We studied the effects of TRH on intracellular pH (pHi) in individual cells of the GH3 pituitary clonal cell line using the seminaphtorhodafluor pH indicator. We show that, in a majority of cells, TRH action on pHi occurs in two phases: first acidification then alkalinization. Acidification and Ca2+ mobilization are related in time. K+ depolarization (KCl, 50 mM), and Ca2+ ionophores, A23187 (10 microM) or ionomycin (5 microM) lead to acidification. We conclude that a marked increase in [Ca2+]i can induce acidification and that the TRH-induced acidification is due to Ca2+ mobilization. TRH-induced alkalinization is due to Na+/H+ exchanger activation, since it is inhibited by amiloride (200 microM) and Na(+)-free medium. We show that this alkalinization does not occur after a 20-h pretreatment with phorbol myristate acetate (1 microM) which depletes protein kinase C. We also show that blocking Ca2+ entry does not affect the TRH-induced alkalinization, but an increase in [Ca2+]i concomitant with the activation of protein kinase C mimics TRH-induced alkalinization. We conclude that both Ca2+ mobilization and protein kinase C activation are necessary for TRH-induced alkalinization. Studies of secretion in Na(+)-free medium or with amiloride (200 microM) show that pHi does not seem to be involved in PRL short-term release (30 min) but suggest that activation of the Na+/H+ exchanger leading to cytoplasmic alkalinization may have an important role in PRL synthesis.