Receptor binding
Macroscopic AChR responses and the Hill slope for AChR activation. (a) Current through AChR ion channels in response to increasing concentraions of ACh was recorded from a Xenopus oocyte which had been injected 3 days previously with cRNA for the alpha, beta, gamma, and delta subunits of the mouse muscle AChR. An inward current through the AChR ion channels is shown as a downward deflection of the trace. Small artifacts on the trace indicate the time when the solution flowing into the bath was changed from control to the indicated voltage concentrations, and then back to control. Currents were recorded with a two-microelectrode voltage clamp. The membrane potential was -60mV and the recordings were made at room temperature, (b) The response (in nA) to increasing concentrations of ACh is plotted against ACh concentration (in nM) on log-log scales. The slope of the line (1.92) is an approximation to the Hill coefficient (when receptor occupancy is small) and suggests that two agonist molecules must bind to the receptor to produce efficient receptor activation.