We have recently shown that two mechanisms are involved in the regulation of pHi in the osteoblastic phenotype cell line UMR-106 (Na(+)-H+ antiporter and a Na(+)-independent Cl(-)-HCO 3(-)-OH- exchanger). In the present work, we used the pH-sensitive dye 2',7'-bis(2-carboxyethyl)-5,6-carboxyfluorescein as well as isotope fluxes to investigate the influence of second messengers on the activity of these transporters. Elevation in intracellular calcium concentration [( Ca2+]in) in UMR-106 cells (measured by fura-2 fluorescence) is followed by stimulation of the Cl(-)-HCO3- exchanger, leading to cytosolic acidification. Subsequently, cell alkalinization, mediated by the Na(+)-H+ exchanger, restores pHi to its resting value. An acute reduction in [Ca2+]in abruptly stops the activity of the anion exchanger while having no influence on the activity of the Na(+)-H+ exchanger. The stimulatory effect of Ca2+in on the anion exchanger is dose dependent and is abrogated by the calmodulin inhibitors N-(6-aminohexyl)-5-chloro-naphthalenesulfonamide and calmidazolium. An increase in intracellular adenosine 3',5'-cyclic monophosphate (cAMP) brought about by forskolin, 8-bromo-cAMP, or prostaglandin E2 leads to inhibition of activity of both the Na(+)-H+ antiporter and the anion exchanger. The suppressive effect of cAMP on Cl(-)-HCO3- exchange could be overcome by elevating [Ca2+]in. We conclude that 1) Ca2+in and cAMP can influence pHi in osteoblasts by altering the activities of pHi regulatory mechanisms and 2) the effect of Ca2+in is probably mediated by calmodulin.