title: A photoisomerizable muscarinic antagonist. Studies of binding and of conductance relaxations in frog heart.
abstract: These experiments employ the photoisomerizable compound, 3,3'-bis-[alpha-(trimethylammonium)methyl]azobenzene (Bis-Q), to study the response to muscarinic agents in frog myocardium. In homogenates from the heart, trans-Bis-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 microM for trans-Bis-Q; the cis configuration is severalfold less potent as a muscarinic blocker. Voltage-clamped preparations were exposed simultaneously to carbachol and Bis-Q and were subjected to appropriately filtered flashes (less than 1 ms duration) from a xenon flashlamp. Trans leads to cis and cis leads to trans photoisomerizations cause small (less than 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)]n. At 23 degrees C, k is approximately 3 s-1 and n is 2. Neither k nor n is affected when: (a) [Bis-Q] is varied between 5 and 100 microM; (b) [carbachol] is varied between 1 and 50 microM; (c) carbachol is replaced by other agonists (muscarine, acetylcholine, or acetyl-beta-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 degrees C, k increases with temperature; the Q10 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 of two (or more) steps with a high activation energy.
mesh_terms: D000818:Animals; D002217:Carbachol / Q000494:pharmacology; D004553:Electric Conductivity; D006321:Heart / Q000502:physiology*; D007536:Isomerism; D007700:Kinetics; D008027:Light; D009116:Muscarine / Q000037:antagonists & inhibitors*; D009206:Myocardium / Q000378:metabolism*; D019832:N-Methylscopolamine; D000644:Quaternary Ammonium Compounds / Q000378:metabolism; D011896:Rana temporaria; D011976:Receptors, Muscarinic / Q000378:metabolism; D012602:Scopolamine Derivatives / Q000378:metabolism; D013696:Temperature
chemical_list: D000644:Quaternary Ammonium Compounds; D011976:Receptors, Muscarinic; D012602:Scopolamine Derivatives; D009116:Muscarine; C000295:Bis-Q; D002217:Carbachol; D019832:N-Methylscopolamine