Attenuation of maitotoxin-induced cytotoxicity in rat aortic smooth muscle cells by inhibitors of Na+/Ca2+ exchange, and calpain activation

The highly potent marine toxin maitotoxin (MTX) evoked an increase in cytosolic Ca2+ levels in fura-2 loaded rat aortic smooth muscle cells, which was dependent on extracellular Ca2+. This increase was almost fully inhibited by KB-R7943, a potent selective inhibitor of the reverse mode of the Na+/Ca2+ exchanger (NCX). Cell viability was assessed using ethidium bromide uptake and the alamarBlue cytotoxicity assay. In both assays MTX-induced toxicity was attenuated by KB-R7943, as well as by MDL 28170, a membrane permeable calpain inhibitor. Maitotoxin-evoked contractions of rat aortic strip preparations in vitro, which persist following washout of the toxin, were relaxed by subsequent addition of KB-R7943 or MDL 28170, either in the presence of, or following washout of MTX. These results suggest that MTX targets the Na+/Ca2+ exchanger and causes it to operate in reverse mode (Na+ efflux/Ca2+ influx), thus leading to calpain activation, NCX cleavage, secondary Ca2+ overload and cell death.