Stereoselective block of cardiac sodium channels by bupivacaine in guinea pig ventricular myocytes

BACKGROUND

Bupivacaine is a potent local anesthetic widely used for prolonged local and regional anesthesia. However, accidental intravascular injection of bupivacaine can produce severe arrhythmias and cardiac depression. Although used clinically as a racemic mixture, S(-)-bupivacaine appears less toxic than the R(+)-enantiomer despite at least equal potency for local anesthesia. If the R(+)-enantiomer is more potent in blocking cardiac sodium channels, then the S(-)-enantiomer could be used with less chance of cardiovascular toxicity. Therefore, we tested whether such stereoselectivity existed in the bupivacaine affinity for the cardiac sodium channel.

METHODS AND RESULTS

The inhibitory effects on the cardiac sodium current (INa) of 10 mumol/L R(+)- and S(-)-bupivacaine were investigated by use of the whole-cell voltage clamp technique in isolated guinea pig ventricular myocytes. Both enantiomers produced similar but limited levels of tonic block (6% and 8%). During long depolarizations (5 seconds at 0 mV), R(+)-bupivacaine induced a significantly larger inhibition of INa: 72 +/- 2% versus 58 +/- 3% for the S(-)-enantiomer (P < .01). Development of block was slow, but its rate was faster for R(+)-bupivacaine [time constant, 1.84 +/- 0.16 versus 2.56 +/- 0.26 seconds for the S(-)-enantiomer, P < .05]. The voltage dependence of the availability of the Na+ current was shifted to more hyperpolarizing potentials compared with the control; R(+)-bupivacaine induced a larger shift than S(-)-bupivacaine (37 +/- 2 versus 30 +/- 2 mV, P < .05). These data indicate stereoselective interactions with the inactivated state. In addition, both enantiomers induced substantial use-dependent block during 2.5-Hz pulse trains with medium (100-ms) and short (10-ms) depolarizations but without stereoselective difference. A stepwise approach was used to model these experimental results and to derive apparent affinities and rate constants. We initially assumed that bupivacaine interacted only with the rested and inactivated states of the Na+ channel. The apparent affinities of the inactivated state for S(-)- and R(+)-bupivacaine were 4.8 and 2.9 mumol/L, respectively. With the derived binding and unbinding rate constants, this model reproduced the stereoselective block during long depolarizations but failed to predict the use-dependent block induced by trains of short (10-ms) depolarizations. To account for the observed use-dependent interactions, it was necessary to include interactions with the activated state, which resulted in adequate reproduction of the experimental results. The apparent affinities of the activated or open state for S(-)- and R(+)-bupivacaine were 4.3 and 3.3 mumol/L, respectively.

CONCLUSIONS

Both the large level of pulse-dependent block and the failure of the pure inactivated-state block model indicate that bupivacaine interacts with the activated (or open) state of the cardiac sodium channel in addition to its block of the inactivated state. The bupivacaine-induced block of the inactivated state of the Na+ channel displayed stereoselectivity, with R(+)-bupivacaine interacting faster and more potently. Both enantiomers also bind with high affinity to the activated or open state of the channel, but this interaction did not display stereoselectivity, although the binding to the activated or open state was faster for S(-)- than for R(+)-bupivacaine. The higher potency of R(+)-bupivacaine to block the inactivated state of the cardiac Na+ channel may explain its higher toxicity because of the large contribution of the inactivated-state block during the plateau phase of the cardiac action potential. These results would support the use of the S(-)-enantiomer to reduce cardiac toxicity.

Stereoselective block of a human cardiac potassium channel (Kv1.5) by bupivacaine enantiomers

Stereoselective drug-channel interactions may help to elucidate the molecular basis of voltage-gated potassium channel block by local anesthetic drugs. We studied the effects of the enantiomers of bupivacaine on a cloned human cardiac potassium channel (hKv1.5). This channel was stably expressed in a mouse Ltk- cell line and studied using the whole-cell configuration of the patch-clamp technique. Both enantiomers modified the time course of this delayed rectifier current. Exposure to 20 microM of either S(-)-bupivacaine or R(+)-bupivacaine did not modify the activation time constant of the current, but reduced the peak outward current and induced a subsequent exponential decline of current with time constants of 18.7 +/- 1.1 and 10.0 +/- 0.9 ms, respectively. Steady-state levels of block (assessed with 250-ms depolarizing pulses to +60 mV) averaged 30.8 +/- 2.5% (n = 6) and 79.5 +/- 3.2% (n = 6) (p < 0.001), for S(-)- and R(+)-bupivacaine, respectively. The concentration dependence of hKv1.5 inhibition revealed apparent KD values of 27.3 +/- 2.8 and 4.1 +/- 0.7 microM for S(-)-bupivacaine and R(+)-bupivacaine, respectively, with Hill coefficients close to unity, suggesting that binding of one enantiomer molecule per channel was sufficient to block potassium permeation. Analysis of the rate constants of association (k) and dissociation (l) yielded similar values for l (24.9 s-1 vs. 23.6 s-1 for S(-)- and R(+)-bupivacaine, respectively) but different association rate constants (1.0 x 10(6) vs. 4.7 x 10(6) M-1 s-1 for S(-)- and R(+)-bupivacaine, respectively). Block induced by either enantiomer displayed a shallow voltage dependence in the voltage range positive to 0 mV, i.e., where the channel is fully open, consistent with an equivalent electrical distance delta of 0.16 +/- 0.01. This suggested that at the binding site, both enantiomers of bupivacaine experienced 16% of the applied transmembrane electrical field, referenced to the inner surface. Both bupivacaine enantiomers reduced the tail current amplitude recorded on return to -40 mV and slowed their time course relative to control, resulting in a "crossover" phenomenon. These data indicate 1) the charged form of both bupivacaine enantiomers block the hKv1.5 channel after it opens, 2) binding occurs within the transmembrane electrical field, 3) unbinding is required before the channel can close, 4) block of hKv1.5 channels by bupivacaine is markedly stereoselective, with the R(+)-enantiomer being the more potent one, 5) this stereoselective block was associated with a 1.11 -kcal/mol difference in binding energy between both enantiomers, and 6) the stereoselectivity derives mainly from a difference in the association rate constants, suggesting that the S(-)-enantiomer is less likely to access the binding site in an optimal configuration.

On the molecular nature of the lidocaine receptor of cardiac Na+ channels. Modification of block by alterations in the alpha-subunit III-IV interdomain

The mechanism of inhibition of Na+ channels by lidocaine has been suggested to involve low-affinity binding to rested states and high-affinity binding to the inactivated state of the channel, implying either multiple receptor sites or allosteric modulation of receptor affinity. Alternatively, the lidocaine receptor may be guarded by the channel gates. To test these distinct hypotheses, inhibition of Na+ channels by lidocaine was studied by voltage-clamp methods in both native and heterologous expression systems. Native Na+ channels were studied in guinea pig ventricular myocytes, and recombinant human heart Na+ channels were expressed in Xenopus laevis oocytes. Fast inactivation was eliminated by mutating three amino acids (isoleucine, phenylalanine, and methionine) in the III-IV interdomain to glutamines or by enzymatic digestion with alpha-chymotrypsin. In channels with intact fast inactivation, lidocaine block developed with a time constant of 589 +/- 42 ms (n = 7) at membrane potentials between -50 and +20 mV, as measured by use of twin pulse protocols. The IC50 was 36 +/- 1.8 mumol/L. Control channels inactivated within 20 ms, and slow inactivation developed much later (time constant of slow inactivation, 6.2 +/- 0.36 s). The major component of block developed long after activated and open channels were no longer available for drug binding. Control channels recovered fully from inactivation in < 50 ms at -120 mV (time constant, 11 +/- 0.5 ms; n = 50).(ABSTRACT TRUNCATED AT 250 WORDS)

Class III antiarrhythmic effects of zatebradine. Time-, state-, use-, and voltage-dependent block of hKv1.5 channels

BACKGROUND

Zatebradine is a bradycardic agent that inhibits the hyperpolarization-activated current (I(f)) in the rabbit sinoatrial node. It also prolongs action potential duration in papillary muscles in guinea pigs and in Purkinje fibers in rabbits. The underlying mechanism by which zatebradine induces this effect has not been explored, but it is likely to involve K+ channel block.

METHODS AND RESULTS

Cloned human cardiac K+ delayed rectifer currents (hKv1.5) were recorded in Ltk- cells transfected with their coding sequence. Zatebradine 10 mumol/L did not modify the initial activation time course of the current but induced a subsequent decline to a lower steady-state current level with a time constant of 109 +/- 16 ms. Zatebradine inhibited hKv1.5 with an apparent KD of 1.86 +/- 0.14 mumol/L. Block was voltage dependent (electrical distance delta = 0.177 +/- 0.003) and accumulated in a use-dependent manner during 0.5- and 1-Hz pulse trains because of slower recovery kinetics in the presence of the drug. Zatebradine reduced the tail current amplitude, recorded at -30 mV, and slowed the deactivation time course, which resulted in a "crossover" phenomenon when control and zatebradine tail currents were superimposed.

CONCLUSIONS

These results indicate that (1) zatebradine is an open-channel blocker of hKv 1.5, (2) binding occurs in the internal mouth of the ion pore, (3) unbinding is required before the channel can close, and (4) zatebradine-induced block is use dependent because of slower recovery kinetics in the presence of the drug. These effects may explain the prolongation of the cardiac action potential and could be clinically relevant.

Molecular determinants of stereoselective bupivacaine block of hKv1.5 channels

Enantiomers of local anesthetics are useful probes of ion channel structure that can reveal three-dimensional relations for drug binding in the channel pore and may have important clinical consequences. Bupivacaine block of open hKv1.5 channels is stereoselective, with the R(+)-enantiomer being 7-fold more potent than the S(-)-enantiomer (Kd = 4.1 mumol/L versus 27.3 mumol/L). Using whole-cell voltage clamp of hKv1.5 channels and site-directed mutants stably expressed in Ltk- cells, we have identified a set of amino acids that determine the stereoselectivity of bupivacaine block. Replacement of threonine 505 by hydrophobic amino acids (isoleucine, valine, or alanine) abolished stereoselective block, whereas a serine substitution preserved it [Kd = 60 mumol/L and 7.4 mumol/L for S(-)- and R(+)-bupivacaine, respectively]. A similar substitution at the internal tetraethylammonium binding site (T477S) reduced the affinity for both enantiomers similarly, thus preserving the stereoselectivity [Kd = 45.5 mumol/L and 7.8 mumol/L for S(-)- and R(+)-bupivacaine, respectively]. Replacement of L508 or V512 by a methionine (L508M and V512M) abolished stereoselective block, whereas substitution of V512 by an alanine (V512A) preserved it. Block of Kv2.1 channels, which carry valine, leucine, and isoleucine residues at T505, L508, and V512 equivalent sites, respectively, was not stereoselective [Kd = 8.3 mumol/L and 13 mumol/L for S(-)- and R(+)-bupivacaine, respectively]. These results suggest that (1) the bupivacaine binding site is located in the inner mouth of the pore, (2) stereoselective block displays subfamily selectivity, and (3) a polar interaction with T505 combined with hydrophobic interactions with L508 and V512 are required for stereoselective block.

Losartan and its metabolite E3174 modify cardiac delayed rectifier K(+) currents

BACKGROUND

The effects of type 1 angiotensin II receptor antagonist losartan and its metabolite E3174 on transmembrane action potentials, hKv1.5, HERG, and I(Ks) currents were analyzed.

METHODS AND RESULTS

Guinea pig ventricular action potentials were recorded with microelectrode techniques and hKv1.5 and HERG currents with the whole-cell patch-clamp technique. I(Ks) was recorded in guinea pig ventricular myocytes with the perforated-nystatin-patch configuration. Losartan and E3174 transiently increased the hKv1.5 current by 8.0+/-1.4% and 7.4+/-1.6%, respectively. Thereafter, they produced a voltage-dependent block, E3174 being more potent than losartan (P<0.05) for this effect. Losartan decreased HERG currents elicited at 0 mV (23.3+/-4.8%), whereas E3174 increased the current (30.5+/-6.2%). Both drugs shifted the midpoint of the activation curve of HERG channels to more negative potentials. In ventricular myocytes, losartan and E3174 inhibited the I(Ks) (18.4+/-3.2% and 6. 5+/-0.7%, respectively). Losartan-induced block was voltage-independent, whereas E3174 shifted the midpoint of the activation curve to more negative potentials. Losartan lengthened the duration of the action potentials at both 50% and 90% of repolarization, whereas E3174 slowed only the final phase of the repolarization process.

CONCLUSIONS

These results demonstrated that at therapeutic concentrations, both losartan and E3174 modified the cardiac delayed rectifier hKv1.5, HERG, and Ks currents.

Arsenic resistant bacteria isolated from arsenic contaminated river in the Atacama Desert (Chile)

In this study, arsenic resistant bacteria were isolated from sediments of an arsenic contaminated river. Arsenic tolerance of bacteria isolated was carried out by serial dilution on agar plate. Redox abilities were investigated using KMnO4. arsC and aox genes were detected by PCR and RT-PCR, respectively. Bacterial populations were identified by RapID system. Forty nine bacterial strains were isolated, of these, 55 % corresponded to the reducing bacteria, 4% to oxidizing bacteria, 8% presented both activities and in 33% of the bacteria none activity was detected. arsC gene was detected in 11 strains and aox genes were not detected. The activity of arsenic transforming microorganisms in river sediment has significant implications for the behavior of the metalloid.

Block of human cardiac Kv1.5 channels by loratadine: voltage-, time- and use-dependent block at concentrations above therapeutic levels

OBJECTIVE

The aim of this study was to analyze the effects of loratadine on a human cardiac K+ channel (hKv1.5) cloned from human ventricle and stably expressed in a mouse cell line.

METHODS

Currents were studied using the whole-cell configuration of the patch-clamp technique in Ltk- cells transfected with the gene encoding hKv1.5 channels.

RESULTS

Loratadine inhibited in a concentration-dependent manner the hKv1.5 current, the apparent affinity being 1.2 +/- 0.2 microM. The blockade increased steeply between -30 and 0 mV which corresponded with the voltage range for channel opening, thus suggesting that the drug binds preferentially to the open state of the channel. The apparent association and dissociation rate constants were (3.6 +/- 0.5) x 10(6).M-1.s-1 and 3.7 +/- 1.6.s-1, respectively. Loratadine, 1 microM, increased the time constant of deactivation of tail currents elicited on return to -40 mV after 500 ms depolarizing pulses to +60 mV from 36.2 +/- 3.4 to 64.9 +/- 3.6 ms (n = 6, P < 0.01), thus inducing a 'crossover' phenomenon. Application of trains of pulses at 1 Hz lead to a progressive increase in the blockade reaching a final value of 48.6 +/- 4.3%. Recovery from loratadine-induced block at -80 mV exhibited a time constant of 743.0 +/- 78.0 ms. Finally, the results of a mathematical stimulation of the effects of loratadine, based on an open-channel block model, reproduced fairly well the main effects of the drug.

CONCLUSIONS

The present results demonstrated that loratadine blocked hKv1.5 channels in a concentration-, voltage-, time- and use-dependent manner but only at concentrations much higher than therapeutic plasma levels in man.

Effects of propafenone and 5-hydroxy-propafenone on hKv1.5 channels

1. The goal of this study was to analyse the effects of propafenone and its major metabolite, 5-hydroxy-propafenone, on a human cardiac K+ channel (hKv1.5) stably expressed in Ltk- cells and using the whole-cell configuration of the patch-clamp technique. 2. Propafenone and 5-hydroxy-propafenone inhibited in a concentration-dependent manner the hKv1.5 current with K(D) values of 4.4+/-0.3 microM and 9.2+/-1.6 microM, respectively. 3. Block induced by both drugs was voltage-dependent consistent with a value of electrical distance (referenced to the cytoplasmic side) of 0.17+/-0.55 (n=10) and 0.16+/-0.81 (n=16). 4. The apparent association (k) and dissociation (l) rate constants for propafenone were (8.9+/-0.9) x 10(6) M(-1) s(-1) and 39.5+/-4.2 s(-1), respectively. For 5-hydroxy-propafenone these values averaged (2.3+/-0.3) x 10(6) M(-1) s(-1) and 21.4+/-3.1 s(-1), respectively. 5. Both drugs reduced the tail current amplitude recorded at -40 mV after 250 ms depolarizing pulses to +60 mV, and slowed the deactivation time course resulting in a 'crossover' phenomenon when the tail currents recorded under control conditions and in the presence of each drug were superimposed. 6. Both compounds induced a small but statistically significant use-dependent block when trains of depolarizations at frequencies between 0.5 and 3 Hz were applied. 7. These results indicate that propafenone and its metabolite block hKv1.5 channels in a concentration-, voltage-, time- and use-dependent manner and the concentrations needed to observe these effects are in the therapeutical range.

Detection and typing of human papillomavirus DNA in benign and malignant tumours of laryngeal epithelium

The role of human papillomaviruses (HPV) in laryngeal squamous cell carcinoma has not yet been established. Thirty-three cases of laryngeal squamous cell carcinoma were analysed for the presence of HPV DNA and compared with 25 cases of normal larynx and 29 cases of laryngeal squamous papilloma in their positivity index. The presence of HPV DNA was analysed by using L1 consensus primers and also by primers specific for the E7 gene of HPV types 16 and 18. Four normal laryngeal samples (16%) were positive for HPV DNA against the 24 samples (82%) (p < 0.001) found for laryngeal papilloma and 16 (48.5%) (p < 0.05) found for laryngeal squamous cell carcinoma. HPV 16 was the type most frequently found in laryngeal carcinoma samples. Our results support an etiologic role for this type of HPV in the pathogenesis of laryngeal carcinoma.

Propafenone preferentially blocks the rapidly activating component of delayed rectifier K+ current in guinea pig ventricular myocytes. Voltage-independent and time-dependent block of the slowly activating component

The effects of propafenone on the delayed rectifier K+ current were studied in guinea pig ventricular myocytes by using the patch-clamp technique. In these myocytes, this current consists of at least two components: a La(3+)-sensitive component activating rapidly with moderate depolarizations and a La(3+)-resistant current slowly activating at more positive potentials. In the absence of La3+ (when both components are present), propafenone inhibited the delayed outward current, its effects being more marked after weak than after strong depolarizations. Propafenone-induced block of the tail currents elicited on return to -30 mV was more marked after short than after long depolarizing pulses. In the presence of 1 mumol/L propafenone, the envelope-of-tails test was satisfied, thus indicating that at this concentration propafenone completely blocks the rapidly activating component. In the presence of La3+ (when only the slow component is present), the steady state inhibition induced by 5 mumol/L propafenone on both the maximum activated and the tail currents was independent of the test pulse voltage. Development of propafenone-induced block on the slowly activating component was very fast and linked to channel opening. In addition, the blockade appeared to be use dependent, with the rate constant of the onset kinetics at 2 Hz being 0.44 +/- 0.1 pulse-1. The recovery process from propafenone-induced block exhibited a time constant of 2.5 +/- 0.4 s. These results indicated that propafenone preferentially inhibits the rapidly activating component of the delayed rectifier and that it blocks in a voltage-independent and time-dependent manner the slow component of this current.

Imipramine blocks rapidly activating and delays slowly activating K+ current activation in guinea pig ventricular myocytes

Imipramine is a tricyclic antidepressant drug that also exhibits antiarrhythmic effects and whose clinical spectrum of activity is similar to that of quinidine. It has been previously demonstrated that imipramine inhibits the aggregate time-dependent outward K+ current (IK). IK is composed of at least two components: a slowly activating La(3+)-resistant delayed rectifying current (IK,s) and a rapidly activating La(3+)-sensitive current (IK,r). To assess the effects of imipramine on IK,r and IK,s, single guinea pig ventricular myocytes were studied using the nystatin-perforated patch-clamp technique in the absence and in the presence of La3+. Imipramine inhibited IK,r and IK,s in a concentration-dependent manner. The effects of imipramine on the aggregate time-dependent outward current were more marked than those on IK,s alone. Thus, 1 mumol/L imipramine decreased the tail currents elicited on return to -30 mV after long depolarizing pulses (5 seconds, from -40 to +50 mV) in the absence and in the presence of La3+ by 27 +/- 4% and 15 +/- 3% (n = 6), respectively. Moreover, the inhibition induced by imipramine was greater after short (0.5-second) pulses than after 5-second depolarizing pulses, both in the absence and in the presence of La3+ (53 +/- 3% and 30 +/- 5%, respectively; n = 6; P < .05). Imipramine did not significantly modify either the activation midpoint or the slope factor of the aggregate IK and IK,s activation curves. The reduction of IK,s by imipramine was voltage dependent and was more marked at negative membrane potentials. In the presence of 1 mumol/L imipramine, the ratio of tail current to time-dependent current remained constant at 0.37 +/- 0.03, regardless of the test pulse duration at +50 mV. Thus, the envelope-of-tails test was satisfied in the presence of 1 mumol/L imipramine, which indicates that imipramine, at this concentration, blocks IK,r. Imipramine (1, 5, and 10 mumol/L) had no effect on the kinetics of the later phase of IK activation but delayed the beginning of the activation of IK,s by 62 +/- 22, 74 +/- 23, and 155 +/- 53 milliseconds in the presence of 1, 5, and 10 mumol/L imipramine, respectively. These results suggest that imipramine preferentially blocks rapidly activating K+ channels. In addition, experiments performed in the presence of 30 mumol/L La3+ suggest that the drug preferentially binds, but maybe not exclusively, to a closed state of the slowly activating K+ channel.

Isolation of arsenite-oxidizing bacteria from arsenic-enriched sediments from Camarones river, Northern Chile

In Northern Chile, high arsenic concentrations are found in natural water, both natural and anthropogenic sources, a significant health risk. Nine bacterial strains were isolated from Camarones river sediments, located in Northern Chile, a river showing arsenic concentrations up to 1,100 microg/L. These strains were identified as Pseudomonas and they can oxidize arsenite (As(III)) to the less mobile arsenate (As(V)). The arsenite oxidase genes were identified in eight out of nine isolates. The arsenite oxidizing ability shown by the nine strains isolated from arsenic enriched sediments open the way to their potential application in biological treatment of effluents contaminated with arsenic.

Effects of ropivacaine on a potassium channel (hKv1.5) cloned from human ventricle

BACKGROUND

Ropivacaine, a new amide local anesthetic agent chemically related to bupivacaine, is able to induce early after depolarizations in isolated cardiac preparations. The underlying mechanism by which ropivacaine induces this effect has not been explored, but it is likely to involve K+ channel block.

METHODS

Cloned human cardiac K+ channels (hKv1.5) were stably transfected in Ltk cells, and the effects of ropivacaine on the expressed hKv1.5 currents were assessed using the whole-cell configuration of the patch-clamp technique.

RESULTS

Ropivacaine (100 microM) did not modify the initial activation time course of the current, but induced a fast subsequent decline to a lower steady-state current level with a time constant of 12.2 +/- 0.6 ms. Ropivacaine inhibited hKv1.5 with an apparent KD of 80 +/- 4 microM. Block displayed an intrinsic voltage-dependent, consistent with an electrical distance for the binding site of 0.153 +/- 0.007 (n = 6) (from the cytoplasmic side). Ropivacaine reduced the tail current amplitude recorded at -40 mV, and slowed the deactivation time course, resulting in a "crossover" phenomenon when control and ropivacaine tail currents were superimposed.

CONCLUSIONS

These results indicate that: (1) ropivacaine is an open channel blocker of hKv1.5; (2) binding occurs in the internal mouth of the ion pore; and (3) unbinding is required before the channel can close. These effects explain the ropivacaine availability of induction early after depolarizations and could be clinically relevant.

Gating of cardiac Na+ channels in excised membrane patches after modification by alpha-chymotrypsin

Single cardiac Na+ channels were investigated after intracellular proteolysis to remove the fast inactivation process in an attempt to elucidate the mechanisms of channel gating and the role of slow inactivation. Na+ channels were studied in inside-out patches excised from guinea-pig ventricular myocytes both before and after very brief exposure (2-4 min) to the endopeptidase, alpha-chymotrypsin. Enzyme exposure times were chosen to maximize removal of fast inactivation and to minimize potential nonspecific damage to the channel. After proteolysis, the single channel current-voltage relationship was approximately linear with a slope conductance of 18 +/- 2.5 pS. Na+ channel reversal potentials measured before and after proteolysis by alpha-chymotrypsin were not changed. The unitary current amplitude was not altered after channel modification suggesting little or no effect on channel conductance. Channel open times were increased after removal of fast inactivation and were voltage-dependent, ranging between 0.7 (-70 mV) and 3.2 (-10 mV) ms. Open times increased with membrane potential reaching a maximum at -10 mV; at more positive membrane potentials, open times decreased again. Fast inactivation appeared to be completely removed by alpha-chymotrypsin and slow inactivation became more apparent suggesting that fast and slow inactivation normally compete, and that fast inactivation dominates in unmodified channels. This finding is not consistent with a slow inactivated state that can only be entered through the fast inactivated state, since removal of fast inactivation does not eliminate slow inactivation. The data indicate that cardiac Na+ channels can enter the slow inactivated state by a pathway that bypasses the fast inactivated state and that the likelihood of entering the slow inactivated state increases after removal of fast inactivation.

Whole genome sequencing of Shigella sonnei through PulseNet Latin America and Caribbean: advancing global surveillance of foodborne illnesses

Objectives

Shigella sonnei is a globally important diarrhoeal pathogen tracked through the surveillance network PulseNet Latin America and Caribbean (PNLA&C), which participates in PulseNet International. PNLA&C laboratories use common molecular techniques to track pathogens causing foodborne illness. We aimed to demonstrate the possibility and advantages of transitioning to whole genome sequencing (WGS) for surveillance within existing networks across a continent where S. sonnei is endemic.

Methods

We applied WGS to representative archive isolates of S. sonnei (n = 323) from laboratories in nine PNLA&C countries to generate a regional phylogenomic reference for S. sonnei and put this in the global context. We used this reference to contextualise 16 S. sonnei from three Argentinian outbreaks, using locally generated sequence data. Assembled genome sequences were used to predict antimicrobial resistance (AMR) phenotypes and identify AMR determinants.

Results

S. sonnei isolates clustered in five Latin American sublineages in the global phylogeny, with many (46%, 149 of 323) belonging to previously undescribed sublineages. Predicted multidrug resistance was common (77%, 249 of 323), and clinically relevant differences in AMR were found among sublineages. The regional overview showed that Argentinian outbreak isolates belonged to distinct sublineages and had different epidemiologic origins.

Conclusions

Latin America contains novel genetic diversity of S. sonnei that is relevant on a global scale and commonly exhibits multidrug resistance. Retrospective passive surveillance with WGS has utility for informing treatment, identifying regionally epidemic sublineages and providing a framework for interpretation of prospective, locally sequenced outbreaks.

Epidemiology of canine distemper and canine parvovirus in domestic dogs in urban and rural areas of the Araucanía region in Chile

To assess whether the seroprevalence of canine distemper virus (CDV) and canine parvovirus (CPV) in domestic dogs is higher in urban versus rural areas of the Araucanía region in Chile and risk factors for exposure, a serosurvey and questionnaire survey at three, urban-rural paired sites was conducted from 2009 to 2012. Overall, 1161 households were interviewed of which 71% were located in urban areas. A total of 501 blood samples were analysed. The overall CDV and CPV seroprevalences were 61% (CI 90%: 58-70%) and 47% (CI 90%: 40-49%), and 89% (CI 90%: 85-92%) and 72% (CI 90%: 68-76%) in urban and rural areas, respectively. The higher seroprevalence in domestic dogs in urban areas suggests that urban domestic dogs might be a maintenance host for both CDV and CPV in this region. Due to the presence of endangered wild canids populations in areas close to these domestic populations, surveillance and control of these pathogens in urban dog populations is needed a priority.

Structural determinants of potency and stereoselective block of hKv1.5 channels induced by local anesthetics

Block of hKv1.5 channels by bupivacaine is stereoselective, with (R)-(+)-bupivacaine being 7-fold more potent than (S)-(-)-bupivacaine. The study of the effects of chemically related enantiomers on these channels may help to elucidate the structural determinants of stereoselective hKv1.5 channels block by local anesthetics. In this study, we analyzed the effects of (R)-(+)-ropivacaine, (R)-(+)-mepivacaine, and (S)-(-)-mepivacaine on hKv1.5 channels stably expressed in Ltk- cells. (R)-(+)-Ropivacaine inhibited hKv1.5 current and induced a fast initial decline superimposed to the slow inactivation during the application of depolarizing pulses, which reached steady state at the end of 250-msec depolarizing pulses. The concentration-dependence block induced by (R)-(+)-ropivacaine yielded a KD value of 32 +/- 1 microM [i.e., 2.5-fold more potent than (S)-(-)-ropivacaine]. (R)-(+)-Ropivacaine block also was voltage dependent, with a fractional electrical distance (delta) of 0.156 +/- 0.003 (n = 14) referred to the inner surface. Both (S)-(-)- and (R)-(+)-mepivacaine blocked hKv1.5 channels, with KD values of 286.8 +/- 34.1 and 379.0 +/- 56.0 microM, respectively [i.e., block was not stereoselective (p > 0.05)]. (S)-(-)-Mepivacaine and (R)-(+)-mepivacaine block displayed no apparent time-dependence due to a very fast dissociation rate constant. However, block by mepivacaine enantiomers was voltage dependent, with delta values of 0.154 +/- 0.015 and 0.160 +/- 0.008 for the (S)-(-)- and (R)-(+)-enantiomers, respectively. We conclude that (1) (R)-(+)-ropivacaine and mepivacaine enantiomers block the open state of hKv1.5 channels and (2) the length of their alkyl substituent at position 1 determines the potency and the degree of stereoselectivity.

Direct effects of candesartan and eprosartan on human cloned potassium channels involved in cardiac repolarization

In the present study, we analyzed the effects of two angiotensin II type 1 receptor antagonists, candesartan (0.1 microM) and eprosartan (1 microM), on hKv1.5, HERG, KvLQT1+minK, and Kv4.3 channels expressed on Ltk(-) or Chinese hamster ovary cells using the patch-clamp technique. Candesartan and eprosartan produced a voltage-dependent block of hKv1.5 channels decreasing the current at +60 mV by 20.9 +/- 2.3% and 14.3 +/- 1.5%, respectively. The blockade was frequency-dependent, suggesting an open-channel interaction. Eprosartan inhibited the tail amplitude of HERG currents elicited on repolarization after pulses to +60 mV from 239 +/- 78 to 179 +/- 72 pA. Candesartan shifted the activation curve of HERG channels in the hyperpolarizing direction, thus increasing the current amplitude elicited by depolarizations to potentials between -50 and 0 mV. Candesartan reduced the KvLQT1+minK currents elicited by 2-s pulses to +60 mV (38.7 +/- 6.3%). In contrast, eprosartan transiently increased (8.8 +/- 2.7%) and thereafter reduced the KvLQT1+minK current amplitude by 17.7 +/- 3.0%. Eprosartan, but not candesartan, blocked Kv4.3 channels in a voltage-dependent manner (22.2 +/- 3.5% at +50 mV) without modifying the voltage-dependence of Kv4.3 channel inactivation. Candesartan slightly prolonged the action potential duration recorded in guinea pig papillary muscles at all driving rates. Eprosartan prolonged the action potential duration in muscles driven at 0.1 to 1 Hz, but it shortened this parameter at faster rates (2--3 Hz). All these results demonstrated that candesartan and eprosartan exert direct effects on Kv1.5, HERG, KvLQT1+minK, and Kv4.3 currents involved in human cardiac repolarization.

Functional expression of an inactivating potassium channel (Kv4.3) in a mammalian cell line

OBJECTIVE

The goal of this study was to characterize the electrophysiological properties of the Kv4.3 channels expressed in a mammalian cell line.

METHODS

Currents were recorded using the whole-cell voltage clamp technique.

RESULTS

The threshold for activation of the expressed Kv4.3 current was approximately -30 mV. The dominant time constant for activation was 1.71 +/- 0.16 ms (n = 10) at +60 mV. The current inactivated, this process being incomplete, resulting in a sustained level which contributed 15 +/- 2% (n = 25) of the total current. The time course of inactivation was fit by a biexponential function, the fast component contributing 74 +/- 5% (n = 9) to the overall inactivation. The fast time constant was voltage-dependent [27.6 +/- 2.0 ms at +60 mV (n = 10) versus 64.0 +/- 3.6 ms at 0 mV (n = 10); P < 0.01], whereas the slow was voltage-independent [142 +/- 15 ms at +60 mV (n = 10) versus 129 +/- 33 ms at 0 mV (n = 6) P > 0.05]. The voltage-dependence of inactivation exhibited midpoint and slope values of -26.9 +/- 1.5 mV and 5.9 +/- 0.3 mV (n = 21). Recovery from inactivation was faster at more negative membrane potentials [203 +/- 17 ms (n = 13) and 170 +/- 19 ms (n = 4), at -90 and -100 mV]. Bupivacaine block of Kv4.3 channels was not stereoselective (KD approximately 31 microM).

CONCLUSIONS

The functional profile of Kv4.3 channels expressed in Ltk- cells corresponds closely to rat ITO, although differences in recovery do not rule out association with accessory subunits. Nevertheless, the sustained component needs to be considered with respect to native ITO.

Effects of the two enantiomers, S-16257-2 and S-16260-2, of a new bradycardic agent on guinea-pig isolated cardiac preparations

1. The electromechanical effects of two enantiomers, S-16257-2 (S57) and S-16260-2 (R60), were studied and compared in guinea-pig isolated atria and ventricular papillary muscles. The possible stereoselectivity of the interaction on the cardiac Na+ channel was analysed by comparing the effects of the two enantiomers on the onset and recovery kinetics of the frequency-dependent Vmax block. 2. In spontaneously beating right atria, S57 and R60 (10(-8)M-10(-4M) exerted a negative chronotropic effect (pIC50 = 5.07 +/- 0.19 and 4.76 +/- 0.18, respectively) and prolonged the sinus node recovery time, this effect being more marked with S57. In electrically driven left atria, S57 decreased (P < 0.05) contractile force only at 10(-4M) and R60 at concentrations > or = 5 x 10(-5M), whereas in papillary muscles the negative inotropic effect appeared at concentrations > 10(-5M). 3. In papillary muscles driven at 1 Hz, S57 and R60 at concentrations higher than 5 x 10(-6M) produced a concentration-dependent decrease in the maximum upstroke velocity (Vmax) and amplitude of the cardiac action potential without altering the resting membrane potential or the action potential duration. S57 and R60 had no effect on the characteristics of the slow action potentials elicited by isoprenaline in ventricular muscle fibres depolarized in high K+ (27 mM) solution. 4. At 5 x 10(-5M), S57 and R60 produced a small tonic Vmax block. However, in muscles driven at rates between 0.5 and 3 Hz both enantiomers produced an exponential decline in Vmax (frequency-dependent Vmax block) which augmented at higher rates of stimulation. The onset and offset rates of the frequency-dependent Vmax block were similar for both drugs. Both S57 and R60 prolonged the recovery time constant from the frequency-dependent block from 20.1 +/- 2.9 ms to 2-3 s.5. At 5 x 10-5 M, S57 and R60 shifted the membrane responsiveness curve in a hyperpolarizing direction.6. It can be concluded that S57 and R60, the two enantiomers of the new bradycardic agent, produced a similar frequency-dependent Vmax block which indicated that the interaction with the Na+ channel was not stereospecific. The analysis of the onset and offset kinetics of the frequency-dependent Vmax block suggested that both enantiomers can be considered as Na+ channel blockers with intermediate kinetics,e.g., class IA antiarrhythmic drugs.

Mechanisms of block of a human cloned potassium channel by the enantiomers of a new bradycardic agent: S-16257-2 and S-16260-2

1. The effects of S-16257-2 (S57) and S-16260-2 (R60), the two enantiomers of a new bradycardic agent, were studied on human cloned K+ channels (hKv1.5) stably expressed in a mouse L cell line using the whole-cell configuration of the patch-clamp technique. 2. S57 and R60 did not modify the sigmoidal activation time course of the current but reduced the amplitude and increased the rate of the decay of the current during the application of depolarizing pulses. Both, S57 and R60 produced a concentration-dependent block of hKv1.5 channels with apparent KD values of 29.0 +/- 1.9 microM and 40.9 +/- 4.0 microM, respectively. Thus, S57 was 1.4 fold more potent than R60 in blocking hKv1.5 channels. 3. The blockade produced by S57 and R60 was voltage-dependent and increased steeply between -30 and 0 mV, which corresponded with the voltage range for channel opening. This result indicated that both enantiomers block the hKv1.5 channels, preferentially, when they are in the open state. Between 0 and +60 mV the blockade exhibited a shallow voltage-dependence which was described by an electrical distance of 0.18 +/- 0.002 and 0.19 +/- 0.004 for S57 and R60, respectively. 4. S57 and R60 also increased the rate of decline of the current during the application of depolarizing pulses. The time constant of such decline (tau Block) was faster in the presence of R60 than in the presence of S57 (16.2 +/- 1.5 ms vs. 24.0 +/- 2.6 ms; P < 0.01). The apparent association rate constants (k) were similar for S57 and R60 ((0.52 +/- 0.13) x 10(6) M-1 s-1 and (0.66 +/- 0.13) x 10(6) M-1 s-1, respectively), whereas the dissociation rate constant (l) was faster for R60 than for S57 (25.8 +/- 1.8 s-1 and 13.0 +/- 2.4 s-1, respectively). 5. Both enantiomers slowed the deactivation of the tail currents elicited upon repolarization to -40 mV, thus inducing a 'crossover' phenomenon. These results suggested that drug unbinding is required before hKv1.5 channels can close. 6. It is concluded that R60 and S57 produced a similar time- voltage- and state-dependent block of hKv1.5 channels that can be interpreted as open channel block by the charged form of each enantiomer. The main difference between R60 and S57 were linked to the apparent dissociation rate constants.

A truncated HCV core protein elicits a potent immune response with a strong participation of cellular immunity components in mice

The immunogenicity of a truncated HCV core protein (Co.120) was studied in BALB/c and C57BL/6 mice, given three intramuscular injections of antigen, adjuvanted with either aluminum hydroxide or Freund's adjuvant. A rapid antibody response was noted after the first dose, with both strains of mice eventually exhibiting comparable levels of anti-core IgG (titers >1:100000), with a mixed IgG1/IgG2a subclass response. Spleen cells from Co.120-immunized mice gave a significant specific proliferative response. IFN-gamma gene expression was also detected after an ex-vivo specific stimulation of spleen cells in all immunized mice. This response was independent of dose, H-2 genetic background or type of adjuvant. The results indicated that immunization with the Co.120 protein elicits a potent anti-HCV humoral and cellular immune response.

Blockade of cardiac potassium and other channels by antihistamines

The use of terfenadine and astemizole, two long-acting nonsedating histamine H1 receptor antagonists, has been associated with prolongation of the QT interval, development of ventricular arrhythmias, particularly torsade de pointes, and sudden cardiac death. Both drugs block the rapidly activating component of the delayed rectifier channel, I(Kr). At much higher concentrations, they also block several other cardiac channels (Na+, Ca2+, K+). Since many other antihistamines can also block one or other of the cardiac ion currents (e.g. loratadine blocks the human cardiac K+ channel, hKv1.5, with the same potency as terfenadine), these results are also reviewed and their clinical relevance discussed. Because of the proarrhythmic risk, some antihistamines should be taken only at the recommended doses and avoided in patients with liver disease or in those taking medications that inhibit oxidative cytochrome P-450 enzymes. These drugs should also be avoided in those with the congenital long QT syndrome or with secondary forms of delayed repolarisation (hypokalaemia, bradycardia, drug-induced QT prolongation). Identification of predisposing factors could enable physicians to anticipate, and thereby avoid, this potentially lethal complication of antihistamine therapy.

Negative inotropic effect of somatostatin in guinea-pig atrial fibres

The effect of somatostatin (SS, 1 X 10(-7) M-5 X 10(-6) M) was studied on the electrical and mechanical properties of isolated atria of the guinea-pig. On spontaneously beating right atria, SS produced a dose-dependent negative inotropic effect which was accompanied by a decrease in atrial rate and a prolongation of the sinus node recovery time. In electrically driven left atria, SS produced a dose-dependent negative inotropic effect which occurred concomitantly with a decrease in the amplitude and duration of the plateau phase of the action potential of atrial fibres. SS also decreased the amplitude and maximum rate of depolarization of the slow action potential as well as the amplitude of the slow contractions induced by isoprenaline and caffeine in K-depolarized atrial fibres. The negative inotropic effect of SS varied with the concentration of Ca and Na in the bathing media and the frequency of stimulation. SS, 1 X 10(-6) M and 5 X 10(-6) M, decreased 45Ca uptake in electrically driven atria. All these results suggest that the negative inotropic effect produced by SS on rat isolated atria is related to its ability to reduce Ca influx via the slow inward Ca current.