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 about 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%). Individual 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 "wide-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). This protection was due to blockade of Ba2+ entry through Ca2+ channels because dotarizine (10 mu M) inhibited the increase in cytosolic [Ba2+] seen in fura-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 sustained elevation of the fura-2 signal. This contrasts with the fast dissipation of the fura-2 signal generated by [Ca2+](i) elevation. Thus, Ba2+ overload can cause cell death by mechanisms similar to those reported for Ca2+ overload and might be used as a novel and convenient tool to search for new cytoprotective compounds. (C) 2000 Elsevier Science B.V. All rights reserved.