Regulation of exocytosis by Ca2+ and cAMP - A study on pancreatic beta- and alpha-cells

Research output: ThesisDoctoral Thesis (compilation)

Abstract

Type-2 diabetes is characterized by impaired insulin secretion associated with excess glucagon release. Exocytosis of insulin- or glucagon-containing granules is initiated by Ca2+-influx through voltage-dependent Ca2+-channels and is modulated by the second messengers such as cAMP. Membrane capacitance measurements were used in this study to investigate the cellular mechanisms behind cAMP-regulated Ca2+-dependent exocytosis in pancreatic beta-and alpha-cells. In pancreatic beta-cells, the secretory granules have been demonstrated to co-localize with L-type Ca2+-channels. Experiments using photorelease of caged Ca2+ revealed that exocytosis of insulin-containing granules requires an increase in [Ca2+]i with a Kd of 17 µM, which is most likely achieved in the vicinity of the Ca2+-channels. It was further proved that a close association between the alpha1c L-type Ca2+-channels and the secretory granules is a prerequisite for rapid exocytosis and ensures maximum usage of Ca2+ influx in a cell with few Ca2+-channels (~500) as is the case in the beta-cell. Pharmacologically, L-type Ca2+-channels are suppressed by organic Ca2+-channel blockers such as dihydropyridines. Glacontryphan- M was isolated from the venom of Conus Marmoreus and was detected to reduce L-type Ca2+ currents and associated exocytosis. It was discovered that this novel antagonist needs the binding of Ca2+ to the Gla-residues for its function. Ca2+-dependent exocytosis in the beta-cell is enhanced by cAMP through both PKA-independent granular priming and PKA-dependent recruitment of granules from a reserve pool. This study revealed a concentration-dependent activation of the two cAMP-signalling pathways and that 5-fold higher [cAMP]i was needed for the PKA-independent mechanism. Interestingly, the sulfonylurea receptor SUR1 was required in the regulation of cAMP-stimulated PKA-independent exocytosis. This is an additional function of SUR1 apart from being a subunit of the KATP channel. Enhancement of exocytosis by cAMP in alpha-cells was discovered to involve similar mechanisms as in the beta-cell. In the present study glucagon was detected to elevate cAMP resulting in an enhanced Ca2+-dependent exocytotic response by binding to the glucagon-receptors in the plasma membrane of the alpha-cell. The stimulatory effect of glucagon was mainly mediated by PKA-dependent mechanisms, possibly due to the inability of glucagon to elevate [cAMP]i to the sufficient level needed for activation of the PKA-independent pathway.

Details

Authors
  • Xiaosong Ma
Organisations
Research areas and keywords

Subject classification (UKÄ) – MANDATORY

  • Developmental Biology

Keywords

  • Endocrinology, secreting systems, type-2 diabetes, SUR1, Ca2+-channel, glucagon, exocytosis, insulin, diabetology, Endokrinologi, sekretion, diabetologi
Original languageEnglish
QualificationDoctor
Awarding Institution
Supervisors/Assistant supervisor
  • [unknown], [unknown], Supervisor, External person
Award date2004 Oct 29
Publisher
  • Xiaosong Ma, Department of Physiological sciences, BMC B11, Tornavägen 10, 221 84 LUND,
Print ISBNs91-628-6259-6
Publication statusPublished - 2004
Publication categoryResearch

Bibliographic note

Defence details Date: 2004-10-29 Time: 10:15 Place: Segerfalkssalen, BMC, Sölvegatan 19 External reviewer(s) Name: Gylfe, Erik Title: Professor Affiliation: Uppsala University --- Article: I. Sebastian Barg, Xiaosong Ma, Lena Eliasson, Juris Galvanovskis, Sven O.Göpel, Stefanie Obermüller, Josef Platzer, Erik Renström, Michel Trus, Daphne Atlas, Jörg Striessnig and Patrik Rorsman (2001). Fast exocytosis with few Ca2+ channels in insulin-secreting mouse pancreatic B cells. Biophys J 81: 3308-3323 Article: II. Karin Hansson, Xiaosong Ma, Lena Eliasson, Eva Czerwiec, Bruce Furie, Barbara C. Furie, Patrik Rorsman and Johan Stenflo (2004). The first γ-carboxyglutamic acid-containing contryphan: A selective L-type calcium ion channel blocker isolated from the venom of conus marmoreus. J Biol Chem 279: 32453-32463 Article: III. Lena Eliasson*, Xiaosong Ma*, Erik Renström*, Sebastian Barg, Per-Olof Berggren, Juris Galvanovskis, Jesper Gromada, Xingjun Jing, Ingmar Lundquist, Albert Salehi, Sabine Sewing and Patrik Rorsman (2003). SUR1 regulates PKA-independent cAMP-induced granule priming in mouse pancreatic B-cells. J Gen Physiol 121: 181-197 Article: IV. Xiaosong Ma, Yang Zhang, Jesper Gromada, Sabine Sewing, Per-Olof Berggren, Karsten Buschard, Albert Salehi, Jenny Vikman, Patrik Rorsman and Lena Eliasson (2004). Glucagon stimulates exocytosis in mouse and rat pancreatic alpha cells by binding to glucagon receptors. Mol Endocrinol, in press

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