta(1)-subunits modulate cardiac sodium channel current, beta(2)-subunit function in heart may be limited to cell adhesion.

Molecular pharmacology of the sodium channel mutation D1790G linked to the long-QT syndrome

BACKGROUND

Multiple mutations of SCN5A, the gene that encodes the human Na(+) channel alpha-subunit, are linked to 1 form of the congenital long-QT syndrome (LQT-3). D1790G (DG), an LQT-3 mutation of the C-terminal region of the Na(+) channel alpha-subunit, alters steady-state inactivation of expressed channels but does not promote sustained Na(+) channel activity. Recently, flecainide, but not lidocaine, has been found to correct the disease phenotype, delayed ventricular repolarization, in DG carriers.

METHODS AND RESULTS

To understand the molecular basis of this difference, we studied both drugs using wild-type (WT) and mutant Na(+) channels expressed in HEK 293 cells. The DG mutation conferred a higher sensitivity to lidocaine (EC(50), WT=894 and DG=205 micromol/L) but not flecainide tonic block in a concentration range that is not clinically relevant. In contrast, in a concentration range that is therapeutically relevant, DG channels are blocked selectively by flecainide (EC(50), WT=11.0 and DG=1.7 micromol/L), but not lidocaine (EC(50), WT=318.0 and DG=176 micromol/L) during repetitive stimulation.

CONCLUSIONS

These results (1) demonstrate that the DG mutation confers a unique pharmacological response on expressed channels; (2) suggest that flecainide use-dependent block of DG channels underlies its therapeutic effects in carriers of this gene mutation; and (3) suggest a role of the Na(+) channel alpha-subunit C-terminus in the flecainide/channel interaction.

Arrhythmogenic mechanism of an LQT-3 mutation of the human heart Na(+) channel alpha-subunit: A computational analysis

BACKGROUND

D1790G, a mutation of SCN5A, the gene that encodes the human Na(+) channel alpha-subunit, is linked to 1 form of the congenital long-QT syndrome (LQT-3). In contrast to other LQT-3-linked SCN5A mutations, D1790G does not promote sustained Na(+) channel activity but instead alters the kinetics and voltage-dependence of the inactivated state.

METHODS AND RESULTS

We modeled the cardiac ventricular action potential (AP) using parameters and techniques described by Luo and Rudy as our control. On this background, we modified only the properties of the voltage-gated Na(+) channel according to our patch-clamp analysis of D1790G channels. Our results indicate that D1790G-induced changes in Na(+) channel activity prolong APs in a steeply heart rate-dependent manner not directly due to changes in Na(+) entry through mutant channels but instead to alterations in the balance of net plateau currents by modulation of calcium-sensitive exchange and ion channel currents.

CONCLUSIONS

We conclude that the D1790G mutation of the Na(+) channel alpha-subunit can prolong the cardiac ventricular AP despite the absence of mutation-induced sustained Na(+) channel current. This prolongation is calcium-dependent, is enhanced at slow heart rates, and at sufficiently slow heart rate triggers arrhythmogenic early afterdepolarizations.

Differentiation of cardiomyocytes in floating embryoid bodies is comparable to fetal cardiomyocytes

Embryonic stem cells will cluster and differentiate into embryoid bodies, which can develop spontaneous rhythmic contractions. From these embryoid bodies, cardiomyocytes can be isolated based on density by a discontinuous Percoll gradient. These cardiomyocytes differentiate into ventricular myocytes, which is demonstrated by the expression of the ventricular specific isoform of the myosin light chain 2 gene. In this study the functional expression of ion channels was compared between fetal cardiomyocytes (in vivo) and stem cell derived cardiomyocytes (in vitro). Sodium and calcium currents together with transient potassium currents could be detected in early developmental stages (<day 14) both in vivo and in vitro. In the early stages, we found a limited number of cells expressing I(Kr)and virtual absence of I(Ks). The characteristics and distribution of currents are similar in both cell types. The current characteristics were identical for ventricular compared to atrial or undifferentiated stem cell derived cardiomyocytes, despite differences in expression of regulatory myosin light chain proteins. The myocyte differentiation was verified in a limited number of cardiomyocytes following the patch clamp procedure by immunocytochemistry.

Effects of flecainide in patients with new SCN5A mutation: mutation-specific therapy for long-QT syndrome?

BACKGROUND

Mutations in the cardiac sodium channel gene (SCN5A) can cause one variant of the congenital long-QT syndrome. The effects of some of these mutations on the alpha-subunit channel properties can be blocked by type Ib antiarrhythmic drugs. Recently, we have described a new SCN5A mutation (D1790G) that affects the channel properties in a manner suggesting that sodium blockers of the Ib type will be ineffective in carriers of this mutation. Hence, the ECG effects of flecainide-acetate, a type Ic sodium blocker, were evaluated in carriers of this mutation.

METHODS AND RESULTS

Eight asymptomatic mutation carriers and 5 control subjects were studied. Intravenous lidocaine was tested first in only 2 mutation carriers and had no significant effect on any ECG parameter. Flecainide significantly shortened all heart rate-corrected repolarization duration parameters only in carriers and not in control subjects: QT(c) shortened by 9.5% (from 517+/-45 to 468+/-36 ms, P=0.011), and the S-offset to T-onset interval shortened by 64.7% (from 187+/-88 to 66+/-50 ms, P=0.0092). Flecainide also normalized the marked baseline repolarization dispersion in most mutation carriers. These effects among carriers were maintained during long-term (9 to 17 months) outpatient flecainide therapy with no adverse effects.

CONCLUSIONS

This report is the first to describe SCN5A mutation carriers who significantly responded to flecainide therapy yet did not respond to lidocaine. These results have important implications for long-QT allele-specific therapeutic strategies.

Distinctions in the molecular determinants of charged and neutral dihydropyridine block of L-type calcium channels

We investigated block of the alpha1Cb subunit of L-type calcium channels by dihydropyridines (DHPs) in which a permanently charged or neutral head group was linked to the active DHP moiety by a spacer chain containing ten methylene (-CH2) groups. We compared the sensitivity of channel modulation by the charged (DHPch) and neutral (DHPn) forms to specific alpha1Cb mutations in domains IIIS5, IIIS6, and IVS6, which had previously been shown to reduce channel modulation by the neutral DHP (+)-isradipine. The effects of these mutations were studied on channel block recorded from polarized (-80 mV) and depolarized (-40 mV) holding potentials (HPs). We found that channel block by DHPn was markedly reduced at both HPs by each mutation studied. In contrast, channel block by DHPch was only modestly reduced by mutations in IIIS6 and IVS6 for block from either -40 mV or -80 mV. Replacement of IIIS5 Thr1061 by Tyr, which abolished block by DHPn in an HP-independent manner, had little effect on channel block by DHPch recorded from -40 mV. However, this mutation markedly reduced DHPch block of currents recorded from a -80 mV HP. Inhibition of current by DHPch was not markedly use-dependent, in contrast with block by verapamil, another charged calcium channel blocker. These results suggest that the presence of a permanently charged head group restricts the access of the attached DHP moiety to a subset of interaction residues on the alpha1C subunit in a voltage-dependent manner. Furthermore, these restricted interactions confer distinct functional properties upon the charged DHP molecules.

Beta2-adrenergic receptor overexpression in the developing mouse heart: evidence for targeted modulation of ion channels

1. We studied the effect of overexpression of the beta2-adrenergic receptor (beta2-AR) in the heart on ion channel currents in single cells isolated from hearts of fetal and neonatal transgenic and wild-type mice. The beta2-AR transgene construct was under the control of the murine alpha-myosin heavy chain (alpha-MHC) promoter, and ion channel activity was measured at distinct developmental stages using whole-cell and perforated patch clamp techniques. 2. We found no change in L-type Ca2+ channel current (ICa) density in early embryonic stages (E11-13) of beta2-AR transgenic positive (TG+) mice, but significant increases in ICa density in intermediate (E14-16, 152 %) and late (E17-19, 173.7 %) fetal and neonatal (1 day post partum, 161 %) TG+ compared with transgenic negative (TG-) mice. This increase in ICa was accompanied by a negative shift in the peak of the current-voltage relationship in TG+ mice. 3. Transient (< 3 min) or prolonged (16-24 h) exposure of TG- neonatal stage myocytes to 8-Br-cAMP (300 microM) increased ICa density and caused a shift in the current-voltage relationship to a similar extent to that seen in TG+ mice. In TG+ myocytes, 8-Br-cAMP had no effect. Exposure of TG+ cells to Rp-cAMPS reversed both the shift in voltage dependence and reduced the peak current density observed in these myocytes. We concluded from these results that the L-type Ca2+ channel is maximally modulated by cAMP-dependent protein kinase (PKA) in TG+ mice and that the alpha-MHC promoter is functional in the ventricle as early as embryonic day 14. 4. In contrast, we found that slow delayed rectifier K+ channels were not changed significantly at any of the developmental stages studied by the overexpression of beta2-ARs compared with TG- mice. The sensitivity of murine slow delayed rectifier K+ channels to cAMP was tested by both transient and prolonged exposure to 8-Br-cAMP (300 microM), which increased the slow delayed rectifier K+ channel current (IK,s) density to a similar extent in both TG- and TG+ neonatal myocytes. In addition, we found that there was no difference in the concentration dependence of the response of ICa and IK,s to 8-Br-cAMP. 5. Thus, overexpression of the beta2-AR in the heart results in distinct modulation of ICa, but not IK,s, and this is not due to differences in the 8-Br-cAMP sensitivity of the two channels. Instead, these results are consistent with both compartmentalization of beta2-AR-controlled cAMP and distinct localization of L-type Ca2+ and slow delayed rectifier K+ channels. This cAMP is targeted preferentially to the L-type Ca2+ channel and is not accessible to the slow delayed rectifier K+ channel.

MinK-KvLQT1 fusion proteins, evidence for multiple stoichiometries of the assembled IsK channel

IsK, a slowly activating delayed rectifier K+ current through channels formed by the assembly of two channel proteins KvLQT1 and MinK, modulates the repolarization of cardiac action potentials. Mutations that map to the KvLQT1 and minK genes account for more than 50% of an inherited cardiac disorder, the Long QT syndrome (Splawski, I., Tristani-Firouzi, M., Lehmann, M. H., Sanguinetti, M. C., and Keating, M. T. (1997) Nat. Genet. 17, 338-340). Despite the importance of these channels to human cardiac function, the molecular basis of their uniquely slow gating properties as well as the stoichiometry and interaction sites of these two subunits are still unclear. We have constructed several fusion channel proteins to begin investigating the stoichiometry of these two subunits and the role of voltage-dependent subunit assembly in channel gating. Functional properties of these constructs were measured using whole cell patch clamp recordings of transiently transfected Chinese hamster ovary cells. The constructs we tested are as follows: MK24 (C terminus of MinK linked to N terminus of KvLQT1); KK40 (a tandem homodimer of KvLQT1); and MKK44 (C terminus of MinK linked to N terminus of KK40). In control experiments (no DNA, control DNA, or only MinK), no time-dependent K+ current was observed. Expression of KvLQT1 or KK40 produced currents that activate and inactivate in a voltage-dependent manner as reported by others for KvLQT1. In contrast, expression of MK24 and MKK44 elicited current with activation kinetics and voltage dependence very similar to native IsK and identical to currents expressed by cells co-transfected with independent MinK and KvLQT1 cDNA. Expression of MK24 plus additional MinK significantly slows current kinetics. Our data raise the possibility 1) of multiple MinK/KvLQT1 stoichiometries and 2) indicate that uniquely slow kinetics of IsK channels is due to voltage-dependent conformational changes of the channel protein and not to assembly of channel subunits.

Overexpression of nerve growth factor in the heart alters ion channel activity and beta-adrenergic signalling in an adult transgenic mouse

1. The electrophysiological and pharmacological properties of cardiac myocytes from the hearts of adult transgenic mice engineered to overexpress nerve growth factor (NGF) in the heart were studied. 2. There was a 12% increase in the ventricular myocyte capacitance in NGF myocytes consistent with cardiac hypertrophy, and action potential duration at 90% repolarization (APD90) was prolonged by 142 % compared with wild-type (WT) myocytes. This was due, at least in part, to a decrease in the density of two K+ currents, Ito and IK(ur), which were significantly reduced in NGF mice with no change in their electrophysiological characteristics. We found no change in the current density or electrophysiological properties of the L-type Ca2+ current. 3. The effect on Ito and IK(ur) of TEA and 4-aminopyridine (4-AP) was not different in cells isolated from WT and NGF mice. The prolongation of APD observed in NGF cells was mimicked in WT cells by exposure to 1 mM 4-AP, which partially blocked Ito, completely blocked IK(ur) and increased APD90 by 157%. 4. The isoprenaline-induced increase in ICa was significantly smaller in NGF myocytes than in WT myocytes. This was not due to a decrease in beta-adrenergic receptor (beta-AR) density, as this was increased in NGF tissue by 55%. Analysis of beta-AR subtypes showed that this increase was entirely due to an increase in beta2-AR density with no change in beta1-ARs. 5. The response of the beta-AR-coupled adenylyl cyclase system to isoprenaline, Gpp(NH)p and forskolin was studied by measuring cAMP production. In NGF tissue, isoprenaline elicited a significantly smaller response than in WT myoyctes and this was not due to reduced adenylyl cyclase activity as the responses of NGF tissue to guanylylimidodiphosphate (Gpp(NH)p) and forskolin were unaffected. 6. In conclusion, the overexpression of NGF in the mouse heart resulted in a decrease in the current density of two K+ channels, which contributed to the prolongation of the cardiac action potential. Despite an increase in beta2-AR density in the hearts of the NGF mice, the response to isoprenaline was diminished, and this was due to an uncoupling of the beta-ARs from the intracellular signalling cascade. These potentially pathological changes may be involved in the occurrence of ventricular arrhythmias in cardiac hypertrophy and failure, and this mouse provides a novel model in which to study such changes.

Novel LQT-3 mutation affects Na+ channel activity through interactions between alpha- and beta1-subunits

The congenital long-QT syndrome (LQT), an inherited cardiac arrhythmia characterized in part by prolonged ventricular repolarization, has been linked to 5 loci, 4 of which have been shown to harbor genes that encode ion channels. Previously studied LQT-3 mutations of SCN5A (or hH1), the gene that encodes the human Na+ channel alpha-subunit, have been shown to encode voltage-gated Na+ channels that reopen during prolonged depolarization and hence directly contribute to the disease phenotype: delayed repolarization. Here, we report the functional consequences of a novel SCN5A mutation discovered in an extended LQT family. The mutation, a single A-->G base substitution at nucleotide 5519 of the SCN5A cDNA, is expected to cause a nonconservative change from an aspartate to a glycine at position 1790 (D1790G) of the SCN5A gene product. We investigated ion channel activity in human embryonic kidney (HEK 293) cells transiently transfected with wild-type (hH1) or mutant (D1790G) cDNA alone or in combination with cDNA encoding the human Na+ channel beta1-subunit (hbeta1) using whole-cell patch-clamp procedures. Heteromeric channels formed by coexpression of alpha- and beta1-subunits are affected: steady-state inactivation is shifted by -16 mV, but there is no D1790G-induced sustained inward current. This effect is independent of the beta1-subunit isoform. We find no significant effect of D1790G on the biophysical properties of monomeric alpha- (hH1) channels. We conclude that the effects of the novel LQT-3 mutation on inactivation of heteromeric channels are due to D1790G-induced changes in alpha- and beta1-interactions.

Molecular pharmacology of UK-118, 434-05, a permanently charged amlodipine analog

We studied the effects of UK-118, 434-05, a permanently charged form of amlodipine, on recombinant smooth muscle and cardiac L-type calcium channels to determine the distinct modulatory properties of the ionized form of amlodipine. We found that the short distance between the permanent charge group and the active dihydropyridine (DHP) ring of UK-118, 434-05 reduces the potency of this compound as an inhibitor of smooth muscle (alpha(1c-b)) L-type channels, and is similar to the effects of other charged DHP derivatives on cardiac (alpha(1c-a)) L-type channels. However, we found surprisingly that the tonic block of cardiac (alpha(1c-a)) L-type channels was more pronounced than the tonic block of smooth muscle (alpha(1c-b)) L-type channels. This result contrasts with the previously reported subunit-specificity of neutral DHP compounds, and suggests that interactions between the amlodipine charge group and site(s) on the L-type channel alpha1 subunit distinguish the action of charged from neutral DHPs and may contribute to amlodipine's unique pharmacological profile.

Decreased density of Ito in left ventricular myocytes from German shepherd dogs with inherited arrhythmias

INTRODUCTION

A colony of inbred German shepherd dogs with inherited ventricular arrhythmias has been established.

METHODS AND RESULTS

The inward rectifier (IK1), the slow delayed rectifier (IKs), and the transient outward current (I(to)) were recorded from epicardial myocytes, and Ito was recorded from Purkinje myocytes isolated from the left ventricles of dogs mildly or severely affected with arrhythmias, and unaffected relatives. There were no differences between unaffected and severely affected dogs in the densities of either IK1 or IKs. Peak Ito density at +40 mV was reduced by 49% in epicardial myocytes from severely affected dogs. I(to) density was also reduced in a subset of Purkinje myocytes. Boltzmann analysis of steady-state inactivation showed no differences between groups in slope factor. V1/2, the half-inactivation voltage, was shifted by +6.2 mV in epicardial cells from severely affected versus unaffected dogs. In addition, the time constant for I(to) decay was reduced in mildly and severely affected dogs compared to unaffected dogs.

CONCLUSION

Altered density and inactivation of I(to) are associated with the presence of severe ventricular arrhythmias in inbred dogs at risk for sudden death.

Lidocaine block of LQT-3 mutant human Na+ channels

In transiently transfected mammalian cells we have identified pharmacological consequences of a naturally occurring deletion mutation, delta KPQ, of the human heart Na+ channel alpha subunit that previously has been linked to one form of the long QT syndrome, an inherited heart disease. Our results show that the Class IB antiarrhythmic agent lidocaine blocks maintained inward current through and slows recovery from inactivation of delta KPQ-encoded Na+ channels. Block is greater for maintained than for peak current. Because incomplete inactivation of mutant Na+ channels is now thought to underlie the prolonged ventricular action potential, which is the phenotype of this disease, and we find that the delta KPQ mutation speeds the recovery from inactivation of drug-free mutant channels, our results provide evidence, for the first time, that clinically relevant dysfunctional properties of an ion channel can be selectively targeted on the basis of the molecular properties conferred on the channel by an inherited genetic disorder.

The utility of fluorescent in vivo reporter genes in molecular cardiology

In vivo reporter genes can be used in different ways in molecular cardiology. In this paper studies are presented using the green fluorescent protein and one of its mutants, S65T-GFP, as in vivo reporter genes. With this new molecular tool we studied cell type specificity of the murine ventricular myosin light chain 2 promoter, positive cell identification prior to patch clamp procedures, and the use of fluorescence activated cell sorting of transiently transfected mammalian cells.

Influence of L-type Ca channel alpha 2/delta-subunit on ionic and gating current in transiently transfected HEK 293 cells

We have measured ionic and gating currents in human embryonic kidney (HEK 293) cells transiently transfected with cDNAs encoding subunits of the cardiac voltage-gated L-type Ca2+ channel. Robust recombinant ionic current and associated nonlinear charge movement could be measured over a broad voltage range without contamination by endogenous channel activity. Coexpression of the alpha 2/delta-subunit along with alpha 1- and beta 2-subunits speeded activation and deactivation kinetics and significantly increased the maximal conductance of ionic current. Charge movement was measured at voltages negative to the threshold for activation of ionic current, and gating charge could be immobilized at positive holding potentials that did not inactivate ionic current. The ratio of maximal ionic conductance to maximal charge moved remained the same in the absence or presence of the alpha 2/delta-subunit. However, the maximal amount of charge moved was increased about twofold in the presence of the alpha 2/delta-subunit. These results suggest that coexpression of the alpha 2/delta-subunit enhances the expression of functional L-type channels and, in addition, provide evidence that most of the L-type channel-associated nonlinear charge movement is caused by transitions between nonconducting states of the channel protein that precede the open and inactivated states.

Dual actions of the novel class III antiarrhythmic drug NE-10064 on delayed potassium channel currents in guinea pig ventricular and sinoatrial node cells

We have investigated the concentration-dependent modulation, by the novel class III antiarrhythmic compound NE-10064, of the delayed potassium channel current Iks in isolated guinea pig sinoatrial nodal (SAN) and ventricular cells. At concentrations greater than 1 micron, the drug potently inhibited Iks in each of the cell types investigated. The concentration-dependent inhibition of Iks (IC50 = 700 nM) was the same in ventricular and SAN cells. At near-threshold drug concentrations, we also observed increases of Iks activity in both SAN and ventricular cells. The NE-10064-induced enhancement of Iks was more pronounced at voltages near the Iks activation threshold (0 mV), than at more positive voltages in both cell types. Furthermore, the agonistic effects of the drug were more prominent before steady-state effects of the compound were attained, which suggests parallel agonistic and antagonistic pathways. Our results demonstrate that Iks channels in cells of the sinoatrial node region of the guinea pig heart respond to NE-10064 in the same manner as cells of the ventricle and that this compound, although a potent inhibitor of Iks activity, possesses an interesting but as yet mechanistically unidentified agonistic action at low concentrations in both cell types.

Developmental changes in beta-adrenergic modulation of L-type Ca2+ channels in embryonic mouse heart

In the adult mammalian myocardium, cellular Ca2+ entry is regulated by the sympathetic nervous system. L-type Ca2+ channel currents are markedly increased by beta-adrenergic (beta-A) agonists, which contribute to changes in pacing and contractile activity of the heart. In the developing mammalian heart, the regulation of Ca2+ entry by this enzyme cascade has not been clearly established, because changes in receptor density and coupling to downstream elements of the signaling cascade are known to occur during embryogenesis. In this study, we systematically investigated the regulation of L-type Ca2+ channel currents during development of the murine embryonic heart. We used conventional whole-cell and perforated-patch-clamp procedures to study modulation of L- type Ca2+ channel currents and to assay functional activity of distinct steps in the beta-A signaling cascade in murine embryonic myocytes at different stages of gestation. Our data indicate that the L-type Ca2+ channels in early-stage (day-11 to -13) myocytes are unresponsive to either isoproterenol or cAMP. L-type Ca2+ channels in late-stage (day-17 to -19) murine myocytes, however, exhibit responses to isoproterenol and cAMP similar to responses in adult cells, providing evidence that the beta-A cascade becomes functionally active during this period of embryonic development. We found that L-type Ca2+ channel activity in early-stage cells is increased by cell dialysis with the catalytic subunit of cAMP-dependent protein kinase (cA-PK) and that dialysis of early-stage cells with the holoenzyme of cA-PK restores functional responses to forskolin and cAMP, but not to isoproterenol. Our results provide strong evidence that a key factor in the early-stage insensitivity of L-type Ca2+ channels to cAMP is the absence, or low expression level, of the holoenzyme of cA-PK but that in addition, another element in the signaling cascade upstream from adenylate cyclase is expressed at a nonfunctional level or is uncoupled from the cascade and thus contributes to L-type Ca2+ channel insensitivity to beta-A agonists in early stages of the developing murine heart.

Developmental changes in ionic channel activity in the embryonic murine heart

We have isolated murine embryonic atrial and ventricular cells derived from timed-pregnant females at different periods and used patch-clamp procedures to investigate age- and chamber-specific expression of ionic channels in the developing fetal mouse. Our data indicate that L-type Ca2+ channels play a dominant role in excitation during early murine cardiac embryogenesis and that Na+ channel expression increases dramatically just before birth. K+ channel expression is particularly sensitive to changes during development. Neither atrial nor ventricular cells express a slowly activating component of delayed rectification (IKs) until just before birth, and inwardly rectifying channel activity, associated with determination of cellular resting potential, is not markedly apparent until late stages of embryogenesis. Instead, we find robust expression of the ATP-regulated K+ channel at early and late states of embryonic development, which may indicate a novel functional role for this channel during morphogenesis of the heart. These results have important implications for the physiology and development of the murine cardiac conduction system and will also serve as a baseline for future studies designed to investigate developmental changes of ion channel expression in the myocardium of both wild-type and genetically modified mice.

Dependence of the GABAA receptor gating kinetics on the alpha-subunit isoform: implications for structure-function relations and synaptic transmission

1. To examine the dependence of gamma-aminobutyric acid (GABAA) receptor gating on the alpha-subunit isoform, we studied the kinetics of GABA-gated currents (IGABA) of receptors that differed in the alpha-subunit subtype, alpha 1 beta 2 gamma 2S and alpha 3 beta 2 gamma 2S. cDNAs encoding rat brain subunits were co-expressed heterologously in HEK-293 cells and the resultant receptors studied with the whole-cell patch clamp technique and rapidly applied GABA pulses (5-10 s). 2. IGABA of both receptors showed a loosely similar dependence on GABA concentration over a wide range (1-5000 microM). Generally, IGABA manifested activation reaching an early current peak, subsequent slower spontaneous desensitization, and deactivation of open channels at pulse termination. Lowering GABA concentrations reduced peak currents and slowed activation and desensitization kinetics. 3. The presence of alpha 3 altered the peak IGABA concentration-response relationship by shifting the fitted Hill equation to tenfold greater GABA concentrations (GABA concentration at half amplitude: alpha 1, 7 microM; and alpha 3, 75 microM) without affecting Hill coefficients (alpha 1, 1.6; alpha 3, 1.5). These findings indicate a reduction in the apparent activating site affinity and are consistent with previous reports. 4. To investigate differences in gating, we normalized for apparent activating site affinities by analysing the time course of macroscopic gating at equi-activating GABA concentrations. The presence of alpha 3 slowed activation fourfold (time to current peak (means +/- S.E.M.): alpha 1, 1.2 +/- 0.06 s (2 microM); alpha 3, 4.7 +/- 0.5 s (20 microM)), desensitization nearly twofold (reciprocal of time to 80% decay: alpha 1, 2.5 +/- 0.48 s-1 (100 microM); alpha 3, 1.5 +/- 0.15 s-1 (1000 microM)) and deactivation threefold (monoexponential decay time constant: alpha 1, 0.22 +/- 0.026 s (2 microM); alpha 3, 0.68 +/- 0.1 s (20 microM)). 5. To gain an insight into the gating mechanisms underlying macroscopic desensitization, we extended a previous gating model of GABAA receptor single-channel activity to include a desensitization pathway. Such a mechanism reproduced empirical alpha 1 beta 2 gamma 2S activation, desensitization and deactivation kinetics. 6. To identify molecular transitions underlying the gating differences between alpha 1 beta 2 gamma 2S and alpha 3 beta 2 gamma 2S receptors, we explored parameter alterations of the alpha 1 beta 2 gamma 2S gating model that provided an accounting of alpha 3 beta 2 gamma 2S empirical responses. Remarkably, alteration of rates and rate constants involved in ligand binding alone allowed reproduction of alpha 3 beta 2 gamma 2S activation, desensitization and deactivation. 7. These results indicate that substitution of the alpha 3 subunit variant in an alpha 1 beta 2 gamma 2S receptor alters transition rates involved in ligand binding that underlie changes in apparent activating site affinity and macroscopic current gating. Furthermore, they argue strongly that the structural determinants of these functional features reside on the alpha-subunit.

Cholinergic inhibition of slow delayed-rectifier K+ current in guinea pig sino-atrial node is not mediated by muscarinic receptors

We studied the effects of cholinergic agonists on slow delayed-rectifier K+ current (IKs) in isolated cells from the sino-atrial node (SAN) region of guinea pig heart, using patch-clamp procedures. Carbachol (5 nM to 10 microM) inhibited IKs in guinea pig SAN cells in the absence of previous beta-adrenergic stimulation and in cells pretreated with 8-(4-chlorophenylthio)-cAMP. Neither the muscarinic antagonist atropine nor the nicotinic antagonist hexamethonium antagonized carbachol inhibition of the current. Similar results were obtained with other cholinergic agonists. Cholinergic stimulation of the muscarinic K+ current was successfully antagonized by atropine in SAN cells where inhibition of IKs persisted. Therefore, the lack of antagonist effects on inhibition of IKs cannot be attributed to either an absence of muscarinic cholinoceptors on SAN cells or a loss of antagonist activity under our experimental conditions. These data demonstrate that cholinergic agonists, including the endogenous neurotransmitter acetylcholine, decrease the amplitude of IKs in guinea pig SAN cells via a non-muscarinic, non-nicotinic, cAMP-independent mechanism. Although the precise nature of this signal transduction pathway has not been elucidated, it is clearly different from those described for regulation of other nodal currents. Differential regulation of IKs in guinea pig SAN and ventricle cannot be attributed to higher basal adenylate cyclase activity in SAN cells. The inhibitory effect of carbachol on IKs was not additive with that of verapamil, a drug that is both an allosteric muscarinic antagonist and a potassium channel-blocking agent. Cholinergic agonists may inhibit IKs in SAN cells via a direct interaction with the SAN IKs channel.

Inhibition of cardiac L-type calcium channels by quaternary amlodipine: implications for pharmacokinetics and access to dihydropyridine binding site

We have used whole cell patch clamp procedures to investigate the inhibition of L-type calcium channel currents in guinea pig ventricular cells by the permanently charged dihydropyridine (DHP)compound UK-118,434-05 (quaternary amlodipine, QA). The location of the charge group of this drug molecule is approximately three times closer to the active DHP moiety than is the case for SDZ-207-180, the only other previously-investigated quaternary DHP molecule. Like SDZ-207-180, QA inhibits channel activity only by external application, consistent with an externally, but not internally, accessible binding site, and once blocked, channels do not recover availability by membrane hyperpolarization independent of extracellular pH. However inhibition by QA occurs at roughly 20 x lower potency than comparable inhibition by SDZ-207-180. Low affinity binding to the DHP binding site was confirmed directly with radioligand binding. The permanently charged amlodipine derivative inhibited radioligand DHP binding in partially purified rabbit skeletal muscle transverse tubule membranes with a pseudo-Hill slope close to unity and an IC50 value of 4.2 +/- 0.6 microM. These results indicate that the characteristically slow pharmacokinetics of tertiary amlodipine are due to the unusually stable inhibition of L-channels caused by the ionized fraction of drug molecules. Furthermore, because the distance between the ionized head group and the DHP moiety is so short, the low affinity binding and channel inhibition by QA suggests that the DHP binding site is not on the extracellular domain of the L-channel alpha 1 subunit, but instead must reside within the bilayer or channel pore at a location closer to the extracellular rather than the intracellular face of the membrane.

Polymorphism of the gene encoding a human minimal potassium ion channel (minK)

A gene (minK) that encodes a minimal potassium channel has been cloned recently. We describe in this paper a human minK sequence which differs from the original sequence with a single A-->G at position 112. This resulted in a change from a Ser codon (AGT) to a Gly codon (GGT) and created a new MspAI restriction site. Of the 32 alleles from 16 subjects studied, 25 had this newly discovered sequence and 7 had the previously described sequence.

L-type calcium channels: asymmetrical intramembrane binding domain revealed by variable length, permanently charged 1,4-dihydropyridines

We have used an homologous series of dihydropyridine (DHP) derivatives to determine the location of the binding domain for DHPs on cardiac L-type calcium channels, relative to the extracellular and intracellular membrane surfaces. The series of test molecules consisted of DHP analogs in which the DHP moiety was linked to either a neutral (-CH2CH3) or permanently charged [(-)+N(CH3)3] headgroup and the distance between the headgroup and the active moiety was systematically varied with alkyl spacer chains containing 2, 6, 8, 10, 12, or 16 methylene (-CH2) groups. These compounds were previously shown, by radioligand binding experiments, to interact with the high affinity DHP binding site in intact neonatal rat heart cells. In the present experiments, access to the DHP binding site was assayed by inhibition of L-type calcium channel currents using whole-cell patch-clamp procedures in guinea pig ventricular myocytes. Intracellular application was achieved by dialysis via charged DHP-containing whole-cell patch pipettes, and cell dialysis was monitored by using a charged DHP labeled with a rhodamine fluorophore. Our results show that access of extracellularly applied charged, but not neutral, DHPs to the DHP binding domain depends markedly on the alkyl spacer chain, with the optimal length being near 10 methylene groups. Intracellular application failed to inhibit channel activity for spacer chain lengths up to 16 methylene groups. From our results, we conclude that the DHP binding domain of cardiac L-type calcium channels is not on the extracellular membrane surface but is probably within the lipid bilayer, approximately 11-14 A from the extracellular surface.

Inhibition of L-type calcium-channel activity by thapsigargin and 2,5-t-butylhydroquinone, but not by cyclopiazonic acid

Thapsigargin (TG), 2,5-t-butylhydroquinone (tBHQ) and cyclopiazonic acid (CPA) all inhibit the initial Ca(2+)-response to thyrotropin-releasing hormone (TRH) by depleting intracellular Ca2+ pools sensitive to inositol 1,4,5-trisphosphate (IP3). Treatment of GH3 pituitary cells for 30 min with 5 nM TG, 500 nM tBHQ or 50 nM CPA completely eliminated the TRH-induced spike in intracellular free Ca2+ ([Ca2+]i). Higher concentrations of TG and tBHQ, but not CPA, were also found to inhibit strongly the activity of L-type calcium channels, as measured by the increase in [Ca2+]i or 45Ca2+ influx stimulated by depolarization. TG and tBHQ blocked high-K(+)-stimulated 45Ca2+ uptake, with IC50 values of 10 and 1 microM respectively. Maximal inhibition of L-channel activity was achieved 15-30 min after drug addition. Inhibition by tBHQ was reversible, whereas inhibition by TG was not. TG and CPA did not affect spontaneous [Ca2+]i oscillations when tested at concentrations adequate to deplete the IP3-sensitive Ca2+ pool. However, 20 microM TG and 10 microM tBHQ blocked [Ca2+]i oscillations completely. The effect of drugs on calcium currents was measured directly by using the patch-clamp technique. When added to the external bath, 10 microM CPA caused a sustained increase in the calcium-channel current amplitude over 8 min, 10 microM tBHQ caused a progressive inhibition, and 10 microM TG caused an enhancement followed by a sustained block of the calcium current over 8 min. In summary, CPA depletes IP3-sensitive Ca2+ stores and does not inhibit voltage-operated calcium channels. At sufficiently low concentrations, TG depletes IP3-sensitive stores without inhibiting L-channel activity, but, for tBHQ, inhibition of calcium channels occurs at concentrations close to those needed to block agonist mobilization of intracellular Ca2+.

Inhibition of pinacidil induced IK(ATP) in heart by changes in extracellular pH

OBJECTIVE

The aim was to determine whether extracellular protons influence the activation of ATP sensitive potassium channels (KATP) in heart by the drug pinacidil.

METHODS

Single channel and whole cell currents were measured in enzymatically dispersed guinea pig ventricular myocytes with patch clamp procedures. ATP sensitive potassium channel activity was induced by pinacidil in whole cell experiments and either by lowering cytoplasmic ATP or pinacidil in membrane patch experiments.

RESULTS

Extracellular acidification inhibited and extracellular alkalisation potentiated ATP sensitive potassium channel currents (IK(ATP)) induced by pinacidil under whole cell conditions. In membrane patches, IK(ATP) induced by low cytoplasmic ATP (1 mM) was not sensitive to changes in extracellular pH, but IK(ATP) induced by cytoplasmic application of pinacidil was inhibited by extracellular acidification.

CONCLUSIONS

Interactions between protons and pinacidil drug molecules underlie the pH dependent changes in IK(ATP) induced by pinacidil. Based on the kinetics of pH dependent changes in pinacidil induced channel activity in inside-out membrane patches, our data suggest that the important inhibitory site of action of this drug is located on the extracellular side of the cell membrane.

Expression of a minimal K+ channel protein in mammalian cells and immunolocalization in guinea pig heart

The minimal K+ channel protein (minK, also called IsK) is structurally dissimilar to other cloned voltage-gated ion channels. minK is a 15-kD polypeptide with only one potential transmembrane helix. Published data suggest that the current associated with minK expression in Xenopus oocytes may be related to the slow cardiac delayed rectifier K+ current (IKs). However, the fact that minK expression has been limited exclusively to Xenopus oocytes has caused continuing concern about the nature of this protein and its molecular link to known mammalian K+ channels. We report in the present study the first expression of minK activity in transiently transfected mammalian (HEK 293) cells and demonstrate that the characteristics of the expressed minK current are similar to those of IKs recorded from guinea pig heart cells under similar experimental conditions. We also show that an antibody directed against the minK channel protein reacts with a surface antigen on adult guinea pig ventricular myocytes and sinoatrial nodal cells, where IKs is the dominant outward K+ current. The data provide strong evidence that a minK-like protein underlies IKs.

Subunit-dependent modulation of recombinant L-type calcium channels. Molecular basis for dihydropyridine tissue selectivity

At least four calcium channel subtypes (P, T, N, and L) have now been classified on the basis of their biophysical and/or pharmacological properties. L-type channels, a channel family particularly important to physiological function of the cardiovascular system, are identified by their slow voltage- and calcium-dependent inactivation as well as their sensitivity to dihydropyridine (DHP) calcium channel antagonists. In this study, we report the results of experiments in which we have measured the DHP modulation of recombinant calcium channel activity in cells transfected with alpha 1 subunits of cardiac and smooth muscle L-type calcium channels. We find subunit-dependent differences in the voltage and concentration dependence of channel modulation. Our results provide evidence for a molecular basis for DHP sensitivity of heart and smooth muscle calcium channels and, additionally, indicate that, even within one family of calcium channels, slight differences in channel structure can cause marked differences in channel pharmacology.

Block of cardiac ATP-sensitive K+ channels by external divalent cations is modulated by intracellular ATP. Evidence for allosteric regulation of the channel protein

We have investigated the interactions between extracellular divalent cations and the ATP-sensitive potassium channel in single guinea pig ventricular cells and found that, under whole-cell patch clamp recording conditions, extracellularly applied Co2+, Cd2+, and Zn2+ block current through the ATP-sensitive K channel (IKATP). The respective Kd's for block of IKATP by Cd2+ and Zn2+ are 28 and 0.46 microM. The Kd for Co2+ is > 200 microM. Extracellular Ca2+ and Mg2+ appear to have no effect at concentrations up to 1 and 2 mM, respectively. Block of IKATP by extracellular cations is not voltage dependent, and both onset and recovery from block occur within seconds. Single-channel experiments using the inside-out patch configuration show that internally applied Cd2+ and Zn2+ are not effective blockers of IKATP. Experiments in the outside-out patch configuration confirm that the divalent cations interact directly with IKATP channel activity. Our study also shows that this block of IKATP is dependent on intracellular ATP concentrations. Under whole-cell conditions, when cells are dialyzed with [ATP]pipette = 0, the degree of cation block is reduced. This dependence on intracellular ATP was confirmed at the single-channel level by experiments in excised, inside-out patch configurations. Our results show that some, but not all, divalent cations inhibit current through IKATP channels by binding to sites that are not within the transmembrane electric field, but are on the extracellular membrane surface. The interdependence of internal ATP and external divalent cation binding is consistent with an allosteric interaction between two binding sites and is highly suggestive of a modulatory mechanism involving conformational change of the channel protein.

Interactions between H+ and Ca2+ near cardiac L-type calcium channels: evidence for independent channel-associated binding sites

Monovalent and divalent ions are known to affect voltage-gated ion channels by the screening of, and/or binding to, negative charges located on the surface of cell membranes within the vicinity of the channel protein. In this investigation, we studied gating shifts of cardiac L-type calcium channels induced by extracellular H+ and Ca2+ to determine whether these cations interact at independent or competitive binding sites. At constant pHo (7.4), Cao-induced gating shifts begin to approach a maximum value (approximately equal to 17 mV) at concentrations of extracellular calcium of > or = 40 mM. A fraction of the calcium-dependent gating shift could be titrated with an effective pKa = 6.9 indicating common and competitive access to H+ and Ca2+ ions for at least one binding site. However, if pHo is lowered when Cao is > or = 40 mM, additional shifts in gating are measured, suggesting a subpopulation of sites to which Ca2+ and H+ bind independently. The interdependence of L-channel gating shifts and Cao and pHo was well described by the predictions of surface potential theory in which two sets of binding sites are postulated; site 1 (pKa = 5.5) is accessible only to H+ ions and site 2 (pKa = 6.9) is accessible to both Ca2+ and H+ ions. Theoretical computations generated with this model are consistent with previously determined data, in which interactions between these two cations were not studied, in addition to the present experiments in which interactions were systematically probed.

Delayed rectifier potassium channels in ventricle and sinoatrial node of the guinea pig: molecular and regulatory properties

We focus on the regulatory properties of delayed rectifier K+ (IK) channels in guinea-pig sinoatrial node (SAN) and compare SAN IK to the better characterized ventricular IK. Despite demonstrated similarities in the properties of IK in guinea-pig ventricle and SAN, the possibility remains that expression of IK channels can vary regionally within the same heart. Like ventricular IK, SAN IK can be enhanced by beta-adrenergic stimulation and exposure to phorbol ester. However, in contrast to ventricular IK, regulation of SAN IK by protein kinases A and C is not temperature dependent. Basal SAN IK can be diminished by muscarinic agonists, while beta-adrenergic stimulation is a precondition for reduction of ventricular IK by cholinergic agonists. Nonstationary state fluctuation analysis predicts a small single-channel current (1 pA) and a large number of functional channels (308) associated with whole-cell SAN IK. The corresponding single-channel conductance of 6 pS is somewhat larger than that estimated for ventricular IK. Overall comparisons of guinea-pig ventricular and SAN IK to the current associated with the minK channel clone suggest that the native guinea-pig cardiac IK channels may be related not only to each other but lso to the minK channel protein.

Insensitivity of guinea pig ventricular delayed rectifier IK to intracellular trypsin: implications for channel structure and function

OBJECTIVE

Intracellular application of proteolytic agents modifies the function of many voltage gated ion channels. The presence of a trypsin sensitive inhibitory domain in a channel protein may be important for G protein dependent activation. Guinea pig ventricular IK is modulated by a direct G protein pathway. The aim was to determine if guinea pig ventricular IK is also modified by intracellularly applied trypsin.

METHODS

Whole cell and excised inside out configurations of patch clamp were used to record IK from guinea pig ventricular myocytes before and after cytosolic application of trypsin (1 mg.ml-1). We used previously reported effects of trypsin on the L type calcium current (ICa) to monitor dialysis time and enzyme activity in whole cell experiments where IK and ICa were measured concomitantly.

RESULTS

Addition of trypsin to the solution bathing the cytosolic face of excised membrane patches had no effect on the amplitude or kinetics of IK. When added to the pipette solution and introduced by cell dialysis, trypsin had no effect on whole cell IK, even when significant effects on the amplitude and kinetics of ICa were evident.

CONCLUSIONS

Guinea pig ventricular IK is not enhanced or otherwise altered by intracellularly applied trypsin. Therefore direct phosphorylation independent enhancement of IK by guanine nucleotides cannot depend on interactions between G protein subunits and trypsin sensitive inhibitory channel domains. The lack of trypsin modification of cardiac ventricular IK suggests that the structure of the endogenous delayed rectifier K+ channel may be different than that of other voltage gated channels.

Activation of protein kinase C reduces L-type calcium channel activity of GH3 pituitary cells

These studies describe the effect of protein kinase C (PKC) activation on the activity of voltage-sensitive L-type Ca2+ channels of GH3 pituitary cells. The rate of 45Ca2+ uptake was stimulated greater than 25-fold by depolarization in the presence of BAY K 8644; the phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA) reduced this response by 70% in a concentration-dependent fashion. Phorbol 12,13-dibutyrate (PDBu) inhibited depolarization-induced 45Ca2+ uptake within 1 min and caused a nearly maximal reduction after 1 h; its effects were rapidly reversible. TPA decreased the high K(+)-stimulated increase in intracellular free calcium ion concentration ([Ca2+]i) from 8.5- to 3.2-fold by 5 min and to 2.0-fold after 18 h without altering the peak [Ca2+]i response to the peptide hormone TRH. Ca2+ channel current, measured directly using the whole cell configuration of the patch-clamp technique, declined an average of 6.4% over 5 min for control cells and 28.9% when TPA was added to the bathing medium for 5 min. Treatment with 100 nM TPA for 24 h dramatically reduced peak current without shifting the peak of the current-voltage relationship. The mean peak Ca2+ channel current was reduced from 423 to 128 pA, although a few cells seemed completely resistant. To determine whether the effects of phorbol esters were due to the activation of PKC we tested the potency of several drugs to inhibit L-channel activity and to shift the affinity of the epidermal growth factor (EGF) receptor, an established PKC response.(ABSTRACT TRUNCATED AT 250 WORDS)

Phosphorylation-independent regulation of cardiac IK by guanine nucleotides and isoproterenol

We tested for direct G protein regulation of delayed rectifier K+ (IK) channels, by measuring IK currents in guinea pig ventricular cells using patch-clamp procedures. In excised inside-out patches, IK was enhanced by adding guanosine triphosphate or guanosine 5'-O-(3-thiotriphosphate) to the cytoplasmic side, even in the presence of phosphorylation inhibitors. Enhancement of patch IK did not require extracellular agonist; however, enhancement of IK was also seen when isoproterenol was included in the pipette solution. Whole cell IK currents were increased by isoproterenol when phosphorylation pathways were blocked. These data demonstrate that guanine nucleotides and beta-adrenergic agonists can enhance IK by a phosphorylation-independent pathway. Our findings are consistent with a direct coupling of the beta-adrenergic receptor to the cardiac IK channel via a membrane-delimited G protein pathway, in addition to the well-established indirect pathway.

Delayed rectification in single cells isolated from guinea pig sinoatrial node

We have studied delayed rectifier K+ currents (IK) in cells isolated from the sinoatrial node (SAN) region of the guinea pig. Using whole cell patch-clamp procedures, we measured the voltage dependence of IK activation and IK kinetics and the IK equilibrium potential in 4.8 mM extracellular K concentration solutions. Experiments were designed to contrast properties of guinea pig SAN IK with those of IK recorded from SAN cells of the rabbit. We find that guinea pig SAN IK differs from IK recorded from single rabbit SAN cells in its activation threshold, and in the absence of inactivation of whole cell currents recorded over a wide voltage range. These results, along with the relative insensitivity of guinea pig SAN IK to E-4031 and lanthanum, suggest that under our experimental conditions, a strongly rectifying IK component (IK,r) is not the major component of delayed rectification in the guinea pig SAN, as it appears to be in SAN cells of the rabbit.