Biophysical Society Thematic Meeting| Padova 2019

Quantitative Aspects of Membrane Fusion and Fission

Poster Abstracts

7-POS Board 7 BURSTING VERSUS SPIKING: SYSTEMATIC INVESTIGATION OF HOW PATTERNS OF ELECTRICAL ACTIVITY CONTROL LOCAL CA 2+ AND HORMONE RELEASE Iulia Martina Bulai 1 ; Morten Gram Pedersen 1,2 ; Joel Tabak 3 ; 1 University of Padova, Department of Information Engineering, Padova, Italy 2 University of Padova, Department of Mathematics "Tullio Levi- Civita" Padova, Italy 3 University of Exeter, Medical School, Exeter, Devon, United Kingdom Exocytosis is the fusion of the membranes of hormone-containing secretory granules with the cell membrane, which releases hormone molecules into the extracellular space. Endocrine pituitary cells secrete different types of hormones. These cells contain a wide range of ion channels and are electrically excitable with some cell types exhibiting spiking (repetitive action potential firing), and others exhibiting bursting (silent phases interspersed by active ones). The different patterns of electrical activity are believed to induce different amounts of hormone release from the different pituitary cell types. We study how different patterns of electrical activity control hormone secretion. We generate spiking patterns characterized by the same action potential duration but different periods of silence, and do the same for bursting. From our numerical simulations we observe that bursting generally results in larger Ca 2+ influx than spiking, since the cell is electrically active for a longer time. Secondly, we find that close to the channel (30 nm) the Ca 2+ concentration reaches tens of µM while far from the channel (500 nm) it reaches units of µM. To measure the efficiency of Ca 2+ in triggering exocytosis, we compare the cumulative number of fused granules (E) versus the integral of the measured Ca 2+ -current (Q) at different distances from the Ca 2+ channel for both spiking and bursting. We observe that close to the channel (30 nm) the E-vs-Q curves for both spiking and bursting are almost superimposed, and conclude that spiking and bursting are equally Ca 2+ efficient in triggering exocytosis. At 500 nm from the channel bursting is slightly more efficient than spiking, since the E-vs-Q bursting curves are above the curves that corresponds to spiking. In conclusion, the pattern of electrical activity is important for controlling exocytosis in subtle ways that depend also on the distance between Ca 2+ channels and granules.

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