Calcium‐pH Crosstalks in the human mast cell line HMC‐1: Intracellular alkalinization activates calcium extrusion through the plasma membrane Ca 2+ ‐ATPase

The human mast cell line (HMC‐1) has been used to study the relationship between intracellular pH and cytosolic calcium (Ca 2+ ) in mast cells. Thapsigargin (TG) caused store‐operated Ca 2+ entry, that is enhanced by the PKC activator PMA. NH 4 Cl‐induced alkalinization showed an inhibitory effect on TG‐sensitive stores depletion (not on TG‐insensitive stores), and also on final cytosolic Ca 2+ levels reached in response to both TG and the ionophore ionomycin. Loperamide, a positive modulator of store‐operated channels, induced a slight Ca 2+ entry by itself, and also increased TG‐induced Ca 2+ entry. This enhancement was not enough to reverse the inhibitory effect of NH 4 Cl‐induced alkalinization. When comparing the effect of NH 4 Cl‐induced alkalinization on Ca 2+ levels, with those observed using Ca 2+ channel blockers (namely Ni 2+ and SKF‐96365), cytosolic profiles for this ion are different, either in modified saline solution or in HCO   3 − ‐free medium. Thus, it seems unlikely that the inhibitory effect of NH 4 Cl‐induced alkalinization on Ca 2+ is taking place by blockage of Ca 2+ entry. Furthermore, inhibition of the plasma membrane Ca 2+ ‐ATPase (an important mechanism for Ca 2+ efflux) with sodium orthovanadate (SO) matches with the inhibition of the negative effect on Ca 2+ levels elicited by NH 4 Cl. Data indicate that NH 4 Cl‐induced alkalinization might be activating Ca 2+ efflux from the cell, by stimulation of the plasma membrane Ca 2+ ‐ATPase, and also confirm our previous finding that Ca 2+ is a secondary signal to activate HMC‐1 cells. J. Cell. Biochem. 99: 1397–1408, 2006. © 2006 Wiley‐Liss, Inc.