A Photoisomerizable Muscarinic Antagonist

These experiments employ the photoisomerizable compound, 3,3'bis-[a-(trimethylammonium)methyl]azobenzene (Bis-Q), to study the response to muscarinic agents in frog myocardium . In homogenates from the heart, transBis-Q blocks the binding of [3H]-N-methylscopolamine to muscarinic receptors . In voltage-clamped atrial trabeculae, trans-Bis-Q blocks the agonist-induced potassium conductance. The equilibrium dose-response curve for carbachol is shifted to the right, suggesting competitive blockade . Both the biochemical and electrophysiological data yield a dissociation constant of 4-5 AM for trans-Bis-Q; the cis configuration is severalfold less potent as a muscarinic blocker . Voltageclamped preparations were exposed simultaneously to carbachol and Bis-Q and were subjected to appropriately filtered flashes (<1 ms duration) from a xenon flashlamp . Trans --+ cis and cis -> trans photoisomerizations cause small (<20%) increases and decreases, respectively, in the agonist-induced current . The relaxation follows an S-shaped time course, including an initial delay or period of zero slope. The entire waveform is described by [1 exp(-kt)]" . At 23 °C, k is -3 s' and n is 2 . Neither knor n is affected when: (a) [Bis-Q] is varied between 5 and 100 AM; (b) [carbachol] is varied between I and 50 AM; (c) carbachol is replaced by other agonists (muscarine, acetylcholine, or acetyl-,8-methylcholine) ; or (d) the voltage is varied between the normal resting potential and a depolarization of 80 mV. However, in the range of 13-30 °C, k increases with temperature ; the Qto is between 2 and 2.5 . In the same range, n does not change significantly . Like other investigators, we conclude that the activation kinetics of the muscarinic K+ conductance are not determined by ligand-receptor binding, but rather by a subsequent sequence oftwo (or more) steps with a high