Depletion of calcium stores by thapsigargin induces membrane depolarization by cation entry in human neutrophils

The ability of various cations to change the electrical potential of the plasma membrane was examined in human neutrophils by the use of the fluorescent cationic dye 3,3'-dipropylthiadicarbocyanine. When the cells were suspended in 140 mM KCl, the fluorescence was high, indicating depolarized neutrophils. Suspension in 145 mM N-methyl-D-glucamine chloride (NMG), replacing sodium and potassium chloride, resulted in hyperpolarized neutrophils. After depletion of the intracellular calcium stores of the NMG-suspended cells with thapsigargin and EDTA or EGTA, the addition of cations depolarized the neutrophils, suggesting the existence of pathways for cation entry. Besides Na+ and K+, several divalent cations were effective in the sequence: Ca2+ > Mn2+ > Ba2+ > Cd2+ > Mg2+ > Co2+ > Zn2+ > Ni2+. Pretreatment of the neutrophils with 0.5 or 1 mM CaCl2, resulting in loading of calcium stores, reduced the ability of some of the cations to depolarize the NMG-suspended cells. From the depolarizing effects of the cations it is concluded that the entries of Ca2+, Mg2+ , Mn2+, Ba2+, probably Co2+, to some extent Na+ and K+, but hardly Cd2+, Zn2+, or Ni2+, are regulated by the filling state of the intracellular calcium stores in human neutrophils. The store-regulated entry pathway may contribute to the control of the membrane potential and become active when the neutrophils are stimulated.