MRI findings in acute Hendra virus meningoencephalitis.

AIM: To describe serial changes in brain magnetic resonance imaging (MRI) in acute human infection from two outbreaks of Hendra virus (HeV), relate these changes to disease prognosis, and compare HeV encephalitis to reported cases of Nipah virus encephalitis. MATERIALS AND METHODS: The MRI images of three human cases (two of which were fatal) of acute HeV meningoencephalitis were reviewed. RESULTS: Cortical selectivity early in the disease is evident in all three patients, while deep white matter involvement appears to be a late and possibly premorbid finding. This apparent early grey matter selectivity may be related to viral biology or ribavirin pharmacokinetics. Neuronal loss is evident at MRI, and the rate of progression of MRI abnormalities can predict the outcome of the infection. In both fatal cases, the serial changes in the MRI picture mirrored the clinical course. CONCLUSION: This is the first comprehensive report of serial MRI findings in acute human cerebral HeV infection from two outbreaks. The cortical selectivity appears to be an early finding while deep white matter involvement a late, and possibly premorbid, finding. In both fatal cases, the serial changes in MRI mirrored the clinical course.

Potassium and calcium current blocking properties of the novel antiarrhythmic agent H 345/52: implications for proarrhythmic potential.

OBJECTIVES: To study the blocking effects of H 345/52 on ionic currents of rabbit ventricular myocytes and how these features translate into a proarrhythmic potential. METHODS: The single electrode voltage clamp technique was used to study the effects of H 345/52 on the rapid component of the delayed rectifying potassium current, I(Kr), and the L-type calcium current (I(Ca)). Differential effects of H 345/52 and almokalant on APD prolongation were studied in a rabbit Purkinje fibre/ventricular muscle preparation. The temporal variability of the action potential duration (APD) and its relation to proarrhythmias was examined in Langendorff-perfused rabbit hearts administered H 345/52 or almokalant. Anaesthetised, methoxamine-sensitised rabbits were used to assess the propensity of intravenous H 345/52 and ibutilide to induce torsades de pointes (TdP). RESULTS: H 345/52 potently blocked I(Kr) (IC(50)=40 nM) without consequential use-dependency. The I(Ca) was also blocked, but at higher concentrations (IC(50)=1.3 microM). Block of I(Ca) was markedly frequency-dependent (positive) and influenced by membrane potential, such that H 345/52 was more effective following clamp steps from plateau potentials than from -80 mV. In the Purkinje fibre-ventricular muscle preparation, almokalant prolonged the Purkinje fibre APD preferentially, whereas H 345/52 homogeneously prolonged APD in both tissue types. In the perfused rabbit heart, H 345/52 (1 microM) and almokalant (0.3 microM) prolonged APD to a similar degree but increased the temporal variability of APD differently, from 3+/-0.4 ms in control hearts to 8+/-1.2 ms and to 38+/-7.5 ms (P<0.001 vs. H 345/52), respectively. Unequivocal early after-depolarisations were seen in 5/6 almokalant-perfused hearts but in no heart administered H 345/52 (P<0.05). In anaesthetised rabbits, H 345/52 (17.4 micromol/kg) or ibutilide (2.6 micromol/kg maximum), maximally lengthened the QT interval from 133+/-4.5 to 177+/-8.0 ms and from 125+/-5.1 to 166+/-9.3 ms (P<0.001, n=8). However, whereas ibutilide induced TdP in all animals at 0.06+/-0.009 micromol/kg, H 345/52 did not induce TdP (P=0.0002) at up to 17.4 micromol/kg. CONCLUSIONS: H 345/52 blocks I(Kr) with high potency and I(Ca) with somewhat lower potency and was found to delay ventricular repolarisation without substantially increasing temporal or spatial dispersion and without inducing early after-depolarisations or TdP.

Effects of the calcium sensitizer [+]-EMD 60263 and its enantiomer [-]-EMD 60264 on cardiac ionic currents of guinea pig and rat ventricular myocytes.

The thiadiazinone enantiomers [+]-EMD 60263 and [-]-EMD 60264 ((+)-5-(1-(alpha-ethylimino-3,4-dimethoxybenzyl)-1,2,3,4-tetrah ydroquinoline-6-yl)-6-methyl-3,6-dihydro-2H-1,3,4-thiadiazine-2 -on) exhibit distinct stereoselectivity for Ca2+-sensitizing action ([+]-enantiomer) and phosphodiesterase inhibition ([-]-enantiomer). However, in isolated guinea pig papillary muscle, both compounds cause an action-potential prolongation that has been related to a nonselective depression of the delayed rectifier potassium current. Because [-]-EMD 60264 did not increase force of contraction despite phosphodiesterase inhibition, we postulated that one or several additional actions may oppose the anticipated positive inotropic effect. Therefore we investigated whether other membrane currents were also affected in voltage-clamped ventricular cardiomyocytes. Both [+]-EMD 60263 and [-]-EMD 60264 reduced sodium current as well as L-type calcium current in guinea pig ventricular myocytes, but steady-state inactivation or conductance curves of I(Na) and I(Ca) were not shifted along the voltage axis. Inward rectifier and transient outward current were studied in rat myocytes, but neither current was affected. We conclude that the positive inotropic action of [+]-EMD 60263 can be explained by prevalence of the Ca2+-sensitizing effect over its inhibitory actions on Na+ and Ca2+ current, whereas the negative inotropic effect of [-]-EMD 60264 may be caused by inhibition of I(Ca) predominating over PDE inhibition.

Mechanism of block by tedisamil of transient outward current in human ventricular subepicardial myocytes.

1. Tedisamil is a new antiarrhythmic drug with predominant class III action. The aim of the present study was to investigate the blocking pattern of the compound on the transient outward current (I(to)) in human subepicardial myocytes isolated from explanted left ventricles. Using the single electrode whole cell voltage clamp technique, I(to) was analysed after appropriate voltage inactivation of sodium current and block of calcium current. 2. Tedisamil reduced the amplitude of peak I(to), but did not affect the amplitude of non-inactivating outward current. The drug accelerated the apparent rate of I(to) inactivation. The reduction in time constant of I(to) inactivation depended on drug concentration, the apparent IC50 value was 4.4 microM. 3. Tedisamil affected I(to) amplitude in a use-dependent manner. After 2 min at -80 mV, maximum block of I(to) was reached after 4-5 clamp steps either at the frequency of 0.2 or 2 Hz, indicating that the block was not frequency-dependent in an experimentally relevant range. Recovery from block was very slow and proceeded with a time constant of 12.1+/-1.8 s. Also in the presence of drug, a fraction of channels recovered from inactivation with a similar time constant as in control myocytes (i.e. 81+/-40 ms and 51+/-8 ms, respectively, n.s.). 4. From the onset of fractional block of I(to) by tedisamil during the initial 60 ms of a clamp step, we calculated k1 = 9 x 10(6) mol(-1) s(-1) for the association rate constant, and k2 = 23 s(-1) for the dissociation rate constant. The resulting apparent KD was 2.6 microM and is similar to the IC50 value. 5. The effects of tedisamil on I(to) could be simulated by assuming a four state channel model where the drug binds to the channel in an open (activated) conformation. It is concluded that in human subepicardial myocytes tedisamil is an open channel blocker of I(to) and that this effect probably contributes to the antiarrhythmic potential of this drug.

Electrophysiological characterization of the prokinetic agents cisapride and mosapride in vivo and in vitro: implications for proarrhythmic potential?

In the present study the electrophysiological characteristics and the proarrhythmic potential of cisapride and a structurally related drug, mosapride, were compared. In the anesthetized guinea pig, cisapride and d-sotalol (0.01-10 micromol/kg i.v., n = 6) dose-dependently prolonged the duration of the monophasic action potential recorded from the left ventricle. The maximal lengthening was 18 +/- 3.2% at 1.0 micromol/kg (mean +/- S.E.M., P < .01 vs. base line) and 19 +/- 2.5% at 10 micromol/kg (P < .001) for cisapride and d-sotalol, respectively. In contrast, mosapride did not increase this variable. In a rabbit model of the acquired long QT syndrome, infusion of cisapride (0.3 micromol/kg/min for 10 min maximum, n = 6), but not mosapride or vehicle, was associated with a significant lengthening of the QTU interval (43 +/- 3.8 ms, P < .01). Furthermore, torsades de pointes appeared in two of the six rabbits given cisapride. In isolated rabbit Purkinje fibers (PF), cisapride increased the action potential duration (48 +/- 5.6% at 0.1 micromol/l, P < .01 vs. control, n = 4). Mosapride did not significantly influence the action potential duration (3 +/- 2.0% increase at 1.0 micromol/l, n = 6). However, after mosapride was washed out, the addition of cisapride (0.1 micromol/l) caused a 46 +/- 3.2% lengthening of the action potential duration (P < .01 vs. 1.0 micromol/l mosapride). Early afterdepolarizations and triggered activity appeared in four of eight cisapride-superfused PF stimulated at a very low frequency (0.1 Hz). In isolated rabbit cardiomyocytes, cisapride concentration-dependently blocked (IC50 = 9 nmol/l) the rapid component of the delayed rectifying K+ current (I(Kr)). Mosapride was approximately 1000-fold less potent in blocking I(Kr) (IC50 = 4 micromol/l). It is concluded that the electrophysiological characteristics of cisapride may explain the recently reported propensity to prolong the QT interval and to induce torsades de pointes in susceptible patients, although a structurally related benzamide, mosapride, did not appear to have electrophysiological features of relevance for induction of torsades de pointes in common with cisapride.

Differences between outward currents of human atrial and subepicardial ventricular myocytes.

1. Outward currents were studied in myocytes isolated from human atrial and subepicardial ventricular myocardium using the whole-cell voltage clamp technique at 22 degrees C. The Na+ current was inactivated with prepulses to -40 mV and the Ca2+ current was eliminated by both reducing extracellular [Ca2+] to 0.5 mM and addition of 100 microM CdCl2 to the bath solution. 2. In human myocytes, three different outward currents were observed. A slowly inactivating sustained outward current, I(so), was found in atrial but not ventricular myocytes. A rapidly inactivating outward current, I(to), of similar current density was observed in cells from the two tissues. An additional uncharacterized non-inactivating background current of similar size was observed in atrial and in ventricular myocytes. 3. I(to) and I(so) could be differentiated in atrial myocytes by their different kinetics and potential dependence of inactivation, and their different sensitivities to block by 4-amino-pyridine, suggesting that two individual channel types were involved. 4. In atrial cells, inactivation of I(to) was more rapid and steady-state inactivation occurred at more negative membrane potentials than in ventricular cells. Furthermore, the recovery of I(to) from inactivation was slower and without overshoot in atrial myocytes. In addition, 4-aminopyridine-induced block of I(to) was more efficient in atrial than in ventricular cells. These observations suggest that the channels responsible for atrial and ventricular I(to) were not identical. 5. We conclude that the differences in outward currents substantially contribute to the particular shapes of human atrial and ventricular action potentials. The existence of I(so) in atrial cells only provides a clinically interesting target for anti-arrhythmic drug action, since blockers of I(so) would selectively prolong the atrial refractory period, leaving ventricular refractoriness unaltered.

Electrophysiology of ion channels of the heart.

The contribution of Na+, Ca2+, and various K+ currents to the shape of the cardiac action potential is outlined based on the relation between electrophysiological properties and structure of channel molecules. These currents have also been found in human ventricular myocytes, where the most prominent K+ current is a transient outward current that is not influenced by methylsulfonanilide antiarrhythmic drugs. Combined knowledge of electrophysiological and molecular properties of ion channels is likely to form the basis for rational design of future drugs.

The inotropic agents DPI 201-106 and BDF 9148 differentially affect potassium currents of guinea-pig ventricular myocytes.

The inotropic agents DPI 201-106 and BDF 9148 increase action potential duration (APD) of heart muscle. This effect can be explained by inhibition of inactivation of sodium current, which is affected by both agents to a similar extent (Ravens et al. 1991, Br J Pharmacol 104:1019-1023). However, as DPI 201-106 prolongs APD of guinea-pig ventricle to a larger extent than BDF 9148, other currents may also be involved. The aim of the present study was to measure the effects of DPI 201-106 and BDF 9148 on the inward rectifier IK1, and the two components of the delayed rectifier, IKs and IKr. The methyl-for-carbonitrile-substituted derivative BDF 8784 was included to study structure-activity relationships. Single-electrode whole-cell voltage-clamp technique was used to measure membrane currents of guinea-pig ventricular myocytes. Only DPI 201-106 reduced IK1 at potentials both negative and positive to the reversal potential. Three microM of DPI 201-106 reduced IKs, whereas 1 microM of BDF 9148 had no effect on this current. These concentrations were equieffective with respect to positive inotropic action (Ravens et al. 1991, Br J Pharmacol 104:1019-1023). BDF 9148 did however block IKs at higher concentrations, as did BDF 8784. It is concluded that block of outward current by DPI 201-106, but insignificant effects of BDF 9148, are responsible for the differential effects of these compounds on APD at equieffective concentrations with respect to inotropy.

Transient outward current in human ventricular myocytes of subepicardial and subendocardial origin.

In various mammalian species, shapes of action potentials vary within the cardiac wall because of differences in transient outward current (Ito). A prominent Ito exists in human ventricular myocytes, but cells have not been separated according to their original localization. Human ventricular myocytes were isolated from separated subepicardial and subendocardial tissue, and regional variations in Ito were studied. Ito was larger in subepicardial than subendocardial cells. Current density at +60 mV was 7.9 +/- 0.7 pA/pF (n = 28) in subepicardial cells and 2.3 +/- 0.3 pA/pF (n = 16) in subendocardial cells. When cells from explanted failing and nonfailing donor hearts were compared, Ito was not different in subepicardial cells; however, it was larger in subendocardial cells from nonfailing hearts. The potential of half-maximal activation (V0.5) was more positive in subendocardial cells (+25.6 +/- 3.5 mV, n = 15) than in subepicardial cells (+9.2 +/- 1.8 mV, n = 28). There was no difference in V0.5 between cells from failing and nonfailing hearts. Ito inactivation was similar in all cell types and independent of membrane depolarization (time constant [tau] = approximately 60 milliseconds at 22 degrees C). The potential of half-maximal steady-state inactivation was similar in all cell types. Recovery from inactivation of Ito was fast in subepicardial cells at -100 mV (tau = 24 +/- 4 milliseconds, n = 6), exceeding control values transiently (overshoot), and slow at -40 mV without overshoot (tau = 638 +/- 91 milliseconds, n = 6). In subendocardial cells, Ito recovered at -100 mV with a fast phase (tau = 25 milliseconds) and a slow phase (tau = 328 milliseconds), and recovery was not complete after 6 seconds at -100 mV. In conclusion, regional differences in Ito between subepicardial and subendocardial cells may have clinical implications with respect to rhythmic disturbance during heart failure.

Comparison of the action potential prolonging and positive inotropic activity of DPI 201-106 and BDF 9148 in human ventricular myocardium.

The aim of our study was to compare the effects on contractile function and action potential duration of the new Na+ channel modulator BDF 9148 with the parent compound DPI 201-106 in human ventricular myocardium. Right ventricular papillary muscles were obtained from explanted hearts of heart transplant recipients or from non-failing hearts not suitable for transplantation. BDF 9148 induced an increase in force of contraction that was accompanied by prolongation of action potential duration. The action potential duration prolonging effect of BDF 9148 was not significantly different to that of DPI 201-106. The effects of BDF 9148 were similar in muscles obtained from non-failing and failing hearts. Using Na(+)-sensitive electrodes, we have demonstrated that the positive inotropic effect of BDF 9148 is accompanied by an increase in intracellular Na+ activity. Our results indicate: (i) that BDF 9148 is as effective as DPI 201-106 in increasing force of contraction and prolonging action potential duration in human ventricular myocardium: (ii) that BDF 9148 is effective in enhancing force of contraction, in spite of heart failure; (iii) that the positive inotropic effect is related to an increased Na+ load; and (iv) due to action potential duration prolongation, changes in Q-T interval of the electrocardiogram could be possible during in vivo use of BDF 9148.

Frequency-dependent effects of E-4031, almokalant, dofetilide and tedisamil on action potential duration: no evidence for "reverse use dependent" block.

Antiarrhythmic drugs with class III action are incriminated by "reverse use dependency" which implies preferential block of resting channels (Hondeghem and Snyders 1990). The purpose of the present study was to investigate the frequency dependence of the effects of four new antiarrhythmic compounds on action potential duration (APD) in guinea-pig papillary muscle and on delayed rectifier in guinea-pig ventricular myocytes in order to scrutinize the concept of reverse use dependency and to obtain evidence for drug-channel interaction. In guinea-pig papillary muscles, E-4031 (1-[2-(6-methyl-2-pyridyl)ethyl]-4- (4-methylsulfonyl-aminobenzoyl)piperidine), almokalant, dofetilide and tedisamil prolonged APD in a concentration-dependent manner. Drug-induced APD prolongation was not affected significantly by low rates of stimulation (0.2 to 0.5 Hz. In order to investigate whether drug-channel interaction takes places during rest, regular stimulation (1 Hz) was interrupted by three 30-min periods of quiescence. Drug was added at the beginning of the second period of rest, the third period was interposed at steady state of drug action. With E-4031 and dofetilide no change in shape of the first AP after the initial 30 min of drug exposure was observed as compared with pre-drug control, but regular stimulation was required for the full effect to develop. APD did not recover to pre-drug values after the third period of quiescence. With almokalant and tedisamil, however, the first APD after wash-in was already prolonged and the effects increased further with regular pacing. Only with almokalant but not with tedisamil did APD recover during rest.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential effects of BDF 9148 and DPI 201-106 on action potential and contractility in rat and guinea pig myocardium.

The effects of BDF 9148 and its parent compound DPI 201-106 were compared in guinea pigs and rat cardiac tissue because these tissues possess long and short action potentials (AP), respectively, and have different excitation-contraction coupling mechanisms. Conventional electrophysiologic techniques were used to study drug effects on AP, sodium current, and force of contraction (Fc). In guinea pig and rat papillary muscle, BDF 9148 and DPI 201-106 increased Fc; BDF 9148 was more potent than DPI 201-106. In guinea pig, but not in rat muscle, DPI 201-106 significantly prolonged AP duration (APD) to a greater degree than did BDF 9148. In rat cardiac muscle, both agents were more potent but increased Fc to a lesser degree than in guinea pig cardiac muscle and led to Ca2+ overload, as evidenced by after-contractions and contracture. BDF 9148 failed to increase Fc at low frequencies in guinea pig but was effective at all frequencies in rat muscle. In isolated myocytes, substance effects on sodium current were similar in both species. In guinea pigs, but not in rats, DPI 201-106 prolongs APD to a greater degree than does BDF 9148. Both agents produce a greater positive inotropic effect in guinea pigs than in rats, which may be due to species differences in excitation-contraction coupling.