Monovalent and divalent cation permeation in acetylcholine receptor channels. Ion transport related to structure.

Abstract

Single channel patch-clamp techniques were used to study nicotinic acetylcholine receptors in cultured rat myotubes . The single channel conductance in pure cesium and sodium levels off at high concentrations, as if a binding site within the channel were saturating . The conductances at very low concentrations, however, are larger than predicted by the simplest one-site transport model fitted to the high-concentration data . At low concentrations, the current-voltage relations are inwardly rectifying, but they become more ohmic if a small amount of divalent cations is added externally . Magnesium and barium are good permeants that have rather high affinities for the channel. Upon adding low millimolar concentrations of these divalent cations externally to a membrane bathed in pure cesium, the inward current carried by cesium is decreased. As more divalent cations are added, the inward-going currents continued to decrease and the divalent cation replaces cesium as the main current carrier. The ion transport data are described by considering the size, shape, and possible net charge of the channel. In that way, even the complex features of transport are explained in a realistic physical framework. The results are consistent with the channel having long, wide, multiply occupied vestibules that serve as transition zones to the short, selective, singly occupied narrow region of the channel. A small amount of net negative charge within the pore could produce concentration-dependent potentials that provide a simple explanation for the more complicated aspects of the permeation properties .

Mutations at two distinct sites within the channel domain M2 alter calcium permeability of neuronal alpha 7 nicotinic receptor.

“…3). This is in agreement with the notion, proposed for endplate channels (2, 33,36) with a glutamine at position 254 or 255 exhibit a higher Ca2+ permeability than with an arginine (Table 2 and Fig. 3).…”

Interaction of Na+ and Mg2+ ions in acetylcholine receptor channels of frog skeletal muscle changes in character with an increase in agonist concentration

“…2B). Since these values are not much different from the IMp, M~./IM+ ratios of about 0.5-0.6 that are found in single-channel measurements [3,8], the responses at these lower agonist levels are in line with what one would expect of aggregate currents formed by uncomplicated summation of unitary single-channel currents [19]. Beginning, however, with tests at 110 gM carbamylcholine and at all These results, as well as revealing a transition point beyond which macroscopic nAChR currents cease to mimic behavior seen in single-channel measurements, raise further questions about how Na + and Mg 2+ affect ion permeation of nAChR which it is appropriate to consider at this point.…”

“…An underlying assumption in the development of the ion-exchange model is that all of the measured inhibitory effect of Mg 2 § on nAChR responses resides in the competition of Mg 2+ and agonist molecules for ligand-binding sites. On the other hand, single-channel measurements [3] show that Mg 2+ and other divalent cations can have a direct influence on channel currents which is separate from any action that limits access of agonists to receptor sites. The application of the ligand-site ion-exchange model to the present results is further complicated, therefore, to the extent that the Mg 2 § effect must be partitioned between receptor and channel sites.…”

“…This interaction is evident in both noise analysis [1,8,9,14] and patch-clamp [3] studies as a decrease of 40% or more in single-channel currents carried by monovalent cations (Na +, Cs +) when permeant divalent cations (Ca 2+, Mg 2+, Ba 2+) are added in the millimolar range. In order to see if the same behavior is exhibited in aggregate whole-cell nAChR currents as well, conditions were devised for estimating agonist-induced currents of Na § and Mg 2+ as the test cations, either singly (I~,Ta+ or IMg2~) or in combination (INa~ Mg--), using conventional voltage-clamp measurements at frog skeletal muscle endplates.…”

Nicotinic acetylcholine receptors in porcine hypophyseal intermediate lobe cells.

“…It has been shown in rat myotubes that ACh channels are permeable to Na+ and Cs+ ions (Dani & Eisenman, 1987). Several other studies revealed calcium permeation in ACh receptor channels in frog endplate and mouse muscles (Bregestovski, Miledi & 94 AChR CHANNELS IN INTERMEDIATE LOBE PITUITARY Parker, 1979;Lewis, 1979;Lorkovic, 1988).…”