Edward Carmeliet

Abrupt rate accelerations or premature beats cause life-threatening arrhythmias in mice with long-QT3 syndrome

Deletion of amino-acid residues 1505–1507 (KPQ) in the cardiac SCN5A Na+ channel causes autosomal dominant prolongation of the electrocardiographic QT interval (long-QT syndrome type 3 or LQT3). Excessive prolongation of the action potential at low heart rates predisposes individuals with LQT3 to fatal arrhythmias, typically at rest or during sleep. Here we report that mice heterozygous for a knock-in KPQ-deletion (SCN5AΔ/+) show the essential LQT3 features and spontaneously develop life-threatening polymorphous ventricular arrhythmias. Unexpectedly, sudden accelerations in heart rate or premature beats caused lengthening of the action potential with early afterdepolarization and triggered arrhythmias in Scn5aΔ/+ mice. Adrenergic agonists normalized the response to rate acceleration in vitro and suppressed arrhythmias upon premature stimulation in vivo. These results show the possible risk of sudden heart-rate accelerations. The Scn5aΔ/+ mouse with its predisposition for pacing-induced arrhythmia might be useful for the development of new treatments for the LQT3 syndrome.

Downregulation of Delayed Rectifier K+ Currents in Dogs With Chronic Complete Atrioventricular Block and Acquired Torsades de Pointes

BACKGROUND Acquired QT prolongation enhances the susceptibility to torsades de pointes (TdP). Clinical and experimental studies indicate ventricular action potential prolongation, increased regional dispersion of repolarization, and early afterdepolarizations as underlying factors. We examined whether K(+)-current alterations contribute to these proarrhythmic responses in an animal model of TdP: the dog with chronic complete atrioventricular block (AVB) and biventricular hypertrophy. METHODS AND RESULTS The whole-cell K(+) currents I(TO1), I(K1), I(Kr), and I(Ks) were recorded in left (LV) and right (RV) ventricular midmyocardial cells from dogs with 9+/-1 weeks of AVB and controls with sinus rhythm. I(TO1) density and kinetics and I(K1) outward current were not different between chronic AVB and control cells. I(Kr) had a similar voltage dependence of activation and time course of deactivation in chronic AVB and control. I(Kr) density was similar in LV myocytes but smaller in RV myocytes (-45%) of chronic AVB versus control. For I(Ks), voltage-dependence of activation and time course of deactivation were similar in chronic AVB and control. However, I(Ks) densities of LV (-50%) and RV (-55%) cells were significantly lower in chronic AVB than control. CONCLUSIONS Significant downregulation of delayed rectifier K(+) current occurs in both ventricles of the dog with chronic AVB. Acquired TdP in this animal model with biventricular hypertrophy is thus related to intrinsic repolarization defects.

Adrenaline and the plateau phase of the cardiac action potential

SummaryConduction block in heart cells by K+ rich, or Na+ depleted solutions can be overcome by adrenaline. In order to explain this phenomenon, the effect of adrenaline on the membrane resting and action potentials of cow Purkinje fibers was measured at various extracellular concentrations of Na+, K+ and Ca++, in the presence of tetrodotoxin, Mn++ and beta-receptor antagonists.It was found that adrenaline specifically increases the amplitude and duration of the plateau phase of the cardiac action potential. Plateu-like action potentials, without preceding Na+-spike, can be generated and conducted in an all-or-nothing way. In K+ rich solutions and under the influence of adrenaline, the depolarization proceeds in two steps. The first step corresponds to the Na+-spike. The second step or secondary depolarization corresponds to the plateau; it was not modified by changes of the membrane potential between −85 and −55 mV, or by reduction of extracellular Na+ ions, but was specifically blocked by Mn++ ions and beta-receptor antagonists. Its amplitude increased by 17 mV for a tenfold change in extracellular Ca++. Tetrodotoxin preferentially blocked the Na+-spike, but also slowed the rate of potential change during the secondary depolarization.The simplest explanation for the observed phenomena can be found in an increase of Ca++ inward current under the influence of adrenaline. The existence of an inward Na++ current, different in characteristics from the Na+ conductance during the fast upstroke, cannot be ruled out. Some data are in accord with a decrease in K+ conductance.

Existence of two transient outward currents in sheep cardiac Purkinje fibers

Voltage clamp analysis of the transient outward (positive dynamic) current was performed in sheep Purkinje fibers at a pulse frequency of 1/min. 4-aminopyridine (4-AP, 1 mM) suppressed most of the transient outward current, thus revealing the slow inward current,isi, and an associated brief outward current,ibo. The long lasting component of the current suppressed by 4-AP was labelledilo. In the presence of 4-AP,ibo was suppressed either by caffeine 10 mM or when Sr was substituted for Ca, both conditions makingisi clearly detectable. Mn ions suppressed bothisi andibo. Current decay was a monoexponential process foribo (τ=12 ms) and a two exponential process forilo (τ1=80−100 ms, τ2=250−400 ms). The peak amplitude-Em relationships were different for the two currents. It was shown that the reversal potential ofilo was not measureble by the usual method probably because of the too fast activation-deactivation kinetics of the current.It is concluded that not one but two transient outward currents with different electrophysiological and pharmacological characteristics exist in the sheep Purkinje fiber. The reason of the caffeine-sensitivity ofibo is discussed.

Inhibition and Rapid Recovery of Ca2+ Current During Ca2+ Release From Sarcoplasmic Reticulum in Guinea Pig Ventricular Myocytes

We have investigated the modulation of the L-type Ca2+ channel by Ca2+ released from the sarcoplasmic reticulum (SR) in single guinea pig ventricular myocytes under whole-cell voltage clamp. [Ca2+]i was monitored by fura 2. By use of impermeant monovalent cations in intracellular and extracellular solutions, the current through Na+ channels, K+ channels, nonspecific cation channels, and the Na+-Ca2+ exchanger was effectively blocked. By altering the amount of Ca2+ loading of the SR, the time course of the Ca2+ current (ICa) could be studied during various amplitudes of Ca2+ release. In the presence of a large Ca2+ release, fast inhibition of ICa occurred, whereas on relaxation of [Ca2+]i, fast recovery was observed. The time course of this transient inhibition of ICa reflected the time course of [Ca2+]i. However, the inhibition seen in the first 50 ms, ie, the time of net Ca2+ release from the SR, exceeded the inhibition observed later during the pulse, suggesting the existence of a higher [Ca2+] near the channel during this time. Transient inhibition of ICa during Ca2+ release was observed to a similar degree at all potentials. It could still be observed in the presence of intracellular ATP-gamma-S and of cAMP. Therefore, we conclude that the modulation of ICa by Ca2+ release from the SR is not related to dephosphorylation. It could be related to a reduction in the driving force and to a direct inhibition of the channel by [Ca2+]i. The observation that the degree of inhibition does not depend on membrane potential suggests that the Ca2+ binding site for this modulation is located outside the pore.(ABSTRACT TRUNCATED AT 250 WORDS)

Stereoselective effects of the enantiomers of bupivacaine on the electrophysiological properties of the guinea‐pig papillary muscle

1 Direct myocardial effects of the S(−)‐ and R(+)‐enantiomers of bupivacaine were compared in the guinea‐pig isolated papillary muscle by recording transmembrane action potentials with the standard microelectrode technique. 2 In 5.4 mm K+, at a stimulation rate of 1 Hz, the maximal rate of depolarization () was reduced to 59.9 ± 1.4% (n = 10) of control (mean ± s.e.mean) in the presence of 10 μm R(+)‐bupivacaine, and to 76.7 ± 1.2% (n = 14) in the presence of the same concentration of S(−)‐bupivacaine. This was mainly due to a difference in time constant at which block dissipated during the diastolic period. Recovery was slower in the presence of R(+)‐bupivacaine. The slower recovery in the presence of R(+)‐bupivacaine resulted also in a more pronounced frequency‐dependent block of . 3 Time constants for recovery from use‐dependent block became significantly faster for both enantiomers on hyperpolarization, while no significant change was observed at depolarization. At all membrane potentials recovery was slower in the presence of R(+)‐bupivacaine. 4 The action potential duration (APD) was shortened to a greater extent in the presence of R(+)‐bupivacaine over a large range of stimulation frequencies. 5 We conclude that S(−)‐bupivacaine affects and APD in the guinea‐pig papillary muscle less than the R(+)‐enantiomer at different rates of stimulation and resting membrane potentials.

Ionic currents during hypoxia in voltage-clamped cat ventricular muscle.

To explore the mechanisms underlying the shortening of the cardiac action potential in hypoxia, we studied the effect of hypoxia on the ionic currents in cat papillary and trabecular muscles using the single sucrose gap-voltage clamp technique. For potentials positive to −70 mV, hypoxia induces an increase in time-independent outward current. The changes in the tail current suggest that time-dependent outward current is not increased but, rather, reduced. Because the time course of IK remains unchanged, we concluded that the shortening of the action potential is not a result of a change in the time-dependent outward current. In the potential range of the plateau, the amplitude of the slow inward current is not affected by hypoxia. Its time constant of inactivation appears slightly decreased. The prolongation of the action potential by epinephrine during hypoxia is accompanied by an increase in the slow inward current. As a result of these studies, we conclude that the shortening of the cardiac action potential in the early stage of hypoxia results from an increase in K+outward background current. Circ Res 47: 501-508, 1980

Use-dependent block and use-dependent unblock of the delayed rectifier K+ current by almokalant in rabbit ventricular myocytes.

A voltage-clamp analysis of the effect of almokalant on the delayed rectifier K+ current (IK) was made in rabbit ventricular myocytes. The two-suction pipette method was used, and appropriate voltage-clamp protocols were used to study more specifically use dependence, block development, and recovery from block. Almokalant interacted with the IK in two ways: it shifted the activation curve in the hyperpolarizing direction (stimulatory effect) and blocked the open IK channel in a use-dependent way (inhibitory effect). For 2-second voltage clamps to +20 mV, half-maximum block was obtained at 5 x 10(-8) mol/L, with a Hill coefficient of 1.76. Use-dependent block was related to an open-channel block that occurred at 0 mV with a time constant of 1.07 second and a rather slow recovery from block: at -50 mV, recovery time constant was approximately 10 seconds; at -75 mV, recovery was practically absent. The absence of an important recovery at negative membrane potentials is consistent with the hypothesis of the drug being trapped in the channel. A limited frequency-dependent block could be demonstrated. Use-dependent unblock was demonstrated by a rapid recovery from block during stimulation following complete washout of the drug. It is concluded that almokalant shifts the activation curve of IK in the hyperpolarizing direction, blocks the open channel, and is trapped by the closure of the activation gate.

Slow inactivation of the sodium current in rabbit cardiac Purkinje fibres

An analysis of the slowly inactivating Na current in rabbit cardiac Purkinje fibres was made, using the twomicroelectrode voltage clamp technique. The existence of the slowly inactivating Na current was demonstrated by recording TTX-sensitive currents. The current was sensitive to Na withdrawal and could be blocked by 0.1 mM Cd. The current-voltage relation extended over a broad range of potentials, as negative as −85 mV. The time course of inactivation consisted of different phases, with time constants differing as much as three orders of magnitude. Time constants of the first phase of slow inactivation increased at more positive potentials. Non-inactivating Na currents were observed in the threshold region. Recovery from inactivation was less complex. The voltage dependency of inactivation could be described by a sigmoidal curve with a half maximum potential of −75.6 mV and a slope factor of 6.3 mV. Deactivation was fast. The results suggest that at the microscopic level the Na channel shows multiple states of inactivation. At the macroscopic level the slowly inactivating Na current plays an important role in determining diastolic potential, pacemaker activity and plateau duration, and is fundamental in explaining the effect of local anesthetics and frequency of stimulation on action potential duration.

T‐type Ca2+ current as a trigger for Ca2+ release from the sarcoplasmic reticulum in guinea‐pig ventricular myocytes

1 We have investigated whether Ca2+ entry through T‐type Ca2+ channels participates in triggering Ca2+ release from the sarcoplasmic reticulum (SR) in single guinea‐pig ventricular myocytes (whole‐cell voltage clamp, K5fura‐2 as [Ca2+]i indicator; all monovalent cations replaced by impermeant ions to record uncontaminated Ca2+ currents; T= 23 or 36 °C). 2 T‐type Ca2+ currents were elicited from a holding potential of ‐90 mV during steps to ‐50 to ‐20 mV. For steps to ‐50 mV, very small [Ca2+]i transients could be recorded with high loading of the SR (peak Δ[Ca2+]i, 67 ± 41 nM; n= 9). 3 For steps to ‐40, ‐30 and ‐20 mV, we compared the amplitude of Ca2+ release for a holding potential of ‐50 mV with L‐type Ca2+ current only to Ca2+ release for a holding potential of ‐90 mV with both T‐ and L‐type Ca2+ current. Significantly more Ca2+ release was observed with T‐type current present, and both the T‐type current and the additional Ca2+ release were suppressed by 50 μM NiCl2. 4 Ca2+ influx through T‐type Ca2+ channels triggered less Ca2+ release than a comparable Ca2+ influx through L‐type Ca2+ channels. 5 Rapid block of T‐type Ca2+ current during the action potential (50 μM NiCl2 during steady‐state stimulation at 1 or 2 Hz) did not immediately reduce Ca2+ release, although a small decrease was observed after longer application. 6 We conclude that T‐type Ca2+ current can trigger Ca2+ release from the SR albeit less efficiently than L‐type Ca2+ current. T‐type current is most likely to provide only a small contribution to the trigger for Ca2+ release in normal conditions. These results support the hypothesis that L‐type Ca2+ channels have a privileged role in excitation‐contraction coupling.