PROSTAGLANDINS STIMULATE SEROTONIN ACETYLATION IN CHICK PINEAL CELLS - INVOLVEMENT OF CYCLIC-AMP DEPENDENT AND CALCIUM CALMODULIN-DEPENDENT MECHANISMS

The effects of prostaglandins (PGs) on the activity of the rate-limiting enzyme of melatonin biosynthesis, arylalkylamine-N-acetyltransferase (NAT) were investigated on primary cultures of dispersed chick pineal cells. In indomethacin-treated cells, PGs caused a four-fold increase in NAT activity. This response was associated with an eight-fold increase in cyclic AMP (cAMP) levels. The potency order of PGs was the same for NAT and for cAMP responses (PGE1 > PGE2 > PGF2alpha much greater than cloprostenol). However, each PG tested was 30- to 200-fold more potent to increase NAT activity than to stimulate cAMP accumulation. As a result, half-maximal stimulation of NAT by PGs was not associated with an increase in cAMP levels. Half-maximal stimulation of NAT by PGE1 was highly sensitive to inhibition by a calcium/calmodulin antagonist (W-7). In contrast, maximal stimulation of NAT by PGE1 as well as stimulations evoked by either forskolin or 8-bromo-cAMP were poorly sensitive to inhibition by W-7. These results indicate that an increase in cAMP levels may be responsible for the maximal stimulation of NAT evoked by PGs, whereas half-maximal stimulation of NAT by PGs would rely principally on a calcium/calmodulin-dependent mechanism.