Ca2+-activated K+ currents in Necturus choroid plexus.
The tight-seal whole-cell recording method has been used to study Necturus choroid plexus epithelium. A cell potential of -59 +/- 2 mV and a whole cell resistance of 56 +/- 6 M omega were measured using this technique. Application of depolarizing step potentials activated voltage-dependent outward currents that developed with time. For example, when the cell was bathed in 110 mM NaCl Ringer solution and the interior of the cell contained a solution of 110 mM KCl and 5 nM Ca2+, stepping the membrane potential from a holding value of -50 to -10 mV evoked outward currents which, after a delay of greater than 50 msec, increased to a steady state in 500 msec. The voltage dependence of the delayed currents suggests that they may be currents through Ca2+-activated K+ channels. Based on the voltage dependence of the activation of Ca2+-activated K+ channels, we have devised a general method to isolate the delayed currents. The delayed currents were highly selective for K+ as their reversal potential at different K+ concentration gradients followed the Nernst potential for K+. These currents were reduced by the addition of TEA+ to the bath solution and were eliminated when Cs+ or Na+ replaced intracellular K+. Increasing the membrane potential to more positive values decreased both the delay and the half-times (t1/2) to the steady value. Increasing the pipette Ca2+ also decreased the delay and decreased t1/2. For instance, when pipette Ca2+ was increased from 5 to 500 nM, the delay and t1/2 decreased from values greater than 50 and 150 msec to values less than 10 and 50 msec. We conclude that the delayed currents are K+ currents through Ca2+-activated K+ channels. At the resting membrane potential of -60 mV, Ca2+-activated K+ channels contribute between 13 to 25% of the total conductance of the cell. The contribution of these channels to cell conductance nearly doubles with membrane depolarization of 20-30 mV. Such depolarizations have been observed when cerebrospinal fluid (CSF) secretion is stimulated by cAMP and with intracellular Ca2+. Thus the Ca2+-activated K+ channels may play a specific role in maintaining intracellular K+ concentrations during CSF secretion.
Loo
D D
DD
Department of Physiology, UCLA School of Medicine 90024-1751.
Brown
P D
PD
Wright
E M
EM
D000818:Animals; D002118:Calcium / Q000494:pharmacology*; D002586:Cesium / Q000494:pharmacology; D002831:Choroid Plexus / Q000166:cytology / Q000502:physiology* / Q000648:ultrastructure; D004553:Electric Conductivity; D004848:Epithelium / Q000502:physiology / Q000648:ultrastructure; D008564:Membrane Potentials; D009337:Necturus / Q000502:physiology*; D009338:Necturus maculosus / Q000502:physiology*; D011188:Potassium / Q000378:metabolism* / Q000494:pharmacology; D015221:Potassium Channels / Q000187:drug effects / Q000502:physiology*; D012964:Sodium / Q000494:pharmacology; D013757:Tetraethylammonium Compounds / Q000494:pharmacology
D015221:Potassium Channels; D013757:Tetraethylammonium Compounds; D002586:Cesium; D012964:Sodium; D011188:Potassium; D002118:Calcium