A photoisomerizable muscarinic antagonist. Studies of binding and of conductance relaxations in frog heart.

Abstract

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 [3 H]-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 k nor n is affected when : (a) [Bis-Q] ature ; 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 of two (or more) steps with a high activation energy .

Photochromic Potassium Channel Blockers: Design and Electrophysiological Characterization

“…1a). Ligand: The ligand can be an agonist (11, 13), a competitive antagonist (14, 23), a pore blocker (7–10, 12), a substrate (16), an inhibitor (15, 17, 18), or even an allosteric modulator, depending on the protein target and the experimental use.Photoswitch: Among the photosensitive groups available, azobenzene has been predominantly used (7–18, 23) due to its advantageous chemical and photochemical properties. Indeed, azobenzene is a small, relatively simple molecule, easy to synthesize and derivatize.…”

“…Ligand: The ligand can be an agonist (11, 13), a competitive antagonist (14, 23), a pore blocker (7–10, 12), a substrate (16), an inhibitor (15, 17, 18), or even an allosteric modulator, depending on the protein target and the experimental use.…”

“…We are focusing on photochromic blockers for K v channels, but the general strategy is applicable to different types of proteins, as already shown for ion channels (7–13), metabotropic receptors (14), and enzymes (15–18). …”

Flipping the Photoswitch: Ion Channels Under Light Control

“…These molecules were mono-or bifunctional with a quaternary ammonium (QA) and were applied to control nicotinic acetylcholine receptors (nAChRs) and acetylcholine esterase to study kinetics of ion channel activation and membrane potential shifts in excitable tissue (Bartels et al 1971;Bieth et al 1969;Deal et al 1969;Chabala et al 1985;Lester et al 1979;Nargeot et al 1982). Despite these early successful examples for the optical control of ion channel function, further design and application of PCLs was not revisited until nearly three decades later.…”

Photochrome Liganden für spannungsgesteuerte Kaliumkanäle

“…[7] So wurden z. B. vor mehr als 30 Jahren photochrome Agonisten [5] und Antagonisten [8] für nikotinische Acetylcholinrezeptoren beschrieben.…”