Photoinduced calcium release from rhodopsin-phospholipid membrane vesicles

Brief blue-green light exposure of rhodopsin-phospholipid membrane vesicles that contained divalent cations released the cations from the vesicles. The photoinduced release is due to an increase in permeability of the membrane. The quantity of ions released depends on the initial ionic concentration inside the vesicles. At 37 degrees C and an internal concentration of 30 mM Ca2+, the initial flux for rhodopsin-egg phosphatidylcholine membrane vesicles was 0.25 +/- 0.11 Ca2+ per bleached rhodopsin per s. Similar fluxes were observed for the release of Co2+, Mn2+, Ni2+, and Mg2+. The addition of proton uncouplers and lipophilic anions accelerated the rate to approximately Ca2+ per bleached rhodopsin per s. The flux was independent of the concentration of rhodopsin in the membranes and sensitive to the head-group composition of the rhodopsin-phospholipid vesicles. Analysis of the fraction of Ca2+ released and the fraction of bleached rhodopsin per vesicle showed that a single bleached rhodopsin per vesicle is necessary and sufficient for Ca2+ release. Ca2+ release was not observed from thermally bleached rhodopsin. These results are discussed with regard to the possible role of Ca2+ as an excitatory transmitter in vision.