Response of acetylcholine receptors to photoisomerizations of bound agonist molecules

In these experiments, agonist-induced conductance is measured while a sudden perturbation is produced at the agonist-receptor binding site. A voltage-clamped Electrophorus electroplaque is exposed to trans-Bis-Q, a potent agonist. Some channels are open; these receptors have bound agonist molecules. A light flash isomerizes 3(-35)% of the trans-Bis-Q molecules to their cis form, a far poorer agonist. This causes a rapid decrease of membrane conductance (phase 1), followed by a slower increase (phase 2). Phase 1 has the amplitude and wavelength dependence expected if the channel closes within 100 mus after a single bound trans-Bis-Q is isomerized, and if the photochemistry of bound Bis-Q resembles that in solution. Therefore, the receptor channel responds rapidly, and with a hundred-fold greater closing rate, after this change in the structure of a bound ligand. Phase 2 (the conductance increase) seems to represent the relaxation back toward equilibrium after phase 1, because (a) phase 2 has the same time constant (1(-5) ms) as a voltage- or concentration-jump relaxation under identical conditions; and (b) phase 2 is smaller if the flash has led to a net decrease in (trans-Bis-Q). Still slower signals follow: phase 3, a decrease of conductance (time constant 5(-10 ms); and phase 4, an equal and opposite increase (several seconds). Phase 3 is abolished by curare and does not depend on the history of the membrane voltage. We consider several mechanisms for phases 3 and 4.