Strontium and barium induce exocytosis in electropermeabilized neutrophils

Abstract

Calcium, strontium and barium induced an exocytotic response in electropermeabilized rabbit neutrophils while magnesium was without any effect. The extent of enzyme release was found to depend upon the concentration of these cations. For all cations, an optimum concentration was found with the same maximum enzyme release. At concentrations higher than optimum a decrease in lysozyme release was observed. Efficiency to induce enzyme release was in the order: Ca 2+>Sr 2+>Ba 2+. Enzyme release was significantly enhanced by guanosine-5′-[γ-thio]triphosphate (GTPγS) resulting in a shift to the left of the dose/response curve. The enhancement by GTPγS was strongest with Ca 2+, was less with Sr 2+, and was very little with Ba 2+. The time course of lysozyme release was the same for Ca 2+, Sr 2+, and Ba 2+ in the presence and absence of GTPγS when suboptimal cation concentrations were used. A decrease in responsiveness to the effectors after electropermeabilization was observed with Ca 2+, Sr 2+ and Ba 2+ in the presence and absence of GTPγS. The lysozyme release induced by the different cations was not inhibited by the protein kinase C inhibitor staurosporine and was slightly affected by pertussis toxin. Ca 2+ and Sr 2+, but not Ba 2+, potentiated formyl-methionyl-leucyl-phenylalanine (fMet-Leu-Phe) induced enzyme release in intact neutrophils. The divalent cation ionophore A23187 induced enzyme release in the presence of Ca 2+ and Sr 2+ but not in the presence of Ba 2+. The results obtained with electropermeabilized neutrophils indicate that Sr 2+ and Ba 2+ can act as substitutes for Ca 2+ in activating exocytosis, and that permeabilized neutrophils provide the best tool to investigate the effects of alkaline earth ions in exocytosis.