Mechanisms of block of a human cloned potassium channel by the enantiomers of a new bradycardic agent: S‐16257‐2 and S‐16260‐2

Abstract

1 The effects of S-16257-2 (S57) and S-16260-2 (R60), the two enantiomers of a new bradycardic agent, were studied on human cloned K+ channels (hKvl.5) stably expressed in a mouse L cell line using the whole-cell configuration of the patch-clamp technique. 2 S57 and R60 did not modify the sigmoidal activation time course of the current but reduced the amplitude and increased the rate of the decay of the current during the application of depolarizing pulses. Both, S57 and R60 produced a concentration-dependent block of hKvl.5 channels with apparent KD values of 29.0 + 1.9 pM and 40.9 +4.0 pM, respectively. Thus, S57 was 1.4 fold more potent than R60 in blocking hKv1.5 channels. 3 The blockade produced by S57 and R60 was voltage-dependent and increased steeply between -30 and 0 mV, which corresponded with the voltage range for channel opening. This result indicated that both enantiomers block the hKvl.5 channels, preferentially, when they are in the open state. Between 0 and + 60 mV the blockade exhibited a shallow voltage-dependence which was described by an electrical distance of 0.18 + 0.002 and 0.19 + 0.004 for S57 and R60, respectively. 4 S57 and R60 also increased the rate of decline of the current during the application of depolarizing pulses. The time constant of such decline (TBI,) was faster in the presence of R60 than in the presence of S57 (16.2+ 1.5 ms vs. 24.0+2.6 ms; P<0.01). The apparent association rate constants (k) were similar for S57 and R60 ((0.52+0.13) x 106 M-1 s-l and (0.66+0.13) x 106 M-l s'l, respectively), whereas the dissociation rate constant (1) was faster for R60 than for S57 (25.8+ 1.8 s-' and 13.0+2.4 sl', respectively). 5 Both enantiomers slowed the deactivation of the tail currents elicited upon repolarization to -40 mV, thus inducing a 'crossover' phenomenon. These results suggested that drug unbinding is required before hKvl.5 channels can close. 6 It is concluded that R60 and S57 produced a similar time-voltage-and state-dependent block of hKvl.5 channels that can be interpreted as open channel block by the charged form of each enantiomer. The main difference between R60 and S57 were linked to the apparent dissociation rate constants.

Class III Antiarrhythmic Effects of Zatebradine

“…45 Internal TEA blocks Shaker K + channels with a voltage dependence described by an electrical distance of 0.15, 45 which suggests that TEA moves approximately 15% into the membrane electrical field. This value is similar to those previously described for quinidine ( =0.18), 31 R(+)-and S(-)-bupivacaine ( =0.16), 41 two enantiomers of a new benzazepin-2-on derivative with specific bradycardic effects ( =0.19), 46 and quinine ( =0.18) 36 and for zatebradine in the present experiments ( =0.18). All these findings may suggest the existence of a common receptor site for TEA, local anesthetics, and antiarrhythmic drugs both in Shaker and hKv1.5 channels.…”

Papaverine Blocks hKv1.5 Channel Current and Human Atrial Ultrarapid Delayed Rectifier K+ Currents

“…These results are also consistent with an open channel block mechanism, because the probability of opening increases at more positive membrane potentials. The ␦ z value based on Woodhull's voltage-dependent block obtained for papaverine is very similar to those described previously for other hKv1.5 blocking agents (Snyders et al, 1992;Rampe et al, 1993a,b;Valenzuela et al, 1995Valenzuela et al, , 1996Valenzuela et al, , 1997Yang et al, 1995;Delpon et al, 1996;Caballero et al, 1997;Franqueza et al, 1998), which suggests that all these compounds share the same receptor site in hKv1.5 channels. Open channel blockers mimic fast inactivation.…”

Effect of descarboethoxyloratadine, the major metabolite of loratadine, on the human cardiac potassium channel Kv1.5

“…The experiments were performed at room temperature in mouse Ltk − cells stably expressing hKv1.5 channels (Snyders et al , 1993). The hKv1.5 currents were recorded by use of the whole‐cell configuration of the patch‐clamp technique as previously described (Delpón et al , 1996; 1997; Valenzuela et al , 1996). Cells were perfused with an external solution containing (in m m ): NaCl 130, KCl 4, CaCl 2 1, MgCl 2 1, HEPES 10 and glucose 10; pH was adjusted to 7.4 with NaOH.…”