Decline of Electrogenic Na ' / K ' Pump Activity in Rod Photoreceptors During Maintained Illumination HIROSHI SHIMAZAKI

Light-evoked changes in membrane voltage were recorded intracellularly from rod photoreceptors in the isolated retina preparation of the toad, Bufo marinus, during superfusion with a solution containing pharmacological agents that blocked voltage-dependent conductances . Under these conditions, the amplitude ofthe hyperpolarizing photoresponse became much greater than under control conditions . The results of several experiments support the conclusion that this increase in photoresponse amplitude was due primarily to a voltage that was produced when the electrogenic current from the rods' Na''/ K+ pump flowed across an increased membrane resistance (Torre, V. 1982. Journal of Physiology . 333:315) . At the onset of a period of continuous illumination, the rod membrane first hyperpolarized and then began to repolarize, and after 180 s of illumination, the membrane voltage had recovered by 6072% of its initial hyperpolarization . There did not appear to be any significant decrease in rod membrane resistance associated with this repolarization . Both the enhanced hyperpolarization at light onset and the slow repolarization during maintained illumination were blocked by superfusion with 10.0 UM strophanthidin . These data support the hypothesis that the activity of the rods' Na'/K' pump declines progressively during maintained illumination . It is likely that the decline in pump activity produces significant changes in [K+] .in the subretinal space during maintained illumination .