title: Ouabain evokes exocytosis dependent on ryanodine and mitochondrial calcium stores that is not followed by compensatory endocytosis at the neuromuscular junction.
abstract: Ouabain is a cardiotonic glycoside that inhibits the sodium potassium ATPase pump leading to sodium accumulation in nerve terminals. At the frog neuromuscular junction, ouabain induces acetylcholine release and a rapid depletion of synaptic vesicles. In the present work, we used FM1-43 vital labeling to dissect the effect of ouabain on synaptic vesicles recycling. We first examined images of nerve-muscle preparations that were stained with FM1-43 by electrical stimulation of the nerve and destained with ouabain. We observed that ouabain induced exocytosis of synaptic vesicles independently of extracellular calcium, implying a mechanism of exocytosis that can bypass the requirement for extracellular calcium. We therefore tested the hypothesis that ouabain induces exocytosis by mobilizing intracellular calcium and we report that calcium release from endoplasmic reticulum through ryanodine receptors is necessary for ouabain-evoked exocytosis. In addition, the ouabain-evoked exocytosis was dependent on calcium released from mitochondria. We also investigated if exocytosis evoked by ouabain is followed by compensatory endocytosis. We observed that muscles incubated with FM1-43 in the presence of ouabain did not present significant staining. In conclusion, our data demonstrate that exocytosis evoked by ouabain is independent on extracellular calcium but dependent on calcium release from endoplasmic reticulum and mitochondrial stores. In addition, we suggest that ouabain can be used as a pharmacological tool to uncouple synaptic vesicles exocytosis from endocytosis at the neuromuscular junction.
authors.affiliation: Department of Morphology, Cell Biology Program, Biological Sciences Institute, Federal University of Minas Gerais, Belo Horizonte, MG 31270-901, Brazil.
authors.forename: Ernani
authors.initials: E
authors.lastname: Amaral
authors.forename: Luciana F
authors.initials: LF
authors.lastname: Leite
authors.forename: Marcus V
authors.initials: MV
authors.lastname: Gomez
authors.forename: Marco A M
authors.initials: MA
authors.lastname: Prado
authors.forename: Cristina
authors.initials: C
authors.lastname: Guatimosim
mesh_terms: D000109:Acetylcholine / Q000378:metabolism; D000818:Animals; D002118:Calcium / Q000378:metabolism; D020013:Calcium Signaling / Q000187:drug effects / Q000502:physiology*; D004558:Electric Stimulation; D004705:Endocytosis / Q000187:drug effects / Q000502:physiology; D004721:Endoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005089:Exocytosis / Q000187:drug effects / Q000502:physiology*; D005456:Fluorescent Dyes; D008561:Membrane Fusion / Q000187:drug effects / Q000502:physiology; D008928:Mitochondria / Q000187:drug effects / Q000378:metabolism*; D009469:Neuromuscular Junction / Q000187:drug effects / Q000378:metabolism*; D010042:Ouabain / Q000494:pharmacology; D017729:Presynaptic Terminals / Q000187:drug effects / Q000378:metabolism*; D011726:Pyridinium Compounds; D000644:Quaternary Ammonium Compounds; D011898:Ranidae; D019837:Ryanodine Receptor Calcium Release Channel / Q000187:drug effects / Q000378:metabolism*; D013572:Synaptic Vesicles / Q000187:drug effects / Q000378:metabolism
chemical_list: D004791:Enzyme Inhibitors; C073804:FM1 43; D005456:Fluorescent Dyes; D011726:Pyridinium Compounds; D000644:Quaternary Ammonium Compounds; D019837:Ryanodine Receptor Calcium Release Channel; D010042:Ouabain; D000109:Acetylcholine; D002118:Calcium