4.1. Introduction
A small, very slowly inactivating, TTX-sensitive sodium current exists in cardiac myocytes was described in Chapter 3. There has been considerable indirect evidence for such a current e.g. the decrease in duration of action potentials in dog Purkinje fibres caused by TTX (Coraboeuf, Deroubaix & Coulombe, 1979). As this current becomes activated close to the resting membrane potential (Chapter 3; Saint, Ju & Gage, 1992), it would contribute to the pacemaker current. Drugs that depress the persistent sodium current could act as antiarrhythmic agents by increasing the interval between action potentials and slowing the heart rate. Indeed, TTX has been shown to have an antiarrhythmic action in vivo (Abraham et at. 1989). But not all of antiarrhythmic drugs action this way, some prolonging the refractory period by blocking potassium currents may contribute to antifibrillatory actions (Woosley,
1991).
Lidocaine and quinidine are widely used in the management of ventricular arrhythmias. Both decrease the maximum rate of rise of ventricular action potentials and have been classified as Class 1 antiarrhythmic agents (Vaughan-Williams, 1981) which share the common property of acting primarily on sodium channels. It is generally considered that these drugs
suppress arrhythmias by blocking the sodium channels that cause rapid depolarization during an action potential and then quickly inactivate. Arrhythmias would be less likely in the presence of such agents because of an increase in threshold or a decrease in conduction velocity of action potentials.
In this chapter, the effects of lidocaine and quinidine on the persistent sodium current are described. Both lidocaine and quinidine block the persistent sodium current at clinically relevant concentrations (Rosen, Hoffman & Wit, 1975; Hoffman, Rosen & Wit, 1975) that have relatively little effect on the transient sodium current.
These observations raise the possibility that the antiarrhythmic effects of these drugs are due, at least in part, to depression of the persistent sodium current.
4.2. Solutions and drugs
For simultanous recording of sodium and potassium currents, the pipette solution contained (mM): KF 120; K-EGTA 10; MgCl2 1; CaCl2 2; ATP 10; TES 10, pH adjusted to 7.4 ± 0.05 with KOH. For recording sodium currents in isolation, the pipette solution contained (mM): CsF 50; NaF 70; K-EGTA 20; CaCl2 2; TES 10; ATP 10, pH adjusted to 7.4 ± 0.05 with KOH. Electrodes typically had resistances of 1.5 to 2.0 Ml) when filled with pipette solution. Experiments were performed at room temperature (22 to 25 °C) in a bath solution normally containing (mM): NaCl 130; KC1 5.4; MgCl2 1; CaCl2 2; CsCl 5; CoCl2 5; TES 10; NaOH 5; glucose 10, pH adjusted to 7.4 ±0.05 with NaOH.
Quinidine sulfate dihydrate (Aldrich-chemie D-7924 Steinheim) and lidocaine (Sigma Chemical, St.louis, MO) were dissolved in test solutions.
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4.3. The effects of drugs on the action potentials
The effects of tetrodotoxin, lidocaine and quinidine on action potentials in isolated rat ventricular myocytes are shown for comparison in Fig. 4-1.
At the concentrations used, tetrodotoxin and lidocaine had little effect on the amplitude of action potentials but affected their plateau phase. It can be seen in Fig. 4-1 that tetrodotoxin (0.1 /xM) and lidocaine (25 /xM) shortened the duration of the action potential. Quinidine (10 /xM) had a more complicated effect, as shown in Fig. 4-1C: the amplitude of the action potential was slightly reduced and repolarization after the peak was initially more rapid but the plateau phase was prolonged. The opposite effects of lidocaine and quinidine on the duration of the plateau phase of action potentials agree with previous observations (e.g. Colatsky, 1982).
The plateau phase of a cardiac action potential is due to a balance of inward and outward currents at the plateau potential. A drug which disturbs the balance of these currents should affect the plateau. For example, TTX would be expected to affect the plateau because it has been shown at this concentration (0.1 /xM) to block a persistent, inward sodium current that would be present at the plateau potential (Chapter 3; Saint, Ju & Gage, 1992). And indeed TTX does shorten the action potential (Fig. 4-1 A). The reduced duration of the plateau phase of the action potential caused by lidocaine (Fig. 4-IB) would be explained if lidocaine also blocked the persistent sodium current. This possibility was therefore tested.
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4.4. Lidocaine and quinidine block the persistent sodium current
The effect of lidocaine on the persistent sodium current is illustrated in Fig. 4-2A. At a concentration of 200 lidocaine clearly blocked the persistent sodium current while not completely blocking the transient sodium current (the peak amplitude of the transient current was reduced from 30.1 to 2.7 nA). It has been proved in choline substitution experiments (see Chapter 3, Fig. 3-2) that this concentration of TTX (50/xM) could completely block the persistent sodium current. Therefore, the degree of block of the persistent sodium current by the lidocaine was checked with 50 /zM TTX. Addition of TTX (50 /zM) caused no further reduction in the amplitude of the persistent sodium current (Fig. 4-2A) indicating that the lidocaine had completely blocked this current. It can be seen in Fig. 4-2A, however, that the addition of TTX caused a further reduction in the amplitude of the transient sodium current.
Quinidine had a similar effect on the persistent sodium current (Fig. 4- 2B). At a concentration of 40 /xM, quinidine caused significant depression of the persistent sodium current whereas the transient current was much less affected (the peak amplitude of the transient current was reduced from -68 to -11 nA).
Again, most of the persistent sodium current must have been blocked by the quinidine because there was little further depression of the persistent current when 50 /zM TTX was added to the solution (Fig. 4-2B).