CALCIUM-ION DEPENDENCY AND THE ROLE OF INOSITOL PHOSPHATES IN MELATONIN-INDUCED ENCYSTMENT OF DINOFLAGELLATES

The unicellular eukaryotic dinoflagellates shed their flagella and for m a new pellicle cyst wall in response to environmental stress, This e ncystment process can also be induced by indoleamines such as melatoni n and 5-methoxytryptamine. To decipher the complex signaling events wh ich lead to encystment, we have investigated the functional roles of C a2+ and inositol phosphates in indoleamine-induced encystment of the d inoflagellates Alexandrium catenella and Crypthecodinium cohnii, Pretr eatment with EGTA, but not with EDTA, effectively blocked the indoleam ine-induced encystment of A. catenella in a dose-dependent manner. Con versely, agents that facilitate the influx of Ca2+ (Bay K 8644, A23187 and ionomycin) dose-dependently induced encystment of A. catenella, E ndoplasmic Ca2+-ATPase inhibitors such as thapsigargin and the peptide toxin melittin also induced encystment of A. catenella, These results suggest that an elevation of intracellular [Ca2+] may be involved in the encystment response. In terms of the regulation of phospholipase C , melatonin dose- and time-dependently stimulated the formation of ino sitol phosphates in C. cohnii, The rank order of potency for several i ndoleamines to stimulate inositol phosphates formation was 2-iodomelat onin > 5-methoxytryptamine greater than or equal to melatonin much gre ater than N-acetylserotonin > 5-hydroxytryptamine. This rank order was the same as for the indoleamine-induced encystment of C. cohnii as pr eviously reported. Our results indicate that indoleamine-induced activ ation of phospholipase C and elevation of intracellular [Ca2+] may be proximal steps in the signal transduction pathway leading to encystmen t in dinoflagellates. Moreover, this is the first demonstration of the possible involvement of Ca2+ and inositol phosphates as second messen gers in dinaflagellates.