An analysis of the dose-response curve at voltage-clamped frog-endplates.

The agonist concentration--endplate conductance relation was examined for a number of agonists (such as carbachol, alkyl trimethylammonium salts, choline and decamethonium). The endplate current evoked varied as Imax [a/(a + K)]2, where a is the agonist concentration and Imax and K are agonist-specific parameters. This finding suggests that the endplate receptor has 2 equivalent subunits which bind agonist approximately non-cooperatively. The liganded subunits then switch to an active conformation with a probability that depends on the nature of the agonist. Both subunits must adopt the active conformation for the channel to open, but the transitions of the subunits could be either independent or concerted.