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This summary article presents an overview of the molecular relationships among the voltage-gated sodium channels and a standard nomenclature for them, which is derived from the IUPHAR Compendium of Voltage-Gated Ion Channels. The complete Compendium, including data tables for each member of the sodium channel family can be found athttp://www.iuphar-db.org/iuphar-ic/.

Epithelial cells from the anterior and equatorial surfaces of the frog lens were isolated and used the same day for studies of the Na/K ATPase. RNase protection assays showed that all cells express α1- and α2-isoforms of the Na/K pump but not the α3-isoform, however the α2-isoform dominates in anterior cells whereas the α1-isoform dominates in equatorial cells. The whole cell patch-clamp technique was used to record functional properties of the Na/K pump current (I P ), defined as the current specifically inhibited by dihydro-ouabain (DHO). DHO-I P blockade data indicate the α1-isoform has a dissociation constant of 100 μm DHO whereas for the α2-isoform it is 0.75 μm DHO. Both α1- and α2-isoforms are half maximally activated at an intracellular Na+-concentration of 9 mm. The α1-isoform is half maximally activated at an extracellular K+-concentration of 3.9 mm whereas for the α2-isoform, half maximal activation occurs at 0.4 mm. Lastly, transport by the α1-isoform is inhibited by a drop in extracellular pH, which does not affect transport by the α2-isoform. Under normal physiological conditions, I P in equatorial cells is approximately 0.23 μA/μF, and in anterior cells it is about 0.14 μA/μF. These current densities refer to the area of cell membrane assuming a capacitance of around 1 μF/cm2. Because cell size and geometry are different at the equatorial vs. anterior surface of the intact lens, we estimate Na/K pump current density per area of lens surface to be around 10 μA/cm2 at the equator vs. 0.5 μA/cm2 at the anterior pole.

The M-current regulates the subthreshold electrical excitability of many neurons, determining their firing properties and responsiveness to synaptic input. To date, however, the genes that encode subunits of this important channel have not been identified. The biophysical properties, sensitivity to pharmacological blockade, and expression pattern of the KCNQ2 and KCNQ3 potassium channels were determined. It is concluded that both these subunits contribute to the native M-current.

The human K + channel gene, HERG, has been linked to the type 2 form of the autosomal dominant long-QT syndrome and has been suggested to encode the fast component of the delayed rectifier K + current (I Kr ) found in heart. To date, the published electrophysiological and pharmacological data on the Xenopus -expressed HERG are very similar but are not identical to those of the endogenous I Kr . In an effort to provide a different type of correlative data on the relationship between erg and I Kr , cDNA fragments of erg homologues from guinea pig, rabbit, human, dog, and rat were cloned and used to test for the presence of erg mRNA in cardiac tissue. RNase protection assays reveal that erg message is found in the hearts of all five species and that it is expressed uniformly throughout the heart. The erg transcript is expressed at relatively high levels, being ≈50% more abundant than the most prevalent Kv-class K + channel transcript in canine ventricle (Kv4.3). Erg transcripts were found to have a wide tissue distribution in rat and are abundant in the brain, retina, thymus, and adrenal gland and are also found in skeletal muscle, lung, and cornea. Since there were no published reports of an I Kr -like current in the rat heart, electrophysiological studies were performed to test whether the significant level of erg message in rat heart was correlated with the presence of an I Kr -like current in rat. In isolated rat ventricular myocytes, an E-4031–sensitive current was observed, which is consistent with the presence of I Kr . These results strengthen the link between erg and the native I Kr in heart and suggest that erg may play an important role in other noncardiac tissues.

The mouse voltage-gated K+ channel gene, Kv1.4, is expressed in brain and heart as approximately 4.5- and approximately 3.5-kilobase (kb) transcripts. Both mRNAs begin at a common site 1338 bp upstream of the initiation codon, contain 3477 and 4411 nucleotides, respectively, and are encoded by two exons; exon 1 contains 0.5 kb of the 5'-noncoding region (NCR), while exon 2 encodes the remaining 0.8 kb of the 5'-NCR, the entire coding region (2 kb), and all of the 3'-NCR. The 3.5-kb transcript terminates at a polyadenylation signal 177 bp 3' of the stop codon, while the 4.5-kb mRNA utilizes a signal 94 bp farther downstream. Although the proteins generated from either transcript are identical, the two mRNAs are functionally different, the 3.5-kb transcript producing approximately 4-5-fold larger currents when expressed in Xenopus oocytes compared to the 4. 5-kb mRNA. The decreased expression of the longer transcript is due to the presence of five ATTTA repeats in the 3'-NCR which inhibit translation; such motifs have also been reported to destabilize the messages of many other genes and might therefore shorten the life of the 4.5-kb transcript during its natural expression. The Kv1.4 basal promoter is GC-rich, contains three SP1 repeats (CCGCCC, -65 to -35), lacks canonical TATAAA and GGCAATCT motifs, and has no apparent tissue specificity. One region enhances activity of this promoter. Thus, transcriptional and post-transcriptional regulation of mKv1.4, coupled with selective usage of the two alternate Kv1.4 mRNAs, may modulate the levels of functional Kv1.4 channels.

A genomic clone encoding the Shaker-related potassium channel gene, Kcna4/mKv1.4, was isolated from mice. Its coding region is contained in a single exon, encodes a protein of 654 amino acids, and shares approximately 91% nucleotide sequence identity with human KCNA4/hKv1.4. We show that 0.8 kb of the 5' noncoding region (NCR), the entire protein coding region (approximately 2.0 kb), and all of the known 3' NCR (approximately 1.1 kb) are contained within a single exon; the remaining 0.5 kb of the 5' NCR is separated from this exon by a 3.4-kb intron. The sequenced genomic region thus accounts for essentially all of the longest known transcript (4.5 kb), although the precise ends of this transcript have not been defined. The 3' NCR contains several ATTTA and ATTTG motifs that are thought to destabilize mRNAs, and these are also present in rat, bovine, and human Kcna4/Kv1.4 cDNAs. It also contains three conserved polyadenylation signals, alternate utilization of which could generate mRNAs of differing stabilities. The 5' NCR of Kcna4/mKv1.4 may also serve to regulate channel expression. This region is approximately 85% identical to KCNA4/hKv1.4 and contains eight consensus translation start sites [(G, A)NNATG] that, based on the 5'-3' scanning model, would lead to a lowering of translational efficiency. The shortest Kcna4/Kv1.4 transcript (2.4 kb) can contain at most 400 bp of NCR and should lack the 3' ATTTAs and most of the 5' ATGs; this transcript might therefore exhibit increased stability and translational efficiency. The Kcna4/mKv1.4 channel exhibited biophysical and pharmacological properties indistinguishable from its rat and human homologues. Kcna4/mKv1.4 lies on mouse chromosome 2, near the Fshb locus, and in humans on the proximal half of chromosome 11p14 near human FSHB. Another K+ channel gene, Kcnc1/mKv3.1, lies approximately 1.8 cM from the Myod-1 gene on mouse chromosome 7, and in situ hybridization localizes KCNC1/hKv3.1 to the homologous region on human chromosome 11p14.3-p15.2. A third gene, KCNA1/hKv1.1, was mapped to human 12p13.

A common feature of tumor cells is the aberrant expression of ion channels on their plasma membrane. The molecular mechanisms regulating ion channel expression in cancer cells are still poorly known. K+ channels that belong to the human ether-a-go-go-related gene 1 (herg1) family are frequently misexpressed in cancer cells compared to their healthy counterparts. We describe here a posttranslational mechanism for the regulation of hERG1 channel surface expression in cancer cells. This mechanism is based on the activity of hERG1 isoforms containing the USO exon. These isoforms (i) are frequently overexpressed in human cancers, (ii) are retained in the endoplasmic reticulum, and (iii) form heterotetramers with different proteins of the hERG family. (iv) The USO-containing heterotetramers are retained intracellularly and undergo ubiquitin-dependent degradation. This process results in decreased hERG1 current (IhERG1) density. We detailed such a mechanism in heterologous systems and confirmed its functioning in tumor cells that endogenously express hERG1 proteins. The silencing of USO-containing hERG1 isoforms induces a higher IhERG1 density in tumors, an effect that apparently regulates neurite outgrowth in neuroblastoma cells and apoptosis in leukemia cells. Copyright © 2008, American Society for Microbiology. All Rights Reserved.

This summary article presents an overview of the molecular relationships among the voltage-gated potassium channels and a standard nomenclature for them, which is derived from the IUPHAR Compendium of Voltage-Gated Ion Channels.1 The complete Compendium, including data tables for each member of the potassium channel family can be found at http://www.iuphar-db.org/iuphar-ic/.

The role of K(+) channel activity during cell cycle progression has become a research topic of considerable interest. Blocking of K(+) channels inhibits the proliferation of many cell types, although the mechanism of this inhibition is unclear. There is speculation that K(+) channels differentially regulate the electrical potential of the plasma membrane (V(m)) during proliferation. We have demonstrated that in tumor cells the value of V(m) is clamped to rather depolarized values by K(+) channels belonging to the HERG family. We report here that tumor cell lines preferentially express the herg1 gene and a truncated, N-deleted form that corresponds to herg1b. This alternative transcript is also expressed in human primary acute myeloid leukemias. Both HERG1 and HERG1B proteins are expressed on the plasma membrane of tumor cells and can form heterotetramers. The expression of HERG protein isoforms is strongly cell cycle-dependent, accounting for variations in HERG currents along the mitotic cycle. Moreover, the blocking of HERG channels dramatically impairs cell growth of HERG-bearing tumor cells. These results suggest that modulated expression of different K(+) channels is the molecular basis of a novel mechanism regulating neoplastic cell proliferation.

Abstract —The HCN family of ion channel subunits underlies the currents I f in heart and I h and I q in the nervous system. In the present study, we demonstrate that minK-related peptide 1 (MiRP1) is a β subunit for the HCN family. As such, it enhances protein and current expression as well as accelerating the kinetics of activation. Because MiRP1 also functions as a β subunit for the cardiac delayed rectifier I Kr , these results suggest that this peptide may have the unique role of regulating both the inward and outward channels that underlie cardiac pacemaker activity. The full text of this article is available at http://www.circresaha.org.

Abstract —The Ca 2+ -independent portion of transient outward K + current ( I to ) exhibits a transmural gradient in ventricle. To investigate control mechanisms for this gradient, we studied canine epicardial and endocardial ventricular myocytes with use of the whole-cell patch-clamp technique. I to was larger in amplitude, had a more negative voltage threshold for activation, and had a more negative midpoint of inactivation in epicardium. Recovery from inactivation was >10-fold slower in endocardium. Incubation of epicardial myocytes with angiotensin II for 2 to 52 hours altered I to to resemble unincubated endocardium and reduced the amplitude of the phase 1 notch of the action potential. In contrast, incubation of endocardial myocytes with losartan for 2 to 52 hours altered I to to resemble unincubated epicardium and induced a phase 1 notch in the action potential. With RNase protection assays, we determined that incubations with angiotensin II or losartan did not alter mRNA levels for either Kv4.3 or Kv1.4; thus, a change in the α subunit for I to is unlikely to be responsible. To test whether posttranslational modification produced the effects of angiotensin II, we coexpressed Kv4.3 and the angiotensin II type 1a receptor in Xenopus oocytes. Incubation with angiotensin II increased the time constant for recovery from inactivation of the expressed current by 2-fold with an incubation time constant of 3.7 hours. No effect on activation or inactivation voltage dependence was observed. These results demonstrate that the properties of I to in endocardium and epicardium are plastic and likely under the tonic-differing influence of the renin-angiotensin system.

Abstract —HCN cation channel mRNA expression was determined in the rabbit heart and neonatal and adult rat ventricle using RNase protection assays. In the rabbit SA node, the dominant HCN transcript is HCN4, representing >81% of the total HCN message. HCN1 is also expressed, representing >18% of the total HCN mRNA. Rabbit Purkinje fibers contained almost equal amounts of HCN1 and HCN4 transcripts with low levels of HCN2, whereas rabbit ventricle contained predominantly HCN2. The SA node contained 25 times the total HCN message of Purkinje fibers and 140 times the total HCN message of ventricle. No reports of hyperpolarization-activated current ( I f ) exist in rabbit Purkinje fibers, and we could not record I f in rabbit ventricular myocytes. To investigate the possible role of isoform switching in determining the voltage dependence of I f , we determined the prevalence of HCN isoforms in neonatal and adult rat ventricle. We had previously determined the threshold for activation of I f to be ≈−70 mV in neonatal rat ventricle and −113 mV in adult rat ventricle. In both neonatal and adult rat ventricle, only HCN2 and HCN4 transcripts are present. The ratio of HCN2 to HCN4 is ≈5:1 in the neonate and 13:1 in the adult. Taken together, these results suggest that different cardiac regions express different isoforms of the HCN family. The HCN1 and HCN4 isoforms are most closely associated with a depolarized threshold for I f activation, whereas the HCN2 isoform is associated with a more negative activation curve. The full text of this article is available at http://www.circresaha.org.

Background —Cardiac memory refers to an altered T-wave morphology induced by ventricular pacing or arrhythmias that persist for variable intervals after resumption of sinus rhythm. Methods and Results —We induced long-term cardiac memory (LTM) in conscious dogs by pacing the ventricles at 120 bpm for 3 weeks. ECGs were recorded daily for 1 hour, during which time pacing was discontinued. At terminal study, the heart was removed and the electrophysiology of left ventricular epicardial myocytes was investigated. Control (C) and LTM ECG did not differ, except for T-wave amplitude, which decreased from 0.12±0.18 to −0.34±0.21 mV (±SEM, P P P I to ) was investigated by whole-cell patch clamp. No difference in capacitance was seen in C and LTM myocytes. I to activated on membrane depolarization to −25±1 mV in C and −7±1 mV ( P I to density was reduced in LTM myocytes, and a decreased mRNA level for Kv4.3 was observed. Recovery of I to from inactivation was significantly prolonged: it was 531±80 ms (n=10) in LTM and 27±6 ms (n=9) in C ( P Conclusions — I to changes are associated with and can provide at least a partial explanation for action-potential and T-wave changes occurring with LTM.

1. Guinea-pig ventricle was used in the RNase protection assays to determine which alpha-isoforms of the Na+-K+ pumps are present, and ventricular myocytes were used in whole cell patch clamp studies to investigate the actions of alpha- and beta-adrenergic agonists on Na+-K+ pump current. 2. RNase protection assays showed that two isoforms of the alpha-subunit of the Na+-K+-ATPase are present in guinea-pig ventricle. The mRNA for the alpha1-isoform comprises 82 % of the total pump message, the rest being the alpha2-isoform. 3. We have previously shown that beta-adrenergic agonists affect Na+-K+ pump current (Ip) through a protein kinase A (PKA)-dependent pathway. We now show that these beta-effects are targeted to the alpha1-isoform of the Na+-K+ pumps. 4. We have also previously shown that alpha-adrenergic agonists increase Ip through a protein kinase C (PKC)-dependent pathway. We now show that these alpha-isoform effects are targeted to the alpha2-isoform of the Na+-K+ pumps. 5. These results suggest the effects of adrenergic activation on Na+-K+ pump activity in the heart can be regionally specific, depending on which alpha-isoform of the Na+-K+ pump is expressed.

Three new members of the EAG potassium channel gene family were identified in rat and the complete coding sequence of one of these genes (elk1) was determined by cDNA cloning. The elk1 gene, when expressed in Xenopus oocytes, encodes a slowly activating and slowly deactivating potassium channel. The elk1 gene is expressed in sympathetic ganglia and is also expressed in sciatic nerve. Six of the seven known EAG genes were found to be expressed in rat sympathetic ganglia, suggesting an important functional role for these channels in the sympathetic nervous system. A family of three related voltage-gated potassium channel genes, which are distinct from the Shaker class of potassium channel genes, have been previously identified in either Drosophila or mammals (Warmke & Ganetzky, 1994). The first gene identified in this family was the ether ago-go gene (eag), which was identified initially in Drosophila (Warmke et al. 1991). Two related genes were subsequently described: the eag-related gene (erg) and the eag-like K+ channel gene (elk) (Warmke & Ganetzky, 1994). This family of channel genes is known as the EAG family (Wei et al. 1996). Mammalian homologues of the eag and erg genes have been identified and the potassium channels that they encode have been expressed and characterized (Warmke & Ganetzky, 1994; Ludwig et al. 1994; Robertson et al. 1996; Shi et al. 1997). To date, however, no mammalian homologue of the Drosophilaelk gene has been described, nor have the biophysical properties of any elk channel been characterized. In this paper, three new members of the mammalian EAG gene family are identified. For one of these genes, elk1, we describe the complete coding sequence and have determined the biophysical and pharmacological properties of the encoded channel. In addition, we have determined the mRNA distribution pattern in sympathetic ganglia and sciatic nerve of all known EAG genes.

Two new potassium channel genes,erg2 anderg3, that are expressed in the nervous system of the rat were identified. These two genes form a small gene family with the previously describederg1 (HERG) gene. Theerg2 anderg3 genes are expressed exclusively in the nervous system, in marked contrast toerg1, which is expressed in both neural and non-neural tissues. All three genes are expressed in peripheral sympathetic ganglia. Theerg3 channel produces a current that has a large transient component at positive potentials, whereas the other two channels are slowly activating delayed rectifiers. Expression of theerg1 gene in the sympathetic nervous system has potential implications for the etiology of the LQT2 form of the human genetic disease long QT syndrome.

The expression of 15 different K + channels in canine heart was examined, and a new K + channel gene (Kv4.3), which encodes a rapidly inactivating K + current, is described. The Kv4.3 channel was found to have biophysical and pharmacological properties similar to the native canine transient outward current (I to ). The Kv4.3 gene is also expressed in human and rat heart. It is concluded that the Kv4.3 channel underlies the bulk of the I to in canine ventricular myocytes, and probably in human myocytes. Both the Kv4.3 and Kv4.2 channels are likely to contribute to the I to in rat heart, and differential expression of these two channels can account for observed differences in the kinetic properties of the I to in different regions of rat ventricle. There are significant differences in the pattern of K + channel expression in canine heart, compared with rat heart, and these differences may be an adaptation to the different requirements for cardiac function in mammals of markedly different sizes. It is possible that the much longer ventricular action potential duration observed in canine heart compared with rat heart is due, in part, to the lower levels of Kv1.2, Kv2.1, and Kv4.2 gene expression in canine heart.