Selective activation of adrenoceptors potentiates IKs current in pulmonary vein cardiomyocytes through the protein kinase A and C signaling pathways.

Delayed rectifier K+ current (IKs) is a key contributor to repolarization of action potentials. This study investigated the mechanisms underlying the adrenoceptor-induced potentiation of IKs in pulmonary vein cardiomyocytes (PVC). PVC were isolated from guinea pig pulmonary vein. The action potentials and IKs current were recorded using perforated and conventional whole-cell patch-clamp techniques. The expression of IKs was examined using immunocytochemistry and Western blotting. KCNQ1, a IKs pore-forming protein was detected as a signal band approximately 100 kDa in size, and its immunofluorescence signal was found to be mainly localized on the cell membrane. The IKs current in PVC was markedly enhanced by both ß1- and ß2-adrenoceptor stimulation with a negative voltage shift in the current activation, although the potentiation was more effectively induced by ß2-adrenoceptor stimulation than ß1-adrenoceptor stimulation. Both ß-adrenoceptor-mediated increases in IKs were attenuated by treatment with the adenylyl cyclase (AC) inhibitor or protein kinase A (PKA) inhibitor. Furthermore, the IKs current was increased by α1-adrenoceptor agonist but attenuated by the protein kinase C (PKC) inhibitor. PVC exhibited action potentials in normal Tyrode solution which was slightly reduced by HMR-1556 a selective IKs blocker. However, HMR-1556 markedly reduced the ß-adrenoceptor-potentiated firing rate. The stimulatory effects of ß- and α1-adrenoceptor on IKs in PVC are mediated via the PKA and PKC signal pathways. HMR-1556 effectively reduced the firing rate under ß-adrenoceptor activation, suggesting that the functional role of IKs might increase during sympathetic excitation under in vivo conditions.

Characterization and functional role of rapid- and slow-activating delayed rectifier K+ currents in atrioventricular node cells of guinea pigs.

The atrioventricular (AV) node is the only conduction pathway where electrical impulse can pass from atria to ventricles and exhibits spontaneous automaticity. This study examined the function of the rapid- and slow-activating delayed rectifier K+ currents (IKr and IKs) in the regulation of AV node automaticity. Isolated AV node cells from guinea pigs were current- and voltage-clamped to record the action potentials and the IKr and IKs current. The expression of IKr or IKs was confirmed in the AV node cells by immunocytochemistry, and the positive signals of both channels were localized mainly on the cell membrane. The basal spontaneous automaticity was equally reduced by E4031 and HMR-1556, selective blockers of IKr and IKs, respectively. The nonselective ß-adrenoceptor agonist isoproterenol markedly increased the firing rate of action potentials. In the presence of isoproterenol, the firing rate of action potentials was more effectively reduced by the IKs inhibitor HMR-1556 than by the IKr inhibitor E4031. Both E4031 and HMR-1556 prolonged the action potential duration and depolarized the maximum diastolic potential under basal and ß-adrenoceptor-stimulated conditions. IKr was not significantly influenced by ß-adrenoceptor stimulation, but IKs was concentration-dependently enhanced by isoproterenol (EC50: 15 nM), with a significant negative voltage shift in the channel activation. These findings suggest that both the IKr and IKs channels might exert similar effects on regulating the repolarization process of AV node action potentials under basal conditions; however, when the ß-adrenoceptor is activated, IKs modulation may become more important.

Dexmedetomidine Exerts a Negative Chronotropic Action on Sinoatrial Node Cells Through the Activation of Imidazoline Receptors.

ABSTRACT: Dexmedetomidine (DEX), an α2-adrenoreceptor (α2-AR) and imidazoline receptor agonist, is most often used for the sedation of patients in the intensive care unit. Its administration is associated with an increased incidence of bradycardia; however, the precise mechanism of DEX-induced bradycardia has yet to be fully elucidated. This study was undertaken to examine whether DEX modifies pacemaker activity and the underlying ionic channel function through α2-AR and imidazoline receptors. The whole-cell patch-clamp techniques were used to record action potentials and related ionic currents of sinoatrial node cells in guinea pigs. DEX (≥10 nM) reduced sinoatrial node automaticity and the diastolic depolarization rate. DEX reduced the amplitude of hyperpolarization-activated cation current (If or Ih) the pacemaker current, even within the physiological pacemaker potential range. DEX slowed the If current activation kinetics and caused a significant shift in the voltage dependence of channel activation to negative potentials. In addition, efaroxan, an α2-AR and imidazoline I1 receptor antagonist, attenuated the inhibitory effects of DEX on sinoatrial node automaticity and If current activity, whereas yohimbine, an α2-AR-selective antagonist, did not. DEX did not affect the current activities of other channels, including rapidly and slowly activating delayed rectifier K+ currents (IKr and IKs), L-type Ca2+ current (ICa,L), Na+/Ca2+ exchange current (INCX), and muscarinic K+ current (IK,ACh). Our results indicate that DEX, at clinically relevant concentrations, induced a negative chronotropic effect on the sinoatrial node function through the downregulation of If current through an imidazoline I1 receptor other than the α2-AR in the clinical setting.

Identification of Verapamil Binding Sites Within Human Kv1.5 Channel Using Mutagenesis and Docking Simulation.

BACKGROUND/AIMS: The phenylalkylamine class of L-type Ca2+ channel antagonist verapamil prolongs the effective refractory period (ERP) of human atrium, which appears to contribute to the efficacy of verapamil in preventing reentrant-based atrial arrhythmias including atrial fibrillation. This study was designed to investigate the molecular and electrophysiological mechanism underlying the action of verapamil on human Kv1.5 (hKv1.5) channel that determines action potential duration and ERP in human atrium. METHODS: Site-directed mutagenesis created 10 single-point mutations within pore region of hKv1.5 channel. Wholecell patch-clamp method investigated the effect of verapamil on wild-type and mutant hKv1.5 channels heterologously expressed in Chinese hamster ovary cells. Docking simulation was conducted using open-state homology model of hKv1.5 channel pore. RESULTS: Verapamil preferentially blocked hKv1.5 channel in its open state with IC50 of 2.4±0.6 µM (n = 6). The blocking effect of verapamil was significantly attenuated in T479A, T480A, I502A, V505A, I508A, L510A, V512A and V516A mutants, compared with wild-type hKv1.5 channel. Computer docking simulation predicted that verapamil is positioned within central cavity of channel pore and has contact with Thr479, Thr480, Val505, Ile508, Ala509, Val512, Pro513 and Val516. CONCLUSION: Verapamil acts as an open-channel blocker of hKv1.5 channel, presumably due to direct binding to specific amino acids within pore region of hKv1.5 channel, such as Thr479, Thr480, Val505, Ile508, Val512 and Val516. This blocking effect of verapamil on hKv1.5 channel appears to contribute at least partly to prolongation of atrial ERP and resultant antiarrhythmic action on atrial fibrillation in humans.

Correction to: Heterogeneous functional expression of the sustained inward Na+ current in guinea pig sinoatrial node cells.

Dr. Wei-Guang Ding's given name and family name were inadvertently interchanged initially. The correct names are as shown above.

Heterogeneous functional expression of the sustained inward Na+ current in guinea pig sinoatrial node cells.

The sustained inward Na+ current (I st) identified in the sinoatrial node (SAN) cell has been suggested to play a pivotal role in cardiac pacemaking. However, the composition of cells in the SAN is heterogeneous and cell-to-cell variability in the magnitude of I st remains to be fully characterized. The present study investigated the current density of I st in morphologically different types of pacemaker cells dissociated from guinea pig SAN. I st was preferentially detected in spontaneously active spindle or spider-shaped cells, but was less well expressed in larger-sized elongated spindle-type cells and practically absent in clearly striated atrial-like cells, despite clear expression of the funny current (I f). The current density of I st in spindle and spider cells varied from 0.7 to 1.6 pA pF-1 and was significantly reduced in non-beating cells with similar morphologies. By linear regression analysis, we identified a positive correlation between the current densities of I st and the L-type Ca2+ current (I Ca,L), which was specifically observed in spindle and spider cells. These cells exhibited a more negative voltage for half maximal I Ca,L activation than atrial-like cells, suggesting a variable ratio between CaV1.2- and CaV1.3-mediated I Ca,L in SAN cells. Consistent single-cell transcript measurements confirmed a higher relative expression of CaV1.3, which activates at more negative potentials, in spindle cells than in atrial-like cells. Taken together, these results can be interpreted as indicating that I st plays a specific role in primary pacemaker cells and that its presence is closely correlated with functional levels of CaV1.3-mediated I Ca,L.

Postnatal developmental changes in the sensitivity of L-type Ca2+ channel to inhibition by verapamil in a mouse heart model.

BackgroundIn the clinical setting, verapamil is contraindicated in neonates and infants, because of the perceived risk of hypotension or bradyarrhythmia. However, it remains unclear whether there is an age-dependent difference in the sensitivity of cardiac L-type Ca2+ channel current (ICa,L) to inhibition by verapamil.MethodsVentricular myocytes were enzymatically dissociated from the hearts of six different age groups (0, 7, 14, 21, 28 days, and 10-15 weeks) of mice, using a similar Langendorff-perfusion method. Whole-cell patch-clamp technique was applied to examine the sensitivity of ICa,L to inhibition, by three classes of structurally different L-type Ca2+ channel antagonists.ResultsVerapamil, nifedipine, and diltiazem concentration-dependently blocked the ventricular ICa,L in all six age groups. However, although nifedipine and diltiazem blocked ventricular ICa,L with a similar potency in all age groups, verapamil more potently blocked ventricular ICa,L in day 0, day 7, day 14, and day 21 mice, than in day 28, and 10-15-week mice.ConclusionIn a mouse heart model, ventricular ICa,L before the weaning age (~21 days of age) exhibited a higher sensitivity to inhibition by verapamil than that after the weaning age, which may explain one possible mechanism associated with the development of verapamil-induced hypotension in human neonates and infants.

Transient Receptor Potential Canonical Channel Blockers Improve Ventricular Contractile Functions After Ischemia/Reperfusion in a Langendorff-perfused Mouse Heart Model.

Reperfusion of ischemic myocardium is accompanied by intracellular Ca overload, leading to cardiac dysfunction. However, the mechanisms underlying intracellular Ca overload have yet to be fully elucidated. The mechanism may involve the activation of store-operated Ca entry, which is primarily mediated through the transient receptor potential canonical (TRPC) channels. This study was undertaken to examine the possible involvement of TRPC channels in the development of contractile dysfunction associated with reperfusion of ischemic myocardium using a mouse heart model. The functional expression of TRPC channels was confirmed in mouse ventricular myocytes using immunocytochemistry, Western blotting, and patch-clamp experiments. The left ventricular functions were assessed by measuring left ventricular end-diastolic pressure, left ventricular developed pressure, and its first derivatives in a Langendorff-perfused mouse heart subjected to 30 minutes of normothermic (37°C) global ischemia followed by 60 minutes of reperfusion. Under control conditions, left ventricular functions were deteriorated during reperfusion, which was significantly ameliorated by administration of the TRPC channel blockers 2-aminoethoxydiphenyl borate and La during initial 5 minutes of reperfusion. Our findings suggest that TRPC channels are involved in mediating contractile dysfunction during reperfusion of ischemic myocardium and detect TRPC channels as a potential therapeutic target for preventing myocardial ischemia/reperfusion injury.

Interactions of Propofol With Human Voltage-gated Kv1.5 Channel Determined by Docking Simulation and Mutagenesis Analyses.

Propofol blocks the voltage-gated human Kv1.5 (hKv1.5) channel by preferentially affecting in its open state. A previous mutational study suggested that several amino acids within the pore region of the hKv1.5 channel are involved in mediating the blocking action of propofol. The present investigation was undertaken to elucidate the predicted binding modes of propofol within the pore cavity of the open-state hKv1.5 channel, using computational docking and mutagenesis approaches. The docking simulation using a homology model of the hKv1.5 channel, constructed based on the crystal structure of the Kv1.2 channel, predicted that propofol was positioned at the base of the pore cavity of hKv1.5 channel, adjacent to 4 amino acids Thr479, Thr480, Val505, and Ile508, and formed arene-H interactions with Val505. The patch-clamp experiments on wild-type and mutant hKv1.5 channels constructed by site-directed mutagenesis revealed that the blocking potency of propofol was significantly reduced in T480A, V505A, and I508A but not in T479A mutants compared with wild-type hKv1.5 channel. These computational docking and experimental mutational analyses suggest that propofol is positioned at the base of the pore cavity and forms functional contact with Thr480, Val505, and Ile508 to directly block the hKv1.5 channel.

Regulation of human cardiac Kv1.5 channels by extracellular acidification.

Human Kv1.5 channels (hKv1.5) conduct the ultra-rapid delayed rectifier potassium current (I Kur), which plays an important role in action potential repolarization of atrial myocytes. The present study was undertaken to examine the effects of acidic pH on hKv1.5 wild-type (WT) and its pore mutant channels heterologously expressed in Chinese hamster ovary (CHO) cells using site-directed mutagenesis combined with whole-cell patch-clamp technique. Both extracellular and intracellular acidifications equally and reversely reduced the amplitude of hKv1.5 currents. The extracellular acidification significantly shifted the voltage dependence of current activation to more depolarized potentials and accelerated deactivation kinetics of the current. The ancillary ß subunits Kvß1.3 and Kvß1.2, known to modify the pharmacological sensitivities of hKv1.5, enhanced the extracellular proton-induced inhibitory effect on hKv1.5 current. In addition, several mutants (T462C, T479A, T480A, and I508A) exhibited significantly higher sensitivity to acidic pH-induced inhibition compared with WT channel, whereas the inhibitory effect of acidic pH was markedly reduced in H463G mutant. These observations indicate that (1) extracellular acidification modifies hKv1.5 gating and activity, (2) ß subunits and several residues (T462, T479, T480, and I508) play critical roles in determining the sensitivity of the channel to acidic exposure, and (3) H463 may be a critical sensor for the channel inhibition by extracellular protons.

Ca2+/calmodulin potentiates I Ks in sinoatrial node cells by activating Ca2+/calmodulin-dependent protein kinase II.

The slow component of the delayed rectifier K(+) current (I Ks) plays an important role in the repolarization of action potentials in cardiac pacemaker cells and ventricular myocytes, and is regulated by various signaling pathways. Recent evidence has shown that calmodulin (CaM) is involved in modulation of diverse ion channels in cardiac myocytes under physiological and pathophysiological conditions. In the present study, we examined regulation of I Ks by Ca(2+)/CaM in guinea pig sinoatrial (SA) node cells using the whole-cell patch-clamp method. The density of I Ks was larger during intracellular dialysis with a higher Ca(2+) concentration (pCa 7, Ca (+)) compared to that with a low Ca(2+) concentration (pCa 10, Ca (-)). Intracellular application of CaM (400 nM) markedly potentiated I Ks with a Ca (+) pipette solution but not with a Ca (-) solution, thus showing that CaM potentiates I Ks in an intracellular Ca(2+)-dependent manner. Intracellular application of a specific Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) inhibitor, autocamtide-2 inhibitory peptide (AIP, 500 nM), markedly reduced I Ks activity in the presence of higher intracellular Ca(2+). Similarly, bath application of another inhibitor, KN-93 (1 µM) also significantly suppressed I Ks. Finally, the stimulatory action on I Ks of Ca(2+)/CaM was abolished by pretreatment with KN-93. Taken together, these observations suggest that Ca(2+)/CaM stimulates I Ks in guinea pig SA node cells through activation of CaMKII. This enhancement of I Ks by CaMKII may be involved in modulation of SA node automaticity under physiological or pathophysiological condition.

Phosphatidylinositol4-phosphate 5-kinase prevents the decrease in the HERG potassium current induced by Gq protein-coupled receptor stimulation.

The human ether-a-go-go-related gene (HERG) potassium current (IHERG) has been shown to decrease in amplitude following stimulation with Gq protein-coupled receptors (GqRs), such as α1-adrenergic and M1-muscarinic receptors (α1R and M1R, respectively), at least partly via the reduction of membrane phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2). The present study was designed to investigate the modulation of HERG channels by PI(4,5)P2 and phosphatidylinositol4-phosphate 5-kinase (PI(4)P5-K), a synthetic enzyme of PI(4,5)P2. Whole-cell patch-clamp recordings were used to examine the activity of HERG channels expressed heterologously in Chinese Hamster Ovary cells. The stimulation of α1R with phenylephrine or M1R with acetylcholine decreased the amplitude of IHERG accompanied by a significant acceleration of deactivation kinetics and the effects on IHERG were significantly attenuated in cells expressing PI(4)P5-K. The density of IHERG in cells expressing GqRs alone was significantly increased by the coexpression of PI(4)P5-K without significant differences in the voltage dependence of activation and deactivation kinetics. The kinase-deficient substitution mutant, PI(4)P5-K-K138A did not have these counteracting effects on the change in IHERG by M1R stimulation. These results suggest that the current density of IHERG is closely dependent on the membrane PI(4,5)P2 level, which is regulated by PI(4)P5-K and GqRs and that replenishing PI(4,5)P2 by PI(4)P5-K recovers IHERG.

Optical mapping of the electrical activity of isolated adult zebrafish hearts: acute effects of temperature.

The zebrafish (Danio rerio) has emerged as an important model for developmental cardiovascular (CV) biology; however, little is known about the cardiac function of the adult zebrafish enabling it to be used as a model of teleost CV biology. Here, we describe electrophysiological parameters, such as heart rate (HR), action potential duration (APD), and atrioventricular (AV) delay, in the zebrafish heart over a range of physiological temperatures (18-28°C). Hearts were isolated and incubated in a potentiometric dye, RH-237, enabling electrical activity assessment in several distinct regions of the heart simultaneously. Integration of a rapid thermoelectric cooling system facilitated the investigation of acute changes in temperature on critical electrophysiological parameters in the zebrafish heart. While intrinsic HR varied considerably between fish, the ex vivo preparation exhibited impressively stable HRs and sinus rhythm for more than 5 h, with a mean HR of 158 ± 9 bpm (means ± SE; n = 20) at 28°C. Atrial and ventricular APDs at 50% repolarization (APD50) were 33 ± 1 ms and 98 ± 2 ms, respectively. Excitation originated in the atrium, and there was an AV delay of 61 ± 3 ms prior to activation of the ventricle at 28°C. APD and AV delay varied between hearts beating at unique HRs; however, APD and AV delay did not appear to be statistically dependent on intrinsic basal HR, likely due to the innate beat-to-beat variability within each heart. As hearts were cooled to 18°C (by 1°C increments), HR decreased by ~40%, and atrial and ventricular APD50 increased by a factor of ~3 and 2, respectively. The increase in APD with cooling was disproportionate at different levels of repolarization, indicating unique temperature sensitivities for ion currents at different phases of the action potential. The effect of temperature was more apparent at lower levels of repolarization and, as a whole, the atrial APD was the cardiac parameter most affected by acute temperature change. In conclusion, this study describes a preparation enabling the in-depth analysis of transmembrane potential dynamics in whole zebrafish hearts. Because the zebrafish offers some critical advantages over the murine model for cardiac electrophysiology, optical mapping studies utilizing zebrafish offer insightful information into the understanding and treatment of human cardiac arrhythmias, as well as serving as a model for other teleosts.

Cardiac channelopathies associated with infantile fatal ventricular arrhythmias: from the cradle to the bench.

BACKGROUND: Fatal ventricular arrhythmias in the early period of life have been associated with cardiac channelopathies for decades, and postmortem analyses in SIDS victims have provided evidence of this association. However, the prevalence and functional properties of cardiac ion channel mutations in infantile fatal arrhythmia cases are not clear. METHODS AND RESULTS: Seven infants with potentially lethal arrhythmias at age < 1 year (5 males, age of onset 44.1 ± 72.1 days) were genetically analyzed for KCNQ1, KCNH2, KCNE1-5, KCNJ2, SCN5A, GJA5, and CALM1 by using denaturing high-performance liquid chromatography and direct sequencing. Whole-cell currents of wildtype and mutant channels were recorded and analyzed in Chinese hamster ovary cells transfected with SCN5A and KCNH2 cDNA. In 5 of 7 patients, we identified 4 mutations (p.N1774D, p.T290fsX53, p.F1486del and p.N406K) in SCN5A, and 1 mutation (p.G628D) in KCNH2. N1774D, F1486del, and N406K in SCN5A displayed tetrodotoxin-sensitive persistent late Na(+) currents. By contrast, SCN5A-T290fsX53 was nonfunctional. KCNH2-G628D exhibited loss of channel function. CONCLUSION: Genetic screening of 7 patients was used to demonstrate the high prevalence of cardiac channelopathies. Functional assays revealed both gain and loss of channel function in SCN5A mutations, as well as loss of function associated with the KCNH2 mutation.

Gain-of-function KCNH2 mutations in patients with Brugada syndrome.

BACKGROUND: Brugada syndrome (BrS) is an inherited disease characterized by right precordial ST segment elevation on electrocardiograms (ECGs) that predisposes patients to sudden cardiac death as a result of polymorphic ventricular tachyarrhythmia or ventricular fibrillation (VF). In BrS patients, except for SCN5A, mutations in other responsible genes are poorly elucidated. METHODS AND RESULTS: We identified 4 KCNH2 mutations, T152I, R164C, W927G, and R1135H, in 236 consecutive probands with BrS or Brugada-like ECG. Three of these mutation carriers showed QTc intervals shorter than 360 milliseconds and 1 experienced VF. We performed patch-clamp analyses on I(Kr) reconstituted with the KCNH2 mutations in Chinese hamster ovary cells and compared the phenotypes of the patients with different genotypes. Three mutations, R164C, W927G, and R1135H, increased I(Kr) densities. Three mutations, T152I, R164C, and W927G, caused a negative shift in voltage-dependent activation curves. Only the R1135H mutant channel prolonged the deactivation time constants. We also identified 20 SCN5A and 5 CACNA1C mutation carriers in our cohort. Comparison of probands' phenotypes with 3 different genotypes revealed that KCNH2 mutation carriers showed shorter QTc intervals and SCN5A mutation carriers had longer QRS durations. CONCLUSIONS: All KCNH2 mutations that we identified in probands with BrS exerted gain-of-function effects on I(Kr) channels, which may partially explain the ECG findings in our patients.

Long QT syndrome type 8: novel CACNA1C mutations causing QT prolongation and variant phenotypes.

AIMS: CACNA1C mutations have been reported to cause LQTS type 8 (LQT8; Timothy syndrome), which exhibits severe phenotypes, although the frequency of patients with LQT8 exhibiting only QT prolongation is unknown. This study aimed to elucidate the frequency of CACNA1C mutations in patients with long QT syndrome (LQTS), except those with Timothy syndrome and investigate phenotypic variants. METHODS AND RESULTS: CACNA1C gene screening was performed in 278 probands negative for LQTS-related gene mutations. Functional analysis of mutant channels using a whole-cell patch-clamp technique was also performed. Using genetic screening, we identified five novel CACNA1C mutations: P381S, M456I, A582D, R858H, and G1783C in seven (2.5%) unrelated probands. Seven mutation carriers showed alternative clinical phenotypes. Biophysical assay of CACNA1C mutations revealed that the peak calcium currents were significantly larger in R858H mutant channels than those of wild-type (WT). In contrast, A582D mutant channels displayed significantly slower inactivation compared with WT. The two mutant channels exerted different gain-of-function effects on calcium currents. CONCLUSION: In patients with LQTS, the frequency of CACNA1C mutations was higher than reported. Even without typical phenotypes of Timothy syndrome, CACNA1C mutations may cause QT prolongation and/or fatal arrhythmia attacks.

A novel HCN4 mutation, G1097W, is associated with atrioventricular block.

BACKGROUND: Loss-of-function mutations in the HCN4 gene have been shown to be associated with sinus dysfunction, but there are no reports on HCN4-mediated atrioventricular (AV) block. A novel missense HCN4 mutation G1097W was identified in a 69 year-old Japanese male with AV block, and we characterized the functional consequences of If-like channels reconstituted with the heterozygous HCN4 mutation. METHODS AND RESULTS: Wild-type (WT) HCN4 or/and HCN4-G1097W were expressed in a heterologous cell expression system. A functional assay using a whole-cell patch-clamp demonstrated that the mutant If-like currents were activated at more negative voltages compared to WT currents, while they retained the sensitivity to changes in intracellular cyclic adenosine monophosphate (cAMP) levels. Co-expression of G1097W with WT channels showed dominant-negative effects, including a reduction in peak currents and a negative voltage shifting on reconstituted currents. CONCLUSIONS: The HCN4-G1097W mutant channels displayed a loss-of-function type modulation on cardiac If channels and thus could predispose them to AV nodal dysfunction. These data provide a novel insight into the genetic basis for the AV block.

Regulatory mechanisms underlying the modulation of GIRK1/GIRK4 heteromeric channels by P2Y receptors.

The muscarinic K(+) channel (I (K,ACh)) is a heterotetramer composed of GIRK1 (Kir3.1) and GIRK4 (Kir3.4) subunits of a G protein-coupled inwardly rectifying channel, and plays an important role in mediating electrical responses to the vagal stimulation in the heart. I (K,ACh) displays biphasic changes (activation followed by inhibition) through the stimulation of the purinergic P2Y receptors, but the regulatory mechanism involved in these modulation of I (K,ACh) by P2Y receptors remains to be fully elucidated. Various P2Y receptor subtypes and GIRK1/GIRK4 (I (GIRK)) were co-expressed in Chinese hamster ovary cells, and the effect of stimulation of P2Y receptor subtypes on I (GIRK) were examined using the whole-cell patch-clamp method. Extracellular application of 10 µM ATP induced a transient activation of I (GIRK) through the P2Y(1) receptor, which was completely abolished by pretreatment with pertussis toxin. ATP initially caused an additive transient increase in ACh-activated I (GIRK) (via M(2) receptor), which was followed by subsequent inhibition. This inhibition of I (GIRK) by ATP was attenuated by co-expression of regulator of G-protein signaling 2, or phosphatidylinositol-4-phosphate-5-kinase, or intracellular phosphatidylinositol 4,5-bisphosphate loading, but not by the exposure to protein kinase C inhibitors. P2Y(4) stimulation also persistently suppressed the ACh-activated I (GIRK). In addition, I (GIRK) evoked by the stimulation of the P2Y(4) receptor exhibited a transient activation, but that evoked by the stimulation of P2Y(2) or P2Y(12) receptor showed a rather persistent activation. These results reveal (1) that P2Y(1) and P2Y(4) are primarily coupled to the G(q)-phospholipase C-pathway, while being weakly linked to G(i/o), and (2) that P2Y(2) and P2Y(12) involve G(i/o) activation.

KCNE3 T4A as the genetic basis of Brugada-pattern electrocardiogram.

BACKGROUND: Brugada syndrome (BrS) is genetically heterogeneous. In Japanese BrS patients, except for SCN5A and KCNE5, mutations in the responsible genes have not yet been identified, and therefore the genetic heterogeneity remains poorly elucidated. METHODS AND RESULTS: Forty consecutive patients with Brugada-pattern electrocardiogram (ECG) underwent comprehensive genetic analysis of BrS-causing genes including SCN5A, SCN1B, SCN3B, CACNA1C, CACNB2, KCNE3 and KCNE5. Besides identifying 8 SCN5A mutations in the present cohort, a KCNE3 T4A mutation was found in a 55-year-old male patient who had experienced several episodes of syncope. A head-up tilt test during passive tilt provoked both hypotension and bradycardia, followed by syncope. He was therefore diagnosed with neurally mediated syncope (NMS). To characterize the functional consequence of the mutant, electrophysiological experiments using whole-cell patch-clamp methods and computer simulations using human right ventricular wall model were carried out. It was found that KCNE3 T4A increased I(to) recapitulated by heterologously coexpressing Kv4.3+KChIP2b+KCNE3-wild type or KCNE3-T4A in CHO cells. CONCLUSIONS: A KCNE3 T4A mutation was identified in a Japanese patient presenting Brugada-pattern ECG and NMS. Its functional consequence was the gain of function of I(to), which could underlie the pathogenesis of Brugada-pattern ECG. The data provide novel insights into the genetic basis of Japanese BrS.

GHGKHKNK octapeptide (P-5m) inhibits metastasis of HCCLM3 cell lines via regulation of MMP-2 expression in in vitro and in vivo studies.

P-5m, an octapeptide derived from domain 5 of HKa, was initially found to inhibit the invasion and migration of melanoma cells. The high metastatic potential of melanoma cells was prevented by the HGK motif in the P-5m peptide in vitro and in an experimental lung metastasis model, suggesting that P-5m may play an important role in the regulation of tumor metastasis. The aim of this study was to measure the effect of P-5m on tumor metastasis of human hepatocarcinoma cell line (HCCLM3) in vitro and in vivo in a nude mouse model of hepatocellular carcinoma (HCC), and detect the mechanisms involved in P-5m-induced anti-metastasis. By gelatin zymography, matrix metallo-proteinases 2 (MMP-2) activity in HCCLM3 was dramatically diminished by P-5m peptide. In addition, the migration and metastasis of HCCLM3 cells was also inhibited by the peptide in vitro. In an orthotopic model of HCC in nude mice, P-5m treatment effectively reduced the lung metastasis as well as the expression of MMP-2 in the tumor tissues. Overall, these observations indicate an important role for P-5m peptide in HCC invasion and metastasis, at least partially through modulation MMP-2 expression. These data suggests that P-5m may have therapeutic potential in metastatic human hepatocarcinoma.