Interaction of different potassium channels in cardiac repolarization in dog ventricular preparations: role of repolarization reserve

Abstract

1 The aim of this study was to investigate the possible role of the interaction of different potassium channels in dog ventricular muscle, by applying the conventional microelectrode and whole cell patch-clamp techniques at 37 degrees C. 2 Complete block of I(Kr) by 1 micro M dofetilide lengthened action potential duration (APD) by 45.6+/-3.6% at 0.2 Hz (n=13). Chromanol 293B applied alone at 10 micro M (a concentration which selectively blocks I(Ks)) did not markedly lengthen APD (<7%), but when repolarization had already been prolonged by complete I(Kr) block with 1 micro M dofetilide, inhibition of I(Ks) with 10 micro M chromanol 293B substantially delayed repolarization by 38.5+/-8.2% at 0.2 Hz (n=6). 3 BaCl(2), at a concentration of 10 micro M which blocks I(Kl) without affecting other currents, lengthened APD by 33.0+/-3.1% (n=11), but when I(Kr) was blocked with 1 micro M dofetilide, 10 micro M BaCl(2) produced a more excessive rate dependent lengthening in APD, frequently (in three out of seven preparations) initiating early afterdepolarizations. 4 These findings indicate that if only one type of potassium channels is inhibited in dog ventricular muscle, excessive APD lengthening is not likely to occur. Dog ventricular myocytes seem to repolarize with a strong safety margin ('repolarization reserve'). However, when this normal 'repolarization reserve' is attenuated, otherwise minimal or moderate potassium current inhibition can result in excessive and potentially proarrhythmic prolongation of the ventricular APD. Therefore, application of drugs which are able to block more than one type of potassium channel is probably more hazardous than the use of a specific inhibitor of one given sort of potassium channel, and when simultaneous blockade of several kinds of potassium channel may be presumed, a detailed study is needed to define the determinants of 'repolarization reserve'.

Figure 1

Effect of chromanol 293B on the inward rectifier (I_Kl), transient outward (I_to), rapid delayed rectifier (I_Kr) and slow delayed rectifier (I_Ks) potassium currents in dog ventricular myocytes. When measuring I_Kr, L-735,821 (0.1 μM) was used to completely block I_Ks, and E-4031 (2 μM) was applied to block I_Kr when measuring I_Ks. The applied voltage protocols are shown in the insets and explained in the text in more detail. In all experiments I_Ca was fully blocked by addition of 1 μM nisoldipine. The dotted lines represent the zero current levels.

Figure 2

Frequency dependent effect of I_Ks block (chromanol 293B) on the action potential duration in dog right ventricular papillary muscle. Note that the error bars are often smaller than the corresponding symbols.

Figure 3

Frequency dependent effect of I_Kr (dofetilide) and combined I_Kr (dofetilide) and I_Ks (chromanol 293B) block on the action potential duration in dog right ventricular papillary muscle. Note that the error bars are often smaller than the corresponding symbols, and the drug combination augmented the reverse rate dependent APD prolongation.

Figure 4

Effect of BaCl₂ on the inward rectifier (I_Kl), transient outward (I_to), rapid delayed rectifier (I_Kr) and slow delayed rectifier (I_Kr) potassium currents in dog ventricular myocytes. For measuring the effect of BaCl₂ on I_Kr the following protocol was used; (1) current recording under control conditions; (2) current recording in the presence of 10 μM BaCl₂; (3) current recording after application of 2 μM E-4031 (in the presence of 10 μM BaCl₂). Subtracting (3) from (2) gave the E-4031 sensitive current (i.e. I_Kr) in the presence of 10 μM BaCl₂. For measuring the effect of BaCl₂ on I_Ks a similar protocol was used but 0.1 μM L-731,821 was applied for determining I_Ks. The applied voltage protocols are shown in the insets, and explained in the text in more detail. In all experiments I_Ca was fully blocked by addition of 1 μM nisoldipine. The dotted lines represent the zero current levels.

Figure 5

Frequency dependent effect of I_Kl block (BaCl₂) on the action potential duration in dog right ventricular papillary muscle. Note that the error bars are often smaller than the corresponding symbols.

Figure 6

Frequency dependent effect of I_Kr (dofetilide) and combined I_Kr (dofetilide) and I_Kl (BaCl₂) block on the action potential duration in dog right ventricular papillary muscle. Note that the error bars are often smaller than the corresponding symbols, and the drug combination augmented the reverse rate dependent APD prolongation.

Figure 7

Early afterdepolarization (EAD) evoked by the combined I_Kr (dofetilide) and I_Kl (BaCl₂) block in dog right ventricular papillary muscle.