Distribution of exponentiality in miniature endplate current decay.

Exponential functions are commonly used to describe miniature endplate current (MEPC) decay; under control conditions a monoexponential is usually regarded as sufficient, whereas in the presence of some drugs a biexponential may be necessary. Using an automated fitting procedure which estimated exponential parameters and the period of decay from peak to baseline, a unimodal distribution of curvature was found for control MEPCs recorded in frog sartorius muscle. The majority of MEPCs were of monoexponential form, while the remainder were biexponential with either less or greater curvature than expected for a simple exponential (hypoexponential or hyperexponential, respectively). The proportion of MEPCs in each of the 3 groups was constant for a given endplate but varied between endplates. A possible explanation for this phenomenon could be differences in synaptic geometry within and between endplates. The increased curvature of individual MEPCs in the hyperexponential group was analyzed by assuming a sequential model for agonist blockade or desensitization, and calculating closing and reopening rate constants. These rate constants were altered by procaine and verapamil (100 microM) in a manner consistent with blockade of the acetylcholine receptor by enhancement of agonist-induced desensitization.