THE EFFECTS OF CHLORAMPHENICOL ISOMERS ON THE MOTOR END-PLATE NICOTINIC RECEPTOR ION CHANNEL COMPLEX

  • 1 January 1986
    • journal article
    • research article
    • Vol. 29  (1) , 52-64
Abstract
Four enantiomers of chloramphenicol have been tested for their effects on end-plate current and miniature end-plate current decay and amplitude characteristics in the voltage-clamped costocutaneous nerve-muscle preparation of the garter snake. All four enantiomers exhibited effects on end-plate current and miniature end-plate current decay at similar concentrations (0.2-1.0 nM), indicating that the measured effect was not related to the antibacterial action of the compounds in which D-threo chloramphenicol is known to be at least 50 times more powerful than the L-threo and D- and L-erythro isomers. The compounds slightly increased end-plate current but not miniature end-plate current amplitude, indicating that they produce an increase in end-plate current quantal content. This effect was verified by an analysis of end-plate current driving functions (see Appendix) for one of the chloramphenicol isomers. In addition to this presynaptic action, all four compounds converted end-plate current and miniature end-plate current decays from single to double exponential functions. This effect was both concentration and voltage dependent. For all four compounds, hyperpolarization resulted in a progressive decrease in .tau.f and an increase in .tau.s. The relative amplitudes of the fast and slow decay components were independent of membrane potential. The results are interpreted in terms of the drugs blocking the open form of the acetylcholine-activated receptor-ion channel complex. However, in addition to affecting decay characteristics, all four compounds increased the charge passed during both end-plate and miniature end-plate currents. This effect was concentration but not voltage dependent and is inconsistent with the predictions of the sequential model for open ion channel blockade. By using an extension of Ruff''s analysis of the sequential model of open end-plated ion channel blockade, we have been able to show that the action of the chloramphenicols on end-plate current amplitude and time course can be explained by the combination of two distinct mechanisms. First is an open channel block conforming to the sequential model and with calculated channel blocking affinity constants ranging from 0.3-1.0 mM. The channel blocking actions of all four isomers were shown to be independent of membrane voltage. Second is an action to slow channel closing, resulting in prolonged open time and hence increased charge passed during the end-plate current. This effect was strongly concentration dependent, but not voltage dependent. We suggest that this action is due to the lipid-soluble chloramphenicols dissolving within the membrane and causing a change in the dielectric constant which leads to a hindering of the conformational change in molecular structure underlying channel closure. Failure to take any effect of channel blocking drugs on closing characteristics into account can lead to inaccurate estimates of the kinetics of the channel block. Despite the deviation of the actions of the compounds from the predictions of the sequential model for end-plate ion channel block, it is clear from the results that the channel binding site is unable to distinguish between D- and L-forms of either threo or erythro chloramphenicol. It is therefore concluded that the channel binding site is not stereoselective in nature.