2 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 inophore A23187; it was blocked by tetraethylammonium (TEA+), quinine, and 4-aminopyridine (4-AP). 4-AP blocked 70% of the outward current with a K(i) 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 I(K)(Ca) indirectly via decreasing Ca2+ entry through T-type Ca2+ channels. Separating Ni2+-sensitive from -insensitive outward conductance gives components that correspond notionally to I(K)(Ca) and I(K)(V), respectively. On this basis I(K)(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: I(K)(V), an inactivating voltage-sensitive K+ current, and I(K)(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, I(K)(Ca) will be the more important in terms of K+ transport and the secretion of aqueous humor by the ciliary epithelium.