Effect of fluoxetine on a neuronal, voltage-dependent potassium channel (Kv1.1)

1. Fluoxetine (Prozac) is widely used as an antidepressant drug and is assumed to be a selective 5-hydroxytryptamine (5-HT) reuptake inhibitor (SSRI). Claims that its beneficial psychotropic effects extend beyond those in treatment of depression have drawn clinical and popular attention to this compound, raising the question of whether there is anything exceptional about the supposed selective actions. 2. We have used the voltage clamp technique to study the effect of fluoxetine on a neuronal, voltage-dependent potassium (K+) channel (RCK1; Kv1.1), expressed in p6nopus laevis oocytes. This channel subunit is abundantly expressed in the central nervous system and K+ channels containing this subunit are involved in the repolarization process of many types of neurones. 3. Blockade of the K+ currents by fluoxetine was found to be use- and dose-dependent. Wash-out of this compound could not be achieved. Fluoxetine did not affect the ion selectivity of this K+ channel, as the reversal potential was unaltered. 4. Slowing of both activation and deactivation kinetics of the channel by fluoxetine was observed, including tail current crossover upon repolarization. 5. Hodgkin-Huxley type of models and more generalized Markov chain models were used to fit the kinetics of the data. Based upon a Markov kinetic scheme, our data can be interpreted to mean that blockade of fluoxetine consists of two components: a voltage-independent occurring in the last closed, but available state of the channel, and a voltage-dependent occurring in the open state. 6. This study describes the first biophysical working model for the mechanism of action of fluoxetine on a neuronal, voltage-dependent K+ channel, RCK1. Although this channel is not very potently blocked by fluoxetine when expressed in oocytes, this study may help us to understand some of the clinical symptoms seen with elevated serum concentrations of this SSRI.

Development and optimization of a purification strategy for the venom of the scorpion Parabuthus transvaalicus

In search for the toxic components constituting the venom of the southern African scorpion Parabuthus transvaalicus we compared and optimized several chromatographic separation protocols resulting in an efficient venom purification strategy. We found that the sequential combination of gelfiltration (Superdex) together with reversed phase chromatography (C2/C18)-but not cation exchange (Mono S)-stood surety for the best purification.

Alternative splicing of ClC-6 (a member of the CIC chloride-channel family) transcripts generates three truncated isoforms one of which, ClC-6c, is kidney-specific

ClC-6 is a protein that structurally belongs to the family of ClC-type chloride channels. We now report the identification of three additional ClC-6 isoforms that are truncated because of alternative splicing. We have isolated, from human K562 cells, four types of ClC-6 cDNAs that encode four distinct ClC-6 protein isoforms. ClC-6a (869 amino acids) corresponds to the previously published ClC-6 protein [Brandt and Jentsch (1995) FEBS Lett. 377, 15-20] and it has a canonical ClC structure. However, ClC-6b (320 amino acids), ClC-6c (353 amino acids) and ClC-6d (308 amino acids) are truncated at their C-termini. Hydropathy-plot analysis indicates that the shortened isoforms contain maximally four (ClC-6b and -6d) or seven (ClC-6c) transmembrane domains. Sequence analysis of a human genomic ClC-6 fragment indicates that the cDNA variability arises from alternative splicing at two different positions: the first alternative site consists of an intron flanked by two alternative donor sites and two alternative acceptor sites, the second being due to an exon that is optionally included or excluded. Reverse-transcription-PCR analysis of ClC-6 expression in human cell lines and tissues shows that the majority (83%) of ClC-6 mRNAs consists of ClC-6a or ClC-6c messengers. Furthermore, in a mouse tissue panel, the ClC-6a mRNA has a relatively broad tissue expression pattern, since it could be detected in brain, kidney, testis, skeletal muscle, thymus and pancreas. In contrast, expression of ClC-6c is more restricted, since it was only detected in kidney.

Detection of 2-amino-5-chloropyridine in urine as a parameter of zopiclone (Imovane) intake using HPLC with diode array detection

A qualitative screening technique was developed for the detection of 2-amino-5-chloropyridine, a newly identified decomposition product of zopiclone and metabolites after alkaline hydrolysis of urine samples. The method was elaborated using a standard operation procedure (Merck Tox Screening System), combining a solid-phase extraction with reversed-phase high-performance liquid chromatography and diode array detection. The limit of detection, expressed as zopiclone concentrations in spiked urine, was found to be 0.5 microg/mL. Field urine samples from a volunteer were positive for 2-amino-5-chloropyridine until 16 h after ingestion of one therapeutic dose of Imovane (7.5 mg zopiclone).

Effect of lanthanum on voltage-dependent gating of a cloned mammalian neuronal potassium channel

The effect of the trivalent cation lanthanum (La3+) on voltage-dependent gating of a cloned mammalian neuronal Kv1.1 potassium channel was studied under whole-cell voltage-clamp conditions in oocytes of Xenopus laevis. La3+ (100 microM) was found to decrease the potassium currents at all test potentials and to shift the midpoint of the fraction open channels/membrane voltage curve by approximately +20 mV. The opening and closing time constants of Kv1.1 channels were empirically fitted with a 4th power Hodgkin-Huxley formalism, or with mono- and multi-exponentials. It was found that La3+ slowed down the kinetics of activation, speeded up those of deactivation, and shifted the opening kinetics by approximately + 60 mV. Interestingly, all these parameters of channel gating were not affected equally by La3+. Furthermore, amplitudes of the inward tail currents evoked at potentials more negative than the potassium equilibrium potential (E(K+)) were more strongly inhibited by La3+ than those of the outward tail currents evoked at potentials more positive than E(K+). This suggests voltage-dependent block and binding of La3+ to the Kv1.1 channel protein. We conclude that these actions cannot be explained in terms of surface charge considerations alone. Our results provide evidence for a direct interaction with the potassium channel protein, shedding new light on the mechanism of action of this lanthanide.

Expression of human pICln and ClC-6 in Xenopus oocytes induces an identical endogenous chloride conductance

pICln is a protein that induces an outwardly rectifying, nucleotide-sensitive chloride current (ICln) when expressed in Xenopus oocytes, but its precise function (plasma-membrane anion channel versus cytosolic regulator of a channel) remains controversial. We now report that a chloride current identical to ICln is induced when Xenopus oocytes are injected with human ClC-6 RNA. Indeed, both the pICln and the ClC-6 induced current are outwardly rectifying, they inactivate slowly at positive potentials and have an anion permeability sequence NO3- > I- > Br- > Cl- > gluconate. Cyclamate, NPPB, and extracellular cAMP block the induced currents. The success rate of current expression is significantly increased when the injected Xenopus oocytes are incubated at a higher temperature (24 or 37 degrees C) prior to the analysis. In addition, the ICln current was detected in 6.2% of noninjected control Xenopus oocytes. We therefore conclude that the ICln current in Xenopus oocytes corresponds to an endogenous conductance that can be activated by expression of structurally unrelated proteins. Furthermore, functional, biochemical, and morphological observations did not support the notion that pICln resides in the plasma membrane either permanently or transiently after cell swelling. Thus, it is unlikely that pICln forms the channel that is responsible for the ICln current in Xenopus oocytes.

Value of axillary ultrasonography and sonographically guided puncture of axillary nodes: a prospective study in 144 consecutive patients

The value of axillary sonography in 144 consecutive patients was prospectively studied. Abnormal lymph nodes were demonstrated in 72 axillae, only half (36 of 72) of which were clinically detected. In comparison with intraoperative findings in 47 patients with breast carcinoma, the sensitivity of sonography in detecting malignant nodes was 92%, the specificity 95%, and the positive and negative predictive value 96% and 91%, respectively. In 42 patients a sonographically guided node puncture was performed. Evidence of malignancy was found in 38 (90.5%). In 14 (33%) of them, cyto-histology pointed to a clinically undetected primary malignancy.

Detection and quantification of the hypnotic zopiclone, connected with an uncommon case of drowning

A fatality, attributed to a suicidal ingestion of zopiclone and subsequent death by drowning, is described. A radioimmunoassay was performed to prescreen for the presence of the analyte. HPLC analysis with fluorescence detection demonstrated a testicular tissue concentration of 2.2 micrograms/g zopiclone. Further toxicological analysis revealed also the presence of diazepam in a concentration of 150 ng/g.

Development of a stereospecific radioimmunoassay for the analysis of zopiclone and metabolites in urine

A sensitive and specific radioimmunoassay has been developed, allowing the stereospecific detection of nanogram amounts of (+)- and (-)-enantiomers of zopiclone and its major metabolites in urine, without prior extraction or purification. Antisera were obtained from two series of four rabbits, immunized with optically pure (+)- and (-)-N-hemisuc-cinyldesmethylzopiclone, conjugated to bovine serum albumin according to the active ester method. The assay was stereospecific, allowing discrimination between the two enantiomers of N-desmethylzopiclone with mutual cross-reactivities below 2%. Substantial cross-reaction was observed with the parent compound, although lower than expected, and to a lesser extent with the N-oxide metabolite. A selection of hypnotics, anxiolytics, antidepressants and some other widely used drugs did not interfere with the assay (< 0.1%), when tested at a concentration level of 10 micrograms/ml. The sensitivity of the assay was 50 pg/ml and 10 pg/ml for the (+)- and (-)-enantiomers, respectively. The binding assay described here was used to evaluate the stereoselective excretion pattern of zopiclone. Analysis of cumulative excretion samples from a volunteer revealed a mean metabolic excretion ratio (+)/(-) of 2.2, ranging from 1.7 (7 h) to 4.4 (36 h) A mean excretion ratio (+)/(-) of 2.5 +/- 1 was calculated after analysis of urine samples from 20 patients receiving zopiclone as a hypnotic daily.

Solvent-free sample preparation by headspace solid-phase microextraction applied to the tracing of n-butyl nitrite abuse

The most common alkyl nitrites encountered in forensic toxicology are iso-butyl, n-butyl and iso-pentyl(amyl) nitrites. All have become popular as an aphrodisiac, especially among the homosexual population. Alkyl nitrites are a volatile and unstable group of compounds, which hydrolyse in aqueous matrices to the alcohol and nitrite ion. Here we describe a fast, clean and sensitive procedure for the detection of hydrolysed n-butyl nitrite in whole human blood using a new, solvent-free sampling technique, the headspace solid-phase micro-extraction (HSPME), combined with GC/FID analysis. Sample preparation was investigated using two different stationary phases (100 microns polydimethylsiloxane and 85 microns polyacrylate), coating a fused silica fibre. The effect of different sampling times at fixed temperatures was also studied. Our results demonstrate that the HSPME/GC/FID procedure allows tracing of n-butyl nitrite abuse and detects hydrolysed n-butyl nitrite, i.e., released n-butanol, in whole blood at the 1 ng/mL level.

The chloride current induced by expression of the protein pICln in Xenopus oocytes differs from the endogenous volume-sensitive chloride current

1. Phenotypical similarities between ICl,swell, the cell-swelling-induced chloride current and ICln, the nucleotide-sensitive chloride current induced by expression of mammalian pICln in Xenopus oocytes, have led to models which identify pICln either as the volume-sensitive chloride channel or as a cytosolic regulator thereof. 2. To investigate critically the relationship between ICl,swell and pICln two-microelectrode voltage clamp experiments were performed on Xenopus oocytes in which either human pICln was expressed or endogenous ICl,swell was activated. 3. Several criteria that clearly differentiated ICln from ICl,swell were detected. Outward rectification and the discrimination between NO3- and Cl- were more pronounced for ICln. Cyclamate blocked ICln but not ICl,swell. In contrast to ICl,swell, inactivation kinetics of ICln were pH independent and extracellular cAMP blocked only the outward ICln component. Finally, ICln was readily expressed in collagenase-defolliculated oocytes and was not modulated by extracellular hypotonicity, whereas ICl,swell could only be triggered in follicle-enclosed or manually defolliculated oocytes. 4. We therefore conclude that ICln and ICl,swell are two different chloride currents. Consequently, any model which invokes a crucial role for pICln in ICl,swell should be critically reviewed.

Do voltage-gated Kv1.1 and inward rectifier Kir2.1 potassium channels form heteromultimers?

Possible heteromultimer formation between Kv- and Kir-type K+ channels was investigated, in connection with the known functional diversity of K+ channels in vivo. Voltage-clamp experiments were performed on Xenopus oocytes, either injected with concatenated Kir2.1-Kv1.1 mRNA, or co-injected with Kv1.1 and Kir2.1 mRNA. K+ currents could be approximated by the algebraic sum of the 2 K+ current types alone. The tandem construct did not show functional expression, although it could be detected by Western blotting. We conclude that Kv1.1 and Kir2.1 alpha-subunit proteins fail to assemble and do not contribute functional diversity to K+ channels.

Fast protein liquid chromatography for the purification of animal venoms

In this paper a methodology is presented for the purification of toxins acting on neuronal potassium channels. These neurotoxins are ideal tools for studying the physiological functioning of ion channels, for classifying them and for exploring them in several neuronal regions. Separation procedures leading to the identification of neurotoxins almost always include gel filtration chromatography as well as other chromatographic techniques such as ion exchange and reversed phase chromatography. The use of a Superdex 30 prep grade gel filtration column, which has the advantage of superior resolution in comparison with traditionally used gel filtration column, which has the advantage of superior resolution in comparison with traditionally used gel filtration columns such as a Sephadex G-50 column, is reported here. The biological assay incorporates the use of Xenopus laevis oocytes, which express potassium channels. Screening of the collected venom fractions was performed by means of the whole-cell voltage clamp technique. The combination of these techniques represents a fast and efficient identification procedure in the search for new and selective neurotoxins for cloned channels and receptors.

Mechanism of L- and T-type Ca2+ channel blockade by flunarizine in ventricular myocytes of the guinea-pig

Flunarizine is a substance known to block voltage-dependent Ca2+ channels in smooth muscle and neuronal cells. Reports on the effect on voltage-dependent cardiac Ca2+ channels are however sparse. Therefore, the mechanism of action of flunarizine on two types of voltage-dependent cardiac Ca2+ channels, the L- and T-type, in single ventricular myocytes of the guinea-pig was investigated using the whole-cell voltage clamp technique. Both channel types can be blocked by flunarizine in a time-, frequency-, voltage-, Ca(2+)-, and proton-dependent way. While the overall mechanism of action on cardiac myocytes is similar to the one reported for other cell types, we found that cardiomyocytes are less susceptible to block (Kd 3.3-11 mM). We also describe a complete analysis of the different components of block, together with evidence for open channel state block and drug-induced changes in channel gating. These findings provide new insights into the mechanism of action of flunarizine on voltage-dependent Ca2+ channels.

An improved fractionation and fast screening method for the identification of new and selective neurotoxins

Neurotoxins have highly specific actions on molecular targets, and thus offer an effective means of characterizing the growing number of identified ion channels and receptors in the nervous system. Separation procedures leading to the identification of neurotoxins almost always include gel filtration chromatography, combined with ion-exchange and/or reversed phase chromatography. We present here an improved fractionation method based on the use of a new Superdex 30 prep grade HiLoad 16/60 FPLC gel filtration column. This single-step gel filtration protocol results in a shortening of the purification process and allows a superior qualitative separation of (neuro-)peptides in crude venoms as compared to any other type of gel filtration column used thus far. Screening of the collected fractions for potential ion channel blocking properties was performed by means of the whole-cell voltage clamp technique. To increase both the amount and speed of expression in Xenopus laevis oocytes of cloned ion channels, we employed a high-expression vector, pGEMHE, wherein the cDNA encoding a neuronal voltage-dependent potassium channel (RCK1) was subcloned. The combination of these techniques represents a fast and efficient identification procedure in the quest for new and selective neurotoxins for cloned channels and receptors.

The alpha-dendrotoxin footprint on a mammalian potassium channel

alpha-Dendrotoxin, a 59-amino acid basic peptide from the venom of Dendroaspis angusticeps (green mamba snake), potently blocks some but not all voltage-dependent potassium channels. Here we have investigated the relative contribution of the individual alpha-subunits constituting functional Kv1.1 potassium channels to alpha-dentroxin binding. Three residues critical for alpha-dentrotoxin binding and located in the loop between domains S5 and S6 were mutated (A352P, E353S, and Y379H), and multimeric cDNAs were constructed encoding homo- and heterotetrameric channels composed of all possible ratios of wild-type and mutant alpha-subunits. Complete mutant channels were about 200-fold less sensitive for the alpha-dendrotoxin block than complete wild-type channels, which is attributable to a smaller association rate. Analysis of the bimolecular reaction between alpha-dendrotoxin and the different homo- and heteromeric channel constructs revealed that the association rate depends on the number of wild-type alpha-subunits in the functional channel. Furthermore, we observed a linear relationship between the number of wild-type alpha-subunits in functional channels and the free energy for alpha-dendrotoxin binding, providing evidence that all four alpha-subunits must interact with alpha-dendrotoxin to produce a high affinity binding site.

Transmembrane segments critical for potassium channel function

The relative contribution of the transmembrane segments in the alpha-subunit of Shaker-type potassium channels was investigated in relation to potassium channel function. Starting from a wild-type Kv1.1 channel, four different deletion mutants were made, missing respectively transmembrane segments S1 and S2, S2 and S3, S1 to S3, and S1 to S4. To ensure the assembly of the different subunits, the hydrophylic N-terminal domain was always conserved. The lack of transmembrane segments S1 to S4 converts a depolarization-activated WT Kv1.1 channel with outward rectification into a hyperpolarization-activated channel with inward rectification. In contrast, mutant channels missing transmembrane segments S1 and S2, S2 and S3, or S1 to S3 did not reveal functional expression.

Reversal of rectification and alteration of selectivity and pharmacology in a mammalian Kv1.1 potassium channel by deletion of domains S1 to S4

1. A possible relation between the family of inwardly rectifying K+ channels and the Shaker superfamily of K+ channels was investigated using a deletion mutant (DelS1-S4) of a delayed rectifier Kv1.1 (RCK1) K+ channel. 2. The mutant DelS1-S4 was made by eliminating the sequence coding for transmembrane domains S1 to S4 of the Kv1.1 K+ channel, and re-ligating the sequence coding for the cytoplasmic amino terminus to transmembrane domain S5. Microelectrode voltage-clamp and patch-clamp experiments were performed on Xenopus laevis oocytes after injection of in vitro transcribed mRNA coding for mutant and wild-type channels. 3. The lack of transmembrane domains S1 to S4 converts a depolarization-activated wild-type Kv1.1 K+ channel with outward rectification into a hyperpolarization-activated channel with inward rectification. Although the pore region of the deletion mutant is identical to the wild-type channel, the mutant channel is a non-selective cation channel and is characterized by an altered pharmacology profile.

Mutations in the P-region of a mammalian potassium channel (RCK1): a comparison with the Shaker potassium channel

Deletion mutants of Shaker K channels lacking the middlemost 2 residues of the amino acid sequence GYGD in their P-region lose K-selectivity and become functionally similar to voltage-activated Ca channels. Ca channel characteristics are also conferred on voltage-activated Na channels when residues K in domain III and/or A in domain IV in the P-region of the Na channel are replaced by E residues. In this study, we have investigated a possible evolutionary connection between voltage-activated K and Na channels. Mutant monomeric and multi-heteromeric RCK1 K channel cDNAs were made to match the residues at equivalent positions in the P-region of the Na channel. We found that in contrast to mutant Shaker K channels, the same mutants in RCK1 K channels did not yield functional expression. Therefore possible Na channel-like ion conduction properties conferred on K channels could not be demonstrated. However, our results show that the same mutations in highly homologous channels can produce different effects and point to hitherto unknown structural differences in the P-region of these homologous K channels. Therefore we conclude that extrapolation of the structural and functional importance of residues should be done with caution, even when ion channels belong to the same family.

How to isolate cardiac myocytes

A complete technique is described for the isolation of myocytes from mammalian hearts using the Langendorff perfusion technique. The use of calcium-free solution containing collagenase and protease, followed by low calcium solution, consistently results in a large number of calcium tolerant myocytes which are well suited for long periods of electrophysiological recording.

Pursuing the voltage sensor of a voltage-gated mammalian potassium channel

Ion channels are generally classified as voltage- or ligand-gated channels based upon their ability to respond to a change in the transmembrane voltage or to a specific ligand interaction. The voltage sensor of voltage-gated ion channels is thought to be contained within the fourth putative transmembrane segment, S4, and is characterized by basic residues typically spaced apart. Thus far, a full understanding of all charges contributing to the voltage sensor of voltage-gated potassium channels has been lacking since several neutralization mutants in S4 could not be functionally expressed. By construction of multimeric cDNAs encoding a mammalian RCK1 (Kv1.1) potassium channel, we now report functional expression of all charge neutralizations in the S4 segment, providing a more comprehensive insight into the functioning of the voltage sensor.