Time-dependent block of the slowly activating delayed rectifier K(+) current by chromanol 293B in guinea-pig ventricular cells

The slowly activating delayed rectifier K(+) current (I(Ks)) was recorded in single myocytes dissociated from guinea-pig ventricles and the mechanism underlying the block of I(Ks) by a chromanol derivative, 293B, was investigated. In the presence of 1 - 100 microM 293B, activation phase of I(Ks) was followed by a slower decay during 10 s depolarizing pulses. Both the rate and extent of the decay were increased in a concentration-dependent manner. The relationship between the concentration of 293B and the block showed a Hill's coefficient of approximately 1. The half-inhibitory concentration was approximately 3.0 microM and did not differ significantly at various membrane potentials from +20 to +80 mV. A mathematical model for the 293B block was constructed on the basis of multiple closed and open states for the I(Ks) channels, and the blocking rate was calculated by fitting the model to the original current traces. The blocking rate constant showed a linear function with the 293B concentration, indicating 1 : 1 binding stoichiometry. At +80 mV the blocking rate was 4x10(4) M(-1) s(-1) and the unblocking rate was 0.2 s(-1). The results indicate that 293B is an open channel blocker with relatively smaller blocking rate than those reported so far for time-dependent blockade of various ionic channels.