alpha-Latrotoxin increases spontaneous and depolarization-evoked exocytosis from pancreatic islet beta-cells.
alpha-Latrotoxin (alpha-LT), a potent excitatory neurotoxin, increases spontaneous, as well as action potential-evoked, quantal release at nerve terminals and increases hormone release from excitable endocrine cells. We have investigated the effects of alpha-LT on single human, mouse and canine beta-cells. In isolated and combined measurements, alpha-LT, at nanomolar concentrations, induces: (i) rises in cytosolic Ca(2+), into the micromolar range, that are dependent on extracellular Ca(2+); (ii) large conductance non-selective cation channels; and (iii) Ca(2+)-dependent insulin granule exocytosis, measured as increases in membrane capacitance and quantal release of preloaded serotonin. Furthermore, at picomolar concentrations, alpha-LT potentiates depolarization-induced exocytosis often without evidence of inducing channel activity or increasing cytosolic Ca(2+). These results strongly support the hypothesis that alpha-LT, after binding to specific receptors, has at least two complementary modes of action on excitable cells. (i) alpha-LT inserts into the plasma membrane to form Ca(2+) permeable channels and promote Ca(2+) entry thereby triggering Ca(2+)-dependent exocytosis in unstimulated cells. (ii) At lower concentrations, where its channel forming activity is hardly evident, alpha-LT augments depolarization-evoked exocytosis probably by second messenger-induced enhancement of the efficiency of the vesicle recruitment or vesicle fusion machinery. We suggest that both modes of action enhance exocytosis from a newly described highly Ca(2+)-sensitive pool of insulin granules activated by global cytosolic Ca(2+) concentrations in the range of approximately 1 microm.
Silva
Amelia M
AM
Department of Internal Medicine, Washington University Medical Center, St Louis, MO 63110, USA. mislers@msnotes.wustl.edu
Liu-Gentry
June
J
Dickey
Adam S
AS
Barnett
David W
DW
Misler
Stanley
S
D000818:Animals; D002118:Calcium / Q000378:metabolism; D015220:Calcium Channels / Q000502:physiology; D003600:Cytosol / Q000378:metabolism; D004285:Dogs; D005089:Exocytosis / Q000187:drug effects* / Q000502:physiology; D006801:Humans; D015640:Ion Channel Gating / Q000187:drug effects / Q000502:physiology; D007515:Islets of Langerhans / Q000187:drug effects* / Q000502:physiology*; D008564:Membrane Potentials / Q000187:drug effects; D051379:Mice; D018408:Patch-Clamp Techniques; D013111:Spider Venoms / Q000494:pharmacology*
D015220:Calcium Channels; D013111:Spider Venoms; C020763:alpha-latrotoxin; D002118:Calcium