Opentime heterogeneity during bursting of sodium channels in frog skeletal muscle.

Single voltage-activated Na+ channel currents were obtained from membrane patches on internally dialyzed skeletal muscle segments of adult frogs. The high channel density in these membranes permitted frequent observation of the "bursting mode" of individual Na+ channels during 400-ms records. We examined the opentimes within and between bursts on individual membrane patches. We used a new nonparametric statistical procedure to test for heterogeneity in the opentime distributions. We found that although 80% of all bursts consisted of opentimes drawn from a single distribution, the opentime distribution varied significantly from burst to burst. Significant heterogeneity was also detected within the remaining 20% of individual bursts. Our results indicate that the gating kinetics of individual Na+ channels are heterogeneous, and that they may occasionally change in a single channel.