ROLE OF CALCIUM AND CAMP MESSENGER SYSTEMS IN INTRACELLULAR PH REGULATION OF OSTEOBLASTIC CELLS

We have recently shown that two mechanisms are involved in the regulation of pH(i) in the osteoblastic phenotype cell line UMR-106 (Na+-H+ antiporter and a Na+- independent Cl--HCO3--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 pH(i) 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-naphthale-nesulfonamide 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 pH(i) in osteoblasts by altering the activities of pH(i) regulatory mechanisms and 2) the effect of Ca2+in is probably mediated by calmodulin.