Activation and desensitization of acetylcholine receptors in fish muscle with a photoisomerizable agonist.

Voltage‐clamped muscle fibres of the fish Xenomystus nigris were bathed in a solution containing 3,3‐bis[alpha‐(trimethylammonium)methyl]azobenzene (cis‐Bis‐Q; 100 nM). A flash of light photoisomerized some of the cis‐Bis‐Q (non‐agonist form) to trans‐Bis‐Q (agonist form). This resulted in agonist‐induced current. Current noise of channels activated by both trans‐Bis‐Q and ACh was analysed to find the channel conductances and open times associated with these two agonists. Channels activated by trans‐Bis‐Q and ACh had similar conductances (20‐30 pS) and open times (3‐4 msec) despite the photolabile azobenzene group of Bis‐Q. Light flashes subsequent to the first flash caused further increases in the ratio of trans‐Bis‐Q to cis‐Bis‐Q and accompanying increases in agonist‐induced current. Eventually, agonist‐induced currents levelled off as a photo‐equilibrium state was reached with a constant trans:cis ratio. After the photo‐equilibrium current level was reached, light flashes caused temporary increases in agonist‐induced current which decayed back to equilibrium in seconds. This result is interpreted according to a model in which trans‐Bis‐Q molecules are bound to a subpopulation of desensitized receptors, preventing recovery to native receptor. A flash of light then converts some trans‐Bis‐Q molecules bound to the desensitized receptor to the cis isomer. The newly formed cis‐Bis‐Q molecule may then unbind, allowing the desensitized receptor to recover. When light was flashed on muscle exposed to 300‐600 nM‐cis‐Bis‐Q, large (100‐200 nA) agonist‐induced currents were produced. These currents decayed exponentially over several seconds as the fibre desensitized. This result confirms that a first‐order process underlies the onset of desensitization.