U-73122, an aminosteroid phospholipase C inhibitor, may also block Ca2+ influx through phospholipase C-independent mechanism in neutrophil activation

1-6-17a-3-Methoxyestra-1,3,5(10)-trien17-yl]amino]hexyl]-1H-pyrrole-2,5-dione (U-73122) has been proven to be a useful tool in investigation of phospholipase C (PLC)-coupled signal transduction during cell activation. In the present studies, the inhibition by U-73122 of cytosolic free Ca²⁺ concentration (Ca 2+]i) of neutrophils was investigated. U-73122 suppressed the Ca²⁺]i elevation of neutrophils suspended in Ca²⁺-containing medium challenged by N-formyl-Met-Leu-Phe (fMLP), cyclopiazonic acid (CPA) and ionomycin. The concentrations of U-73122 required for inhibition of CPA- and ionomycin-induced changes with IC₅₀ values 4.06 ± 0.27 µM and 4.04 ± 0.44 µM, respectively, is almost 10-times that required for inhibition of the fMLP-induced response (IC₅₀ value 0.62 ± 0.04 µM) U-73122 also reduced the intracellular Ca²⁺ mobilization of neutrophils suspended in Ca ²⁺-free medium stimulated by fMLP and CPA, but not by ionomycin, with IC₅₀ values 0.52 ± 0.02 µM and 6.82 ± 0.74 µM, respectively. 1-6-17f3-Methoxyestra-1,3,5(10)-trien-l7-yl]amino]hexyl]2,5-pyrrolidinedione (U-73343), a close analog of U-73122 that does not inhibit PLC activity, suppressed the Ca²⁺]_i elevation of neutrophils challenged by fMLP in Ca²⁺-containing medium, but not in Ca²⁺-free medium, with IC₅₀ value 22.30 ± 1.61 µM. In Mn²⁺-quench studies, U-73122 suppressed the Mn²⁺ influx in CPA-activated neutrophils (IC₅₀ value was 7.16 ± 0.28 µM) as well as in resting neutrophils (IC₅₀ value was 6.72 ± 0.30 M). U-73343 also suppressed the Mn²⁺ influx in resting neutrophils in a concentration-dependent manner. These results suggest that the inhibitory effect of U-73122 on Ca²⁺]i of activated neutrophils is attributed partly to the suppression of Ca²⁺ release from the intracellular Ca²⁺ stores through PLC inhibition, and partly to the blockade, especially at higher concentrations, of Ca²⁺ entry from the extracellular space through PLC-independent processes.