Thapsigargin activates univalent- and bivalent-cation entry in human neutrophils by a SK&F 96365- and Gd³⁺-sensitive pathway and is a partial secretagogue: Involvement of pertussis-toxin-sensitive G-proteins and protein phosphatases 1/2A and 2B in the signal-transduction pathway

The Ca²⁺-ATPase inhibitor thapsigargin (TG) activates bivalent-cation entry in human neutrophils via depletion of intracellular Ca²⁺ stores, but little is known about the underlying mechanism and the functional role of TG-induced cation entry. We studied the effects of TG on univalent- and bivalent-cation entry, lysozyme release and superoxide-anion (O₂^-) formation in human neutrophils. TG, like the chemotactic peptide, N-formyl-L-methionyl-L-leucyl-L-phenylalanine (fMLP), stimulated entry of Ca²⁺, Mn²⁺, Ba²⁺, Sr²⁺ and Na⁺ in a 1-{β-[3-(4-methoxyphenyl)propoxy]-4-methoxyphenethyl}-1H-imidazole hydrochloride (SK&F 96365)- and Gd³⁺-sensitive manner. The inhibitors of protein phosphatases 1/2A, calyculin A and okadaic acid, diminished TG-induced cation influxes, whereas the inhibitors of protein phosphatase 2B, cyclosporin A and FK-506, were potentiators. Pertussis toxin (PTX) partially inhibited the effects of TG on Ca²⁺ and Mn²⁺ entry. TG and fMLP activated inward currents with a linear current-voltage relationship and a reversal potential at about 0 mV. TG activated lysozyme release and potentiated fMLP-induced O₂^- formation. TG-induced lysozyme release was inhibited by SK&F 96365, PTX and the removal of extracellular Ca²⁺ or Na⁺. Our data show that TG activates a non-selective and SK&F 96365- and Gd³⁺-sensitive cation entry pathway and is a partial secretagogue. TG-stimulated cation entry involves PTX-sensitive G-proteins and protein phosphatases, with protein phosphatases 1/2A and 2B playing opposite roles.