Effects of Onium Derivatives on Na-, Cl-, and K-dependent Acetylcholine Receptors

Various types of acetylcholine (ACh) receptor in the pleural or abdominal ganglion cells of Aplysia were classified into three types depending on the ion species involved in the electrogenesis of their responses. The Na-dependent receptor was designated the D_Na-type since the response was a depolarization (D) produced by an increase in permeability, mainly toward Na⁺, although other ions were partially involved. Similarly, the Cl^- or K-dependent receptor was designated the H_Cl- or H_K-type since the response was a hyperpolarization (H) produced by an exclusive increase in permeability toward Cl^- or K⁺. ACh-induced responses were evaluated by an increase in membrane conductance. The D_Na-type receptor could be readily activated by tetramethylammonium (TMA) and choline, but not by decamethonium (Deca). Instead, it was blocked competitively either by hexamethonium (Hexa) or by Deca. The H_Cl-type receptor could be readily activated by TMA, choline, and Deca. Hexa did not block this type of receptor. The H_K-type receptor was neither activated nor blocked by TMA, choline, Hexa, and Deca. The above findings suggested that there is a certain structural similarity in the ACh-binding site between D_Na-type and H_Cl-type receptors, although the ion species involved in the electrogenesis of each response were totally different. The structure of the ACh-binding site at the H_K-type receptor seemed considerably different from that of either D_Na-type or H_Cl-type.