Light activated electrogenic Na⁺-Ca²⁺-exchange in fly photoreceptors: Modulation by Na⁺/K⁺-pump activity

Abstract

We found recently that illumination of white-eyed Musca photoreceptors following application of ruthenium red (RR), (a known inhibitor of Ca²⁺-uptake into intracellular organelles) induced a transient post-illumination-afterdepolarization (TA). We found that the TA depended strongly on Na⁺- and Ca²⁺-gradients across the cell membrane suggesting that it arose from activation of electrogenic Na⁺-Ca²⁺-exchange. Since the magnitude of the TA was dependent on the Na⁺-gradient, in this work we studied the effects of the Na⁺/K⁺-pump, which controls the Na⁺-gradient, on the TA. Increasing extracellular K⁺-concentration which largely increased pump activity also increased the TA. Application of 100 ΜM ouabain, for a short time, depressed the TA considerably without causing any noticeable effect on the resting or the receptor potential. Only after longer exposure to ouabain, when the TA was totally depressed, were the resting and receptor potentials abolished by only a few strong light flashes. Trains of strong light pulses either facilitated or inhibited the TA depending on the degree of increase in internal Ca²⁺-ions by light and on the activity of the Na⁺/K⁺-pump.