Maitotoxin-induced calcium entry in human lymphocytes

We have studied the effect of the ciguatera-related toxin maitotoxin (MTX) on the cytosolic free calcium concentration ([Ca2+]i) of human peripheral blood lymphocytes loaded with the fluorescent probe Fura2 and the regulation of MTX action by different drugs known to interfere in cellular Ca2+ signalling mechanisms and by the marine phycotoxin yessotoxin (YTX). MTX produced a concentration-dependent elevation of [Ca2+]i in a Ca2+-containing medium. This effect was stimulated by pretreatment with YTX 1 μM and NiCl2 15 μM. The voltage-independent Ca2+ channel antagonist 1-[β-[3-(4-methoxyphenyl)propoxyl]-4-methoxyphenyl]-1H-imidazole hydrochloride (SKF96365) blocked the MTX-induced [Ca2+]i elevation, while the L-type channel blocker nifedipine had no effect. Pretreatment with NiCl2 or nifedipine did not modify YTX-induced potentiation of MTX effect, and SKF96365-induced inhibition was reduced in the presence of YTX, which suggest different pathways to act on [Ca2+]i. Preincubation with N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide.2HCl (H-89) or genistein (10 μM) also had no effect on the MTX-induced [Ca2+]i increment. In contrast, the PKC inhibitor bisindolilmaleimide I (GF109203X 1 μM) potentiated the MTX effect, whereas phosphatidylinositol (PI) 3-kinase inhibition with wortmannin (10 nM) reduced the MTX-elicited Ca2+ entry. In summary, MTX produced Ca2+ influx into human lymphocytes through a SKF96365-sensitive, nifedipine-insensitive pathway. The MTX-induced [Ca2+]i elevation was stimulated by the marine toxin YTX through a mechanism insensitive to SKF96365, nifedipine or NiCl2. It was also stimulated by the divalent cation Ni2+ and PKC inhibition and was partially inhibited by PI 3-kinase inhibition.