Effects of adapting lights on the time course of the receptor potential of the anuran retinal rod.

1. The intracellular receptor potential of the retinal rod cell was recorded in the unperfused, isolated retina of Rana catesbiana and in the perfused, isolated retina of Bufo marinus. Qualitatively, the responses from the two preparations were similar. 2. The rate at which the receptor potential returned to the dark level at the termination of a pulse of light (Voff) was measured at a fixed potential chosen to be about 0-6 of the way from the dark level to the peak of the response. 3. When the light intensity was such that less than about 10-minus 5 of the photopigment was bleached per second, Voff increased as the duration of the pulse was increased, reaching a maximum in 50-100 s. 4. When a brief test flash was presented at various intervals after an adapting pulse lasting about 50 s, Voff for the test flash was greater than the value in the dark adapted state for times up to about 80 s after the adapting pulse. 5. It has been hypothesized that in the vertebrate rod light causes release from the disk sacs of particles which block conducting channels in the surface membrane (Yoshikami & Hagins, 1971, 1973). A modification is proposed in which the blocking particles are converted to an inactive state can be increased by light adaptation. 6. This modified hypothesis will account qualitatively for the further observations that (a) during the response to illumination lasting several seconds the membrane potential recovers part of the way to the dark level and (b) if a second light pulse is superimposed on this background illumination then after the superimposed pulse the depolarization is increased.