Maitotoxin induces a concentration-dependent 45Ca uptake in primary cultures of rabbit tracheal epithelial cells. This response is insensitive to the calcium channel antagonists nifedipine, diltiazem and verapamil up to 20 microM. However, verapamil at 200 microM completely prevents 45Ca uptake. Measurements of indo-1 fluorescence show that MTX induces a very sustained (> or = 2 h) [Ca]i rise, which is completely inhibited by 200 microM of verapamil. Genistein (110 microM) (an inhibitor of tyrosine kinases) also strongly inhibits it. The inhibitory effect of 50 microM miconazole (an inhibitor of cytochrome P450) is only partial. Okadaic acid (inhibitor of protein-phosphatases) primarily delays the response to the toxin without decreasing its magnitude. MTX induces the formation of (1,4,5) inositol trisphosphate (IP3). The MTX response curve is biphasic. Stimulation is transient (< or = 10 min) and is not inhibited by chelation of intracellular Cai with BAPTA, nor by verapamil (200 microM) or U73122 (10 microM) (an inhibitor of activation of PLC beta 1 through a trimeric G protein). Results suggest that MTX independently activates a calcium transport process (which might imply phosphorylation on tyrosine residues) and a PLC not linked to a trimeric G protein.
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