Pharmacological block of the slow component of the outward delayed rectifier current (I(Ks)) fails to lengthen rabbit ventricular muscle QT(c) and action potential duration

Abstract

1. The effects of I(Ks) block by chromanol 293B and L-735,821 on rabbit QT-interval, action potential duration (APD), and membrane current were compared to those of E-4031, a recognized I(Kr) blocker. Measurements were made in rabbit Langendorff-perfused whole hearts, isolated papillary muscle, and single isolated ventricular myocytes. 2. Neither chromanol 293B (10 microM) nor L-735,821 (100 nM) had a significant effect on QTc interval in Langendorff-perfused hearts. E-4031 (100 nM), on the other hand, significantly increased QTc interval (35.6+/-3.9%, n=8, P<0.05). 3. Similarly both chromanol 293B (10 microM) and L-735,821 (100 nM) produced little increase in papillary muscle APD (less than 7%) while pacing at cycle lengths between 300 and 5000 ms. In contrast, E-4031 (100 nM) markedly increased (30 - 60%) APD in a reverse frequency-dependent manner. 4. In ventricular myocytes, the same concentrations of chromanol 293B (10 microM), L-735,821 (100 nM) and E-4031 (1 microM) markedly or totally blocked I(Ks) and I(Kr), respectively. 5. I(Ks) tail currents activated slowly (at +30 mV, tau=888.1+/-48.2 ms, n=21) and deactivated rapidly (at -40 mV, tau=157.1+/-4.7 ms, n=22), while I(Kr) tail currents activated rapidly (at +30 mV, tau=35.5+/-3.1 ms, n=26) and deactivated slowly (at -40 mV, tau(1)=641.5+/-29.0 ms, tau(2)=6531+/-343, n=35). I(Kr) was estimated to contribute substantially more to total current density during normal ventricular muscle action potentials (i.e., after a 150 ms square pulse to +30 mV) than does I(Ks). 6. These findings indicate that block of I(Ks) is not likely to provide antiarrhythmic benefit by lengthening normal ventricular muscle QTc, APD, and refractoriness over a wide range of frequencies.

Figure 1

Effect of chromanol 293B (10 μM, A), L-735,821 (100 nM, B) and E-4031 (100 nM, C) after 40 min of perfusion on surface electrocardiogram recorded in spontaneously beating isolated Langendorff-perfused rabbit hearts. Note that chromanol caused only a 2 ms and L-735,821 only an 8 ms increase in the QTc interval. The I_Ks blockers chromanol 293B and L-735,821 increased spontaneous RR interval by 1.88±2.93% (n=7) and by 3.12±0.71*% (n=7, ^*P<0.01), respectively. The I_Kr blocker E 4031 increased QTc by 67 ms and prolonged spontaneous the RR interval by 10.61±3.5*% (n=8, ^*P<0.05).

Figure 2

Rabbit ventricular papillary muscle action potential recordings before and after 40 min exposure to chromanol 293 B (10 μM, A), L-735,821 (100 nM, B), E-4031 (100 nM, C). Stimulation frequency was 1 Hz.

Figure 3

Frequency dependent effect of the I_Ks blockers chromanol 293B (10 μM, n=7) and L-735,821 (100 nM, n=7) on the action potential duration measured at 90% repolarization (APD₉₀) compared to that of the I_Kr blocker E-4031 (100 nM, n=8) in isolated rabbit ventricular papillary muscles. E-4031, the I_Kr blocker, lengthened APD in a reverse-frequency dependent manner while the two I_Ks blockers had little effect. On the abscissa CL=pacing cycle length in ms, and on the ordinate percentile change in APD₉₀ is given. Bars represent±s.e.mean.

Figure 4

The effect of I_Ks block on the action potentials in the presence of adenylcyclase activation by forskolin (1 μM). (A) Action potential recorded from rabbit ventricular papillary muscle before and after forskolin (1 μM) and after administration of 10 μM chromanol 293B in the continuous presence of forskolin. (B) Result of a similar experiment using 100 nM L-735,821 to block I_Ks. Stimulation frequency was 1 Hz.

Figure 5

Effect of L-735821 (100 nM, A) and E-4031 (1 μM, B) on I_Ks and I_Kr current-voltage relations in rabbit ventricular myocytes. I_Ks was assessed by measuring peak outward tail current densities following a 5000 ms test pulses to between −20 and +50 mV from a holding potential of −40 mV in seven myocytes in the presence of E-4031 (5 μM) to totally block I_Kr. Pulse frequency was 0.1 Hz. Similarly, I_Kr was assessed by measuring peak outward tail current densities following a 1000 ms test pulses to between −20 and +50 mV from a holding potential of −40 mV in 10 myocytes in the presence of 100 nM L-735,821 to block I_Ks. Pulse frequency was 0.05 Hz. In all studies nisoldipine (1 μM) was used to block inward calcium current (I_Ca). Bars represent±s.e.mean.

Figure 6

Activation and deactivation kinetics for I_Ks (A) and I_Kr (B) in rabbit ventricular myocytes. Activation kinetics (left) measured as peak tail currents at −40 mV after test pulses from a holding potential of −40 to +30 mV with sequentially increasing durations between 10 and 5000 ms. Deactivation kinetics (right) measured from outward tail current upon return to −40 mV after test pulse to +30 mV lasting for 5000 ms with I_Ks or for 1000 ms with I_Kr.

Figure 7

Membrane currents in rabbit ventricular myocytes observed during and after a 150 ms depolarizing pulse to +30 mV from a holding potential of −40 mV before and after I_Kr (1 μM E-4031, A) and I_Ks (100 nM L-735,821, B) block. (C) Bar graph shows mean±s.e.mean difference current magnitudes developed during depolarization to +30 mV and peak tail current on return to −40 mV after I_Kr (left) and I_Ks (right) block.