Printed in &eat Britain DOSE-RESPONSE OF ACETYLCHOLINE RECEPTOR CHANNELS OPENED BY A FLASH-ACTIVATED AGONIST IN VOLTAGE-CLAMPED RAT MYOBALLS

1. Whole-cell or single-channel currents through acetylcholine ( ACh) receptor channels were studied in voltage-clamped rat myoballs or in excised membrane patches from myoballs. The recording pipette contained CsCl to suppress outward currents, and tetrodotoxin was used to help suppress Na+ currents. 2. To minimize problems associated with bath applied agonists, myoballs were bathed in a solution containing the inactive (cis) isomer of the photo-isomerizable azobenzene derivative, Bis-Q. Calibrated light flashes of varying intensity were presented to produce concentration jumps of agonist, trans-Bis-Q. The resulting whole-cell current relaxations through ACh channels approach a steady state along an exponential time course, then decline as the newly created agonist diffuses away over the next few seconds. 3. The dose-response relationship was inferred from Hill (double-log) plots for myoballs bathed in 500 n~-cis-Bis-Q at three membrane potentials. At low agonist concentrations (< 300 n~-trans-Bis-Q), the slope of the Hill plot averaged 1-62 at- 150 mV, 1.89 at- 100 mV, and 2.05 at + 80 mV. These results are consistent with an apparent agonist affinity constant that decreases with membrane depolarizatio