Toxin of the marine alga Prymnesium patelliferum enhances voltage dependent Ca2+-currents,elevates the cytosolic Ca2+-concentration and facilitates hormone release in clonal rat pituitary cells

The marine flagellate Prymnesium patelliferum produces toxins lethal to fish. The toxin extracted from the alga has haemolytic, cytotoxic and neurotoxic effects, but the action mechanisms of the toxin are not known in detail. We have examined the toxin effects on the voltage sensitive Ca²⁺-currents, the cytosolic Ca²⁺-level ([Ca²⁺]₁) and the prolactin release in clonal rat anterior pituitary GH₄C₁ cells, which possess T- and L-type Ca²⁺-channels. The trans-membrane Ca²⁺-current was recorded using whole-cell voltage clamp. After 5–15 min exposure to the algal toxin at a final concentration of 50000–100000 cells mL^-1, the Ca²⁺-currents through both the T- and L-channels showed a 2–3-fold enhancement. The voltage sensitivity of the Ca²⁺-currents was not affected by the algal toxin, and the toxin-induced currents were inhibited by 100 μM of the Ca²⁺-channel blocker D-600. In toxin-exposed cells microfluorometric measurements based on fura-2 revealed an increase of [Ca²⁺]₁ from 100–150 to 300–500 nM. This elevation was delayed and partially inhibited by 100 μM D-600. The algal toxin induced prolactin release in a dose-dependent manner, and this effect was inhibited by the Ca²⁺-channel blocker verapamil. We therefore conclude that the toxin of P. patelliferum affects the Ca²⁺ homeostasis of the pituitary cells by increasing the leak through voltage sensitive Ca²⁺-channels, resulting in increased [Ca²⁺]₁ and secretion of prolactin.