Relative inhibitory effects of ATP depletion, ouabain and calcium on retinal photoreceptors

Abstract

The relative inhibitory effects of ATP depletion, ouabain and calcium on the receptor potential of the isolated rat retina were investigated. The principal experimental test used to assess these effects was the rate and extent of recovery of the receptor potential after re-establishing the transmembrane sodium gradient. Retinas were incubated in 25 m m-sodium for 10–20 min and then the external concentration of sodium was increased to 155 m m in control retinas, in retinas exposed to 10 −4 m-ouabain, and in retinas depleted of ATP (glucose-free medium plus 10 m m 2-deoxyglucose). Measurements showed that by 10–20 min ouabain completely inhibited the activity of the sodium-potassium ATPase and 2-deoxyglucose caused almost total loss of retinal ATP. In control retinas, the increase in sodium led to a rapid, full recovery of the receptor potential. When the sodium-potassium pump was blocked by ouabain, a return to the normal level of sodium led within seconds to a small (30%) recovery of the potential which decayed over 5 min. In ATP depleted retinas no recovery of the receptor potential was observed. However, if glucose was added at the time of changing to 155 m m-sodium in depleted retinas then a substantial recovery of the receptor potential was observed, but the return of the potential was delayed relative to the instantaneous re-establishment of the sodium gradient. After ouabain caused the receptor potential to decay in the presence of 2 m m-calcium and 155 m m-sodium, the potential was restored within seconds upon a reduction of external calcium to 10 −7 m. At this low calcium concentration, but not at 2 m m-calcium, an inverted receptor potential was observed transiently when the membrane sodium gradient was reversed during ouabain treatment. No recovery of the receptor potential was seen if the concentrations of calcium and sodium were raised to their control levels after incubation of the retina in a low sodium, low calcium, ouabain-containing medium. These results show that the receptor potential is elicited by changing the sodium gradient when the sodium-potassium pump is blocked, but is not evoked when either retinal ATP content is low or the external level of calcium is increased. The similarity in the inhibitory effects of ATP depletion and calcium suggest that high energy phosphates play a role in maintaining a low intracellular concentration of calcium, possibly by influencing sodium-calcium exchange or calcium pumps.