Regulation of [3H]perhydrohistrionicotoxin binding to Torpedo ocellata electroplax by effectors of the acetylcholine receptor.

The specific binding of [3H]perhydrohistrionicotoxin ([3H]H12-HTX) to the ionic channel of the nicotinic receptor in membranes from the electric organ of the electric ray Torpedo ocellata was studied by use of a rapid filter assay. The time course of the binding was monitored and the initial rate of binding (i.e. within 30 s) was found to be increased up to several hundredfold by the presence of several receptor agonists in a dose-dependent manner. Receptor antagonists blocked this agonist-increased initial rate of binding. The presence of receptor antagonists, with the exception of alpha-bungarotoxin, also increased the initial rate of [3H]H12-HTX binding, though to a much lesser degree than agonists. Preincubation of the membranes with carbamylcholine reduced the initial rate of binding in a time-dependent manner. This was proposed to be due to receptor desensitization. Thus, it is suggested that the time course of [3H]H12-HTX binding to the ionic channel sites is influenced by the conformational state of the receptor-channel complex such that receptor activation, inactivation, and desensitization appear to be reflected in [3H]H12-HTX binding.