2 The role of Ca+ as an internal messenger in visual transduction of
vertebrate and invertebrate organisms has been explored intensely in the
recent past. Since the 2 early 1970s, calcium ions and cyclic GMP (whose
levels are controlled by Ca+ in vertebrates) have been recognized as
important second messengers. Particularly in 2 the last decade, however,
the role of Ca+ in visual transduction has been re-evalu- ated and a
proliferation of research has documented a multiplicity of roles. 2 It
is now evident that Ca+ modulates phototransduction by acting at several
2 sites through a host of small Ca+ -binding proteins. For example, in
phototransduction 2 of vertebrates, Ca+-free forms of guanylate cyclase
activating proteins (GCAPs) activate guanylate cyclase, modulating
levels of cOMP, a key event in the return of photoreceptors to
pre-bleach conditions. Defects in genes encoding guanylate cy- clase or
guanylate cyclase activating proteins lead to severe diseases of the
retina (e. g., Leber congenital amaurosis, rod/cone dystrophy, or cone
dystrophy), thus em- phasizing the important role of these proteins in
phototransduction. Similarly, mu- 2 tant genes encoding cation or Ca+
channels (cyclic nucleotide-gated cation chan- 2 nels located in the
cell membrane and L-type voltage-gated Ca+ channels located at the
synapse of photo receptors) lead to retinitis pigmentosa or congenital
stationary night blindness. In phototransduction of invertebrate
organisms (e. g., Drosophila 2 and Limulus), the role of Ca+ is
similarly central, but distinct, from that of vertebrates.
