Modulation of muscle nicotinic acetylcholine receptors by the glucocorticoid hydrocortisone. Possible allosteric mechanism of channel blockade.

Mechanisms of ion channel blockade by noncompetitive inhibitors of the nicotinic acetylcholine receptor (AChR) have been particularly difficult to elucidate. We have combined here transient expression of embryonic, adult, and a mutated adult muscle AChR associated with a slow channel syndrome (Ohno, K., Hutchinson, D. O., Milone, M., Brengman, J. M., Bouzat, C., Sine, S., and Engel, A. (1995) Proc. Natl. Acad. Sci. U. S. A. 92, 758-762) with single channel recordings to determine subunit specificity and mechanisms of action of the prototype glucocorticoid hydrocortisone (HC). HC affected in a similar manner the gating kinetics of all types of muscle AChR, producing briefer openings with normal amplitudes. We postulate that this steroid acts as a noncompetitive inhibitor of the AChR and that its mechanism of action can be interpreted in terms of blocking models. The forward rate constant for the blocking process was also similar for all channel types, indicating that the structural differences between them are not responsible for the effect. The reduction in the channel open time was not dependent on agonist concentration; it was slightly voltage dependent, suggesting that HC binds to a site located inside the membrane that senses the electric field. Recordings at high acetylcholine concentration in the presence of HC showed a reduced number of openings per activation period and the long closed times typically observed in the desensitization phenomenon. In competition studies with the classical open channel blocker QX-222, HC induced an early termination of the burst, suggesting that the two act at different sites. Taken together the results support the existence of specific sites sensed by the membrane field, different from those of open channel blockers and probably located at the lipid-protein interface. From this site(s), glucocorticoids and other hydrophobic noncompetitive inhibitors could allosterically mediate channel blockade.