Mitogenic effect of alkaline pH on quiescent, serum-starved cells

The effect of environmental pH on the proliferative activity of sparse 3T3 cell cultures was investigated. When quiescent (serum-starved) cells were transferred to a serum-depleted medium in which pH had been elevated, cell proliferation was stimulated during the first 24 hr after the medium change, as reflected by an increase in cell number and in the proportion of cells synthesizing DNA (measured by autoradiography and flow cytophotometry). The growth-stimulatory effect was greater with increasing pH. At pH 8.2 the effect was approximately 50% of that seen in serum-stimulated cells (i.e., upon the addition of 10% serum). Above pH 8.2 toxic effects and cell death were observed. However, the stimulatory effect could be performed without interference of toxic effects by exposing the cells to the high alkaline environment for a short period. A maximum stimulatory effect on the quiescent cells--without any observed toxic effects--was seen after exposure to an alkaline pulse with a duration of 2-10 min at a pH between 8.5 and 10. More than 80% of the cells synthesized DNA during the first 24 hr after the alkaline-pulse stimulation, which is an almost similar response to that seen after stimulation with serum. The growth-stimulatory effect of the alkaline-pulse treatment could not be prevented by a subsequent treatment at acid pH. Kinetic analysis revealed that DNA synthesis was not initiated until 9-12 hr after the alkaline-pulse treatment. This lag period was of the same length as that seen after stimulation with serum. Alkaline-pulse treatment only resulted in stimulation of DNA synthesis and mitosis during one cell cycle. When the alkaline treatment was repeated, only a small proportion of cells could proceed through a second cell cycle.