Metarhodopsin I/metarhodopsin II transition triggers light-induced change in calcium binding at rod disk membranes

The hypothesis of Yoshikami and Hagins that calcium ions act as diffusible transmitter molecules between the photochemistry of rhodopsin and the subsequent electrical events at the outer plasma membrane of rods initiated many investigations on light-stimulated calcium release in vertebrate photoreceptor cells (see refs 2, 3). Although it not seems firmly established that light has some effect on the redistribution of calcium in various disk preparations, reconstituted systems and intact rod outer segments, the physiological significance remained unclear. We previously reported a rapid, light-triggered calcium release from binding sites at the disk membrane in the presence of calcium ionophore A23187 (refs 3, 8). However, there is no evidence for rapid calcium release into the cytosol in the absence of ionophore. On fragmentation of intact rod outer segments, calcium release due to a light-requlated change of calcium binding appeared almost completely abolished. We describe here experiments with sonicated rod outer segments in which the previously observed loss of the calcium release capacity has been prevented. Calcium release in sonicated disks in the presence of A23187 kinetically follows the metarhodopsin I/metarhodopsin II transition (tau 1/2 = 10 ms, activation energy EA = 34 kcal mol-1), suggesting that calcium release is triggered by this photochemical transition.