Effect of low extracellular Ca 2+ on growth spreading area, cytoplasmic Ca 2+ concentration, and intracellular pH in normal and transformed human fibroblasts

The transformation of certain cells reduces the requirement of extracellular Ca 2+ for growth. The SV‐40 transformed human lung fibroblasts, WI‐38 VA13, require less Ca 2+ than normal WI‐38 cells. Spreading area of normal cells decreases when cultured in 10 μM Ca 2+ medium. Intracellular calcium concentration ([Ca 2+ ] i ), of the normal and transformed cells cultured in 10μM and 2 mM Ca 2+ media was measured by the fluorescence microscope technique using fura‐2 as a probe. The [Ca 2+ ], is measured in the resting state and during mobilization by serum or bradykinin stimulation. The lowering of extracellular calcium concentration results in a decrease in the resting state [Ca 2+ ], i of both normal and transformed cells. Although the total decrease in [Ca 2+ ] i is the same for both cell, the rate of decrease is much faster in normal cells than in transformed cells. Low extracellular Ca 2+ reduces the number of cells responsive to the serum or bradykinin stimulation and decreases the peak [Ca 2+ ] i value in both cells. In addition, we investigated, using BCECF as a fluorecent probe, the intracellular pH (pH i ) of normal and transformed cells maintained at low and normal Ca 2+ . The low Ca 2+ condition makes pH i acidic in normal cells but not in transformed cells. The acidification of the normal cell is accompanied by a decrease in the spreading area of the cells. The decrease of the cell attacment, followed by the reduced spreading area, induced the acidic pH i . These results suggest that the reduced Ca 2+ requirement of transformed cells for growth is related to the mechanism of pH i regulation rather than Ca 2+ homeostasis and, possibly, to the anchorage‐independent growth, which is a unique feature of transformed cells. © 1993 Wiley‐Liss, Inc.