Acetylcholine receptor (from Electrophorus electricus): a comparison of single-channel current recordings and chemical kinetic measurements

Abstract

We report a direct comparison between two types of measurements of the dynamic properties of the acetylcholine receptor: single-channel currents recorded using the patch-clamp technique and chemical kinetic measurements. Electrophorus electricus electroplax cells, and membrane vesicles prepared from these cells, were used. Such a comparison, and single-channel currents recorded from these cells, have not previously been reported. We first give the theoretical basis for the comparison and define the conditions under which the comparisons are elegantly simple. We relate (i) measurements of currents through receptor channels in the cell membranes to measurements of the rates of ion translocation through the receptor channels in vesicles and (ii) measurements of the lifetimes of receptor states (for instance, the lifetime of the active state of the receptor--i.e., the state in which it can form open channels) to rate coefficients obtained in chemical kinetic measurements (for instance, those for the interconversions between different states of the receptor). In eel Ringer's solution we have found the single-channel conductance (gamma) of the receptor in E. electricus electroplax cells to be 53 pS. From this value, a specific reaction rate for ion translocation, J, of 5 X 10(7) M-1 X sec-1 was calculated. When membrane vesicles prepared from the electroplax cells and the same solution compositions were used, chemical kinetic measurements gave a J value of 3 X 10(7) M-1 X sec-1. The agreement between the two measurements is important because (i) they reflect different experimental conditions, which require different assumptions in interpreting the results, and (ii) it indicates that the two techniques can be used to obtain complementary information: the methods have different time resolutions and can be used in different ranges of acetylcholine concentrations.