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Fast exocytosis with few Ca(2+) channels in insulin-secreting mouse pancreatic B cells
Department of Molecular and Cellular Physiology, Institute of Physiological Sciences, Lund University, Lund.
Department of Molecular and Cellular Physiology, Institute of Physiological Sciences, Lund University, Lund.
Department of Molecular and Cellular Physiology, Institute of Physiological Sciences, Lund University, Lund.
Department of Molecular and Cellular Physiology, Institute of Physiological Sciences, Lund University, Lund.
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2001 (English)In: Biophysical Journal, ISSN 0006-3495, E-ISSN 1542-0086, Vol. 81, no 6, 3308-23 p.Article in journal (Refereed) Published
Abstract [en]

The association of L-type Ca(2+) channels to the secretory granules and its functional significance to secretion was investigated in mouse pancreatic B cells. Nonstationary fluctuation analysis showed that the B cell is equipped with <500 alpha1(C) L-type Ca(2+) channels, corresponding to a Ca(2+) channel density of 0.9 channels per microm(2). Analysis of the kinetics of exocytosis during voltage-clamp depolarizations revealed an early component that reached a peak rate of 1.1 pFs(-1) (approximately 650 granules/s) 25 ms after onset of the pulse and is completed within approximately 100 ms. This component represents a subset of approximately 60 granules situated in the immediate vicinity of the L-type Ca(2+) channels, corresponding to approximately 10% of the readily releasable pool of granules. Experiments involving photorelease of caged Ca(2+) revealed that the rate of exocytosis was half-maximal at a cytoplasmic Ca(2+) concentration of 17 microM, and concentrations >25 microM are required to attain the rate of exocytosis observed during voltage-clamp depolarizations. The rapid component of exocytosis was not affected by inclusion of millimolar concentrations of the Ca(2+) buffer EGTA but abolished by addition of exogenous L(C753-893), the 140 amino acids of the cytoplasmic loop connecting the 2(nd) and 3(rd) transmembrane region of the alpha1(C) L-type Ca(2+) channel, which has been proposed to tether the Ca(2+) channels to the secretory granules. In keeping with the idea that secretion is determined by Ca(2+) influx through individual Ca(2+) channels, exocytosis triggered by brief (15 ms) depolarizations was enhanced 2.5-fold by the Ca(2+) channel agonist BayK8644 and 3.5-fold by elevating extracellular Ca(2+) from 2.6 to 10 mM. Recordings of single Ca(2+) channel activity revealed that patches predominantly contained no channels or many active channels. We propose that several Ca(2+) channels associate with a single granule thus forming a functional unit. This arrangement is important in a cell with few Ca(2+) channels as it ensures maximum usage of the Ca(2+) entering the cell while minimizing the influence of stochastic variations of the Ca(2+) channel activity.

Place, publisher, year, edition, pages
2001. Vol. 81, no 6, 3308-23 p.
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Medical and Health Sciences
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URN: urn:nbn:se:uu:diva-87027DOI: 10.1016/S0006-3495(01)75964-4PubMedID: 11720994OAI: oai:DiVA.org:uu-87027DiVA: diva2:128150
Available from: 2008-12-12 Created: 2008-12-12 Last updated: 2017-12-14

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Barg, Sebastian

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