Phospholipase A2 Is Involved in Thapsigargin-Induced Sodium Influx in Human Lymphocytes

Previously, we reported that emptying of intracellular Ca2+ pools with endoplasmatic Ca2+-ATP-ase inhibitor thapsigargin leads to the Na+ influx in human lymphocytes (M. Tepel et al., 1994, J. Biol. Chem. 269, 26239–26242). In the present study we examined the mechanism underlying the thapsigargin-induced Na+ entry. We found that the thapsigargin-induced increase in Na+ concentration was effectively inhibited by three structurally unrelated phospholipase A2 (PLA2) inhibitors, p-bromophenacyl bromide, 3-(4-octadecyl)-benzoylacrylic acid (OBAA), and bromoenol lactone (BEL). The thapsigargin-induced Na+ influx could be mimicked by PLA2 exogenously added to the lymphocyte suspension. In addition, thapsigargin stimulated formation of arachidonic acid (AA), the physiological PLA2 product. AA induced Na+ entry in a time- and concentration-dependent fashion. Both, thapsigargin-induced Na+ influx and AA liberation were completely inhibited in the presence of tyrosine kinase inhibitor genistein but not in the absence of extracellular Ca2+. Collectively, these data show that thapsigargin-induced Na+ entry is associated with tyrosine kinase-dependent stimulation of PLA2.