Roles of agonist-binding sites in nicotinic acetylcholine receptor function

Under equilibrium conditions, the nicotinic acetylcholine receptor from Tor pedo electroplax carries two high affinity-binding sites for agonists. It i s generally assumed that these are the only agonist sites on the receptor a nd that their occupancy results in rapid channel activation followed by slo wer conformational transitions that lead to the high affinity equilibrium s tate. These slow transitions are thought to reflect the physiological proce ss of desensitization. Here me show that preequilibration of the high affin ity sites with saturating concentrations of carbamylcholine does not dimini sh the ion flux response to subsequent exposure to higher (activating) conc entrations of this agonist. This finding has profound implications with res pect to receptor function: (1) occupancy of the high affinity sites per se does not desensitize the receptor and (2) these sites cannot be directly in volved in receptor activation. It is thus necessary to invoke the presence of additional binding sites in channel opening. (C) 2000 Academic Press.