Light activation of the sodium pump in blowfly photoreceptors

Summary

• 1. The oxygen consumption of the compound eye in the blowfly was determined during light exposure and in darkness by a manometric measuring technique. Within the first 10 s of exposure to bright white light the oxygen uptake increased up to 20 × the resting value in darkness, which is 2.4 to 3×10⁻⁵ml oxygen x min⁻¹ × eye⁻¹.

• 2. The time course of oxygen consumption during 3-min light stimulation proceeded in two phases: an initial dynamic phase, followed by a decline to a maintained phase of almost constant amplitude. The time course of the dynamic phase varied with the intensity and wavelength of the stimulating light. In contrast, within the intensity range used, oxygen consumption during the maintained phase was about double the resting value in darkness, irrespective of intensity and wavelength of the light stimulus. The time course of the decline in oxygen consumption during light exposure paralleled the electrophysiologically recorded time course of decrease in amplitude of the photoreceptor response.

• 3. Light-induced oxygen consumption decreased continuously after replacement of the extracellular fluid in the eye by solutions of increasing potassium concentration. Oxygen uptake in darkness was influenced less. It is concluded that the additional oxygen consumption during light exposure is mainly caused by activation of the sodium-potassium pump.

• 4. Replacing extracellular sodium by choline did not markedly reduce the oxygen uptake. This result indicates that, after perfusion of the eye, sodium is pumped out of the cell, causing a sodium gradient to be re-established. Alternatively, the ion pump can accept choline and thus is not highly specific for sodium.

• 5. Complete exchange of extracellular sodium by calcium or magnesium 150 mmol/l caused only a small reduction in light-induced oxygen consumption. This observation can be explained by an ion exchange coupled to the ion transport mechanism. The intracellular store of sodium after the replacement is sufficient to form a small sodium gradient, which drives a sodium-calcium exchanger. Sodium itself is actively pumped out. This coupled transport mechanism seems to be sensitive to the extracellular anion composition, since chloride markedly reduced the transport rate.

• 6. The oxygen consumption above steady state, following excitation by blue light, was about six times higher in rhodopsin-rich eyes than in rhodopsin-poor eyes. Oxygen uptake following light exposure thus depends on the absolute rhodopsin content in the rhabdomeres. Sequential stimulations by blue-red or red-blue light showed that the higher oxygen uptake following light exposure in rhodopsin-rich eyes is caused by the excess in excited metarhodopsin molecules, which is strongly correlated with the PDA signal.

• 7. ATP consumption per absorbed light quantum decreases supralinearly with increasing stimulus intensity. Light intensities eliciting photoreceptor responses of a maximal amplitude of 0.64 induce an initial ATP consumption of about 54000 molecules per absorbed quantum. Exposure to bright white light, hitting all rhodopsin molecules many times within 10 s, initially induces a consumption of only 90 ATP molecules per quantum. After 3-min exposure, consumption is reduced to only 7 ATP molecules per quantum. This dramatic reduction in ATP consumption per absorbed quantum occurs within the upper range of physiological light intensities. The results demonstrate that, at low light intensities, most of the energy is used for pump activity. At high physiological intensities, the fraction of energy consumption used for biochemical amplification and adaptation processes becomes more important.