The kinetics of strychnine block of single Na channels in bovine chromaffin cells were studied using the gigohm seal, patch‐clamp technique, under the condition in which the Na current inactivation had been eliminated by treatment with N‐bromoacetamide (NBA). Strychnine, applied to the cytoplasmic face of Na channels at concentrations ranging from 25 to 100 microM, caused repetitive rapid transitions (flickering) between open and blocked states within single openings of Na channels, without affecting the amplitude of the single‐channel current. Average currents in the presence of strychnine accompanied 'hooked' tail currents upon repolarization. The histograms for blocked times and the histograms for open times could be fitted with a single‐exponential function. The mean open time (to) became shorter as the drug concentration was increased, while the mean blocked time (tb) was concentration independent. The association (blocking) rate constant, k, calculated from the slope of the curve relating the reciprocal mean open time to strychnine concentration was 7.6 X 10(6) M‐1 S‐1 at ‐40 mV in a typical experiment. to‐1 and tb‐1 had opposite voltage dependences: tb‐1 became larger as the membrane was hyperpolarized whereas to‐1 became smaller. The voltage dependence suggests that a first‐order blocking site is located 35% of the way through the membrane electric field from the cytoplasmic surface. An increase in the external Na+ concentration greatly decreased to‐1 without affecting tb‐1. The voltage dependence of both to‐1 and tb‐1 did not change when the external Na concentration was changed. It is suggested that the strychnine block of single Na channels is not 'current‐dependent'. All of the features of strychnine block of single Na channels are compatible with the sequential model, in which strychnine molecules block open Na channels, and the blocked channels could not close until strychnine molecules had left the blocking site in the channels.
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