title: Bidirectional insulin granule turnover in the submembrane space during K(+) depolarization-induced secretion.
abstract: Like primary mouse islets, MIN6 pseudoislets responded to the depolarization by 40 mm KCl and the resulting increase in the free cytosolic Ca(2+) concentration ([Ca(2+) ](i) ) with a massive increase in insulin secretion, whereas 15 mm KCl had little effect in spite of a clear increase in [Ca(2+) ](i) . Analysis of insulin-enhanced green fluorescent protein (EGFP)-labeled granules in MIN6 cells by total internal reflection fluorescence (TIRF) microscopy showed that 40 mm KCl increased the number of short-term resident granules (<1 second presence in the submembrane space), while the total granule number and the number of long-term resident granules decreased. The rates of granule arrival at and departure from the submembrane space changed in parallel and were two orders of magnitude higher than the release rates, suggesting a back-and-forth movement of the granules as the primary determinant of the submembrane granule number. The effect of 15 mm KCl resembled that of 40 mm but did not achieve significance. Both 15 and 40 mm KCl evoked a [Ca(2+) ](i) increase, which was antagonized by 10 µm nifedipine. Nifedipine also antagonized the effect on secretion and on granule number and mobility. In conclusion, during KCl depolarization L-type Ca(2+) channels seem to regulate two processes, insulin granule turnover in the submembrane space and granule exocytosis.
authors.affiliation: Institute of Pharmacology and Toxicology, University of Braunschweig, Mendelssohnstr. 1, 38106 Braunschweig, Germany.
authors.forename: Kathrin
authors.initials: K
authors.lastname: Hatlapatka
authors.forename: Magnus
authors.initials: M
authors.lastname: Matz
authors.forename: Kirstin
authors.initials: K
authors.lastname: Schumacher
authors.forename: Knut
authors.initials: K
authors.lastname: Baumann
authors.forename: Ingo
authors.initials: I
authors.lastname: Rustenbeck
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism; D002121:Calcium Channel Blockers / Q000494:pharmacology; D020746:Calcium Channels, L-Type / Q000378:metabolism; D002462:Cell Membrane / Q000378:metabolism; D002478:Cells, Cultured; D003594:Cytoplasmic Granules / Q000378:metabolism*; D005089:Exocytosis / Q000187:drug effects*; D049452:Green Fluorescent Proteins / Q000378:metabolism; D007328:Insulin / Q000378:metabolism*; D000078790:Insulin Secretion; D008564:Membrane Potentials / Q000187:drug effects*; D051379:Mice; D009543:Nifedipine / Q000494:pharmacology; D011188:Potassium / Q000378:metabolism; D011189:Potassium Chloride / Q000494:pharmacology*
chemical_list: D002121:Calcium Channel Blockers; D020746:Calcium Channels, L-Type; D007328:Insulin; C485184:enhanced green fluorescent protein; D049452:Green Fluorescent Proteins; D011189:Potassium Chloride; D009543:Nifedipine; D011188:Potassium; D002118:Calcium