pH and the effect of NH4Cl on human gingival fibroblasts.

pH variations in the range from 6.9 to 7.9 modified the growth rate of human gingival fibroblasts in culture, a pH optimum being found at 7.6-7.8. A pH dependent growth inhibition by ammonia was evident, being particularly prominent in the presence of 2 mM NH4Cl. At this concentration an effect ranging from a complete cessation of growth at pH 8.0 to an indication of a slight stimulatory effect at pH 7.2 was found. Concomitant with this amplification of the growth inhibitory effect, a state of unbalanced growth in the sense of an increased cellular protein content was induced by ammonia upon increasing the pH of the medium. The incorporation of 14C-proline into the total protein fraction decreased rapidly, particularly in the control cultures, with pH; an inhibition by ammonia being found at pH 7.0 and 7.5 with no effect at pH 7.1. Except for part of the inhibition by ammonia, changes in specific activity of 14C-proline in the cellular amino acid pool could ac count for these pH effects. Ammonia reduced the proportion of radiolabeled protein recovered as collagen both from the cells and medium at the lower pHs, whereas the only significant effect on noncollagen protein was an increased fraction being secreted at pH 7.1 in the presence of 10 mM NH4Cl.