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 diffic ult to elucidate, We have combined here transient expression of embryo nic, adult, and a mutated adult muscle AChR associated with a slow cha nnel 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 subuni t specificity and mechanisms of action of the prototype glucocorticoid hydrocortisone (HC), HC affected in a similar manner the gating kinet ics of all types of muscle AChR, producing briefer openings with norma l amplitudes, We postulate that this steroid acts as a noncompetitive inhibitor of the AChR and that its mechanism of action can be interpre ted in terms of blocking models, The forward rate constant for the blo cking process was also similar for all channel types, indicating that the structural differences between them are not responsible for the ef fect, The reduction in the channel open time was not dependent on agon ist concentration; it was slightly voltage dependent, suggesting that HC binds to a site located inside the membrane that senses the electri c field, Recordings at high acetylcholine concentration in the presenc e of HC showed a reduced number of openings per activation period and the long closed times typically observed in the desensitization phenom enon, In competition studies with the classical open channel blocker Q X-222, HC induced an early termination of the burst, suggesting that t he two act at different sites, Taken together the results support the existence of specific sites sensed by the membrane field, different fr om those of open channel blockers and probably located at the lipid-pr otein interface, From this site(s), glucocorticoids and other hydropho bic noncompetitive inhibitors could allosterically mediate channel blo ckade.