Kinetic analysis of acetylcholine-induced current in isolated frog sympathetic ganglion cells

Abstract

1. Kinetic properties of activation and inactivation phases of the ACh-gated nicotinic current were investigated in isolated frog sympathetic ganglion cells using "concentration-clamp" technique under voltage-clamp conditions. This technique combines internal perfusion with a rapid change of the external solution within a few milliseconds. 2. The dose-response curve for the peak current induced by ACh showed a sigmoidal increase, in which the apparent dissociation constant Kd and Hill coefficient were 2.6 x 10(-5) M and 2.0, respectively. 3. The current-voltage relationship of ACh-induced currents were linear at potentials more negative than the reversal potential (EACh) of -5.5 +/- 1.3 mV (mean +/- SE) but showed a slight inward rectification at positive potentials over +20 mV. Since this current could be generated predominantly by an increase of Na+ and K+ conductances, the value of EACh was close to the theoretical potential, -1.3 mV, for the total amount of both Na+ and K+ or Cs+ in the extracellular and intracellular solutions. 4. There was no difference between the dose-response curves of ACh- and nicotine-induced currents. The ACh-induced current was suppressed in a competitive manner by the nicotinic antagonists, d-tubocurarine and hexamethonium, but muscarine did not induce any response, indicating that the ACh-gated current might be mediated by the nicotinic ACh receptor-ionophore complex. 5. There was a latent period of the order of milliseconds in the ACh receptor activation phase before attaining exponential increase of activation process.(ABSTRACT TRUNCATED AT 250 WORDS)