Effects of maitotoxin on calcium entry and phosphoinositidebreakdown in the rabbit ciliated tracheal epithelium

Summary Maitotoxin induces a concentration-dependent 45 Ca 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 μM. However, verapamil at 200 μM completely prevents 45 Ca uptake. Measurements of indo-1 fluorescence show that MTX induces a very sustained (≥ 2 h) [Ca] i rise, which is completely inhibited by 200 μM of verapamil. Genistein (110 μM) (an inhibitor of tyrosine kinases) also strongly inhibits it. The inhibitory effect of 50 μM 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 (5 10 min) and is not inhibited by chelation of intracellular Ca i with BAPTA, nor by verapamil (200 μM) or U73122 (10 μM) (an inhibitor of activation of PLCPI 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.