The calcium-dependent potassium conductance in rat sympathetic neurones

1. Adult and intact sympathetic neurones of isolated rat superior cervical ganglia were subjected to a two-electrode voltage-clamp analysis at 37 degrees C in order to investigate the Ca2(+)-dependent K+ conductance. 2. At each potential a Ca2(+)-dependent K+ current, IKCa, was determined as the difference between the current that could be attributed to the voltage-dependent K+ current, IKV, following Ca2+ channel blockade by Cd2+ and the total current generated. The final IKCa curves were obtained after correcting the experimental tracings for the underlying ICa current component. 3. IKCa became detectable during commands to -30 mV. About 3.6 x 10(5) Ca2+ ions are required to enter the cell before IKCa is initiated. The current was modelled on the basis of a 0.4-0.6 ms delay followed by an exponential activation of a fast component, IKCaf, simultaneously with a much slower exponential activation, IKCas. Experiments indicate a sigmoidal activation curve for the fast conductance, gKCf, with half-maximal activation at -13.0 mV and a slope factor of 4.7 mV (for 5 mM-Ca2+ in the bath). The associated time constant, tau kcf, ranged from 0.8 to 2.0 ms. The slow conductance exhibited a similar steady-state activation curve but an activation time constant in the 48-280 ms range. The maximum mean gKC was 0.32 microS per neurone for either the fast or slow component. 4. Excess K+ ions accumulate in the perineuronal space during K+ current flow giving rise to rapidly occurring, large K+ reversal potential (EK) modifications (up to -45 mV for the largest currents). The kinetics of K+ extracellular load can be described satisfactorily by a simple exponential function (tau = 0.9-2.8 ms). The characteristics of K+ wash-out appear similar to those of accumulation. 5. The immediate effect of such an extracellular K+ build-up is to make the apparent IKCa activation kinetics faster and to reduce (up to 50%) the true value of the K+ conductance. We simulated the predictions of a K+ diffusion model and generated new functions describing the IKCa steady-state activation, activation rate and maximum conductance values which satisfactorily reconstruct the IKCa current tracings together with the K+ accumulation process near the membrane. 6. A small component of the Ca2(+)-dependent K+ current, IAHP, was observed which survived at membrane potential levels negative to -40 mV. Increasing Ca2+ influx by applying longer pulses enhanced IAHP, which on the other hand was also activated by depolarizations of short duration.(ABSTRACT TRUNCATED AT 400 WORDS)