Calcium and the mechanism of light adaptation in vertebrate photoreceptors.

Vertebrate photoreceptors transduce the absorption of light into a hyperpolarizing change in membrane potential. The mechanism of transduction is becoming fairly well understood and has been shown to occur via a G protein-coupled decrease in cyclic GMP. Attention is now turning to the way the enzymatic machinery in the outer segment of the photoreceptor cell is modulated during light adaptation. Recent studies show that light adaptation cannot occur if changes in the concentration of cytoplasmic free calcium in the outer segment are prevented, suggesting that calcium functions as a second messenger in sensitivity regulation.