Intracellular recordings from photoreceptors of the squid (Loligo pealii)

Abstract

Intracellular recordings were made from squid photoreceptors using micropipettes filled with 4 M potassium acetate. Resting potentials were about −55 mV, and light stimuli induced membrane voltage to become more positive (depolarized). For an intense stimulus there was a transient peak depolarization that declined to a plateau; membrane potential exceeded 0 V during the peak depolarization. The magnitude of the light response was decreased either by passing steady depolarizing current through the recording pipette or by bathing the retina in low Na+ sea water. The waveform of the response to an intense stimulus was altered by bathing the retina in low Ca++ sea water or intracellular injection of EGTA; for both of these treatments, the decline from the transient peak depolarization to the plateau was lessened and the responses became more “square”. Squid photoreceptors were found to be very sensitive to hypoxia. These results, taken together, are consistent with the interpretations that (1) the majority of the light-induced current is carried by Na+, and (2) a rise in (Ca++)in is a step in the mechanism underlying the decline from the peak transient depolarization to the plateau.