Voltage dependence of desensitization. Influence of calcium and activation kinetics

The voltage dependence of carbachol-induced desensitization has been analyzed in potassium-depolarized frog sartorius muscle preparations with voltage clamp techniques over a wide voltage range (-120 to +40 mV). Desensitization developed exponentially at all voltages with tau, the time constant of desensitization onset, varying as a logarithmic function of membrane voltage. The voltage dependence of tau remained in calcium-deficient solutions and was not altered by elevating either the level of extracellular or intracellular calcium. We have analyzed our results according to a simple sequential kinetic scheme in which the rate-limiting step in the development of desensitization is a transition of the receptor channel complex from the activated conducting state to a desensitized, nonconducting state. We conclude (a) that the observed voltage sensitivity of desensitization primarily resides in the voltage dependence of this transition, and (b) the kinetics of activation appear to have a greater influence on the observed rate of desensitization than on its voltage dependence. The magnitude of the voltage dependence suggests that a greater change in free energy is required for the transition to the desensitized state than for the transition between the open and closed states of the receptor channel complex.