The role of calcium in the regulation of melatonin biosynthesis in the retina.

Vertebrate retina rhythmically produces melatonin, a hormone involved in the regulation of several intraocular processes cued by environmental lighting conditions. Calcium ions play an important role in the induction process of serotonin N-acetyltransferase (NAT), a key regulatory enzyme in melatonin biosynthetic pathway. The physiological, i.e. nocturnal, increase of NAT activity in the retina depends on transmembrane transport of Ca2+ through the L-type of voltage-sensitive calcium channels. It is suggested that Ca2+ may regulate NAT activity indirectly, by affecting the intracellular cyclic AMP content, which is, in turn, critical in the regulation of melatonin biosynthesis. The mode and mechanisms of Ca2+ action on processes governing melatonin formation in the retina are discussed.