The nature of the acetylcholine receptor in a Buccinum proboscis muscle examined by the sucrose-gap voltage clamp technique.

1. ACh dose-response curves for the radicular retractor muscle of Buccinum showed maximum force and membrane depolarisation of 3.3 mV at 50 mumol l-1 ACh. 2. PCh was found to be almost a full agonist for force and induced higher membrane depolarisations than ACh while BCh was only a partial agonist of very low potency. This suggests an AChR neither muscarinic nor nicotinic in mammalian terminology. 3. Neither muscarine nor nicotine had any direct agonistic effects on the muscle but pre-exposure to nicotine inhibited both force and membrane depolarisation induced by a subsequent dose of ACh. 4. The specific muscarinic and nicotinic antagonists atropine, d-tubocurarine and gallamine all inhibited ACh responses in a dose-dependent manner. 5. Single sucrose-gap recording showed that ACh induced a depolarisation resulting in a contracture. Double sucrose-gap voltage clamp recording showed that 10 mumol l-1 ACh induced an inward transmembrane current of ca 2 microA. Both ACh-induced depolarisation and inward current were abolished in Na-free media. 6. When clamped at a series of membrane voltages between natural Em and positive potentials the ACh-induced Na-dependent inward current declined as Em was reduced and was abolished at -10 mV. This current showed no reversal even at strong positive membrane voltages. 7. The AChR of this muscle appears to be neither exclusively nicotinic nor muscarinic but a hybrid and shows characteristics of voltage inactivation.