Cytosolic Ca2+ homeostasis in Ehrlich and Yoshida carcinomas. A new, membrane-permeant chelator of heavy metals reveals that these ascites tumor cell lines have normal cytosolic free Ca2+.

The intracellularly trappable fluorescent Ca2+ indicator quin-2 was used to measure free cytosolic Ca2+, Ca2+i, in the two highly dedifferentiated tumor cell lines, Ehrlich and Yoshida ascites carcinomas. It was found that these carcinoma cells can trap quin-2 similarly to normal cells, but Ca2+i was apparently significantly lower than in any normal cell tested previously with this method. By using a new lipid-soluble heavy metal chelator TPEN (N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine), which crosses artificial and natural membranes, it was found that endogenous heavy metals are responsible for partially quenching quin-2 fluorescence trapped inside the cells. Although the quenching of intracellular quin-2 fluorescence is quantitatively more relevant in these ascites carcinomas, TPEN was effective also in normal cells like lymphocytes and granulocytes. Both in the normal and especially in the malignant cell lines Ca2+i can be grossly underestimated at low intracellular quin-2 concentrations. Endogenous heavy metal quenching is thus a potential source of artifact when Ca2+i is measured with quin-2. When corrected for quin-2 fluorescence quenching by intracellular heavy metals, Ca2+i and basic regulatory mechanisms of Ca2+i homeostasis in Ehrlich and Yoshida carcinomas are similar to those of nontransformed cells.