Distinct mechanisms of neurotransmitter release from PC12 cells exposed to lead

Abstract

Two enzymes, protein kinase C and microsomal Ca(2+)-ATPase help regulate levels of Ca2+ in many types of cells. Since proteins that regulate Ca2+ often influence sensitivity to Pb2+, we determined the possible roles played by protein kinase C and microsomal Ca(2+)-ATPase for the Pb(2+)-evoked release of norepinephrine (NOR) in PC cells. NOR release was observed at 10 microM Pb2+ when PC 12 cells were stimulated with inhibitors of microsomal Ca(2+)-ATPase such as thapsigargin, cyclopiazonic acid, or 2,5-di-(t-butyl)-hydroquinone. At 5 microM, Pb2+ evoked the release of NOR in PC 12 cells stimulated with activators of protein kinase C such as phorbol 12-myristate 13-acetate (PMA) or (-)-7-octylindolactam. NOR release was observed at 1 microM Pb2+ in the presence of both PMA and thapsigargin. Ni2+ and Cd2+ blocked NOR release stimulated by Pb2+ in the presence of thapsigargin but not by PMA. NOR released by thapsigargin stimulation was not altered in PC 12 cells depleted of protein kinase C. Two proteins found in vesicles, chromogranin B and secretogranin-II were released with NOR. Our results indicate that in PC 12 cells, PB(2+)-evokes the release of neurotransmitters. Furthermore, thapsigargin and PMA increase the cell's sensitivity to Pb2+ by different pathways.

The PC12 cell as model for neurosecretion

“…The neurotoxic heavy metal Pb 2+ was shown to not only block VGCCs (Shafer 1998), but also to enhance spontaneous neurotransmitter release in populations of PC12 cells (Bressler et al. 1996).…”

Ca2+‐independent vesicular catecholamine release in PC12 cells by nanomolar concentrations of Pb2+

“…The first effect observed is inhibition of evoked neurotransmitter release, which has been attributed to a reduced influx of Ca 2+ due to Ca 2+ channel block. The second, delayed effect of Pb 2+ is an enhancement of spontaneous neurotransmitter release, which occurs irrespective of the presence of extracellular Ca 2+ and has been attributed to intracellular effects of Pb 2+ (Manalis and Cooper 1973; Tomsig and Suszkiw 1993; Bressler et al . 1996; Braga et al .…”

“…2000). Previous reports demonstrate that addition of > 5 µ m Pb 2+ to external saline is required to induce neurotransmitter release from undifferentiated PC12 cell populations (Bressler et al . 1996).…”

Targeting Exocytosis: Ins and Outs of the Modulation of Quantal Dopamine Release

“…Depolarization-evoked DA overflow in NAcc is increased after chronic exposure to low concentrations of Pb 2+ , whereas DA clearance time is reduced [303]. On the other hand, acute exposure to Pb 2+ inhibits evoked neurotransmitter release, at least partly by inhibition of VSCCs, whereas spontaneous neurotransmitter release is enhanced following a short delay [301,[304][305][306]. Recently, amperometry was used to demonstrate that Pb 2+ -induced exocytosis in neuroendocrine PC12 cells is mediated through direct effects on intracellular mechanisms, without apparent changes in the intracellular [Ca 2+ ] and in vesicle contents.…”