EVIDENCE FOR A NA+ CA2+ EXCHANGER IN NEUROBLASTOMA X GLIOMA HYBRID NG108-15 CELLS/

To determine whether NG108-15 cells contain a functional Na+/Ca2+ exch anger, we isotonically replaced extracellular Na+ with N-methyl-D-gluc amine (NMG) and measured the effect on cytosolic Ca2+ concentration ([ Ca2+](i)) using the fluorescent Ca2+ indicator fura 2. Replacement wit h NMG alone had no effect on basal [Ca2+](i) or the rise in [Ca2+](i) evoked by 80 mM K+ or 10 mu M bradykinin, but caused a larger [Ca2+](i ) increase when thapsigargin and carbonyl cyanide p-trifluoromethoxyph enylhydrazone (FCCP) were added to the cells; this enhanced [Ca2+](i) increase could be reversed by adding Na+ back to the bathing buffer. T he elevation in [Ca2+](i) induced by thapsigargin and FCCP was inverse ly proportional to extracellular Na+ concentration. Furthermore, the e xchanger operated in the reverse mode, as measured by either [Ca2+](i) change or Ca-45(2+) uptake. An 810 bp cDNA fragment of the exchanger was amplified by PCR; it differed by a single amino acid residue from the corresponding segment of the rat brain Na+/Ca2+ exchanger. These d ata suggest that a functioning Na+/Ca2+ exchanger exists in NG108-15 c ells.