title: The contribution of the electrogenic sodium-potassium pump to the electrical activity of toad rods.
abstract: 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.
mesh_terms: D000818:Animals; D001693:Biological Transport, Active / Q000187:drug effects; D002118:Calcium / Q000494:pharmacology; D002586:Cesium / Q000494:pharmacology; D066298:In Vitro Techniques; D007700:Kinetics; D008027:Light; D008564:Membrane Potentials / Q000187:drug effects; D010786:Photoreceptor Cells / Q000502:physiology*; D011188:Potassium / Q000502:physiology*; D012964:Sodium / Q000502:physiology*; D013327:Strophanthidin / Q000494:pharmacology; D013997:Time Factors
chemical_list: D002586:Cesium; D013327:Strophanthidin; D012964:Sodium; D011188:Potassium; D002118:Calcium