The contribution of the electrogenic sodium-potassium pump to the electrical activity of toad rods.

1. The membrane potential of rods in the isolated toad retina was recorded while changing the ionic composition of the extracellular medium.2. Caesium (Cs(+)) at a concentration of 1 mM was sufficient to completely block the sag from the peak to the plateau in the bright-flash voltage response.3. In the presence of 10 mM-Cs(+) the bright-flash response increased in amplitude to about 90 mV, thus reaching an absolute membrane potential of between -110 and -135 mV. These responses consisted of an initial fast component of about 35 mV followed by a much slower component which could be as large as 50 mV.4. At the peak of the initial fast component the rod membrane conformed closely to the behaviour of a K(+) electrode with a P(Na)/P(K) ratio of 0.023. On average the amplitude of the slow component was about 35 mV in the presence of 2.6 mM-K(+) and was reduced to about 25 mV in a K(+)-free Ringer.5. Addition of 100 muM-strophanthidin to the perfusate induced several reversible changes in the electrical activity of rods. The dark resting membrane potential depolarized by about 5 mV and the kinetics of the voltage response to dim flashes of light slowed down. The voltage sensitivity initially increased by about 30%, but the peak of the response to a bright flash of light was reduced by about 13 mV.6. In rods treated with 10 mM-Cs(+) the slow component present in the bright flash response was abolished by strophanthidin with an apparent K(m) of 3 muM.7. The amplitude of the slow component decreased with a time lag of about 2 min when external Na(+) was reduced. A previous exposure of the retina to a Na(+)-free Ringer solution for at least 3 min modified the voltage photoresponse in a way similar to that observed in the presence of 100 muM-strophanthidin.8. When external Ca(2+) concentration ([Ca(2+)](o)) was increased from 2 to 5 mM the slow component decreased by about 30%. When [Ca(2+)](o) was reduced the slow component increased. A twofold increase was observed when [Ca(2+)](o) was lower than 10(-4) M.9. It is suggested that the slow component of the voltage response in the presence of external Cs(+) is caused by an electrogenic current driven by the Na(+)-K(+) transport system, during a voltage-dependent block of external Cs(+) of some K(+) channels.