A potassium contribution to the response of the barnacle photoreceptor.

1. Intracellular recording from photoreceptors in the lateral eye of the barnacle show a brief negative-going 'dip' shortly after the onset of the late receptor potential. This phase can sometimes result in a hyperpolarization relative to the resting membrane potential. 2. The dip is prominent in light-adapted cells and is reduced by dark-adaptation. Low extracellular Ca2+ also reduces it. 3. The amplitude of the dip changes inversely with the K+ concentration in the saline. 4. The amplitude of the dip depends on the membrane potential, with a reversal potential near - 80 mV. 5. K+ blocking agents such as quinine and quinidine reduce or abolish the dip. 6. These observations indicate that the dip is due to a brief increase in K+ conductance which may be dependent on an influx of Ca ions. The fast decay of this phase may be brought about by a rapid uptake of Ca2+ by an intracellular mechanism.