A light-dependent increase in free Ca2+ concentration in the salamander rod outer segment

1. The Ca2+ indicator dye fluo-5F was excited by an argon ion laser to meas ure changes in free Ca2+ concentration ([Ca2+](i)) in the outer segments of isolated salamander rods rapidly exposed to a 0 Ca2+, 0 Na+ solution desig ned to minimise surface membrane Ca2+ fluxes. Over 30-60 s of laser illumin ation, the fluorescence first increased rapidly and then declined at a rate that was much slower than in Ringer solution and consistent with previous physiological evidence that 0 Ca2+, 0 Na+ solution greatly retards light-in duced changes in [Ca2+](i). 2. The initial increase in fluorescence was investigated with a sequence of 100 ms laser flashes presented at 5 s intervals. The fluorescence evoked b y the second laser flash was on average 30 % larger than the first, and sub sequent responses exhibited a slow decline like that measured with continuo us laser exposures. The initial increase in fluorescence did not depend upo n the timing of exposure to 0 Ca2+, 0 Na+ solution but appeared to be evoke d by exposure to the laser light. 3. Both the increase and subsequent decline in fluorescence measured with b rief laser flashes could be reduced by incorporation of the Ca2+ chelator B APTA. This and other results indicate that the fluorescence increase was un likely to have been caused by a change in the affinity of fluo-5F for Ca2or an increase in the quantity of incorporated dye available to bind Ca2+ b ut reflects an actual release of intracellular C2+ within the outer segment . 4. The pool of Ca2+ available to be released could be decreased if, before the first laser flash, the rod was exposed to light bright enough to bleach a substantial fraction of the photopigment. The releasable pool could also be depleted by exposure to saturating light of much lower intensity if del ivered in Ringer solution but not if delivered in 0 Ca2+, 0 Na+ solution. W e conclude that Ca2+ can be released within the outer segment both by the b leaching of rhodopsin and by the reduction in [Ca2+](i) which normally acco mpanies illumination in Ringer solution. 5. The activation of rhodopsin appears somehow to induce the release of Ca2 + from a binding site or store within the outer segment. Substantial releas e, however, required stimulating light of an intensity sufficient to bleach a considerable fraction of the visual pigment. It therefore seems unlikely that such release contributes to the normal Ca2+-mediated modulation of tr ansduction during light adaptation. The mechanism and physiological functio n of light-induced Ca2+ release are unknown.