Selective effects of estradiol on human corneal endothelial cells

In Fuchs endothelial corneal dystrophy (FECD), mitochondrial and oxidative stresses in corneal endothelial cells (HCEnCs) contribute to cell demise and disease progression. FECD is more common in women than men, but the basis for this observation is poorly understood. To understand the sex disparity in FECD prevalence, we studied the effects of the sex hormone 17-β estradiol (E2) on growth, oxidative stress, and metabolism in primary cultures of HCEnCs grown under physiologic ([O2]2.5) and hyperoxic ([O2]A) conditions. We hypothesized that E2 would counter the damage of oxidative stress generated at [O2]A. HCEnCs were treated with or without E2 (10 nM) for 7-10 days under both conditions. Treatment with E2 did not significantly alter HCEnC density, viability, ROS levels, oxidative DNA damage, oxygen consumption rates, or extracellular acidification rates in either condition. E2 disrupted mitochondrial morphology in HCEnCs solely from female donors in the [O2]A condition. ATP levels were significantly higher at [O2]2.5 than at [O2]A in HCEnCs from female donors only, but were not affected by E2. Our findings demonstrate the resilience of HCEnCs against hyperoxic stress. The effects of hyperoxia and E2 on HCEnCs from female donors suggest cell sex-specific mechanisms of toxicity and hormonal influences.

Loss of Astrocytic µ Opioid Receptors Exacerbates Aversion Associated with Morphine Withdrawal in Mice: Role of Mitochondrial Respiration

Astrocytes express mu/µ opioid receptors, but the function of these receptors remains poorly understood. We evaluated the effects of astrocyte-restricted knockout of µ opioid receptors on reward- and aversion-associated behaviors in mice chronically exposed to morphine. Specifically, one of the floxed alleles of the Oprm1 gene encoding µ opioid receptor 1 was selectively deleted from brain astrocytes in Oprm1 inducible conditional knockout (icKO) mice. These mice did not exhibit changes in locomotor activity, anxiety, or novel object recognition, or in their responses to the acute analgesic effects of morphine. Oprm1 icKO mice displayed increased locomotor activity in response to acute morphine administration but unaltered locomotor sensitization. Oprm1 icKO mice showed normal morphine-induced conditioned place preference but exhibited stronger conditioned place aversion associated with naloxone-precipitated morphine withdrawal. Notably, elevated conditioned place aversion lasted up to 6 weeks in Oprm1 icKO mice. Astrocytes isolated from the brains of Oprm1 icKO mice had unchanged levels of glycolysis but had elevated oxidative phosphorylation. The basal augmentation of oxidative phosphorylation in Oprm1 icKO mice was further exacerbated by naloxone-precipitated withdrawal from morphine and, similar to that for conditioned place aversion, was still present 6 weeks later. Our findings suggest that µ opioid receptors in astrocytes are linked to oxidative phosphorylation and they contribute to long-term changes associated with opioid withdrawal.

Selective effects of estradiol on human corneal endothelial cells

Fuchs endothelial corneal dystrophy (FECD) results from genetic and environmental factors triggering mitochondrial and oxidative stress in corneal endothelial cells (CEnCs) leading to CEnC death and corneal opacification. FECD is more common in women than men, but the basis for this observation is unknown. Because FECD is commonly diagnosed around the time of the menopausal transition in women when estrogen levels decrease precipitously, we studied the effects of the potent estrogen,17-β estradiol (E2) on growth, oxidative stress, and metabolism in primary cultures of human CEnCs (HCEnCs) under conditions of physiologic 2.5% O 2 ([O 2 ] 2.5 ) and under hyperoxic stress ([O 2 ] A : room air + 5% CO 2 ). We hypothesized that E2 would counter the stresses of the hyperoxic environment in HCEnCs. HCEnCs were treated ± 10 nM E2 for 7-10 days at [O 2 ] 2.5 and [O 2 ] A followed by measurements of cell density, viability, reactive oxygen species (ROS), mitochondrial morphology, oxidative DNA damage, ATP levels, mitochondrial respiration (O 2 consumption rate [OCR]), and glycolysis (extracellular acidification rate [ECAR]). There were no significant changes in HCEnC density, viability, ROS levels, oxidative DNA damage, OCR, and ECAR in response to E2 under either O 2 condition. We found that E2 disrupted mitochondrial morphology in HCEnCs from female donors but not male donors at the [O 2 ] A condition. ATP levels were significantly higher at [O 2 ] 2.5 compared to [O 2 ] A in HCEnCs from female donors only, but were not affected by E2. Our findings demonstrate the overall resilience of primary HCEnCs against hyperoxic stress. The selective detrimental effects of hyperoxia and estradiol on HCEnCs from female but not male donors suggests mechanisms of toxicity based upon cell-sex in addition to hormonal environment.

Deficient mitochondrial respiration in astrocytes impairs trace fear conditioning and increases naloxone-precipitated aversion in morphine-dependent mice

Mitochondria are abundant in the fine processes of astrocytes, however, potential roles for astrocyte mitochondria remain poorly understood. In the present study, we performed a systematic examination of the effects of abnormal oxidative phosphorylation in astrocytes on several mouse behaviors. Impaired astrocyte oxidative phosphorylation was produced by astrocyte-specific deletion of the nuclear mitochondrial gene, Cox10, that encodes an accessory protein of complex IV, the protoheme:heme-O-farnesyl transferase. As expected, conditional deletion of the Cox10 gene in mice (cKO mice) significantly reduced expression of COX10 and Cytochrome c oxidase subunit I (MTCO1) of Complex IV, resulting in decreased oxidative phosphorylation without significantly affecting glycolysis. No effects of the deletion were observed on locomotor activity, anxiety-like behavior, nociception, or spontaneous alternation. Cox10 cKO female mice exhibited mildly impaired novel object recognition, while Cox10 cKO male mice were moderately deficient in trace fear conditioning. No group-related changes were observed in conditional place preference (CPP) that assessed effects of morphine on reward. In contrast to CPP, Cox10 cKO mice demonstrated significantly increased aversive behaviors produced by naloxone-precipitated withdrawal following chronic exposure to morphine, that is, jumping and avoidance behavior as assessed by conditional place aversion (CPA). Our study suggests that astrocyte oxidative phosphorylation may contribute to behaviors associated with greater cognitive load and/or aversive and stressful conditions.

Astrocyte Bioenergetics and Major Psychiatric Disorders

Ongoing research continues to add new elements to the emerging picture of involvement of astrocyte energy metabolism in the pathophysiology of major psychiatric disorders, including schizophrenia, mood disorders, and addictions. This review outlines what is known about the energy metabolism in astrocytes, the most numerous cell type in the brain, and summarizes the recent work on how specific perturbations of astrocyte bioenergetics may contribute to the neuropsychiatric conditions. The role of astrocyte energy metabolism in mental health and disease is reviewed on the organism, organ, and cell level. Data arising from genomic, metabolomic, in vitro, and neurobehavioral studies is critically analyzed to suggest future directions in research and possible metabolism-focused therapeutic interventions.

Depletion of Stercobilin in Fecal Matter from a Mouse Model of Autism Spectrum Disorders

INTRODUCTION

Autism spectrum disorders (ASD) are a group of neurodevelopmental disorders lacking a clinical biomarker for diagnosis. Emerging evidence shows that intestinal microflora from ASD subjects can be distinguished from controls, suggesting metabolite differences due to the action of intestinal microbes may provide a means for identifying potential biomarkers for ASD.

OBJECTIVES

The aim of this study was to determine if quantitative differences in levels of stercobilin and stercobilinogen, metabolites produced by biological action of intestinal microflora, exist in the fecal matter between an ASD mouse model population and controls.

METHODS

Pairs of fecal samples were collected from two mouse groups, an ASD model group with Timothy syndrome 2 (TS2-NEO) and a gender-matched control group. After centrifugation, supernatant was spiked with an ¹⁸O-labeled stercobilin isotopomer and subjected to solid phase extraction for processing. Extracted samples were spotted on a stainless steel plate and subjected to matrix-assisted laser desorption and ionization mass spectrometry using dihydroxybenzoic acid as the matrix (n = 5). Peak areas for bilins and ¹⁸O-stercobilin isotopomers were determined in each fecal sample.

RESULTS

A 40-45% depletion in stercobilin in TS2-NEO fecal samples compared with controls was observed with p < 0.05; a less dramatic depletion was observed for stercobilinogen.

CONCLUSIONS

The results show that stercobilin depletion in feces is observed for an ASD mouse model vs. controls. This may help to explain recent observations of a less diverse microbiome in humans with ASD and may prove helpful in developing a clinical ASD biomarker.

Kv4.3-Encoded Fast Transient Outward Current Is Presented in Kv4.2 Knockout Mouse Cardiomyocytes

Gradients of the fast transient outward K⁺ current (I_to,f) contribute to heterogeneity of ventricular repolarization in a number of species. Cardiac I_to,f levels and gradients change notably with heart disease. Human cardiac I_to,f appears to be encoded by the Kv4.3 pore-forming α-subunit plus the auxiliary KChIP2 β-subunit while mouse cardiac I_to,f requires Kv4.2 and Kv4.3 α-subunits plus KChIP2. Regional differences in cardiac I_to,f are associated with expression differences in Kv4.2 and KChIP2. Although I_to,f was reported to be absent in mouse ventricular cardiomyocytes lacking the Kv4.2 gene (Kv4.2-/-) when short depolarizing voltage pulses were used to activate voltage-gated K⁺ currents, in the present study, we showed that the use of long depolarization steps revealed a heteropodatoxin-sensitive I_to,f (at ~40% of the wild-type levels). Immunohistological studies further demonstrated membrane expression of Kv4.3 in Kv4.2-/- cardiomyocytes. Transmural I_to,f gradients across the left ventricular wall were reduced by ~3.5-fold in Kv4.2-/- heart, compared to wild-type. The I_to,f gradient in Kv4.2-/- hearts was associated with gradients in KChIP2 mRNA expression while in wild-type there was also a gradient in Kv4.2 expression. In conclusion, we found that Kv4.3-based I_to,f exists in the absence of Kv4.2, although with a reduced transmural gradient. Kv4.2-/- mice may be a useful animal model for studying Kv4.3-based I_to,f as observed in humans.

Interaction of DPP10a with Kv4.3 channel complex results in a sustained current component of human transient outward current Ito

The sustained component of the K(+) outward current in human atrial myocytes is believed to be due to the slowly inactivating ultra-rapid potassium current I Kur and not to the fast inactivating transient outward current Ito. Here we provide evidence for contribution of Ito to this late current due to the effects of dipeptidyl peptidase-like protein (DPP) 10 (DPP10a) interacting with Kv4.3 channels. We studied the late current component of Ito in human atrial myocytes and CHO cells co-expressing Kv4.3 or Kv4.3/KChIP2 (control) and DPP proteins using voltage-clamp technique and a pharmacological approach. A voltage dependent and slowly inactivating late current (43% of peak amplitude) could be observed in atrial myocytes. We found a similar current in CHO cells expressing Kv4.3/KChIP2 + DPP10a, but not in cells co-expressing Kv4.3 + DPP or Kv4.3/KChIP2 + DPP6-S. Assuming that DPP10a influences atrial Ito, we detected DPP10 expression of three alternatively spliced mRNAs, DPP10 protein and colocalization of Kv4.3 and DPP10 proteins in human atrial myocytes. DPP10a did not affect properties of expressed Kv1.5 excluding a contribution to the sustained IKur in atrial cells. To test for the contribution of Kv4-based Ito on sustained K(+) outward currents in human atrial myocytes, we used 4-AP to block IKur, in combination with Heteropoda toxin 2 to block Kv4 channels. We could clearly separate an Ito fraction of about 19% contributing to the late current in atrial myocytes. Thus, the interaction of DPP10a, expressed in human atrium, with Kv4.3 channels generates a sustained current component of Ito, which may affect late repolarization phase of atrial action potentials.

Heteropoda toxin 2 interaction with Kv4.3 and Kv4.1 reveals differences in gating modification

Kv4 (Shal) potassium channels are responsible for the transient outward K(+) currents in mammalian hearts and central nervous systems. Heteropoda toxin 2 (HpTx2) is an inhibitor cysteine knot peptide toxin specific for Kv4 channels that inhibits gating of Kv4.3 in the voltage-dependent manner typical for this type of toxin. HpTx2 interacts with four independent binding sites containing two conserved hydrophobic amino acids in the S3b transmembrane segments of Kv4.3 and the closely related Kv4.1. Despite these similarities, HpTx2 interaction with Kv4.1 is considerably less voltage-dependent, has smaller shifts in the voltage-dependences of conductance and steady-state inactivation, and a 3-fold higher K(d) value. Swapping four nonconserved amino acids in S3b between the two channels exchanges the phenotypic response to HpTx2. To understand these differences in gating modification, we constructed Markov models of Kv4.3 and Kv4.1 activation gating in the presence of HpTx2. Both models feature a series of voltage-dependent steps leading to a final voltage-independent transition to the open state and closely replicate the experimental data. Interaction with HpTx2 increases the energy barrier for channel opening by slowing activation and accelerating deactivation. The greater degree of voltage-dependence in Kv4.3 occurs because it is the voltage-dependent transitions that are most affected by HpTx2; in contrast, it is the voltage-independent step in Kv4.1 that is most affected by the presence of toxin. These data demonstrate the basis for subtype-specificity of HpTx2 and point the way to a general model of gating modifier toxin interaction with voltage-gated ion channels.

Expression and distribution of voltage-gated ion channels in ferret sinoatrial node

Spontaneous diastolic depolarization in the sinoatrial (SA) node enables it to serve as pacemaker of the heart. The variable cell morphology within the SA node predicts that ion channel expression would be heterogeneous and different from that in the atrium. To evaluate ion channel heterogeneity within the SA node, we used fluorescent in situ hybridization to examine ion channel expression in the ferret SA node region and atrial appendage. SA nodal cells were distinguished from surrounding cardiac myocytes by expression of the slow (SA node) and cardiac (surrounding tissue) forms of troponin I. Nerve cells in the sections were identified by detection of GAP-43 and cytoskeletal middle neurofilament. Transcript expression was characterized for the 4 hyperpolarization-activated cation channels, 6 voltage-gated Na(+) channels, 3 voltage-gated Ca(2+) channels, 24 voltage-gated K(+) channel α-subunits, and 3 ancillary subunits. To ensure that transcript expression was representative of protein expression, immunofluorescence was used to verify localization patterns of voltage-dependent K(+) channels. Colocalizations were performed to observe any preferential patterns. Some overlapping and nonoverlapping binding patterns were observed. Measurement of different cation channel transcripts showed heterogeneous expression with many different patterns of expression, attesting to the complexity of electrical activity in the SA node. This study provides insight into the possible role ion channel heterogeneity plays in SA node pacemaker activity.

Closed-state inactivation in Kv4.3 isoforms is differentially modulated by protein kinase C

Kv4.3, with its complex open- and closed-state inactivation (CSI) characteristics, is a primary contributor to early cardiac repolarization. The two alternatively spliced forms, Kv4.3-short (Kv4.3-S) and Kv4.3-long (Kv4.3-L), differ by the presence of a 19-amino acid insert downstream from the sixth transmembrane segment. The isoforms are similar kinetically; however, the longer form has a unique PKC phosphorylation site. To test the possibility that inactivation is differentially regulated by phosphorylation, we expressed the Kv4.3 isoforms in Xenopus oocytes and examined changes in their inactivation properties after stimulation of PKC activity. Whereas there was no difference in open-state inactivation, there were profound differences in CSI. In Kv4.3-S, PMA reduced the magnitude of CSI by 24% after 14.4 s at -50 mV. In contrast, the magnitude of CSI in Kv4.3-L increased by 25% under the same conditions. Mutation of a putatively phosphorylated threonine (T504) to aspartic acid within a PKC consensus recognition sequence unique to Kv4.3-L eliminated the PMA response. The change in CSI was independent of the intervention used to increase PKC activity; identical results were obtained with either PMA or injected purified PKC. Our previously published 11-state model closely simulated our experimental data. Our data demonstrate isoform-specific regulation of CSI by PKC in Kv4.3 and show that the carboxy terminus of Kv4.3 plays an important role in regulation of CSI.

S3b amino acid substitutions and ancillary subunits alter the affinity of Heteropoda venatoria toxin 2 for Kv4.3

Heteropoda venatoria toxin 2 (HpTx2) is an inhibitor cystine knot (ICK)-gating modifier toxin that selectively inhibits Kv4 channels. To characterize the molecular determinants of interaction, we performed alanine scanning of the Kv4.3 S3b region. HpTx2-Kv4.3 interaction had an apparent K(d) value of 2.3 microM. Two alanine mutants in Kv4.3 increased K(d) values to 6.4 microM for V276A and 25 microM for L275A. Simultaneous mutation of both amino acids to alanine nearly eliminated toxin interaction. Unlike Hanatoxin and other well characterized ICK toxins, HpTx2 binding does not require a charged amino acid for interaction. To determine whether the identity of the S3b binding site amino acids altered HpTx2 specificity, we constructed Kv4.3 [LV275IF]. This mutation decreased the K(d) value to 0.54 microM, suggesting that the hydrophobic character of the putative binding site is the most important property for interaction with HpTx2. One mutant, N280A, caused stronger interaction of HpTx2 with Kv4.3; the K(d) value for Kv4.3 [N280A] was 0.26 microM. To understand Kv4.3-based transient outward currents in native tissues, we tested the affinity of HpTx2 for Kv4.3 coexpressed with KChIP2b. The toxin's K(d) value for Kv4.3 + KChIP2b was 0.95 microM. KChIP2b stabilizes the closed state of Kv4.3, suggesting that the increased toxin affinity is due to increased stabilization of the closed state. These data show that HpTx2 binding to Kv4.3 has aspects in common with other ICK gating modifier toxins but that the interventions that increase toxin affinity suggest flexibility toward channel binding that belies its unusual specificity for Kv4 channels.

Is lipid bilayer binding a common property of inhibitor cysteine knot ion-channel blockers?

Recent studies of several ICK ion-channel blockers suggest that lipid bilayer interactions play a prominent role in their actions. Structural similarities led to the hypothesis that bilayer interactions are important for the entire ICK family. We have tested this hypothesis by performing direct measurements of the free energy of bilayer partitioning (DeltaG) of several peptide blockers using our novel quenching-enhanced fluorescence titration protocol. We show that various ICK peptides demonstrate markedly different modes of interaction with large unilamellar lipid vesicles. The mechanosensitive channel blocker, GsMTx4, and its active diastereomeric analog, D-GsMTx4, bind strongly to both anionic and zwitterionic membranes. One potassium channel gating modifier, rHpTx2gs, interacts negligibly with both types of vesicles at physiological pH, whereas another, SGTx1, interacts only with anionic lipids. The slope of DeltaG dependence on surface potential is very shallow for both GsMTx4 and D-GsMTx4, indicating complex interplay of their hydrophobic and electrostatic interactions with lipid. In contrast, a cell-volume regulator, GsMTx1, and SGTx1 exhibit a very steep DeltaG dependence on surface potential, resulting in a strong binding only for membranes rich in anionic lipids. The high variability of 5 kcal/mole in observed DeltaG shows that bilayer partitioning is not a universal property of the ICK peptides interacting with ion channels.

W-7 modulates Kv4.3: pore block and Ca2+-calmodulin inhibition

Ca(+)-calmodulin (Ca(2+)-CaM)-dependent protein kinase II (Ca(2+)/CaMKII) is an important regulator of cardiac ion channels, and its inhibition may be an approach for treatment of ventricular arrhythmias. Using the two-electrode voltage-clamp technique, we investigated the role of W-7, an inhibitor of Ca(2+)-occupied CaM, and KN-93, an inhibitor of Ca(2+)/CaMKII, on the K(v)4.3 channel in Xenopus laevis oocytes. W-7 caused a voltage- and concentration-dependent decrease in peak current, with IC(50) of 92.4 muM. The block was voltage dependent, with an effective electrical distance of 0.18 +/- 0.05, and use dependence was observed, suggesting that a component of W-7 inhibition of K(v)4.3 current was due to open-channel block. W-7 made recovery from open-state inactivation a biexponential process, also suggesting open-channel block. We compared the effects of W-7 with those of KN-93 after washout of 500 muM BAPTA-AM. KN-93 reduced peak current without evidence of voltage or use dependence. Both W-7 and KN-93 accelerated all components of inactivation. We used wild-type and mutated K(v)4.3 channels with mutant CaMKII consensus phosphorylation sites to examine the effects of W-7 and KN-93. In contrast to W-7, KN-93 at 35 muM selectively accelerated open-state inactivation in the wild-type vs. the mutant channel. W-7 had a significantly greater effect on recovery from inactivation in wild-type than in mutant channels. We conclude that, at certain concentrations, KN-93 selectively inhibits Ca(2+)/CaMKII activity in Xenopus oocytes and that the effects of W-7 are mediated by direct interaction with the channel pore and inhibition of Ca(2+)-CaM, as well as a change in activity of Ca(2+)-CaM-dependent enzymes, including Ca(2+)/CaMKII.

DPP10 is an inactivation modulatory protein of Kv4.3 and Kv1.4

Voltage-gated K+ channels exist in vivo as multiprotein complexes made up of pore-forming and ancillary subunits. To further our understanding of the role of a dipeptidyl peptidase-related ancillary subunit, DPP10, we expressed it with Kv4.3 and Kv1.4, two channels responsible for fast-inactivating K+ currents. Previously, DPP10 has been shown to effect Kv4 channels. However, Kv1.4, when expressed with DPP10, showed many of the same effects as Kv4.3, such as faster time to peak current and negative shifts in the half-inactivation potential of steady-state activation and inactivation. The exception was recovery from inactivation, which is slowed by DPP10. DPP10 expressed with Kv4.3 caused negative shifts in both steady-state activation and inactivation of Kv4.3, but no significant shifts were detected when DPP10 was expressed with Kv4.3 + KChIP2b (Kv channel interacting protein). DPP10 and KChIP2b had different effects on closed-state inactivation. At −60 mV, KChIP2b nearly abolishes closed-state inactivation in Kv4.3, whereas it developed to a much greater extent in the presence of DPP10. Finally, expression of a DPP10 mutant consisting of its transmembrane and cytoplasmic 58 amino acids resulted in effects on Kv4.3 gating that were nearly identical to those of wild-type DPP10. These data show that DPP10 and KChIP2b both modulate Kv4.3 inactivation but that their primary effects are on different inactivation states. Thus DPP10 may be a general modulator of voltage-gated K+ channel inactivation; understanding its mechanism of action may lead to deeper understanding of the inactivation of a broad range of K+ channels.

Ancillary subunits and stimulation frequency determine the potency of chromanol 293B block of the KCNQ1 potassium channel

KCNQ1 (Kv7.1 or KvLQT1) encodes the alpha-subunit of a voltage-gated potassium channel found in tissues including heart, brain, epithelia and smooth muscle. Tissue-specific characteristics of KCNQ1 current are diverse, due to modification by ancillary subunits. In heart, KCNQ1 associates with KCNE1 (MinK), producing a slowly activating voltage-dependent channel. In epithelia, KCNQ1 co-assembles with KCNE3 (Mirp2) producing a constitutively open channel. Chromanol 293B is a selective KCNQ1 blocker. We studied drug binding and frequency dependence of 293B on KCNQ1 and ancillary subunits expressed in Xenopus oocytes. Ancillary subunits altered 293B potency up to 100-fold (IC(50) for KCNQ1 = 65.4 +/- 1.7 microm; KCNQ1/KCNE1 = 15.1 +/- 3.3 microm; KCNQ1/KCNE3 = 0.54 +/- 0.18 microm). Block of KCNQ1 and KCNQ1/KCNE3 was time independent, but 293B altered KCNQ1/KCNE1 activation. We therefore studied frequency-dependent block of KCNQ1/KCNE1. Repetitive rapid stimulation increased KCNQ1/KCNE1 current biphasically, and 293B abolished the slow component. KCNQ1/KCNE3[V72T] activates slowly with a KCNQ1/KCNE1-like phenotype, but retains the high affinity binding of KCNQ1/KCNE3, demonstrating that subunit-mediated changes in gating can be dissociated from subunit-mediated changes in affinity. This study demonstrates the KCNQ1 pharmacology is significantly altered by ancillary subunits. The response of KCNQ1 to specific blockers will therefore be critically dependent on the electrical stimulation pattern of the target organ. Furthermore, the dissociation between gating and overall affinity suggests that mutations in ancillary subunits can potentially strongly alter drug sensitivity without obvious functional changes in gating behaviour, giving rise to unexpected side-effects such as a predisposition to acquired long QT syndrome.

KChIP2b modulates the affinity and use-dependent block of Kv4.3 by nifedipine

Rapidly activating Kv4 voltage-gated ion channels are found in heart, brain, and diverse other tissues including colon and uterus. Kv4.3 can co-assemble with KChIP ancillary subunits, which modify kinetic behavior. We examined the affinity and use dependence of nifedipine block on Kv4.3 and its modulation by KChIP2b. Nifedipine (150 microM) reduced peak Kv4.3 current approximately 50%, but Kv4.3/KChIP2b current only approximately 27%. Nifedipine produced a very rapid component of open channel block in both Kv4.3 and Kv4.3/KChIP2b. However, recovery from the blocked/inactivated state was strongly sensitive to KChIP2b. Kv4.3 Thalf,recovery was slowed significantly by nifedipine (120.0+/-12.4 ms vs. 213.1+/-18.2 ms), whereas KChIP2b eliminated nifedipine's effect on recovery: Kv4.3/KChIP2b Thalf,recovery was 45.3+/-7.2 ms (control) and 47.8+/-8.2 ms (nifedipine). Consequently, Kv4.3 exhibited use-dependent nifedipine block in response to a series of depolarizing pulses which was abolished by KChIP2b. KChIPs alter drug affinity and use dependence of Kv4.3.

Time- and voltage-dependent components of Kv4.3 inactivation

Kv4.3 inactivation is a complex multiexponential process, which can occur from both closed and open states. The fast component of inactivation is modulated by the N-terminus, but the mechanisms mediating the other components of inactivation are controversial. We studied inactivation of Kv4.3 expressed in Xenopus laevis oocytes, using the two-electrode voltage-clamp technique. Inactivation during 2000 ms pulses at potentials positive to the activation threshold was described by three exponents (46 +/- 3, 152 +/- 13, and 930 +/- 50 ms at +50 mV, n = 7) whereas closed-state inactivation (at potentials below threshold) was described by two exponents (1079 +/- 119 and 3719 +/- 307 ms at -40 mV, n = 9). The fast component of open-state inactivation was dominant at potentials positive to -20 mV. Negative to -30 mV, the intermediate and slow components dominated inactivation. Inactivation properties were dependent on pulse duration. Recovery from inactivation was strongly dependent on voltage and pulse duration. We developed an 11-state Markov model of Kv4.3 gating that incorporated a direct transition from the open-inactivated state to the closed-inactivated state. Simulations with this model reproduced open- and closed-state inactivation, isochronal inactivation relationships, and reopening currents. Our data suggest that inactivation can proceed primarily from the open state and that multiple inactivation components can be identified.

Heteropoda toxin 2 is a gating modifier toxin specific for voltage-gated K+ channels of the Kv4 family

Kv4 voltage-gated K(+) channels are responsible for transient K(+) currents in the central nervous system and in the heart. HpTx2 is a peptide toxin that selectively inhibits these currents; making it a useful probe for understanding Kv4 channel structure and drug binding. Therefore, we developed a method to produce large amounts of recombinant HpTx2. Recombinant toxin inhibits all three Kv4 isoforms to the same degree; however, the voltage-dependence of inhibition is less apparent for Kv4.1 than for Kv4.3. Similarly, recombinant HpTx2(GS) effects gating characteristics of both channels, but Kv4.1 to a much lesser degree. The toxin lacks affinity for Kv1.4, Kv2.1, and Kv3.4. To locate the binding site, the amino acids linking the third and forth membrane spanning segments of Kv4.3 were replaced with analogous amino acids of Kv1.4. The chimeric K(+) channel was completely insensitive to block by rHpTx2, suggesting that its binding site is near the channel's voltage sensor. These data show that rHpTx2(GS) is a gating modifier toxin that binds to a site remote from the pore.

Activation properties of Kv4.3 channels: time, voltage and [K+]o dependence

Rapidly inactivating, voltage-dependent K(+) currents play important roles in both neurones and cardiac myocytes. Kv4 channels form the basis of these currents in many neurones and cardiac myocytes and their mechanism of inactivation appears to differ significantly from that reported for Shaker and Kv1.4 channels. In most channel gating models, inactivation is coupled to the kinetics of activation. Hence, there is a need for a rigorous model based on comprehensive experimental data on Kv4.3 channel activation. To develop a gating model of Kv4.3 channel activation, we studied the properties of Kv4.3 channels in Xenopus oocytes, without endogenous KChIP2 ancillary subunits, using the perforated cut-open oocyte voltage clamp and two-electrode voltage clamp techniques. We obtained high-frequency resolution measurements of the activation and deactivation properties of Kv4.3 channels. Activation was sigmoid and well described by a fourth power exponential function. The voltage dependence of the activation time constants was best described by a biexponential function corresponding to at least two different equivalent charges for activation. Deactivation kinetics are voltage dependent and monoexponential. In contrast to other voltage-sensitive K(+) channels such as HERG and Shaker, we found that elevated extracellular [K(+)] modulated the activation process by slowing deactivation and stabilizing the open state. Using these data we developed a model with five closed states and voltage-dependent transitions between the first four closed states coupled to a voltage-insensitive transition between the final closed (partially activated) state and the open state. Our model closely simulates steady-state and kinetic activation and deactivation data.

Kv1.4 channel block by quinidine: evidence for a drug-induced allosteric effect

We studied quinidine block of Kv1.4DeltaN, a K(+) channel lacking N-type inactivation, expressed in Xenopus ooctyes. Initially, quinidine intracellularly blocked the open channel so rapidly it overlapped with activation. This rapid open channel block was reduced (non-additively) by interventions that slow C-type inactivation: [K(+)](o) elevation and an extracellular lysine to tyrosine mutation (K532Y). These manipulations reduced the affinity of rapid open channel block ~10-fold, but left the effective electrical distance unchanged at ~0.15. Following rapid open channel block, there were time-dependent quinidine effects: the rate of inactivation during a single depolarisation was increased, and repetitive pulsing showed use dependence. The rate of recovery from the time-dependent aspect of quinidine block was similar to recovery from normal C-type inactivation. Manipulations that prevented the channel from entering the C-type inactivated state (i.e. high [K(+)](o) or the K532Y mutation) prevented the development of the time-dependent quinidine-induced inactivation. The concentration dependence of the rapid block and the time-dependent quinidine-induced inactivation were similar, but the time-dependent component was strongly voltage sensitive, with an effective electrical distance of 2. Clearly, this cannot reflect the permeation of quinidine through the electric field, but must be the result of some other voltage-sensitive change in the channel. We propose that quinidine promotes the entry of the channel into a C-type inactivated state in a time- and voltage-dependent manner. We developed a mathematical model based on these results to test the hypothesis that, following rapid open channel block, quinidine promotes development of the C-type inactivated state through a voltage-dependent conformational change.

Regulation of N- and C-type inactivation of Kv1.4 by pHo and K+: evidence for transmembrane communication

Kv1.4 encodes a slowly recovering transient outward current (I(to)), which inactivates by a fast N-type (intracellular ball and chain) mechanism but has slow recovery due to C-type inactivation. C-type inactivation of the NH(2)-terminal deletion mutant (fKv1.4DeltaN) was inhibited by 98 mM extracellular K(+) concentration ([K(+)](o)), whereas N-type was unaffected. In 98 mM [K(+)](o), removal of intracellular K(+) concentration ([K(+)](i)) speeded C-type inactivation but had no effect on N-type inactivation, suggesting that C-type inactivation is sensitive to K(+) binding to intracellular sites. C-type inactivation is thought to involve closure of the extracellular pore mouth. However, a valine to alanine mutation on the intracellular side of S6 (V561A) of fKv1.4DeltaN alters recovery and results in anomalous speeding of C-type inactivation with increasing [K(+)](o). Extracellular pH (pH(o)) modulated both N- and C-type inactivation through an S5-H5 linker histidine (H508) with acidosis speeding both N- and C-type inactivation. Mutation of an extracellular lysine to a tyrosine (K532Y) slowed C-type inactivation and inhibited the pH dependence of both N- and C-type inactivation. These results suggest that mutations, [K(+)], and pH modulate inactivation through membrane-spanning mechanisms involving S6.

Kinetic properties of Kv4.3 and their modulation by KChIP2b

KChIPs are a family of Kv4 K(+) channel ancillary subunits whose effects usually include slowing of inactivation, speeding of recovery from inactivation, and increasing channel surface expression. We compared the effects of the 270 amino acid KChIP2b on Kv4.3 and a Kv4.3 inner pore mutant [V(399, 401)I]. Kv4.3 showed fast inactivation with a bi-exponential time course in which the fast time constant predominated. KChIP2b expressed with wild-type Kv4.3 slowed the fast time constant of inactivation; however, the overall rate of inactivation was faster due to reduction of the contribution of the slow inactivation phase. Introduction of [V(399, 401)I] slowed both time constants of inactivation less than 2-fold. Inactivation was incomplete after 20s pulse durations. Co-expression of KChIP2b with Kv4.3 [V(399, 401)I] slowed inactivation dramatically. KChIP2b increased the rate of recovery from inactivation 7.6-fold in the wild-type channel and 5.7-fold in Kv4.3 [V(399,401)I]. These data suggest that inner pore structure is an important factor in the modulatory effects of KChIP2b on Kv4.3 K(+) channels.

Heterogeneous expression of KChIP2 isoforms in the ferret heart

Kv4 channels are believed to underlie the rapidly recovering cardiac transient outward current (I(to)) phenotype. However, heterologously expressed Kv4 channels fail to fully reconstitute the native current. Kv channel interacting proteins (KChIPs) have been shown to modulate Kv4 channel function. To determine the potential involvement of KChIPs in the rapidly recovering I(to), we cloned three KChIP2 isoforms (designated fKChIP2, 2a and 2b) from the ferret heart. Based upon immunoblot data suggesting the presence of a potential endogenous KChIP-like protein in HEK 293, CHO and COS cells but absence in Xenopus oocytes, we coexpressed Kv4.3 and the fKChIP2 isoforms in Xenopus oocytes. Functional analysis showed that while all fKChIP2 isoforms produced a fourfold acceleration of recovery kinetics compared to Kv4.3 expressed alone, only fKChIP2a produced large depolarizing shifts in the V(1/2) of steady-state activation and inactivation as seen for the native rapidly recovering I(to). Analysis of RNA and protein expression of the three fKChIP2 isoforms in ferret ventricles showed that fKChIP2b was most abundant and was expressed in a gradient paralleling the rapidly recovering I(to) distribution. Ferret KChIP2 and 2a were expressed at very low levels. The ventricular expression distribution suggests that fKChIP2 isoforms are involved in modulation of the rapidly recovering I(to); however, additional regulatory factors are also likely to be involved in generating the native current.

Distinct transient outward potassium current (Ito) phenotypes and distribution of fast-inactivating potassium channel alpha subunits in ferret left ventricular myocytes

The biophysical characteristics and alpha subunits underlying calcium-independent transient outward potassium current (Ito) phenotypes expressed in ferret left ventricular epicardial (LV epi) and endocardial (LV endo) myocytes were analyzed using patch clamp, fluorescent in situ hybridization (FISH), and immunofluorescent (IF) techniques. Two distinct Ito phenotypes were measured (21-22 degrees C) in the majority of LV epi and LV endo myocytes studied. The two Ito phenotypes displayed marked differences in peak current densities, activation thresholds, inactivation characteristics, and recovery kinetics. Ito,epi recovered rapidly [taurec, -70 mV = 51 +/- 3 ms] with minimal cumulative inactivation, while Ito,endo recovered slowly [taurec, -70 mV = 3,002 +/- 447 ms] with marked cumulative inactivation. Heteropoda toxin 2 (150 nM) blocked Ito,epi in a voltage-dependent manner, but had no effect on Ito,endo. Parallel FISH and IF measurements conducted on isolated LV epi and LV endo myocytes demonstrated that Kv1.4, Kv4.2, and Kv4.3 alpha subunit expression in LV myocyte types was quite heterogenous: (a) Kv4.2 and Kv4.3 were more predominantly expressed in LV epi than LV endo myocytes, and (b) Kv1.4 was expressed in the majority of LV endo myocytes but was essentially absent in LV epi myocytes. In combination with previous measurements on recovery kinetics (Kv1.4, slow; Kv4.2/4.3, relatively rapid) and Heteropoda toxin block (Kv1.4, insensitive; Kv4.2, sensitive), our results strongly support the hypothesis that, in ferret heart, Kv4.2/Kv4.3 and Kv1.4 alpha subunits, respectively, are the molecular substrates underlying the Ito,epi and Ito,endo phenotypes. FISH and IF measurements were also conducted on ferret ventricular tissue sections. The three Ito alpha subunits again showed distinct patterns of distribution: (a) Kv1.4 was localized primarily to the apical portion of the LV septum, LV endocardium, and approximate inner 75% of the LV free wall; (b) Kv4. 2 was localized primarily to the right ventricular free wall, epicardial layers of the LV, and base of the heart; and (c) Kv4.3 was localized primarily to epicardial layers of the LV apex and diffusely distributed in the LV free wall and septum. Therefore, in intact ventricular tissue, a heterogeneous distribution of candidate Ito alpha subunits not only exists from LV epicardium to endocardium but also from apex to base.

Autogenous mandibular bone grafts in the treatment of the resorbed maxillary anterior alveolar ridge: rationale and approach

Patients can present with a variety of conditions that limit, or prevent, the ideal placement of endosteal implants. One of these situations is depicted by an insufficient amount of bone for implant placement in the maxillary anterior alveolar ridge. Autogenous bone grafting can be used to augment the ridge to provide a sufficient amount of bone for implant placement. Using the mandibular symphysis as a donor site offers ease of access, good bone quality for localized repair, a corticocancellous block graft morphology, low morbidity, and minimal graft resorption. A procedure for harvesting and placing an autogenous mandibular bone graft is presented; as are the advantages of using this location as a donor site compared with other locations.

Lateralization of the inferior alveolar nerve with simultaneous implant placement: surgical techniques

In the event of moderate to severe mandibular bone resorption posterior to the mental foramen, repositioning of the inferior alveolar nerve provides a greater amount of available bone for implant placement and reduces the risk of nerve injury. While neural paresthesia may initially occur, this altered sensation generally resolves spontaneously. Alveolar nerve repositioning may be possible in cases in which other procedures cannot be performed due to the extent of atrophy of the posterior mandibular alveolar crest. This article presents the surgical technique to achieve this objective.

Inactivation of voltage-gated cardiac K+ channels

Inactivation is the process by which an open channel enters a stable nonconducting conformation after a depolarizing change in membrane potential. Inactivation is a widespread property of many different types of voltage-gated ion channels. Recent advances in the molecular biology of K+ channels have elucidated two mechanistically distinct types of inactivation, N-type and C-type. N-type inactivation involves occlusion of the intracellular mouth of the pore through binding of a short segment of residues at the extreme N-terminal. In contrast to this "tethered ball" mechanism of N-type inactivation, C-type inactivation involves movement of conserved core domain residues that result in closure of the external mouth of the pore. Although C-type inactivation can show rapid kinetics that approach those observed for N-type inactivation, it is often thought of as a slowly developing and slowly recovering process. Current models of C-type inactivation also suggest that this process involves a relatively localized change in conformation of residues near the external mouth of the permeation pathway. The rate of C-type inactivation and recovery can be strongly influenced by other factors, such as N-type inactivation, drug binding, and changes in [K+]o. These interactions make C-type inactivation an important biophysical process in determining such physiologically important properties as refractoriness and drug binding. C-type inactivation is currently viewed as arising from small-scale rearrangements at the external mouth of the pore. This review will examine the multiplicity of interactions of C-type inactivation with N-terminal-mediated inactivation and drug binding that suggest that our current view of C-type inactivation is incomplete. This review will suggest that C-type inactivation must involve larger-scale movements of transmembrane-spanning domains and that such movements contribute to the diversity of kinetic properties observed for C-type inactivation.

The use of implants in the reconstruction of oral cancer patients

The spectrum of osseointegrated implant applications in cancer patients is quite varied today. In the future, it will be even wider and more varied, limited only by the ingenuity of practitioners, skills of reconstructive surgeons, biology of the reconstruction, and cost restraint efforts. Today's practitioners are challenged to preserve and enhance the benefits of osseointegrated implants in some of the patients who need them most, the cancer patient. It is done by improving one's own skills by research and by education.

Modulation of HERG affinity for E-4031 by [K+]o and C-type inactivation

Rectification of HERG is due to a rapid inactivation process that has been labeled C-type inactivation and is believed to be due to closure of the external mouth of the pore. We examined the effects of mutation of extracellular residues that remove C-type inactivation on binding of the intracellularly acting methanesulfonanilide drug E-4031. Removal of inactivation through mutation reduced drug affinity by more than an order of magnitude. Elevation of [K+]o in the wild-type channel reduces channel affinity for E-4031. Elevation of [K+]o also interferes with the extracellular pore mouth closure associated with C-type inactivation through a 'foot in the door' mechanism. We examined the possibility that [K+]o elevation reduces drug binding through inhibition of C-type inactivation by comparing drug block in the wild-type and inactivation-removed mutant channels. Elevation of [K+]o decreased affinity in both channel constructs by a roughly equal amount. These results suggest that [K+]o alters drug binding affinity independently of its effects on C-type inactivation. They further suggest that inhibition of pore mouth closure by elevated [K+]o does not have same effect on drug affinity as mutations removing C-type inactivation.

The beta subunit, Kv beta 1.2, acts as a rapid open channel blocker of NH2-terminal deleted Kv1.4 alpha-subunits

A recently discovered class of ancillary subunits has been shown to modify the inactivation properties of alpha-subunits belonging to the Kv1 family of potassium channels. One of these subunits, Kv beta 1.2, modifies intrinsic alpha-subunit C-type inactivation. N-type inactivation and open channel block have been proposed to increase the rate of development of C-type inactivation. We demonstrate here that Kv beta 1.2 has kinetic properties which are consistent with rapid open channel block.

A quantitative analysis of the activation and inactivation kinetics of HERG expressed in Xenopus oocytes

1. The human ether à-go-go-related gene (HERG) encodes a K+ channel that is believed to be the basis of the delayed rectified current, IKr, in cardiac muscle. We studied HERG expressed in Xenopus oocytes using a two-electrode and cut-open oocyte clamp technique with [K+]0 of 2 and 98 mM. 2. The time course of activation of the channel was measured using an envelope of tails protocol and demonstrated that activation of the heterologously expressed HERG current (IHERG) was sigmoidal in onset. At least three closed states were required to reproduce the sigmoid time course. 3. The voltage dependence of the activation process and its saturation at positive voltages suggested the existence of at least one relatively voltage-insensitive step. A three closed state activation model with a single voltage-insensitive intermediate closed state was able to reproduce the time and voltage dependence of activation, deactivation and steady-state activation. Activation was insensitive to changes in [K+]0. 4. Both inactivation and recovery time constants increased with a change of [K+]0 from 2 to 98 mM. Steady-state inactivation shifted by approximately 30 mV in the depolarized direction with a change from 2 to 98 mM K+0. 5. Simulations showed that modulation of inactivation is a minimal component of the increase of this current by [K+]0, and that a large increase in total conductance must also occur.

Regional localization of ERG, the channel protein responsible for the rapid component of the delayed rectifier, K+ current in the ferret heart

Repolarization of the cardiac action potential varies widely throughout the heart. This could be due to the differential distribution of ion channels responsible for repolarization, especially the K+ channels. We have therefore studied the cardiac localization of ERG, a channel protein known to play an important role in generation of the rapid component of the delayed rectifier K+ current (IKr), an important determinant of the repolarization waveform, Cryosections of the ferret atrium and ventricle were prepared to determine the localization of ERG by fluorescence in situ hybridization (FISH) and immunofluorescence. We found that in the ferret, ERG transcript and protein expression was most abundant in the epicardial cell layers throughout most of the ventricle, except at the base. In the atrium, we found that ERG is most abundant in the medial right atrium, especially in the trabeculae and the crista terminalis of the right atrial appendage. It also is present in areas within the sinoatrial node. In all regions studied, FISH and immunofluorescence showed concordant localization patterns. These data suggest that repolarization mediated by IKr is not uniform throughout the ferret heart and provide a molecular explanation for heterogeneity in action potential repolarization throughout the mammalian heart.

Heterogeneity in K+ channel transcript expression detected in isolated ferret cardiac myocytes

The molecular basis of the potassium ion (K+) channels that generate repolarization in heart tissue remains uncertain, in part because of the molecular diversity of the voltage-gated K+ channel family. In our investigation, we used fluorescent labeled oligonucleotide probes to perform in situ hybridization studies on enzymatically isolated myocytes to determine the identity, regional distribution, and cellular distribution of voltage-gated K+ channel, alpha-subunit mRNA expressed in ferret heart. The regions studied were from the sinoatrial node (SA), right and left atrium, right and left ventricle, and interatrial and interventricular septa. Kv1.5 and Kv1.4 were the most widely distributed K+ channel transcripts in the ferret heart (present in approximately 70%-86% and approximately 46%-95% of tested myocytes, respectively), followed by Kv1.2, Kv2.1, and Kv4.2. In addition, many myocytes contain transcripts for Kv1.3, Kv2.2, Kv4.1, Kv5.1, and members of the Kv3 family. Kv1.1, Kv1.6, and Kv6.1 were rarely expressed in working myocytes, but were more commonly expressed in SA nodal cells. Two other transcripts whose genes have been implicated in the long QT syndrome, erg and KvLQT1, were common in all regions (approximately 41%-58% and 52%-72%, respectively). These results show that both the diversity and heterogeneity of K+ channel mRNA in heart tissue is greater than previously suspected.

Immunomodulatory assays to study structure-activity relationships of thalidomide

Thalidomide, which has a long history of tragedy because of its ability to cause severe birth defects, is very effective in alleviating erythema nodosum leprosum in leprosy patients and aphthous ulcers in AIDS patients. The causes of these inflammatory diseases and the mechanism by which thalidomide diminishes them are unknown. It has been suggested that modulation of the immune response plays an important role. We found that thalidomide exerts immunomodulatory activity in three bioassays. It suppresses an IgM plaque forming cell response in mice injected with sheep erythrocytes: it inhibits TNF-alpha production by LPS stimulated human mononuclear cells: and it enhances IL-2 production by Con-A stimulated human mononuclear cells. We employed these bioassays to compare the activity of 15 analogs of thalidomide with thalidomide itself. Eight of the compounds were derivatives of the glutarimide moiety of thalidomide and the others were phthalimide or derivatives of the phthalimide moiety of thalidomide. N-hydroxyphthalimide, a simple derivative of phthalimide, was more effective than thalidomide and was also the most effective of the compounds assayed in suppressing the IgM plaque and TNF-alpha responses, but it did not enhance the IL-2 response, instead, it significantly suppressed it.

The N-terminal domain of a K+ channel beta subunit increases the rate of C-type inactivation from the cytoplasmic side of the channel

Voltage-gated K+ channels are complexes of membrane-bound, ion-conducting alpha and cytoplasmic ancillary (beta) subunits. The primary physiologic effect of coexpression of alpha and beta subunits is to increase the intrinsic rate of inactivation of the alpha subunit. For one beta subunit, Kv beta 1.1, inactivation is enhanced through an N-type mechanism. A second beta subunit, Kv beta 1.2, has been shown to increase inactivation, but through a distinct mechanism. Here we show that the degree of enhancement of Kv beta 1.2 inactivation is dependent on the amino acid composition in the pore mouth of the alpha subunit and the concentration of extracellular K+. Experimental conditions that promote C-type inactivation also enhance the stimulation of inactivation by Kv beta 1.2, showing that this beta subunit directly stimulates C-type inactivation. Chimeric constructs containing just the nonconserved N-terminal region of Kv beta 1.2 fused with an alpha subunit behave in a similar fashion to coexpressed Kv beta 1.2 and alpha subunit. This shows that it is the N-terminal domain of Kv beta 1.2 that mediates the increase in C-type inactivation from the cytoplasmic side of the pore. We propose a model whereby the N terminus of Kv beta 1.2 acts as a weakly binding "ball" domain that associates with the intracellular vestibule of the alpha subunit to effect a conformational change leading to enhancement of C-type inactivation.

Time, voltage and ionic concentration dependence of rectification of h-erg expressed in Xenopus oocytes

The rapid delayed rectifier, IKr, is believed to have h-erg (human ether-à-go-go related gene) as its molecular basis. A recent study has shown that rectification of h-erg involves a rapid inactivation process that involves rapid closure of the external mouth of the pore or C-type inactivation. We measured the instantaneous current to voltage relationship for h-erg channels using the saponin permeabilized variation of the cut-open oocyte clamp technique. In contrast to C-type inactivation in other voltage-gated K+ channels, the rate of inactivation was strongly voltage dependent at depolarized potentials. This voltage dependence could be modulated independently of activation by increasing [K+]0 from 2 to 98 mM. These results suggest that inactivation of h-erg has its own intrinsic voltage sensor.

In situ hybridization reveals extensive diversity of K+ channel mRNA in isolated ferret cardiac myocytes

The molecular basis of K+ currents that generate repolarization in the heart is uncertain. In part, this reflects the similar functional properties different K+ channel clones display when heterologously expressed, in addition to the molecular diversity of the voltage-gated K+ channel family. To determine the identity, regional distribution, and cellular distribution of voltage-sensitive K+ channel mRNA subunits expressed in ferret heart, we used fluorescent labeled oligonucleotide probes to perform in situ hybridization studies on enzymatically isolated myocytes from the sinoatrial (SA) node, right and left atria, right and left ventricles, and interatrial and interventricular septa. The most widely distributed K+ channel transcripts in the ferret heart were Kv1.5 (present in 69.3% to 85.6% of myocytes tested, depending on the anatomic region from which myocytes were isolated) and Kv1.4 (46.1% to 93.7%), followed by kv1.2, Kv2.1, and Kv4.2. Surprisingly, many myocytes contain transcripts for Kv1.3, Kv2.2, Kv4.1, Kv5.1, and members of the Kv3 family. Kv1.1, Kv1.6, and Kv6.1, which were rarely expressed in working myocytes, were more commonly expressed in SA nodal cells. IRK was expressed in ventricular (84.3% to 92.8%) and atrial (52.4% to 64.0%) cells but was nearly absent (6.6%) in SA nodal cells; minK was most frequently expressed in SA nodal cells (33.7%) as opposed to working myocytes (10.3% to 29.3%). Two gene products implicated in long-QT syndrome, ERG and KvLQT1, were common in all anatomic regions (41.1% to 58.2% and 52.1% to 71.8%, respectively). These results show that the diversity of K+ channel mRNA in heart is greater than previously suspected and that the molecular basis of K+ channels may vary from cell to cell within distinct regions of the heart and also between major anatomic regions.

C-type inactivation controls recovery in a fast inactivating cardiac K+ channel (Kv1.4) expressed in Xenopus oocytes

1. A fast inactivating transient K+ current (FK1) cloned from ferret ventricle and expressed in Xenopus oocytes was studied using the two-electrode voltage clamp technique. Removal of the NH2-terminal domain of FK1 (FK1 delta 2-146) removed fast inactivation consistent with previous findings in Kv1.4 channels. The NH2-terminal deletion mutation revealed a slow inactivation process, which matches the criteria for C-type inactivation described for Shaker B channels. 2. Inactivation of FK1 delta 2-146 at depolarized potentials was well described by a single exponential process with a voltage-insensitive time constant. In the range -90 to +20 mV, steady-state C-type inactivation was well described by a Boltzmann relationship that compares closely with inactivation measured in the presence of the NH2-terminus. These results suggest that C-type inactivation is coupled to activation. 3. The coupling of C-type inactivation to activation was assessed by mutation of the fourth positively charged residue (arginine 454) in the S4 voltage sensor to glutamine (R454Q). This mutation produced a hyperpolarizing shift in the inactivation relationship of both FK1 and FK1 delta 2-146 without altering the rate of inactivation of either clone. 4. The rates of recovery from inactivation are nearly identical in FK1 and FK1 delta 2-146. 5. To assess the mechanisms underlying recovery from inactivation the effects of elevated [K+]o and selective mutations in the extracellular pore and the S4 voltage sensor were compared in FK1 and FK1 delta 2-146. The similarity in recovery rates in response to these perturbations suggests that recovery from C-type inactivation governs the overall rate of recovery of inactivated channels for both FK1 and FK1 delta 2-146. 6. Analysis of the rate of recovery of FK1 channels for inactivating pulses of different durations (70-2000 ms) indicates that recovery rate is insensitive to the duration of the inactivating pulse.

Time- and voltage-dependent modulation of a Kv1.4 channel by a beta-subunit (Kv beta 3) cloned from ferret ventricle

In mammals, voltage-gated K+ channels can be made of complexes containing alpha-subunits similar to the Shaker K+ channel and smaller cytoplasmic beta-subunits. Recent studies have suggested that these ancillary beta-subunits can modulate K+ channel gating properties. We studied the effects of a K+ channel beta-subunit, Kv beta 3, coexpressed with a Kv1.4 alpha-subunit, FK1, on the time and voltage dependence of channel activation, inactivation, recovery from inactivation, and deactivation, using an oocyte expression system. Kv beta 3 was found to accelerate both the fast and the slow component of Kv1.4 inactivation. Kv beta 3 also altered the relative contributions of the two components of inactivation by increasing the contribution of the slow component to the inactivation process. Kv beta 3 slowed recovery from inactivation for Kv1.4, but not for a Kv1.4 deletion mutant lacking N-type inactivation. Finally, steady-state activation and the time course of Kv1.4 current activation were not strongly influenced by Kv beta 3; however, deactivation was slowed in the presence of Kv beta 3. This study suggests that Kv beta 3 alters channel states which follow activation.