A patch-clamp study of the partial agonist actions of tubocurarine on rat myotubes.
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.
D000109:Acetylcholine / Q000494:pharmacology; D000818:Animals; D003472:Curare / Q000031:analogs & derivatives; D004553:Electric Conductivity; D066298:In Vitro Techniques; D007473:Ion Channels / Q000187:drug effects* / Q000502:physiology; D008564:Membrane Potentials / Q000187:drug effects; D008954:Models, Biological; D009132:Muscles / Q000187:drug effects* / Q000196:embryology / Q000502:physiology; D011336:Probability; D051381:Rats; D014403:Tubocurarine / Q000494:pharmacology*
D007473:Ion Channels; D003472:Curare; D000109:Acetylcholine; D014403:Tubocurarine