ACTIVATION KINETICS OF THE DELAYED RECTIFIER POTASSIUM CURRENT OF BULLFROG SYMPATHETIC NEURONS

We examined the activation kinetics of the delayed rectifier K+ curren t of bullfrog sympathetic neurons, primarily using whole cell recordin g. On depolarization, currents activated with a sigmoid delay but did not show a Cole-Moore shift. The time course of activation differed sy stematically from an exponential raised to a power. At most voltages, a power of 2 gave the best overall fit but a power of 3 better describ ed the initial delay. After the delay, the time course could be fitted by a single exponential. Time constants were 15-20 ms at 0 mV and dec reased to a limiting tau = 7 ms at +50 to +100 mV. Tail currents were well fitted by single exponential functions and accelerated with hyper polarization, from tau = 15-20 ms at 0 mV to tau = 2 ms at -110 mV (e- fold for 40 mV). Eleven kinetic models were evaluated for their abilit y to describe the activation kinetics of the delayed rectifier. Hodgki n-Huxley-like models did not fit the data well. A linear model where v oltage sensor movement is followed by a distinct channel opening step, allosteric models based an the Monod-Wyman-Changeux model, and an unc onstrained C-C-C-O model could describe whole cell data from -100 to 40 mV. After including whole cell data at +60 and +80 mV, and a maxima l p(open) of 0.8 from noise analysis of cell-attached patches, an allo steric model fit the data best, as the other models had difficulty des cribing qualitative features of the data. However, some more complex s chemes (with additional free parameters) cannot be excluded. We propos e the allosteric model as an empirical description of macroscopic ioni c currents, and as a model worth considering in future studies on the molecular mechanism of potassium channel gating.