CHOLERA-TOXIN AND ITS B-SUBUNIT DO NOT CHANGE CYTOSOLIC FREE CALCIUM-CONCENTRATION

Using the fluorescent Ca2+ probe Quin-2 it has been reported that cholera toxin (CT) and its B subunit (B-CT) increase cytosolic free Ca2+ concentration ([Ca2+]i) in entherocytes [1], thymocytes [2] and fibroblasts [3]. In this work we show, however, that the fluorescence increases of Quin-2-loaded cells (rat thymocytes, mouse splenocytes, P-388 macrophages and 3T3 fibroblasts) observed upon addition of CT or B-CT are not caused by an increase in [Ca2+]i. The observed effect appears to be accounted for by EDTA·2Na admixtures (present as conservation agent in all CT and B-CT preparations) which 'unquenches' the fluorescence of Quin-2 acid leaked out from the cells into the extracellular medium and produces influorescent complexes with contaminating heavy metal ions. Thus the mitogenic effect of B-CT [2, 3] is not obviously connected with the cytosolic free Ca2+ increase but is probably due to ganglioside-mediated protein phosphorylation [4]. © 1990.