Ba2+-induced chromaffin cell death: cytoprotection by Ca2+ channel antagonists

Exposure of bovine adrenal medullary chromaffin cells to Ba2+ ions (in the absence of Ca2+ ions) caused their death, measured as lactate dehydrogenase (LDH) release. The concentration of Ba2+ required to damage the cells by a bout 65% ranged between 1 and 10 mM (no Ca2+ added); the required exposure time was rather brief (15 min-4 h). The simultaneous presence of Ca2+, Mg2 or Zn2+ together with Ba2+ (2 mM, 4 h) afforded cyprotection (60-80%). Ind ividual selective blockers of Ca2+ channel subtypes afforded no protection. However, combined nifedipine (3 mu M) plus omega-conotoxin MVIIC (3 mu M) offered full protection. Substantial protection was also seen with the "wid e-spectrum" Ca2+ channel blockers penfluridol (0.3 mu M), lubeluzole (3 mu M), dotarizine (3 mu M), flunarizine (3 mu M), and mibefradil (3 mu M). Thi s protection was due to blockade of Ba2+ entry through Ca2+ channels becaus e dotarizine (10 mu M) inhibited the increase in cytosolic [Ba2+] seen in f ura-2-loaded chromaffin cells. Once Ba2+ accumulated in the cytosol, it was not extruded by the Na+/Ca2+ exchanger, as shown by the prolonged and sust ained elevation of the fura-2 signal. This contrasts with the fast dissipat ion of the fura-2 signal generated by [Ca2+](i) elevation. Thus, Ba2+ overl oad can cause cell death by mechanisms similar to those reported for Ca2+ o verload and might be used as a novel and convenient tool to search for new cytoprotective compounds. (C) 2000 Elsevier Science B.V. All rights reserve d.