title: Calcium current inactivation rather than pool depletion explains reduced exocytotic rate with prolonged stimulation in insulin-secreting INS-1 832/13 cells.
abstract: Impairment in beta-cell exocytosis is associated with reduced insulin secretion and diabetes. Here we aimed to investigate the dynamics of Ca2+-dependent insulin exocytosis with respect to pool depletion and Ca2+-current inactivation. We studied exocytosis, measured as increase in membrane capacitance (ΔCm), as a function of calcium entry (Q) in insulin secreting INS-1 832/13 cells using patch clamp and mixed-effects statistical analysis. The observed linear relationship between ΔCm and Q suggests that Ca2+-channel inactivation rather than granule pool restrictions is responsible for the decline in exocytosis observed at longer depolarizations. INS-1 832/13 cells possess an immediately releasable pool (IRP) of ∼10 granules and most exocytosis of granules occurs from a large pool. The latter is attenuated by the calcium-buffer EGTA, while IRP is unaffected. These findings suggest that most insulin release occurs away from Ca2+-channels, and that pool depletion plays a minor role in the decline of exocytosis upon prolonged stimulation.
authors.affiliation: Islet Cell Exocytosis, Lund University Diabetes Centre, Department of Clinical Sciences Malmö, Lund University, Malmö, Sweden.
authors.forename: Morten Gram
authors.initials: MG
authors.lastname: Pedersen
authors.affiliation: Islet Cell Exocytosis, Lund University Diabetes Centre, Department of Clinical Sciences Malmö, Lund University, Malmö, Sweden.
authors.forename: Vishal Ashok
authors.initials: VA
authors.lastname: Salunkhe
authors.affiliation: Center for Infectious Medicine, Department of Medicine, The Karolinska Institute, Huddinge University Hospital, Stockholm, Sweden.
authors.forename: Emma
authors.initials: E
authors.lastname: Svedin
authors.affiliation: Islet Cell Exocytosis, Lund University Diabetes Centre, Department of Clinical Sciences Malmö, Lund University, Malmö, Sweden.
authors.forename: Anna
authors.initials: A
authors.lastname: Edlund
authors.affiliation: Islet Cell Exocytosis, Lund University Diabetes Centre, Department of Clinical Sciences Malmö, Lund University, Malmö, Sweden.
authors.forename: Lena
authors.initials: L
authors.lastname: Eliasson
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism*; D045744:Cell Line, Tumor; D002462:Cell Membrane / Q000502:physiology; D025802:Electric Capacitance; D005089:Exocytosis / Q000502:physiology*; D007328:Insulin / Q000378:metabolism*; D050417:Insulin-Secreting Cells / Q000502:physiology*; D008954:Models, Biological; D018408:Patch-Clamp Techniques; D051381:Rats
chemical_list: D007328:Insulin; D002118:Calcium
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