Effects of the semisynthetic bis-indole derivative KAR-2 on store-operated calcium entry in human neutrophils

We studied the effect of KAR-2 on cytosolic Ca2+ level in human neutrophils by using a fluorescent dye (Fura-2) trapped in the cells. KAR-2 is a semisynthetic bis-indole derivative that shares vinblastine anti-microtubular properties, but does not share the vinblastine antagonistic effect on calmodulin. Therefore KAR-2 offers a convenient mean of studying the effect of microtubule destabilization, without concomitant calmodulin alterations. We found that KAR-2 induces Ca2+ release from intracellular stores, whereby the stores are depleted. In addition KAR-2 reduces store-operated entry of extracellular Ca2+ induced by agonists such as thapsigargin or ATP. On the other hand, in Ca2+ refilled cells, KAR-2 promotes limited entry of extracellular Ca2+ in the absence of agonist, but still interferes prominently with Ca2+ entry triggered by ATP and with Ca2+ uptake by intracellular stores. We suggest that Ca2+ traffic through the plasma membrane is operated by two diverse pathways: the prominent pathway is interfered with by microtubule destabilization, while an alternate and minor pathway is actually favored (or uncovered) following microtubule destabilization.