The effects of isoflurane on acetylcholine receptor channels.: 2. Currents elicited by rapid perfusion of acetylcholine

Abstract

We studied the effects of the volatile anesthetic isoflurane on nicotinic acetylcholine (ACh) receptor channels using a technique for rapid perfusion of ACh to outside-out patches. Channels were activated by ACh, at concentrations ranging from 1 microM to 10 mM, and the macroscopic current flowing through tens or hundreds of channels was measured. Isoflurane reduced the peak current response to saturating concentrations of ACh, increased the current decay rate due to desensitization, and decreased the rate of recovery from desensitization. The effect of isoflurane on peak currents was concentration dependent; at 2% isoflurane, the peak current was reduced by half. The effect of isoflurane on the peak current induced by nonsaturating concentrations of ACh was smaller. We measured the onset and recovery of current inhibition by isoflurane, by rapidly applying and removing isoflurane to the patch within 100 microseconds. 2% isoflurane, currents were inhibited with a time constant of 200-300 microseconds and recovered with a time constant of 500-700 microseconds. We interpreted our results in terms of a kinetic model in which isoflurane binds directly to both open and closed channels (not necessarily within the pore of the channel) and stops the flow of ions through open channels. This model provides a quantitative explanation for the kinetic and equilibrium effects of isoflurane on peak currents activated by saturating concentrations of ACh. Our data support the idea that the flickering effect of isoflurane on single ACh receptor channels is caused by rapid binding and dissociation of isoflurane to an inhibitory binding site on the protein. The effects of isoflurane on the apparent affinity of ACh and on desensitization are not predicted by the model. These effects may arise from the binding of isoflurane to other sites, not necessarily on the protein itself.