title: Calcium ion dependency and the role of inositol phosphates in melatonin-induced encystment of dinoflagellates.
abstract: The unicellular eukaryotic dinoflagellates shed their flagella and form a new pellicle cyst wall in response to environmental stress. This encystment process can also be induced by indoleamines such as melatonin and 5-methoxytryptamine. To decipher the complex signaling events which lead to encystment, we have investigated the functional roles of Ca2+ and inositol phosphates in indoleamine-induced encystment of the dinoflagellates Alexandrium catenella and Crypthecodinium cohnii. Pretreatment with EGTA, but not with EDTA, effectively blocked the indoleamine-induced encystment of A. catenella in a dose-dependent manner. Conversely, agents that facilitate the influx of Ca2+ (Bay K 8644, A23187 and ionomycin) dose-dependently induced encystment of A. catenella. Endoplasmic 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 inositol phosphates in C. cohnii. The rank order of potency for several indoleamines to stimulate inositol phosphates formation was 2-iodomelatonin > 5-methoxytryptamine > or = melatonin >> N-acetylserotonin > 5-hydroxytryptamine. This rank order was the same as for the indoleamine-induced encystment of C. cohnii as previously reported. Our results indicate that indoleamine-induced activation of phospholipase C and elevation of intracellular [Ca2+] may be proximal steps in the signal transduction pathway leading to encystment in dinoflagellates. Moreover, this is the first demonstration of the possible involvement of Ca2+ and inositol phosphates as second messengers in dinoflagellates.
authors.affiliation: Department of Biology, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon.
authors.forename: S T
authors.initials: ST
authors.lastname: Tsim
authors.forename: J T
authors.initials: JT
authors.lastname: Wong
authors.forename: Y H
authors.initials: YH
authors.lastname: Wong
mesh_terms: D008735:5-Methoxytryptamine / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000502:physiology*; D003560:Cysts; D004141:Dinoflagellida / Q000502:physiology*; D004305:Dose-Response Relationship, Drug; D007295:Inositol Phosphates / Q000502:physiology*; D008550:Melatonin / Q000494:pharmacology; D015398:Signal Transduction / Q000502:physiology*; D013997:Time Factors; D010738:Type C Phospholipases / Q000502:physiology
chemical_list: D007295:Inositol Phosphates; D008735:5-Methoxytryptamine; D010738:Type C Phospholipases; D008550:Melatonin; D002118:Calcium