Lipoxygenase inhibitors suppress intracellular calcium rise induced by ionomycin in rat thymocytes

The lipoxygenase (LO) inhibitors nordihydroguaiaretic acid (NDGA) and 15S-hydroxy-5,8,11,13-(Z,Z,Z,E)-eicosatetraenoic acid (15-HETE) have been found to suppress the rise in free cytoplasmic Ca2+ concentration ( Ca2+]i) induced by the Ca2+ ionophores ionomycin and A23187 in rat thymocytes. Bromophenacyl bromide (BPB), a phospholipase A2 (PLA2) inhibitor, produced a much weaker inhibitory effect, and indomethacin, a cyclo-oxygenase inhibitor, practically did not influence the Ca2+]i response to ionomycin. These findings implicate the involvement of LO product(s) in the Ca2+]i rise triggered by the Ca2+ ionophores. The contribution of the NDGA-sensitive component to the ionomycin-induced Ca2+]i rise was significant in the ionomycin concentration range of 0.1 nM to 0.1 microM whereas at higher doses of the ionophore it gradually diminished. By contrast, the Ca2+]i rise induced by exogenous arachidonic acid (AA) or melittin, a PLA2 activator, was not suppressed but potentiated by NDGA. Ionomycin and exogenous AA also elicited opposite changes in thymocyte cytoplasmic pH (pHi): the former elevated the pHi while the latter induced a pronounced acidification of the cytoplasm. This difference in the pHi responses may account for the different sensitivity of ionomycin- and AA-elicited Ca2+]i signal to LO inhibitors.