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

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 (hKv1.5) 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 hKv1.5 channels with apparent KD values of 29.0 +/- 1.9 microM and 40.9 +/- 4.0 microM, 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 hKv1.5 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 (tau Block) 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 10(6) M-1 s-1 and (0.66 +/- 0.13) x 10(6) M-1 s-1, respectively), whereas the dissociation rate constant (l) was faster for R60 than for S57 (25.8 +/- 1.8 s-1 and 13.0 +/- 2.4 s-1, 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 hKv1.5 channels can close. 6. It is concluded that R60 and S57 produced a similar time- voltage- and state-dependent block of hKv1.5 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.