Ionophore-mediated calcium influx effects on the post-synaptic muscle fibre membrane

Abstract

THE physiological chemotransmitter acetylcholine and its structural analogues such as carbamylcholine are capable of producing a sustained blockade of neuromuscular transmission when applied to the neuromuscular junction for prolonged periods. This neuromuscular blockade is not due to a depolarisation of the postsynaptic muscle fibre membrane but rather is generally ascribed to the inactivation, or ‘desensitisation’ of cholinergic receptor molecules in the postsynaptic membrane^1,2. Various kinetic models involving agonist, and receptor molecules postulated to exist in different conformational states, have been proposed to account for the time course of the desensitisation process^2,3. These models do not incorporate the acceleration of desensitisation by calcium ions^4,5 and they do not predict correctly the observed effects of certain cholinergic antagonists on desensitisation. Consequently, alternative models for desensitisation have been proposed. In one of them calcium ions accumulate at the interior surface of the post-synaptic membrane and bind to the cholinergic receptors, thereby causing desensitisation by producing or sustaining some inactive receptor conformation⁶. We tested the model for desensitisation which involves internal calcium binding. This was accomplished by facilitating calcium ion flux across the post-synaptic membrane using the divalent cation ionophore A23187, while simultaneously producing a rapid desensitisation with iontophoretic application of carbamylcholine. Our results provide further evidence that calcium ions are a significant factor in the molecular mechanism of desensitisation.