Voltage-dependent effect of curare at the frog neuromuscular junction

Abstract

CONDUCTANCE measurements on various cholinergic post-junctional membranes have demonstrated voltage-dependent effects for acetylcholine (ACh) and its agonists^1–5, for local anaesthetics⁶, and for curare⁵. It is not yet clear which of the steps in drug–receptor interaction are rate-limiting for voltage-dependent effects of ACh. Kordaš⁷ considers this step to be the initial binding between ACh and its receptor, whereas Magleby and Stevens⁸ consider the conformational changes between open and closed states of the receptor to be rate-limiting. Distinct from the initial binding reaction and the subsequent conformational change is the possibility of a drug binding to a receptor that is in its open, rather than closed, state, thereby forming a complex consisting of three molecular species. This idea comes from Steinbach's model⁹ for the action of local anaesthetics at the frog endplate. Such multi-molecular complexes have been included in reaction schemes which describe voltage-dependent effects for local anaesthetics at the frog endplate¹⁰ and for curare in Aplysia neurones⁵. Marty et al.⁵ found that during hyperpolarisation of molluscan neurones, curare became a more effective blocker of the response to slowly applied ACh. Such an effect of curare was not seen when ACh was applied quickly to the receptors, that is when there was insufficient time for ACh to equilibrate with its postsynaptic effect. They suggested that the binding between curare and the cholinergic receptor is voltage-dependent but that this particular kind of binding can only occur between curare and a receptor which is already in its open state. Here I show that curare has a very similar effect at the frog neuromuscular junction. The efficacy of curare as a postsynaptic blocker at the frog neuromuscular junction increases when the muscle fibre is hyperpolarised. This effect is seen only when ACh is applied slowly, however, and not when it is applied rapidly during neural release of ACh.