Developmental analysis reveals mismatches in the expression of K+ channel alpha subunits and voltage-gated K+ channel currents in rat ventricular myocytes

Effect of allisartan on blood pressure and left ventricular hypertrophy through Kv1.5 channels in hypertensive rats

BACKGROUND

The objective of the present work was to study the anti-hypertensive effect of allisartan on blood pressure (BP) and in facilitating left ventricular remodeling through voltage-gated potassium channels (Kv) 1.5 channels.

METHODS

A total of 30 SD rats were randomly divided into sham operation group, hypertension control group, and allisartan treatment group. Hypertension was induced by renal artery stenosis. The animals of treatment group were administered with allisartan once a day at a dose of 30 mg/kg body weight through an oral gavage for 4 weeks. The heart function of animals post 4 weeks of treatment was evaluated by echocardiography, and the degree of ventricular hypertrophy and cardiomyocyte hypertrophy were evaluated by histomorphology. The expression of Kv1.5 is detected by real-time quantitative polymerase chain reaction while Western blotting was used to detect the protein expression.

RESULTS

Four weeks after renal artery stenosis, a significant difference was observed in the whole heart ratio, left heart ratio, and cardiomyocyte area between allisartan treatment group and the hypertension control group (P< .01). A significant decrease in BP of allisartan treatment group compared to hypertension control group (P< .01) was observed. The expression of Kv1.5 mRNA was increased significantly (P< .01) in allisartan treatment group compared to hypertension control group. Western blot analysis also confirmed the increased expression of Kv1.5 channel.

CONCLUSION

The results showed that allisartan lowers BP and improves left ventricular remodeling through increased expression of Kv1.5 mRNA.

Sound Localization in Preweanling Mice Was More Severely Affected by Deleting the Kcna1 Gene Compared to Deleting Kcna2, and a Curious Inverted-U Course of Development That Appeared to Exceed Adult Performance Was Observed in All Groups

The submillisecond acuity for detecting rapid spatial and temporal fluctuations in acoustic stimuli observed in humans and laboratory animals depends in part on select groups of auditory neurons that preserve synchrony from the ears to the binaural nuclei in the brainstem. These fibers have specialized synapses and axons that use a low-threshold voltage-activated outward current, IKL, conducted through Kv1 potassium ion channels. These are in turn coupled with HCN channels that express a mixed cation inward mixed current, IH, to support precise synchronized firing. The behavioral evidence is that their respective Kcna1 or HCN1 genes are absent in adult mice; the results are weak startle reflexes, slow responding to noise offsets, and poor sound localization. The present behavioral experiments were motivated by an in vitro study reporting increased IKL in an auditory nucleus in Kcna2-/- mice lacking the Kv1.2 subunit, suggesting that Kcna2-/- mice might perform better than Kcna2+/+ mice. Because Kcna2-/- mice have only a 17-18-day lifespan, we compared both preweanling Kcna2-/- vs. Kcna2+/+ mice and Kcna1-/- vs. Kcna1+/+ mice at P12-P17/18; then, the remaining mice were tested at P23/P25. Both null mutant strains had a stunted physique, but the Kcna1-/- mice had severe behavioral deficits while those in Kcna2-/- mice were relatively few and minor. The in vitro increase of IKL could have resulted from Kv1.1 subunits substituting for Kv1.2 units and the loss of the inhibitory "managerial" effect of Kv1.2 on Kv1.1. However, any increased neuronal synchronicity that accompanies increased IKL may not have been enough to affect behavior. All mice performed unusually well on the early spatial tests, but then, they fell towards adult levels. This unexpected effect may reflect a shift from summated independent monaural pathways to integrated binaural processing, as has been suggested for similar observations for human infants.

IK1 channel agonist zacopride suppresses ventricular arrhythmias in conscious rats with healing myocardial infarction

AIMS

Arrhythmogenesis of chronic myocardial infarction (MI) is associated with the prolongation of action potential, reduction of inward rectifier potassium (I_K1, Kir) channels and hyper-activity of Calcium/calmodulin-dependent kinase II (CaMKII) in cardiomyocytes. Zacopride, a selective I_K1 agonist, was applied to clarify the cardioprotection of I_K1 agonism via a CaMKII signaling on arrhythmias post-MI.

METHODS

Male SD rats were implanted wireless transmitter in the abdominal cavity and subjected to left main coronary artery ligation or sham operation. The telemetric ECGs were monitored per day throughout 4 weeks. At the endpoint, isoproterenol (1.28 mg/kg, i.v.) was administered for provocation test. The expressions of Kir2.1 (dominant subunit of I_K1 in ventricle) and CaMKII were detected by Western-blotting.

KEY FINDINGS

In the telemetric rats post-MI, zacopride significantly reduced the episodes of atrioventricular conduction block (AVB), premature ventricular contraction (PVC), ventricular tachycardia (VT) and ventricular fibrillation (VF), without significant effect on superventricular premature contraction (SPVC). In provocation test, zacopride suppressed the onset of ventricular arrhythmias in conscious PMI or sham rats. The expression of Kir2.1 was significantly downregulated and p-CaMKII was upregulated post-MI, whereas both were restored by zacopride treatment.

SIGNIFICANCE

I_K1/Kir2.1 might be an attractive target for pharmacological controlling of lethal arrhythmias post MI.

Transmural gradients in ion channel and auxiliary subunit expression

Evolution has acted to shape the action potential in different regions of the heart in order to produce a maximally stable and efficient pump. This has been achieved by creating regional differences in ion channel expression levels within the heart as well as differences between equivalent cardiac tissues in different species. These region- and species-dependent differences in channel expression are established by regulatory evolution, evolution of the regulatory mechanisms that control channel expression levels. Ion channel auxiliary subunits are obvious targets for regulatory evolution, in order to change channel expression levels and/or modify channel function. This review focuses on the transmural gradients of ion channel expression in the heart and the role that regulation of auxiliary subunit expression plays in generating and shaping these gradients.

The electrophysiological development of cardiomyocytes

The generation of human cardiomyocytes (CMs) from human pluripotent stem cells (hPSCs) has become an important resource for modeling human cardiac disease and for drug screening, and also holds significant potential for cardiac regeneration. Many challenges remain to be overcome however, before innovation in this field can translate into a change in the morbidity and mortality associated with heart disease. Of particular importance for the future application of this technology is an improved understanding of the electrophysiologic characteristics of CMs, so that better protocols can be developed and optimized for generating hPSC-CMs. Many different cell culture protocols are currently utilized to generate CMs from hPSCs and all appear to yield relatively “developmentally” immature CMs with highly heterogeneous electrical properties. These hPSC-CMs are characterized by spontaneous beating at highly variable rates with a broad range of depolarization-repolarization patterns, suggestive of mixed populations containing atrial, ventricular and nodal cells. Many recent studies have attempted to introduce approaches to promote maturation and to create cells with specific functional properties. In this review, we summarize the studies in which the electrical properties of CMs derived from stem cells have been examined. In order to place this information in a useful context, we also review the electrical properties of CMs as they transition from the developing embryo to the adult human heart. The signal pathways involved in the regulation of ion channel expression during development are also briefly considered.

Ion Channels in the Heart

Optimal cardiac function depends on proper timing of excitation and contraction in various regions of the heart, as well as on appropriate heart rate. This is accomplished via specialized electrical properties of various components of the system, including the sinoatrial node, atria, atrioventricular node, His-Purkinje system, and ventricles. Here we review the major regionally determined electrical properties of these cardiac regions and present the available data regarding the molecular and ionic bases of regional cardiac function and dysfunction. Understanding these differences is of fundamental importance for the investigation of arrhythmia mechanisms and pharmacotherapy.

Reduced sialylation impacts ventricular repolarization by modulating specific K+ channel isoforms distinctly

Voltage-gated K(+) channels (Kv) are responsible for repolarizing excitable cells and can be heavily glycosylated. Cardiac Kv activity is indispensable where even minimal reductions in function can extend action potential duration, prolong QT intervals, and ultimately contribute to life-threatening arrhythmias. Diseases such as congenital disorders of glycosylation often cause significant cardiac phenotypes that can include arrhythmias. Here we investigated the impact of reduced sialylation on ventricular repolarization through gene deletion of the sialyltransferase ST3Gal4. ST3Gal4-deficient mice (ST3Gal4(-/-)) had prolonged QT intervals with a concomitant increase in ventricular action potential duration. Ventricular apex myocytes isolated from ST3Gal4(-/-) mice demonstrated depolarizing shifts in activation gating of the transient outward (Ito) and delayed rectifier (IKslow) components of K(+) current with no change in maximum current densities. Consistently, similar protein expression levels of the three Kv isoforms responsible for Ito and IKslow were measured for ST3Gal4(-/-) versus controls. However, novel non-enzymatic sialic acid labeling indicated a reduction in sialylation of ST3Gal4(-/-) ventricular Kv4.2 and Kv1.5, which contribute to Ito and IKslow, respectively. Thus, we describe here a novel form of regulating cardiac function through the activities of a specific glycogene product. Namely, reduced ST3Gal4 activity leads to a loss of isoform-specific Kv sialylation and function, thereby limiting Kv activity during the action potential and decreasing repolarization rate, which likely contributes to prolonged ventricular repolarization. These studies elucidate a novel role for individual glycogene products in contributing to a complex network of cardiac regulation under normal and pathologic conditions.

Altered functional differentiation of mesoangioblasts in a genetic myopathy

Mutations underlying genetic cardiomyopathies might affect differentiation commitment of resident progenitor cells. Cardiac mesoangioblasts (cMabs) are multipotent progenitor cells resident in the myocardium. A switch from cardiac to skeletal muscle differentiation has been recently described in cMabs from β-sarcoglycan-null mice (βSG^−/−), a murine model of genetic myopathy with early myocardial involvement. Although complementation with βSG gene was inconsequential, knock-in of miRNA669a (missing in βSG^−/− cMabs) partially rescued the mutation-induced molecular phenotype. Here, we undertook a detailed evaluation of functional differentiation of βSG^−/− cMabs and tested the effects of miRNA669a-induced rescue in vitro. To this end, cMabs were compared with neonatal cardiomyocytes (CMs) and skeletal muscle C2C12 cells, representative of cardiac and skeletal muscle respectively. Consistent with previous data on molecular patterns, electrophysiological and Ca²⁺-handling properties of βSG^−/− cMabs were closer to C2C12 cells than to CM ones. Nevertheless, subtler aspects, including action potential contour, Ca²⁺-spark properties and RyR isoform expression, distinguished βSG^−/− cMabs from C2C12 cells. Contrary to previous reports, wild-type cMabs failed to show functional differentiation towards either cell type. Knock-in of miRNA669a in βSG^−/− cMabs rescued the wild-type functional phenotype, i.e. it completely prevented development of skeletal muscle functional responses. We conclude that miRNA669a expression, ablated by βSG deletion, may prevent functional differentiation of cMabs towards the skeletal muscle phenotype.

Developmental changes in the expression of Κv1 potassium channels in rat vestibular ganglion cells

The primary afferent neurons of the vestibular ganglion convey sensory information from hair cells in the semicircular canals and otolith organs to the vestibular nuclei, the adjacent brainstem and the cerebellum. The intrinsic firing properties of vestibular ganglion cells (VGCs) are heterogeneous and have been classified into phasic, intermediate and tonic firing types on the basis of their response to injected depolarizing currents. A previous study from our group showed that the proportion of phasic discharging VGCs decreased during the first postnatal weeks. Moreover, α-dendrotoxin (α-DTX), a Kv1 potassium channels antagonist, turned neuron phasic firing to tonic, thus suggesting that these channels play an important role in the developmental changes of VGCs firing patterns. Here, by using immunohistochemistry, Western blotting and quantitative real-time reverse transcriptase-polymerase chain reaction (RT-PCR), we explored the change in the expression of α-DTX-sensitive K(+) channels, Kv1.1, Kv1.2 and Kv1.6 in rat VGCs during early postnatal periods. We showed that expression of Kv1.6 protein is down-regulated together with expression of Kv1.6 mRNA after postnatal day 7 in rat VGCs whereas expression of Kv1.1 and Kv1.2 proteins did not change during the same developmental period. Our results suggest that down-regulation of the Kv1.6 protein and mRNA may be associated with maturation of excitable properties of primary vestibular neurons.

A new Kv1.2 channelopathy underlying cerebellar ataxia

A forward genetic screen of mice treated with the mutagen ENU identified a mutant mouse with chronic motor incoordination. This mutant, named Pingu (Pgu), carries a missense mutation, an I402T substitution in the S6 segment of the voltage-gated potassium channel Kcna2. The gene Kcna2 encodes the voltage-gated potassium channel α-subunit Kv1.2, which is abundantly expressed in the large axon terminals of basket cells that make powerful axo-somatic synapses onto Purkinje cells. Patch clamp recordings from cerebellar slices revealed an increased frequency and amplitude of spontaneous GABAergic inhibitory postsynaptic currents and reduced action potential firing frequency in Purkinje cells, suggesting that an increase in GABA release from basket cells is involved in the motor incoordination in Pgu mice. In line with immunochemical analyses showing a significant reduction in the expression of Kv1 channels in the basket cell terminals of Pgu mice, expression of homomeric and heteromeric channels containing the Kv1.2(I402T) α-subunit in cultured CHO cells revealed subtle changes in biophysical properties but a dramatic decrease in the amount of functional Kv1 channels. Pharmacological treatment with acetazolamide or transgenic complementation with wild-type Kcna2 cDNA partially rescued the motor incoordination in Pgu mice. These results suggest that independent of known mutations in Kcna1 encoding Kv1.1, Kcna2 mutations may be important molecular correlates underlying human cerebellar ataxic disease.

High-mobility group box 1 (HMGB1) downregulates cardiac transient outward potassium current (Ito) through downregulation of Kv4.2 and Kv4.3 channel transcripts and proteins

Transient outward potassium currents (I(to)) are major early repolarization currents in shaping cardiac action potential (AP). Downregulation of I(to) contributes to AP configuration alteration in myocardial infarction (MI) and numerous other heart diseases. High-mobility group box 1 (HMGB1), a proinflammatory cytokine, has been reported to increase dramatically in the serum of patients with MI, participating in ischemia-reperfusion injury and recovery of post-infarction failing heart. This study investigated the possible role of HMGB1 in regulating cardiac I(to) and electrical stability. HMGB1 treatment for 24h significantly inhibited the current densities of heterologously expressed Kv4.3 and Kv4.2 in COS-7 cells and native I(to) in neonatal rat ventricular myocytes (NRVMs) in a dose-dependent manner. HMGB1 decreased the mRNA and protein levels of the I(to) alpha subunits Kv4.2 and Kv4.3 channels, but not the beta subunit KChIP2 and KCNE2 in NRVMs. The receptor binding domain (150-186 amino acid residues) responsible for receptor of advanced glycation end product (RAGE) binding similarly inhibited I(to)(,) while treatment with soluble RAGE that blocks binding of ligands to cell-surface RAGE partially restored I(to) current density and Kv4 protein expressions. Box A which possesses no proinflammatory activity of HMGB1 still remained part of the I(to) suppression effect. In addition to downregulating I(to), HMGB1 modestly inhibited L-type Ca(2+) current, but not I(K1). The AP duration (APD) was slightly prolonged by HMGB1 treatment. These results collectively establish HMGB1 as a novel pathological factor downregulating I(to) partially through HMGB1-RAGE interaction, providing new insights into the potential molecular mechanisms underlying the electrical remodeling in MI.

Inhibitory effects of isoliquiritigenin and licorice extract on voltage-dependent K(+) currents in H9c2 cells

The effect of isoliquiritigenin (ISL), a component of licorice, on the voltage-dependent, ultra-rapidly activating delayed-rectifier K(+) current (IKur) was examined in H9c2 cells, a cell-line derived from rat cardiac myoblasts. IKur was recorded using the whole-cell patch clamp method with a pipette solution containing 140 mM K(+). Depolarizing voltage pulses of 200-ms duration were given with 10-mV steps every 10 s from -40 mV holding potential. ISL inhibited IKur in a concentration-dependent manner. The median inhibitory concentration (IC(50)) of ISL was approximately 0.11 microM and the Hill coefficient was 0.71. Using CHO cells expressing Kv1.5 IKur channels, ISL also inhibited Kv1.5 IKur, but less potently than the IKur current in H9c2 cells. Furthermore, in H9c2 cells, the licorice extract itself inhibited IKur in a manner similar to ISL. We conclude that ISL, one component of licorice, is a potent inhibitor of K(+) channels, which specifically in H9c2 cells could be Kv2.1, and that this inhibition may be involved in various pharmacological effects of licorice.

Voltage gated K+ channel expression in arteries of Wistar-Kyoto and spontaneously hypertensive rats

BACKGROUND

We have previously demonstrated differences in the gene expression of voltage-gated K v1.X channel alpha-subunits in arteries from Wistar-Kyoto rats (WKYs) and spontaneously hypertensive rats (SHRs). The purpose of this study was to test the hypothesis that these differences are also present at the protein level.

METHODS

Proteins were isolated from the aorta, mesenteric (MAs) and tail arteries (TAs) of 12- to 15-week-old male WKY and SHR, and analyzed by immunoblotting. K(v) currents were recorded from MA myocytes by patch clamp methods.

RESULTS

Expression of Kv1.2, Kv1.5, and Kv2.1 was higher in MAs but was not different in aortas of SHRs as compared to WKYs. In the TA, expression of Kv1.2 and Kv1.5 was higher while that of Kv2.1 was lower in SHR compared to WKY. In the MA, the larger expression of an 80 kDa species of Kv1.2 in SHRs was associated with a lower expression of a 60 kDa species. Kv2.1 gene expression was larger in MAs from SHRs but not different in TAs. K(v) currents associated with Kv1.X and Kv2.1 channels were both larger in MA myocytes from SHRs but less than expected based upon differences in K(v) alpha-subunit protein expression.

CONCLUSIONS

For the MA, K(v) protein expression and current components between WKYs and SHRs were qualitatively consistent, but differences in gene and protein expression were not closely correlated. The higher expression of K(v) subunits in small mesenteric arteries (SMAs) of SHR would tend to maintain normal myogenic activity and vasoconstrictor reserve, and could be viewed as a form of homeostatic remodeling.

Neonatal rat cardiac fibroblasts express three types of voltage-gated K+ channels: regulation of a transient outward current by protein kinase C

Cardiac fibroblasts regulate myocardial development via mechanical, chemical, and electrical interactions with associated cardiomyocytes. The goal of this study was to identify and characterize voltage-gated K(+) (Kv) channels in neonatal rat ventricular fibroblasts. With the use of the whole cell arrangement of the patch-clamp technique, three types of voltage-gated, outward K(+) currents were measured in the cultured fibroblasts. The majority of cells expressed a transient outward K(+) current (I(to)) that activated at potentials positive to -40 mV and partially inactivated during depolarizing voltage steps. I(to) was inhibited by the antiarrhythmic agent flecainide (100 microM) and BaCl(2) (1 mM) but was unaffected by 4-aminopyridine (4-AP; 0.5 and 1 mM). A smaller number of cells expressed one of two types of kinetically distinct, delayed-rectifier K(+) currents [I(K) fast (I(Kf)) and I(K) slow (I(Ks))] that were strongly blocked by 4-AP. Application of phorbol 12-myristate 13-acetate, to stimulate protein kinase C (PKC), inhibited I(to) but had no effect on I(Kf) and I(Ks). Immunoblot analysis revealed the presence of Kv1.4, Kv1.2, Kv1.5, and Kv2.1 alpha-subunits but not Kv4.2 or Kv1.6 alpha-subunits in the fibroblasts. Finally, pretreatment of the cells with 4-AP inhibited angiotensin II-induced intracellular Ca(2+) mobilization. Thus neonatal cardiac fibroblasts express at least three different Kv channels that may contribute to electrical/chemical signaling in these cells.

Reduced delayed rectifier K+ current, altered electrophysiology, and increased ventricular vulnerability in MLP-deficient mice

BACKGROUND

Mice with a knockout (KO) of muscle LIM protein (MLP) exhibit many morphologic and clinical features of human cardiomyopathy. In humans, MLP-expression is downregulated both in ischemic and dilative cardiomyopathy. In this study, we investigated the effects of MLP on the electrophysiologic phenotype in vivo and on outward potassium currents.

METHODS AND RESULTS

MLP-deficient (MLPKO) and wild-type (MLPWT) mice were subjected to long-term electrocardiogram (ECG) recording and in vivo electrophysiologic study. The whole-cell, patch-clamp technique was applied to measure voltage dependent outward K+ currents in isolated cardiomyocytes. Long-term ECG revealed a significant prolongation of RR mean (108 +/- 9 versus 99 +/- 5 ms), P (16 +/- 3 versus 14 +/- 1 ms), QRS (17 +/- 3 versus 13 +/- 1 ms), QT (68 +/- 8 versus 46 +/- 7 ms), QTc (66 +/- 6 versus 46 +/- 7 ms), JT (51 +/- 7 versus 34 +/- 7 ms), and JTc (49 +/- 5 versus 33 +/- 7 ms) in MLPKO versus MLPWT mice (P < .05). During EP study, QT (80 +/- 8 versus 58 +/- 7 ms), QTc (61 +/- 6 versus 45 +/- 5 ms), JT (62 +/- 9 versus 43 +/- 6 ms), and JTc (47 +/- 5 versus 34 +/- 5 ms) were also significantly prolonged in MLPKO mice (P < .05). Nonsustained VT was inducible in 9/16 MLPKO versus 2/15 MLPWT mice (P < .05). Analysis of outward K+ currents in revealed a significantly reduced density of the slowly inactivating outward K+ current IK, slow in MLPKO mice (11 +/- 5 pA/pF versus 18 +/- 7 pA/pF; P < .05).

CONCLUSION

Mice with KO of MLP exhibit significant prolongation of atrial and ventricular conduction and an increased ventricular vulnerability. A reduction in repolarizing outward K+ currents may be responsible for these alterations.

Alternative polyadenylation signals in the 3' non-coding region of a voltage-gated potassium channel gene are major determinants of mRNA isoform expression

We investigated the role of the 3' non-coding region of a mouse voltage-gated potassium channel mRNA (mKv1.4 mRNA) in post-transcriptional regulation of gene expression. In contrast to an earlier report from studies carried out in Xenopus oocytes, we found that 3' non-coding region sequences of mKv1.4 mRNAs did not significantly affect expression of a heterologous reporter RNA in vitro or in mammalian cells/cell lines. Instead, our data revealed a possible role for alternative polyadenylation mediated by distinct determinants approximately 0.2 kb and approximately 1.2 kb downstream of the Kv1.4 coding region. The use of the downstream polyadenylation signal correlated with the synthesis of a larger Kv1.4 mRNA isoform that was more abundantly expressed than the smaller mRNA species, whose expression was regulated by the upstream polyadenylation signal. Our results suggest that the relative strengths of the polyadenylation signals are major determinants of overall Kv1.4 mRNA abundance in cells.

Postnatal development has a marked effect on ventricular repolarization in mice

To better understand the mechanisms that underlie cardiac repolarization abnormalities in the immature heart, this study characterized and compared K(+) currents in mouse ventricular myocytes from day 1, day 7, day 20, and adult CD1 mice to determine the effects of postnatal development on ventricular repolarization. Current- and patch-clamp techniques were used to examine action potentials and the K(+) currents underlying repolarization in isolated myocytes. RT-PCR was used to quantify mRNA expression for the K(+) channels of interest. This study found that action potential duration (APD) decreased as age increased, with the shortest APDs observed in adult myocytes. This study also showed that K(+) currents and the mRNA relative abundance for the various K(+) channels were significantly greater in adult myocytes compared with day 1 myocytes. Examination of the individual components of total K(+) current revealed that the inward rectifier K(+) current (I(K1)) developed by day 7, both the Ca(2+)-independent transient outward current (I(to)) and the steady-state outward K(+) current (I(ss)) developed by day 20, and the ultrarapid delayed rectifier K(+) current (I(Kur)) did not fully develop until the mouse reached maturity. Interestingly, the increase in I(Kur) was not associated with a decrease in APD. Comparison of atrial and ventricular K(+) currents showed that I(to) and I(Kur) density were significantly greater in day 7, day 20, and adult myocytes compared with age-matched atrial cells. Overall, it appears that, in mouse ventricle, developmental changes in APD are likely attributable to increases in I(to), I(ss), and I(K1), whereas the role of I(Kur) during postnatal development appears to be less critical to APD.

Voltage-gated and two-pore-domain potassium channels in murine spiral ganglion neurons

The systematically varied firing features of spiral ganglion neurons provide an excellent model system for the exploration of how graded ion channel distributions can be used to organize neuronal firing across a population of neurons. Elucidating the underlying mechanisms that determine neuronal response properties requires a complete understanding of the combination of ion channels, auxiliary proteins, modulators, and second messengers that form this highly organized system in the auditory periphery. Toward this goal, we built upon previous studies of voltage-gated K+-selective ion channels (Kv), and expanded our analysis to K+-selective leak channels (KCNK), which can play a major role in setting the basic firing characteristics of spiral ganglion neurons. To begin a more comprehensive analysis of Kv and KCNK channels, a screening approach was employed. RT-PCR was utilized to examine gene expression, the major results of which were confirmed with immunocytochemistry. Initial studies validated this approach by accurately detecting voltage-dependent K+ channels that were documented previously in the spiral ganglion. Furthermore, an additional channel type within the Kv3 family, Kv3.3, was identified and further characterized. The major focus of the study, however, was to systematically examine gene expression levels of the KCNK family of K+-selective leak channels. These channel types determine the resting membrane potential which has a major impact on setting the level of neuronal excitation. TWIK-1, TASK-3, TASK-1, and TREK-1 were expressed in the spiral ganglion; TWIK-1 was specifically localized with immunocytochemistry to the neuronal somata and initial processes of spiral ganglion neurons in vitro.

Mitogen-activated protein kinases control cardiac KChIP2 gene expression

Hypertrophied myocardium is associated with reductions in the transient outward K(+) current (Ito) and expression of pore-forming Kv4.2/4.3 and auxiliary KChIP2 subunits. Here we show that KChIP2 mRNA and protein levels are dramatically decreased to 10% to 30% of control levels in the left ventricle of aorta-constricted rats in vivo and phenylephrine (PE)-treated myocytes in vitro. PE also markedly decreases Ito density. Inhibition of protein kinase Cs (PKCs) does not affect the PE-induced reduction in KChIP2 mRNA level, whereas activation of PKC with phorbol ester (phorbol myristate [PMA]) causes a marked reduction in KChIP2 mRNA level. Pharmacological inhibition of MEKs or overexpression of a dominant-negative MEK1 increases the basal KChIP2 mRNA expression and blocks the PMA-induced decrease in auxiliary subunit mRNA level. In addition, a constitutively active MEK1 decreases the basal KChIP2 mRNA level, and PMA causes no further reduction in auxiliary subunit mRNA level in active MEK1-expressing cells. Furthermore, pharmacological inhibition of JNKs or overexpression of a dominant-negative JNK1 prevents the PE-induced, but not PMA-induced, reduction in KChIP2 mRNA expression. These results suggest that downregulation of KChIP2 expression significantly contributes to the hypertrophy-associated reduction in Ito density. They also indicate that the expression of KChIP2 mRNA is controlled by the 2 branches of mitogen-activated protein kinase pathways: JNKs play a predominant role in mediating the PE-induced reduction, whereas the MEK-ERK pathway influences the basal expression and mediates the PKC-mediated downregulation.

The Mechanism of Atrial Antiarrhythmic Action of RSD1235

INTRODUCTION

RSD1235 is a novel drug recently shown to convert AF rapidly and safely in patients.(1) Its mechanism of action has been investigated in a rat model of ischemic arrhythmia, along with changes in action potential (AP) morphology in isolated rat ventricular myocytes and effects on cloned channels.

METHODS AND RESULTS

Ischemic arrhythmias were inhibited with an ED50 of 1.5 micromol/kg/min, and repolarization times increased with non-significant effects on PR and QRS durations. AP prolongation was observed in rat myocytes at low doses, with plateau elevation and a reduction in the AP overshoot at higher doses. RSD1235 showed selectivity for voltage-gated K+ channels with IC50 values of 13 microM on hKv1.5 (1 Hz) versus 38 and 30 microM on Kv4.2 and Kv4.3, respectively, and 21 microM on hERG channels. RSD1235 did not block IK1 (IC50 > 1 mM) nor ICa,L (IC50= 220 microM) at 1 Hz in guinea pig ventricular myocytes (n = 4-5). The drug displayed mild (IC50= 43 microM at 1 Hz) open-channel blockade of Nav1.5 with rapid recovery kinetics after rate reduction (10-->1 Hz, 75% recovery with tau= 320 msec). Nav1.5 blocking potency increased with stimulus frequency from an IC50= 40 microM at 0.25 Hz, to an IC50= 9 microM at 20 Hz, and with depolarization increasing from 107 microM at -120 mV to 31 microM at -60 mV (1 Hz).

CONCLUSIONS

These data suggest that RSD1235's clinical selectivity and AF conversion efficacy result from block of potassium channels combined with frequency- and voltage-dependent block of INa.

Molecular physiology of cardiac repolarization

The heart is a rhythmic electromechanical pump, the functioning of which depends on action potential generation and propagation, followed by relaxation and a period of refractoriness until the next impulse is generated. Myocardial action potentials reflect the sequential activation and inactivation of inward (Na(+) and Ca(2+)) and outward (K(+)) current carrying ion channels. In different regions of the heart, action potential waveforms are distinct, owing to differences in Na(+), Ca(2+), and K(+) channel expression, and these differences contribute to the normal, unidirectional propagation of activity and to the generation of normal cardiac rhythms. Changes in channel functioning, resulting from inherited or acquired disease, affect action potential repolarization and can lead to the generation of life-threatening arrhythmias. There is, therefore, considerable interest in understanding the mechanisms that control cardiac repolarization and rhythm generation. Electrophysiological studies have detailed the properties of the Na(+), Ca(2+), and K(+) currents that generate cardiac action potentials, and molecular cloning has revealed a large number of pore forming (alpha) and accessory (beta, delta, and gamma) subunits thought to contribute to the formation of these channels. Considerable progress has been made in defining the functional roles of the various channels and in identifying the alpha-subunits encoding these channels. Much less is known, however, about the functioning of channel accessory subunits and/or posttranslational processing of the channel proteins. It has also become clear that cardiac ion channels function as components of macromolecular complexes, comprising the alpha-subunits, one or more accessory subunit, and a variety of other regulatory proteins. In addition, these macromolecular channel protein complexes appear to interact with the actin cytoskeleton and/or the extracellular matrix, suggesting important functional links between channel complexes, as well as between cardiac structure and electrical functioning. Important areas of future research will be the identification of (all of) the molecular components of functional cardiac ion channels and delineation of the molecular mechanisms involved in regulating the expression and the functioning of these channels in the normal and the diseased myocardium.

O2 sensing in the human ductus arteriosus: redox-sensitive K+ channels are regulated by mitochondria-derived hydrogen peroxide

The ductus arteriosus (DA) is a fetal artery that allows blood ejected from the right ventricle to bypass the pulmonary circulation in utero. At birth, functional closure of the DA is initiated by an O2-induced, vasoconstrictor mechanism which, though modulated by endothelial-derived endothelin and prostaglandins, is intrinsic to the smooth muscle cell (DASMC) [Michelakis et al., Circ. Res. 91 (2002); pp. 478-486]. As pO2 increases, a mitochondrial O2-sensor (electron transport chain complexes I or III) is activated, which generates a diffusible redox mediator (H2O2). H2O2 inhibits voltage-gated K+ channels (Kv) in DASMC. The resulting membrane depolarization activates L-type Ca2+ channels, thereby promoting vasoconstriction. Conversely, inhibiting mitochondrial ETC complexes I or III mimics hypoxia, depolarizing mitochondria, and decreasing H2O2 levels. The resulting increase in K+ current hyperpolarizes the DASMC and relaxes the DA. We have developed two models for study of the DA's O2-sensor pathway, both characterized by decreased O2-constriction and Kv expression: (i) preterm rabbit DA, (ii) ionically-remodeled, human term DA. The O2-sensitive channels Kv1.5 and Kv2.1 are important to DA O2-constriction and overexpression of either channel enhances DA constriction in these models. Understanding this O2-sensing pathway offers therapeutic targets to modulate the tone and patency of human DA in vivo, thereby addressing a common form of congenital heart disease in preterm infants.

Contribution of KChIP2 to the developmental increase in transient outward current of rat cardiomyocytes

The Ca(2+)-independent, voltage-gated transient outward current (I(to)) displays a marked increase during development of cardiomyocytes. However, the molecular mechanism remained unclear. In rat adult ventricular myocytes, I(to) can be divided into a fast (I(to,f)) and a slow (I(to,s)) component by recovery process from inactivation. Voltage-gated K(+) channel-interacting proteins 2 (KChIP2) has recently been shown to modify membrane expressions and current densities of I(to,f). Here we examined the developmental change of I(to) and the putative molecular correlates of I(to,f) (Kv4.2 and Kv4.3) and KChIP2 in rat ventricular myocytes. Even in rat embryonic day 12 (E12) myocytes, we detected I(to). However, I(to) in E12 was solely composed of I(to,s). In postnatal day 10 (P10), we recorded much increased I(to) composed of two components (I(to,f) and I(to,s)), and I(to,f) was dominant. Thus, the developmental increase of I(to) from E12 to P10 can be explained by the dramatic appearance of I(to,f). Real-time RT-PCR revealed that Kv4.2 and Kv4.3 mRNA levels were slightly changed. By contrast, KChIP2 mRNA level increased from E12 to P10 by 731-fold. Therefore, the huge increase of KChIP2 expression was likely to be the cause of the great increase of I(to,f). In order to confirm that KChIP2 is crucial to induce I(to,f), we used adenoviral gene transfer technique. When KChIP2 was over-expressed in E12 myocytes, a great amplitude of I(to,f) appeared. Immunocytochemical experiments also demonstrated that KChIP2 enhanced the trafficking of Kv4.2 channels to cell surface. These results indicate that KChIP2 plays an important role in the generation of functional I(to,f) channels during development.

Quantitative relationship between Kv4.2 mRNA and A-type K+ current in rat striatal cholinergic interneurons during development

Channel density is a fundamental factor in determining neuronal firing and is primarily regulated during development through transcriptional and translational regulation. In adult rats, striatal cholinergic interneurons have a prominent A-type current and co-express Kv4.1 and Kv4.2 mRNAs. There is evidence that Kv4.2 plays a primary role in producing the current in adult neurons. The contribution of Kv4.2 and Kv4.1 to the A-type current in cholinergic interneurons during development, however, is not known. Here, using patch-clamp recording and semi-quantitative single-cell reverse transcription-polymerase chain reaction (RT-PCR) techniques, we have examined the postnatal development of A-type current and the expression of Kv4.2 and Kv4.1 in rat striatal cholinergic interneurons. A-type current was detectable at birth, and its amplitude was up-regulated with age, reaching a plateau at about 3 wk after birth. At all ages, the current inactivated with two time constants: one ranging from 15 to 27 ms and the other ranging from 99 to 142 ms. Kv4.2 mRNA was detectable at birth, and the expression level increased exponentially with age, reaching a plateau by 3 wk postnatal. In contrast, Kv4.1 mRNA was not detectable during the first week after birth, and the expression level did not show a clear tendency with age. Taken together, our results suggest that Kv4.2 plays an essential role in producing the A-type current in striatal cholinergic interneurons during the entire course of postnatal development.

Developmental expression of NCS-1 (frequenin), a regulator of Kv4 K+ channels, in mouse heart

The channel proteins responsible for the cardiac transient outward K+ current (Ito) of human and rodent heart are composed, in part, of pore-forming Kv4.3 or Kv4.2 principal subunits. Recent reports implicate K+ channel interacting proteins (members of the neuronal Ca2+-binding protein family) as subunits of the Ito channel complex. We reported that another Ca2+-binding protein, frequenin [or neuronal calcium center protein-1 (NCS-1)], also functions as a Kv4 auxiliary subunit in the brain. By examining cardiac expression of NCS-1, the aim of this study was to examine the potential physiologic relevance of this protein as an additional regulator of cardiac Ito. Immunoblot analysis demonstrates NCS-1 protein to be expressed in adult mouse ventricle at levels comparable to that found in some brain regions. Cardiac NCS-1 protein expression levels are much higher in fetal and neonatal mouse hearts when compared with the adult. Immunocytochemical analysis of isolated neonatal mouse ventricular myocytes demonstrates co-localization of NCS-1 and Kv4.2 proteins at the sarcolemma. Given its high levels of expression in the heart, NCS-1 should be considered an important potential Kv4 regulatory subunit, particularly in the immature heart.

KV2.1 K+ channels underlie major voltage-gated K+ outward current in H9c2 myoblasts

The H9c2 clonal cell line derived from embryonic rat ventricle is an in vitro model system for cardiac and skeletal myocytes. We used the whole-cell patch clamp technique to characterize the electrophysiological and pharmacological properties of an outward K+ current (IK(V)) and determined its molecular correlate in H9c2 myoblasts. IK(V) was activated by threshold depolarization to -30 mV, and its current amplitude and rate of activation increased with further depolarizations. IK(V) inactivated slowly with a time constant of 1-2 s, and the V(0.5) for steady-state inactivation was -37.9 +/- 4.6 mV (n = 10). Tetraethylammonium and quinidine suppressed IK(V) with IC(50)'s of 3.7 mM and 11.6 microM, respectively. Using RT-PCR analysis we found that the K(V )2.1 gene is the most abundantly expressed among genes for K(V)1.2, 1.4, 1.5, 2.1, 4.2, and 4.3, and by Western blotting we confirmed the synthesis of the K(V)2.1 alpha-subunit protein. We conclude that IK(V), the predominant voltage-gated outward current in H9c2 myoblasts, flows through the channels comprised of the K(V)2.1-subunit gene product.

Modulation of Kv4-encoded K(+) currents in the mammalian myocardium by neuronal calcium sensor-1

Voltage-gated K(+) channels are multimeric proteins, consisting of four pore-forming alpha-subunits alone or in association with accessory subunits. Recently, for example, it was shown that the accessory Kv channel interacting proteins form complexes with Kv4 alpha-subunits and modulate Kv4 channel activity. The experiments reported here demonstrate that the neuronal calcium sensor protein-1 (NCS-1), another member of the recoverin-neuronal calcium sensor superfamily, is expressed in adult mouse ventricles and that NCS-1 co-immunoprecipitates with Kv4.3 from (adult mouse) ventricular extracts. In addition, co-expression studies in HEK-293 cells reveal that NCS-1 increases membrane expression of Kv4 alpha-subunits and functional Kv4-encoded K(+) current densities. Co-expression of NCS-1 also decreases the rate of inactivation of Kv4 alpha-subunit-encoded K(+) currents. In contrast to the pronounced effects of Kv channel interacting proteins on Kv4 channel gating, however, NCS-1 co-expression does not measurably affect the voltage dependence of steady-state inactivation or the rate of recovery from inactivation of Kv4-encoded K(+) currents. Taken together, these results suggest that NCS-1 is an accessory subunit of Kv4-encoded I(to,f) channels that functions to regulate I(to,f) density in the mammalian myocardium.

Elimination of fast inactivation in Kv4 A-type potassium channels by an auxiliary subunit domain

The Kv4 A-type potassium currents contribute to controlling the frequency of slow repetitive firing and back-propagation of action potentials in neurons and shape the action potential in heart. Kv4 currents exhibit rapid activation and inactivation and are specifically modulated by K-channel interacting proteins (KChIPs). Here we report the discovery and functional characterization of a modular K-channel inactivation suppressor (KIS) domain located in the first 34 aa of an additional KChIP (KChIP4a). Coexpression of KChIP4a with Kv4 alpha-subunits abolishes fast inactivation of the Kv4 currents in various cell types, including cerebellar granule neurons. Kinetic analysis shows that the KIS domain delays Kv4.3 opening, but once the channel is open, it disrupts rapid inactivation and slows Kv4.3 closing. Accordingly, KChIP4a increases the open probability of single Kv4.3 channels. The net effects of KChIP4a and KChIP1-3 on Kv4 gating are quite different. When both KChIP4a and KChIP1 are present, the Kv4.3 current shows mixed inactivation profiles dependent on KChIP4a/KChIP1 ratios. The KIS domain effectively converts the A-type Kv4 current to a slowly inactivating delayed rectifier-type potassium current. This conversion is opposite to that mediated by the Kv1-specific "ball" domain of the Kv beta 1 subunit. Together, these results demonstrate that specific auxiliary subunits with distinct functions actively modulate gating of potassium channels that govern membrane excitability.

Effect of tetramethyl pyrazine on L-type calcium channel in rat ventricular myocytes

To elucidate possible ionic mechanisms of antimyocardial ischemia and antiarrythmia of tetramethyl pyrazine (TP), we studied L-type Ca2+ currents (I(Ca.L)) in adult rat ventricular myocytes using the whole-cell patch-clamp technique. The results showed: (i) under physiological conditions, 0.25 mmol/L TP decreased amplitude of I(Ca.L) to 60.6% and this inhibition was increased with increasing concentration of TP. ID50 was 0.20 mmol/L. (ii) The Ca2+-antagonistic effect of TP was voltage-dependent. A marked negative shift of the steady-state inactivation curve was observed with long (10 s) conditioning prepulses, but not with short (350 ms) ones. (iii) The time course of inhibition during TP treatment was increased with an increase in drug concentration, and recovery from TP-induced inactivation of I(Ca.L) was slower than in control cases. (iv) Tonic block and use-dependent block with TP treatment, which was induced by increasing the frequency of stimulation, occurred. We suggest that TP inhibits the I(Ca.L) mainly by binding to inactivated Ca2+ channels. The high affinity of TP for the inactivated state of I(Ca.L) may play an important role in developing therapies for pathological conditions.

Electrical and structural remodeling of the failing ventricle

Heart failure (HF) is a complex disease that presents a major public health challenge to Western society. The prevalence of HF increases with age in the elderly population, and the societal disease burden will increase with prolongation of life expectancy. HF is initially characterized by an adaptive increase of neurohumoral activation to compensate for reduction of cardiac output. This leads to a combination of neurohumoral activation and mechanical stress in the failing heart that trigger a cascade of maladaptive electrical and structural events that impair both the systolic and diastolic function of the heart.

Functional expression of a GFP-tagged Kv1.5 alpha-subunit in mouse ventricle

The experiments here were undertaken to determine the feasibility of increasing the cell surface expression of voltage-gated ion channels in cardiac cells in vivo and to explore the functional consequences of ectopic channel expression. Transgenic mice expressing a green fluorescent protein (GFP)-tagged, voltage-gated K+ (Kv) channel alpha-subunit, Kv1.5-GFP, driven by the cardiac-specific alpha-MHC promoter, were generated. In recent studies, Kv1.5 has been shown to encode the micromolar 4-aminopyridine (4-AP)-sensitive delayed rectifier K+ current (I(K,slow)) in mouse myocardium. Unexpectedly, Kv1.5-GFP expression is heterogeneous in the ventricles of these animals. Although no electrocardiographic abnormalities were evident, expression of Kv1.5-GFP results in marked decreases in action potential durations in GFP-positive ventricular myocytes. In voltage-clamp recordings from GFP-positive ventricular myocytes, peak outward K+ currents are significantly higher, and their waveforms are distinct from those recorded from wild-type cells. Pharmacological experiments revealed a selective increase in a micromolar 4-AP-sensitive current, similar to the 4-AP-sensitive component of I(K,slow) in wild-type cells. The inactivation rate of the "overexpressed" current, however, is significantly slower than the Kv1.5-encoded component of I(K,slow) in wild-type cells, suggesting differences in association with accessory subunits and/or posttranslational processing.

Altered K(+) channel gene expression in diabetic rat ventricle: isoform switching between Kv4.2 and Kv1.4

Expression of voltage-gated K(+) channels encoding the K(+) independent transient outward current in the streptozocin-induced diabetic (DM) rat ventricle was studied to determine the basis for slowed cardiac repolarization in diabetes mellitus. Although hypertrophy was not detected in diabetic rats at 12 wk after streptozocin treatment, ventricular Kv4.2 mRNA levels decreased 41% relative to nondiabetic controls. Kv1.4 mRNA levels increased 179% relative to controls, whereas Kv4.3 mRNA levels were unaffected. Immunohistochemistry and Western blot analysis of the diabetic heart showed that the density of the Kv4.2 protein decreased, whereas Kv1.4 protein increased. Thus isoform switching from Kv4.2 to Kv1.4 is most likely the mechanism underlying the slower kinetics of transient outward K(+) current observed in the diabetic ventricle. Brain Kv1.4, Kv4.2, or Kv4.3 mRNA levels were unaffected by diabetes. Myosin heavy chain (MHC) gene expression was altered with a 32% decrease in alpha-MHC mRNA and a 259% increase in beta-MHC mRNA levels in diabetic ventricle. Low-dose insulin-like growth factor-II (IGF-II) treatment during the last 6 of the 12 wk of diabetes (DM + IGF) protected against these changes in MHC mRNAs despite continued hyperglycemia and body weight loss. IGF-II treatment did not change K(+) channel mRNA levels in DM or control rat ventricles. Thus IGF treatment may prevent some, but not all, biochemical abnormalities in the diabetic heart.

Kinetic modulation of Kv4-mediated A-current by arachidonic acid is dependent on potassium channel interacting proteins

The Kv4 subfamily of voltage-gated potassium channels is responsible for the transient A-type potassium current that operates at subthreshold membrane potentials to control membrane excitability. Arachidonic acid was shown recently to modulate both the peak amplitude and kinetics of the hippocampal A-current. However, in Xenopus oocytes, arachidonic acid only inhibited the peak amplitude of Kv4 current without modifying its kinetics. These results suggest the existence of Kv4 auxiliary subunit(s) in native cells. We report here a K-channel interacting protein (KChIP)-dependent kinetic modulation of Kv4.2 current in Chinese hamster ovary cells and Kv4.2 and Kv4.3 currents in Xenopus oocytes by arachidonic acid at physiological concentrations. This concentration-dependent effect of arachidonic acid resembled that observed in cerebellar granule neurons and was fully reversible. Other fatty acids, including a nonhydrolyzable inhibitor of both lipooxygenase and cyclooxygenase, 5,8,11,14-eicosatetraynoic acid (ETYA), also mimicked arachidonic acid in modulating Kv4.3 and Kv4.3/KChIP1 currents. Compared with another transient potassium current formed by Kv1.1/Kvbeta1, Kv4.3/KChIP1 current was much more sensitive to arachidonic acid. Association between KChIP1 and Kv4.2 or Kv4.3 was not altered in the presence of 10 microm ETYA as measured by immunoprecipitation and association-dependent growth in yeast. Our data suggest that the KChIP proteins represent a molecular entity for the observed difference between arachidonic acid effects on A-current kinetics in heterologous cells and in native cells and are consistent with the notion that KChIP proteins modulate the subthreshold A-current in neurons.

KChAP/Kvbeta1.2 interactions and their effects on cardiac Kv channel expression

KChAP and voltage-dependent K+ (Kv) beta-subunits are two different types of cytoplasmic proteins that interact with Kv channels. KChAP acts as a chaperone for Kv2.1 and Kv4.3 channels. It also binds to Kv1.x channels but, with the exception of Kv1.3, does not increase Kv1.x currents. Kvbeta-subunits are assembled with Kv1.x channels; they exhibit "chaperone-like" behavior and change gating properties. In addition, KChAP and Kvbeta-subunits interact with each other. Here we examine the consequences of this interaction on Kv currents in Xenopus oocytes injected with different combinations of cRNAs, including Kvbeta1.2, KChAP, and either Kv1.4, Kv1.5, Kv2.1, or Kv4.3. We found that KChAP attenuated the depression of Kv1.5 currents produced by Kvbeta1.2, and Kvbeta1.2 eliminated the increase of Kv2.1 and Kv4.3 currents produced by KChAP. Both KChAP and Kvbeta1.2 are expressed in cardiomyocytes, where Kv1.5 and Kv2.1 produce sustained outward currents and Kv4.3 and Kv1.4 generate transient outward currents. Because they interact, either KChAP or Kvbeta1.2 may alter both sustained and transient cardiac Kv currents. The interaction of these two different classes of modulatory proteins may constitute a novel mechanism for regulating cardiac K+ currents.

Kinetic modulation of Kv4-mediated A-current by arachidonic acid is dependent on potassium channel interacting proteins

The Kv4 subfamily of voltage-gated potassium channels is responsible for the transient A-type potassium current that operates at subthreshold membrane potentials to control membrane excitability. Arachidonic acid was shown recently to modulate both the peak amplitude and kinetics of the hippocampal A-current. However, in Xenopus oocytes, arachidonic acid only inhibited the peak amplitude of Kv4 current without modifying its kinetics. These results suggest the existence of Kv4 auxiliary subunit(s) in native cells. We report here a K-channel interacting protein (KChIP)-dependent kinetic modulation of Kv4.2 current in Chinese hamster ovary cells and Kv4.2 and Kv4.3 currents in Xenopus oocytes by arachidonic acid at physiological concentrations. This concentration-dependent effect of arachidonic acid resembled that observed in cerebellar granule neurons and was fully reversible. Other fatty acids, including a nonhydrolyzable inhibitor of both lipooxygenase and cyclooxygenase, 5,8,11,14-eicosatetraynoic acid (ETYA), also mimicked arachidonic acid in modulating Kv4.3 and Kv4.3/KChIP1 currents. Compared with another transient potassium current formed by Kv1.1/Kvbeta1, Kv4.3/KChIP1 current was much more sensitive to arachidonic acid. Association between KChIP1 and Kv4.2 or Kv4.3 was not altered in the presence of 10 microm ETYA as measured by immunoprecipitation and association-dependent growth in yeast. Our data suggest that the KChIP proteins represent a molecular entity for the observed difference between arachidonic acid effects on A-current kinetics in heterologous cells and in native cells and are consistent with the notion that KChIP proteins modulate the subthreshold A-current in neurons.

The consequences of disrupting cardiac inwardly rectifying K(+) current (I(K1)) as revealed by the targeted deletion of the murine Kir2.1 and Kir2.2 genes

1. Ventricular myocytes demonstrate a steeply inwardly rectifying K(+) current termed I(K1). We investigated the molecular basis for murine I(K1) by removing the genes encoding Kir2.1 and Kir2.2. The physiological consequences of the loss of these genes were studied in newborn animals because mice lacking Kir2.1 have a cleft palate and die shortly after birth. 2. Kir2.1 (-/-) ventricular myocytes lack detectable I(K1) in whole-cell recordings in 4 mM external K(+). In 60 mM external K(+) a small, slower, residual current is observed. Thus Kir2.1 is the major determinant of I(K1). Sustained outward K(+) currents and Ba(2+) currents through L- and T-type channels were not significantly altered by the mutation. A 50 % reduction in I(K1) was observed in Kir2.2 (-/-) mice, raising the possibility that Kir2.2 can also contribute to the native I(K1). 3. Kir2.1 (-/-) myocytes showed significantly broader action potentials and more frequent spontaneous action potentials than wild-type myocytes. 4. In electrocardiograms of Kir2.1 (-/-) neonates, neither ectopic beats nor re-entry arrhythmias were observed. Thus the increased automaticity and prolonged action potential of the mutant ventricular myocytes were not sufficiently severe to disrupt the sinus pacing of the heart. The Kir2.1 (-/-) mice, however, had consistently slower heart rates and this phenotype is likely to arise indirectly from the influence of Kir2.1 outside the heart. 5. Thus Kir2.1 is the major component of murine I(K1) and the Kir2.1 (-/-) mouse provides a model in which the functional consequences of removing I(K1) can be studied at both cellular and organismal levels.

Modulation of outward potassium currents in aligned cultures of neonatal rat ventricular myocytes during phorbol ester-induced hypertrophy

Protein kinase C-stimulating phorbol esters induce a strong hypertrophic response when applied in vitro to cardiac ventricular myocytes. The aim of this study was to determine if this in vitro model of hypertrophy is associated with changes in the expression of voltage-gated K(+)channels. Myocytes were isolated from 3--4-day-old neonatal rats and cultured on aligned collagen thin gels. Membrane currents were measured with the use of the whole-cell arrangement of the patch clamp technique and the expression levels of the Kv1.4, Kv4.2 and Kv2.1 alpha subunits quantified using Western blot analysis. Voltage steps positive to -30 mV resulted in the activation of both a transient (I(to)) and a sustained (I(sus)) component of outward K(+)current in the aligned myocytes. Overnight exposure to phorbol 12-myristate 13-acetate (PMA) caused a 55% increase in myocyte size and a three-fold reduction in the peak amplitude of I(to). No differences in the half-maximal voltages required for activation and steady-state inactivation were observed between I(to)measured in control and PMA-treated myocytes. In contrast, PMA treatment resulted in a 62% increase in a tetraethylammonium-sensitive component of I(sus)(TEA-I(sus)) and was associated with the appearance of a slow component of current decay. Expression levels of the Kv1.4 and Kv4.2 alpha subunits were strongly depressed in the hypertrophic myocytes, while the density of the Kv2.1 alpha subunit was enhanced. PMA-induced changes in the Kv alpha subunits were partially prevented through inhibition of the mitogen-activated protein kinase (MAPK) pathway. Thus, PMA-induced hypertrophy of cultured ventricular myocytes is associated with an altered expression of voltage-gated K(+)channels.

The molecular physiology of the cardiac transient outward potassium current (I(to)) in normal and diseased myocardium

G. Y. Oudit, Z. Kassiri, R. Sah, R. J. Ramirez, C. Zobel and P. H. Backx. The Molecular Physiology of the Cardiac Transient Outward Potassium Current (I(to)) in Normal and Diseased Myocardium. Journal of Molecular and Cellular Cardiology (2001) 33, 851-872. The Ca(2+)-independent transient outward potassium current (I(to)) plays an important role in early repolarization of the cardiac action potential. I(to)has been clearly demonstrated in myocytes from different cardiac regions and species. Two kinetic variants of cardiac I(to)have been identified: fast I(to), called I(to,f), and slow I(to), called I(to,s). Recent findings suggest that I(to,f)is formed by assembly of K(v4.2)and/or K(v4.3)alpha pore-forming voltage-gated subunits while I(to,s)is comprised of K(v1.4)and possibly K(v1.7)subunits. In addition, several regulatory subunits and pathways modulating the level and biophysical properties of cardiac I(to)have been identified. Experimental findings and data from computer modeling of cardiac action potentials have conclusively established an important physiological role of I(to)in rodents, with its role in large mammals being less well defined due to complex interplay between a multitude of cardiac ionic currents. A central and consistent electrophysiological change in cardiac disease is the reduction in I(to)density with a loss of heterogeneity of I(to)expression and associated action potential prolongation. Alterations of I(to)in rodent cardiac disease have been linked to repolarization abnormalities and alterations in intracellular Ca(2+)homeostasis, while in larger mammals the link with functional changes is far less certain. We review the current literature on the molecular basis for cardiac I(to)and the functional consequences of changes in I(to)that occur in cardiovascular disease.

Kv2 channels form delayed-rectifier potassium channels in situ

A non inactivating potassium current known as the delayed rectifier plays a major role in membrane repolarization during an action potential. Whereas several candidate genes exist that code for potassium current, the identities of the molecular isotypes that are responsible in situ for membrane repolarization remain unidentified. We report that Kv2 channels play a major role in action potential repolarization. Kv2 channel elimination resulted in a reduction of the density of noninactivating potassium current and a prolonged impulse duration. In contrast, suppression of noninactivating current carried by Kv1 channels was much less effective in increasing action potential durations. Thus, whereas different potassium channels encode sustained potassium current, their contributions to action potential repolarization vary and require direct examination in situ. Our results indicate that Kv2 subunits function as classic delayed-rectifier channels in vertebrate neurons.

Alternation of inwardly rectifying background K+ channel during development of rat fetal cardiomyocytes

The resting membrane potential of rat ventricular myocytes dramatically hyperpolarizes in the late phase of the fetal period. In order to investigate the mechanisms of this hyperpolarization, we examined the electrophysiological properties and molecular structure of the inwardly rectifying background K+ channels of rat fetal ventricular myocytes. In a patch-clamp experiment the whole-cell current of the inwardly rectifying background K+ channel increased 12-fold from between 12 and 18 days after impregnation. In the single channel recording, the large-conductance (35 pS) channels were mainly observed in the 18-day fetal ventricular myocytes. In the 12-day cells, the large-conductance channel was not observed although the low-conductance channels (11 and 16 pS) were infrequently observed. These data of single channel recordings suggested that channel proteins conducting the inwardly rectifying background K+ current were altered during the fetal development. Therefore, we compared the expression of Kir 2.1 mRNA and Kir 2.2 mRNA between 12 days and 18 days using the RT-PCR method, in order to investigate the possible molecular regulation which contributes to the electrophysiological changes. During the fetal period, the expression of Kir 2.2 mRNA increased tremendously (17 times), whereas the increase in the expression of Kir 2.1 mRNA (two times) was not so great. These results show that hyperpolarization in the late fetal period seems to be mainly due to the dramatic increase in expression of Kir 2.2 mRNA rather than expression of Kir 2.1 mRNA.

Kv2 channels form delayed-rectifier potassium channels in situ

A non inactivating potassium current known as the delayed rectifier plays a major role in membrane repolarization during an action potential. Whereas several candidate genes exist that code for potassium current, the identities of the molecular isotypes that are responsible in situ for membrane repolarization remain unidentified. We report that Kv2 channels play a major role in action potential repolarization. Kv2 channel elimination resulted in a reduction of the density of noninactivating potassium current and a prolonged impulse duration. In contrast, suppression of noninactivating current carried by Kv1 channels was much less effective in increasing action potential durations. Thus, whereas different potassium channels encode sustained potassium current, their contributions to action potential repolarization vary and require direct examination in situ. Our results indicate that Kv2 subunits function as classic delayed-rectifier channels in vertebrate neurons.

MiRP2 forms potassium channels in skeletal muscle with Kv3.4 and is associated with periodic paralysis

The subthreshold, voltage-gated potassium channel of skeletal muscle is shown to contain MinK-related peptide 2 (MiRP2) and the pore-forming subunit Kv3.4. MiRP2-Kv3.4 channels differ from Kv3.4 channels in unitary conductance, voltage-dependent activation, recovery from inactivation, steady-state open probability, and block by a peptide toxin. Thus, MiRP2-Kv3.4 channels set resting membrane potential (RMP) and do not produce afterhyperpolarization or cumulative inactivation to limit action potential frequency. A missense mutation is identified in the gene for MiRP2 (KCNE3) in two families with periodic paralysis and found to segregate with the disease. Mutant MiRP2-Kv3.4 complexes exhibit reduced current density and diminished capacity to set RMP. Thus, MiRP2 operates with a classical potassium channel subunit to govern skeletal muscle function and pathophysiology.

Modulation of potassium channels in the hearts of transgenic and mutant mice with altered polyamine biosynthesis

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

Molecular underpinnings of motor pattern generation: differential targeting of shal and shaker in the pyloric motor system

The patterned activity generated by the pyloric circuit in the stomatogastric ganglion of the spiny lobster, Panulirus interruptus, results not only from the synaptic connectivity between the 14 component neurons but also from differences in the intrinsic properties of the neurons. Presumably, differences in the complement and distribution of expressed ion channels endow these neurons with many of their distinct attributes. Each pyloric cell type possesses a unique, modulatable transient potassium current, or A-current (I(A)), that is instrumental in determining the output of the network. Two genes encode A-channels in this system, shaker and shal. We examined the hypothesis that cell-specific differences in shaker and shal channel distribution contribute to diversity among pyloric neurons. We found a stereotypic distribution of channels in the cells, such that each channel type could contribute to different aspects of the firing properties of a cell. Shal is predominantly found in the somatodendritic compartment in which it influences oscillatory behavior and spike frequency. Shaker channels are exclusively localized to the membranes of the distal axonal compartments and most likely affect distal spike propagation. Neither channel is detectably inserted into the preaxonal or proximal portions of the axonal membrane. Both channel types are targeted to synaptic contacts at the neuromuscular junction. We conclude that the differential targeting of shaker and shal to different compartments is conserved among all the pyloric neurons and that the channels most likely subserve different functions in the neuron.

Expression of Shal potassium channel subunits in the adult and developing cochlear nucleus of the mouse

The pattern of expression of potassium (K(+)) channel subunits is thought to contribute to the establishment of the unique discharge characteristics exhibited by cochlear nucleus (CN) neurons. This study describes the developmental distribution of mRNA for the three Shal channel subunits Kv4.1, Kv4.2 and Kv4.3 within the mouse CN, as assessed with in situ hybridization and RT-PCR techniques. Kv4.1 was not present in CN at any age. Kv4.2 mRNA was detectable as early as postnatal day 2 (P2) in all CN subdivisions, and continued to be constitutively expressed throughout development. Kv4.2 was abundantly expressed in a variety of CN cell types, including all of the major projection neuron classes (i.e., octopus, bushy, stellate, fusiform, and giant cells). In contrast, Kv4.3 was expressed at lower levels and by fewer cell types. Kv4.3-labeled cells were more prevalent in ventral subdivisions than in the dorsal CN. Kv4.3 expression was significantly delayed developmentally in comparison to Kv4.2, as it was detectable only after P14. Although the techniques employed in this study detect mRNA and not protein, it can be inferred from the differential distribution of Kv4 transcripts that CN neurons selectively regulate the expression of Shal K(+) channels among individual neurons throughout development.