Voltage-sensitive potassium channels in Limulus ventral photoreceptors.

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Abstract
The steady-state slope conductance of Limulus ventral photoreceptors increases markedly when the membrane is depolarized from rest. The ionic basis of this rectification was examined with a voltage-clamp technique. Tail currents that occur when membrane potential is repolarized after being depolarized were identified. Tail currents reverse direction at a voltage that becomes more positive when Ko [outside] is increased. Rectification is reduced by extracellular 4-aminopyridine and by intracellular injection of tetraethylammonium (TEA). Membrane rectification around resting potential seems due primarly to voltage-sensitive K+ channels. Increase in gK caused by depolarization is not mediated by a voltage-dependent rise in Cai2+, [internal], since intracellular injection of Ca2+ causes a decrease rather than an increase in slope conductance. TEA can be used to examine the functional role of the K+ channels because it blocks them without substantially affecting the light-activated Na+ conductance. The effect of TEA on response-intensity curves shows that the K+ channels serve to compress the voltage range of receptor potentials.