Light-dependent calcium release from photoreceptors measured by laser micro-mass analysis

Abstract

Yoshikami and Hagins¹ first suggested that calcium is sequestered within membranous disks in the outer segments of vertebrate rods and that the bleaching of visual pigment molecules by light causes the release of Ca from the disks. Once released, the Ca was postulated to bind to Na⁺ channels or carrier molecules in the plasma membrane to produce the electrical response. This theory, termed the ‘calcium hypothesis’, is supported by much evidence^2–5 but remains controversial, largely because of the difficulty in measuring calcium in rods and of demonstrating light-induced release^6–14. Here we describe direct measurements of total rod Ca using a new microprobe method, called laser micro-mass analysis, or LAMMA. Using this technique, we show that rods contain large amounts of Ca concentrated in their outer segments. Physiological levels of illumination produce a graded efflux of rod Ca content, amounting to about 10⁴ ions per rhodopsin molecule bleached in dim light. As light does not change the rate of Ca influx, the total Ca content of the rod decreases. In bright light, as much as half the total Ca leaves the rod during only l min of illumination.