Role of extracellular calcium on the regulation of melatonin synthesis in the Syrian hamster pineal gland

The role of extracellular calcium on the induction of cyclic AMP production , N-acetyltransferase (NAT) activity and melatonin synthesis, induced by fo rskolin, was investigated in Syrian hamster pineal glands. Pineals were rem oved from hamsters killed either in the first half of the normal dark perio d or late in the dark period. Forskolin immediately increased NAT activity and melatonin levels only when the glands were collected late in the dark p eriod, while in the first half of the dark phase, hamster pineals responded to forskolin with an increase of NAT activity and melatonin production onl y after 6 hr of incubation. The absence of calcium prevented the induction of melatonin synthesis by forskolin only when glands were collected early i n the dark phase and incubated for 6 hr. In the second half of the normal d ark period removal of calcium markedly decreased NAT activity and melatonin levels in glands incubated with forskolin for either 4 or 6 hr. However. t he absence of extracellular calcium had no significant effect on the induct ion of cyclic AMP production by forskolin in pineals collected either early in the dark period or late in the dark phase. These data indicate that at least part of the action of extracellular calcium is indirect and that it a ffects steps in the induction of melatonin synthesis beyond the accumulatio n of cAMP.