Kinetics of activation of acetylcholine receptors in a mouse muscle cell line under a range of acetylcholine concentrations.

We studied, using the patch-clamp technique, the kinetics of single acetylcholine (ACh)-activated channels in a mouse muscle cell line. In the presence of high ACh concentrations we estimated the rate of channel isomerization into the open state (beta) from the dwell time between openings. Also, we obtained estimates for beta under low agonist concentrations by assuming a linear sequential model of channel activation and applying burst analysis. If the linear model is correct, then the two estimates of beta should agree since beta should be independent of ACh concentration. However, the estimates of beta obtained under low ACh concentrations were slower than those obtained independently under high ACh concentrations. The discrepancy in the estimates of beta suggests that the linear model is inadequate, but the discrepancy can be explained if open channels can close through two separate pathways. Two alternative kinetic models that can account for our data are discussed.