Activation, inactivation, and desensitization of acetylcholine receptor channel complex detected by binding of perhydrohistrionicotoxin

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The effects of receptor activation were studied on the interaction of perhydrohistrionicotoxin (H12-HTX) with the ionic channel of the nicotinic acetylcholine (AcCho) receptor in membranes from the electric organ of Torpedo ocellata and with the endplate region of the soleus muscle of the rat. In Torpedo membranes, the initial rate (i.e., within 30 sec) of [3H]H12-HTX bindings to the ionic channel of the AcCho receptor was accelerated 102- to 103-fold in the presence of carbamoylcholine (Carb). H12-HTX also inhibited Carb-activated 22Na+ influx, over 95% inhibition at 10 μM H12-HTX. At this concentration H12-HTX did not inhibit [3H]AcCho binding to the AcCho-receptor sites. There was good correspondence between the degree of acceleration of [3H]H12-HTX binding and the stimulation of 22Na+ influx over a wide range of Carb concentrations (up to 100 μM). Preincubation of Torpedo membranes with Carb decreased the initial rate of [3H]H12-HTX binding, as well as the rate of 22Na+ influx, which may reflect desensitization of the AcCho-receptor. d-Tubocurarine inhibited the agonist-mediated acceleration of [3H]H12-HTX binding and 22Na+ influx. In the soleus muscle endplate, H12-HTX inhibited the transient depolarization induced by microiontophoretic application of AcCho; the more receptors activated and channels opened, the stronger was the inhibition by H12-HTX. These findings suggest that H12-HTX binds to closed and open ionic channels, with a preference for the latter conformation. It is also suggested that the conformational changes associated with activation or desensitization of the receptor can be monitored by studying binding of [3H]H12-HTX to the ionic channel sites as well as by the AcCho-receptor-regulated 22Na+ influx.

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