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Argiopine blocks glutamate‐activated single‐channel currents on crayfish muscle by two mechanisms.

1. The effect of the spider venom argiopine on L‐glutamate‐activated membrane channels of crayfish muscle was investigated using the patch‐clamp technique. 2. When 10(‐2) M‐glutamate and 10(‐9) M‐argiopine were contained in the pipette solution of a cell‐attached patch, bursts of openings of excitatory channels appeared after formation of the patch. These bursts ceased abruptly after variable periods of time in the range of 5 min. Higher concentrations of argiopine (up to 10(‐6) M) blocked more rapidly, approximately in proportion to concentration. 3. The block of excitatory channels could be partially or completely reversed by hyperpolarizing the membrane by up to ‐190 mV from the resting potential. The time constant of the recovery of channel opening decreased with increasing hyperpolarization and was 2 ms with ‐160 mV hyperpolarization. Switching back from the hyperpolarized level to the resting potential, the time constant for the resulting block was about 3 s (10(‐7) M‐argiopine). Potential‐dependent block by argiopine with similar characteristics was also observed in outside‐out patches. 4. Up to argiopine concentrations of 10(‐7) M the kinetics of channel openings and of bursts measured in pre‐block periods or during reversal of the block by hyperpolarization were indistinguishable from controls. 5. When the potential‐dependent block observed in the presence of 10(‐6) M‐argiopine and 10(‐2) M‐glutamate was reversed by hyperpolarization, additional short closings occurred during bursts. This ‘flickering block’ did not change burst length appreciably, but an additional open time component (tau = 0.1 ms) appeared and the average open time per burst was reduced. 6. At least two reaction steps seem necessary to model the behaviour of the potential‐dependent block. The flickering block may be described as intermittent blocking of the channel which does not interfere with the reactions between glutamate and the channel.