A patch‐clamp study of the partial agonist actions of tubocurarine on rat myotubes.

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Abstract
Single channels activated by (+)‐tubocurarine (curare) were recorded from rat myotubes using the patch‐clamp technique. The agonist‐like action of curare does not result from a contaminant molecule, as the same effects were observed with curare purified by high‐performance liquid chromatography. A curare‐activated channel can adopt two levels of conductance: full (F) or partial (P). The F state has a slope conductance of 40 pS (identical to that of the acetylcholine (ACh)‐activated channel) and the P state has a conductance of 13 pS. At low concentrations of agonist (ACh or curare), the distribution of channel open times is biphasic. The briefer channels may result from the binding of a single agonist molecule whereas the longer‐lived channels probably occur following the binding of two agonist molecules. The mean open time of the F state decreases with increasing curare concentration. It is shown that band‐width limitations are likely to account for only a very small part of this observed reduction. In contrast, the mean open time of the P state is independent of the concentration of curare. A simple interpretation is that the F state is susceptible to channel blockade by curare, whereas the P state is not. The P state preceded the F state almost as often as it followed the F state; it can also be observed separately from the F state. The fraction of events including a P state increases from about 4% in the presence of 1 microM‐curare to 30% at 100 microM‐curare. This fraction is also increased by hyperpolarization. When the curare concentration is increased, the F‐state frequency first increases and then decreases at higher concentration. This frequency is also decreased by hyperpolarization. The decrease in F‐state frequency is probably related to channel blockade by curare; it cannot be wholly accounted for by problems associated with limited time resolution. A synthetic analogue of curare, (+)‐ tubocurine dimethiodide presents an agonist activity similar to that of curare but with a faster closing rate for both F and P states. The various actions of curare are summarized in two possible models where the P state is interpreted as either a partially open channel or a channel which is partially blocked.