REGULATION OF CALCIUM INFLUX AND PHOSPHOLIPASE-C ACTIVITY BY INDOLEAMINES IN DINOFLAGELLATE CRYPTHECODINIUM-COHNII

Exogenous indoleamines such as melatonin and 5-methoxytryptamine have been shown to induce cyst formation (encystment) in many species of di noflagellate. Induction of inositol phosphates formation by indoleamin e has previously been demonstrated in Crypthecodinium cohnii. In addit ion, depletion of extracellular Ca2+ blocks the indoleamine-induced en cystment. In the present study, 12 indoleamines (including melatonin a nd related compounds) were examined for their abilities to induce Ca2 influx, inositol phosphates formation, and encystment in C. cohnii. T he results showed that melatonin, 5-methoxytryptamine, and the peptide toxin mastoparan stimulated Ca-45(2+) influxes in dose-and time-depen dent manners. The EC50 values of 5-methoxytrypramine and mastoparan to stimulate Ca-45(2+) uptake were 2 mM and 35 mu M, respectively. The 5 -methoxytryptamine- and mastoparan-induced Ca-45(2+) influx were parti ally attenuated by the calcium channel blockers, verapamil and rutheni um red. A series of indoleamines were examined for their structure-act ivity relationship on the induction of encystment and formation of ino sitol phosphates. Melatonin-induced inositol phosphates formation was completely blocked by U73122, indicating the possible involvement of p hospholipase C. Taken together, we conclude that indoleamines may indu ce encystment of the dinoflagellate C. cohnii via parallel activation of phospholipase C and Ca2+ influx signaling pathways. However, activa tion of phospholipase C and Ca2+ influx are not always necessary or su fficient for inducing encystment. Also, these data provided the first direct evidence of a Ca2+ influx regulating mechanism in dinoflagellat e C. cohnii.