DISTINCT MECHANISMS OF NEUROTRANSMITTER RELEASE FROM PC-12 CELLS EXPOSED TO LEAD

Two enzymes, protein kinase C and microsomal Ca2+-ATPase help regulate levels of Ca2+ in many types of cells. Since proteins that regulate C a2+ often influence sensitivity to Pb2+, we determined the possible ro les played by protein kinase C and microsomal Ca2+-ATPase for the Pb2-evoked release of norepinephrine (NOR) in PC 12 cells. NOR release wa s observed at 10 mu M Pb2+ when PC 12 cells were stimulated with inhib itors of microsomal Ca2+-ATPase such as thapsigargin, cyclopiazonic ac id, or 2,5-di-(t-butyl)-hydroquinone. At 5 mu M, Pb2+ evoked the relea se of NOR in PC 12 cells stimulated with activators of protein kinase C such as phorbol 12-myristate 13-acetate (PMA) or (-)-7-octylindolact am. NOR release was observed at 1 mu M Pb2+ in the presence of both PM A and thapsigargin. Ni2+ and Cd2+ blocked NOR release stimulated by Pb 2+ in the presence of thapsigargin but not by PMA. NOR released by tha psigargin stimulation was not altered in PC 12 cells depleted of prote in kinase C. Two proteins found in vesicles, chromogranin B and secret ogranin-II were released with NOR. Our results indicate that in PC 12 cells, Pb2+-evokes the release of neurotransmitters. Furthermore, thap sigargin and PMA increase the cell's sensitivity to Pb2+ by different pathways. (C) 1996 Wiley-Liss, Inc.