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 dinoflagellate. Induction of inositol phosphates formation by indoleamine has previously been demonstrated in Crypthecodinium cohnii. In addition, depletion of extracellular Ca²⁺ blocks the indoleamine-induced encystment. In the present study, 12 indoleamines (including melatonin and related compounds) were examined for their abilities to induce Ca²⁺ influx, inositol phosphates formation, and encystment in C. cohnii. The results showed that melatonin, 5-methoxytryptamine, and the peptide toxin mastoparan stimulated ⁴⁵Ca²⁺ influxes in dose- and time-dependent manners. The EC₅₀ values of 5-methoxytrypramine and mastoparan to stimulate ⁴⁵Ca²⁺ uptake were 2 mM and 35 μM, respectively. The 5-methoxytryptamine- and mastoparan-induced ⁴⁵Ca²⁺ influx were partially attenuated by the calcium channel blockers, verapamil and ruthenium red. A series of indoleamines were examined for their structure-activity relationship on the induction of encystment and formation of inositol phosphates. Melatonin-induced inositol phosphates formation was completely blocked by U73122, indicating the possible involvement of phospholipase C. Taken together, we conclude that indoleamines may induce encystment of the dinoflagellate C. cohnii via parallel activation of phospholipase C and Ca²⁺ influx signaling pathways. However, activation of phospholipase C and Ca²⁺ influx are not always necessary or sufficient for inducing encystment. Also, these data provided the first direct evidence of a Ca²⁺ influx regulating mechanism in dinoflagellate C. cohnii.