Two outward K+ currents in bovine pigmented ciliary epithelial cells: IK(Ca) and IK(V).
Pigmented ciliary epithelial cells were studied using the whole cell voltage-clamp technique. Depolarizing steps from a holding potential of -80 mV resulted in a small initial inward current followed by a large outward current. Prolonged depolarizing voltage steps revealed inactivating and noninactivating components of outward current. Outward current was sensitive to the level of Ca2+ in the pipette and was increased by the calcium ionophore A23187; it was blocked by tetraethylammonium (TEA+), quinine, and 4-aminopyridine (4-AP). 4-AP blocked 70% of the outward current with a Ki of 7 x 10(-5) M, and part of the remaining current was abolished by Ni2+. Ni2+ caused a reduction in outward current by blocking IK(Ca) indirectly via decreasing Ca2+ entry through T-type Ca2+ channels. Separating Ni(2+)-sensitive from -insensitive outward conductance gives components that correspond notionally to IK(Ca) and IK(V), respectively. On this basis IK(Ca) represents approximately 28% of K+ outward current. Charybdotoxin blocked 26% of the outward conductance at very depolarized voltage steps as calculated from the slope of the current-voltage curve in this region. It is concluded that there are two major components to the outward current: IK(V), an inactivating voltage-sensitive K+ current, and IK(Ca), which is dependent on the entry of Ca2+ through T-type Ca2+ channels and comprises approximately a quarter of the total K+ outward current under the conditions described. Because of their relative voltage-activation properties, IK(Ca) will be the more important in terms of K+ transport and the secretion of aqueous humor by the ciliary epithelium.
Jacob
T J
TJ
Department of Physiology, University of Wales, Cardiff, United Kingdom.
D000818:Animals; D002417:Cattle; D002478:Cells, Cultured; D018999:Charybdotoxin; D002924:Ciliary Body / Q000166:cytology / Q000378:metabolism*; D004553:Electric Conductivity; D004847:Epithelial Cells; D004848:Epithelium / Q000378:metabolism; D009532:Nickel / Q000494:pharmacology; D010858:Pigmentation; D011188:Potassium / Q000378:metabolism*; D015221:Potassium Channels / Q000378:metabolism*; D012604:Scorpion Venoms / Q000494:pharmacology
D015221:Potassium Channels; D012604:Scorpion Venoms; D018999:Charybdotoxin; D009532:Nickel; D011188:Potassium