Differential effects of extra- and intracellular anions on GABA-activated currents in bullfrog sensory neurons

1. Kinetic properties of gamma-aminobutyric acid (GABA)-gated inward and outward anion currents were investigated in the frog sensory neurons perfused internally and externally with various anions with the use of a rapid concentration-jump (termed as 'concentration-clamp') technique. 2. Extracellular Br- [( Br-]o) shifted the dose-response curves of GABA-induced inward anion currents to the left without affecting the maximum values, whereas [Cl-]o, [I-]o, [No3-]o, [HCOO-]o, and [CH3COO-]o altered the rate of desensitization differently without shifting the GABA dose-response curves, indicating that the kinetics of desensitization phase are affected differently by various extracellular anions. 3. [CH3COO-]o suppressed the maximum current of the dose-response curve of the GABA-induced inward ICl without affecting Kd. 4. Both activation and desensitization phases of GABA-induced ICl consisted of fast and slow components, respectively. [Br-]o, [I-]o, and [NO3-]o significantly prolonged the slow desensitization component, whereas both [HCOO-]o and [CH3COO-]o shortened it. The fast desensitization and the fast and slow activation components were also affected by these foreign anions. 5. GABA dose-response curves of inward currents carried by various intracellular anions (Cl-, Br-, NO3-, I-, SCN-, HCOO-, F-, CH3COO-, CH3CH2COO-, BrO3-, and ClO3-) while keeping a constant [Cl-]o had a constant Kd value but different saturating maximum currents. There were no marked differences among their current kinetics except in the case of SCN-, indicating that the current kinetics is not affected by replacing intracellular Cl- [( Cl-]i) with various foreign anions. 6. The configuration and amplitude of GABA-gated outward anion currents at a constant [Cl-]i reflected the extracellular action of individual anions on the anion-binding site of GABA receptor associated with the anion-selective channel. 7. The relative conductances of the various anions, calculated from the maximum peak currents in dose-response curves of the GABA-induced inward anion currents at a constant [Cl-]o, was in the sequence: I- greater than Br- greater than or equal to NO3- greater than ClO3- greater than SCN- greater than or equal to Cl- greater than HCOO- greater than BrO3- greater than CH3COO- greater than F- greater than CH3CH2COO-.(ABSTRACT TRUNCATED AT 400 WORDS)

Activation kinetics of the acetylcholine-gated potassium current in isolated atrial cells

Processes involved in activation of the acetylcholine (ACh) receptor-operated K+ current (IK) were examined in atrial cells isolated from guinea pig using a "concentration-clamp" technique. This approach allows for the intracellular perfusion and the rapid change of external solution (time constant 4 ms) with cell-attached condition under a single-electrode voltage-clamp condition. The ACh-induced IK increased in a sigmoidal fashion with increasing concentrations of ACh. The Ka value estimated from the concentration-response curve was 3 X 10(-7) M, and the Hill coefficient was 1.0. The activation phase was accelerated not only by increasing the concentration of ACh but also by elevating the temperature. Before activation of the current, there was a brief latent period after the application of ACh. The latent period was shortened considerably with the increase in ACh concentration and in temperature, i.e., 267 +/- 20 ms at 18 degrees C, 98 +/- 11 ms at 26 degrees C, and 44 +/- 6 ms at 37 degrees C for 10(-6) M ACh. These results suggest that the latent period of ACh response seems to be the time lag needed for the activation of K+ channels using remote sensor.