Rapid calcium release by passively loaded retinal discs on photoexcitation.

In vertebrate retinal rods, rhodopsin undergoes a conformational change upon absorption of a quantum of light. The primary photoprocess is followed by a transient depolarisation of the outer receptor membrane. It is generally accepted that the depolarisation is due to the closing of Na’channels [l-4]. As the rhodopsin containing inner discs and the plasma membrane of the retinal rods seem to be topographically and electrically isolated, it is unlikely that rhodopsin controls the sodium channels directly [ 5,6]. It has been suggested that the primary photoisomerisation and the electrical events at the outer cell membrane are mediated by a transmitter molecule, with calcium as the most likely candidate [7-91. According to Yoshikami and Hagins [ 10,l I], calcium is pumped into the discs and released into the cytoplasm upon photoexcitation of rhodopsin. The reports about calcium accumulation and release respectively in vertebrate photoreceptors are conflicting. Hendricks et al. [12], Liebman[ 131 and Szuts and Cone [ 141 used atomic absorption measurements to detect calcium release. Szuts and Cone [ 141 reported stoichiometries ranging from lo1000 calcium released/rhodopsin bleached. Liebman [ 131 and Hendricks et al. [ 121 reported a somewhat lower release. Sorbi and Cavaggioni [ 151, Smith et al. [ 161, Mason et al. [17], Weller et al. [18], Hemminki [19] and Bownds et al. [20] used radioactive calcium to detect calcium release. Mason et al. [ 171 and Smith et al. [ 161 reported a one-shot-carrier relation of 1: 1. Hemminki [ 191 found a stoichiometry of 1 calcium released/6 rhodopsin bleached and Weller et al. [ 181