IS THERE A NA+ CA2+ EXCHANGER IN MACROPHAGES AND IN LYMPHOCYTES/

In two blood cell types, peritoneal murine macrophages and Jurkat cell s (a human T cell line), we have examined whether a Na+/Ca2+ exchange was present and what could be its functional importance. In non-stimul ated macrophages, the intracellular Ca2+ concentration, [Ca2+]i, was u nchanged when Li+ was substituted for external Na+. However, after sti mulation by platelet-activating factor (PAF), the Ca2+ response was la rger when the extracellular solution contained Li+ rather than Na+ ion s. In stimulated macrophages, the rate of Ca2+ extrusion was smaller i n a Li+-than in a Na+-containing medium. The net electrochemical gradi ent for ionic movements through the Na+/Ca2+ exchanger, during the cou rse of the response of macrophages to PAF, was determined by combining the measurements of membrane potential (in patch-clamp), of [Ca2+]i ( with fura-2), and of the intracellular Na+ concentration (with sodium- binding benzofuran isophthalate). These results show that macrophages possess a Na+/Ca2+ exchange that only functions as a Ca2+ extruder, an d this only when [Ca2+]i has been increased, for instance following PA F stimulation. In T lymphocytes, before or after stimulation by an ant i-CD3 antibody, no Na+/Ca2+ activity could be detected by measuring ei ther [Ca2+]i, or the rate of Ca2+ extrusion. Even if a Na+/Ca2+ exchan ger was present in these cells, its equilibrium potential would be suc h that it would not allow Ca2+ influx but only Ca2+ extrusion.