Effects of changing external potassium and chloride concentrations on the photoresponses of Bufo bufo rods.

1. Intracellular responses to light were recorded from Bufo bufo rods in different ionic media. 2. The exposure of the retina to high external [K+] depolarized the rod and modified the time course of the photoresponse. The prominent initial transient of rod responses to bright flashes was drastically reduced in 5 mM-external [K+] and completely disappeared in 26 mM. In high external [K+] the kinetics of responses to dim flashes were considerably slower than in control conditions. 3. When external [Cl-] was changed from 120.6 to 10.6 mM the resting membrane potential decreased and the size of photoresponses increased. Changes in the kinetics similar to those described in high external [K+] were also observed. 4. In many cases exposure of the retina to low external [Cl-] induced oscillations of the resting membrane potential that sometimes became sustained. This instability of the membrane completely disappeared upon restoring to normal conditions. 5. The present results may be explained by assuming the existence of a voltage- and time-dependent conductance active near the dark level of membrane potential. This hypothesis can be represented by an equivalent electrical circuit that includes an inductance (Detwiler, Hodgkin & McNaughton, 1980).