Calcium-dependent intracellular acidification dominates the pH response to mitogen in human T cells.

The effects of a phorol ester and a mitogenic lectin on the intracellular pH (pHi) of human T lymphocytes was investigated. In contrast to the cytoplasmic alkalinization induced by 12-0-tetradecanoylphorbol-13-acetate, an acidification was recorded in cells treated with phytohemagglutinin. This decrease in pHi was magnified in Na+-free medium or in the presence of amiloride analogues, suggesting that activation of Na+/H+ exchange partially counteracts the phytohemagglutinin-induced acidification. The decrease in pHi was dependent on a sustained increase in cytosolic free Ca2+ and could be mimicked by addition of the divalent cation ionophore, ionomycin. The elevation of cytosolic free Ca2+ leads to metabolic H+ (equivalent) generation with consequent cytoplasmic acidification, which in human T cells predominates over the concurrent activation of the Na+/H+ antiport. These findings argue against the notion that activation of Na+/H+ exchange is a signal for the initiation of proliferation.