ABSTRACT
Mechanisms of the inhibition of evoked multiquantal endplate currents (EPC) by chlorhexidine (CHX) were studied in electrophysiological experiments and by mathematical modeling to discriminate between possible channel, receptor, and non-receptor effects of this common antiseptic drug. Experiments were carried out on the isolated neuromuscular preparation of the cut m. sartorius of the frog Rana ridibunda. The nerve-stimulation-evoked endplate currents were measured by standard double microelectrode technique. For the mathematical simulation, a method based on the solution of a system of ordinary differential equations was used. CHX in milimolar concentrations suppressed the amplitude and shortened the evoked EPC. Recovery of the EPC amplitude was very slow, and EPC shortening persisted during 30-40 min washout of the drug. There is no indication that CHX competes for acetylcholine or carbachol binding site(s). A comparison of the experimental data with mathematical simulation made it possible to construct a reliable kinetic scheme, which describes the action of CHX. CHX induces a combined slow blockade of the open ionic channel and long-lasting allosteric inhibition of the nicotinic acetylcholine receptor. The very slow washout of the drug in terms of EPC amplitude and virtually no recovery of the shortened EPC time course might substantiate certain caution to avoid unintentional high-dose application during its antibacterial application.
Subject(s)
Anti-Infective Agents, Local/pharmacology , Chlorhexidine/pharmacology , Receptors, Cholinergic/drug effects , Allosteric Regulation/drug effects , Animals , Anti-Infective Agents, Local/administration & dosage , Chlorhexidine/administration & dosage , Cholinergic Antagonists/administration & dosage , Cholinergic Antagonists/pharmacology , Computer Simulation , Dose-Response Relationship, Drug , Electric Stimulation , Electrophysiology , In Vitro Techniques , Models, Theoretical , Motor Endplate/metabolism , Muscle, Skeletal/drug effects , Muscle, Skeletal/metabolism , Rana ridibunda , Receptors, Cholinergic/metabolism , Time FactorsABSTRACT
A model of concentration changes across the synaptic cleft during a single quantum release is presented that can be used for description and characterization of the kinetic in postsynaptic current development under the influence of different antagonists, modulators, desensitization promoters or complex channel blockers. The model enables the calculation of the relative number of open channels as a function of time for two standard cases - when acetylcholinesterase (AChE) is either active or inhibited. One outcome of the present model is that the variable part of AChE activity is zero at the moment of acetylcholine (Ach) release and then increases. This is in contrast to common view that the activity of AChE at the initial moment of release of quanta is maximal and decreases over the time course of quantum action. However, the model explains why non-quantal ACh leakage from the nerve terminal creating a concentration of approximately 10(-8) mol.l(-1) in the cleft can escape hydrolysis by intrasynaptically located cholinesterase and reach the subsynaptic membrane. The model can also be used for theoretical considerations of time and amplitude changes during repetitive nerve-evoke quanta release.
