A dual effect of arachidonic acid on Ca2+ transport systems in lymphocytes

A sustained increase of cytoplasmic Ca2+ concentration ([Ca2+]i) in lymphocytes was induced by a potent inhibitor of the intracellular Ca(2+)-pump 2,5-di-(tert-butyl)-1,4-hydroquinone (BHQ) that effectively depletes Ca2+ stores and produces Ca2+ influx. The addition of low concentrations of arachidonic acid (AA) (less than 10 microM) in the presence of a BHQ-induced Ca2+ response produced a [Ca2+]i decrease. At the same time at higher concentrations, AA induced an additional increase of [Ca2+]i. The unidirectional uptake of Mn2+ into fura-2 loaded thymocytes was used to examine the effects of AA and BHQ on Ca2+ permeability of the plasma membrane. Judging by the rate of Mn(2+)-induced fura-2 fluorescence quenching, AA causes a concentration-dependent increase of the divalent cations' permeability both in the control solution and in the presence of BHQ. These results allowed us to suggest that the decrease of BHQ-induced [Ca2+]i stationary levels caused by low AA concentrations is due to the enhancement of Ca2+ extrusion from the cell, whereas the additional [Ca2+]i increase at high AA concentrations results from an elevation of cell membrane Ca2+ permeability.