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)
The Journal of Physiology – Wiley
Published: Mar 1, 1990
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