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Modeling buffered Ca2+ diffusion near the membrane: implications for secretion in neuroendocrine cells
1Department of Membrane Biophysics, Max-Planck-Institute for Biophysical Chemistry, Göttingen, Germany.
Biophysical Journal
|February 1, 1997
Summary
Calcium buffering and diffusion dynamics significantly delay catecholamine secretion from neuroendocrine cells. Models show Ca2+ redistribution near channels modulates release timing and quantal yield during stimulation.
Area of Science:
- Neuroendocrinology
- Cellular Physiology
- Biophysics
Background:
- Catecholamine secretion from neuroendocrine cells is slow, suggesting calcium (Ca2+) dynamics play a key role in delaying the response.
- Previous studies indicate intracellular calcium concentration ([Ca2+]i) peaks below 10 microM and decays over milliseconds, influencing exocytosis.
- Understanding Ca2+ buffering and diffusion is crucial for explaining the kinetics of neuroendocrine secretion.
Purpose of the Study:
- To model buffered diffusion of Ca2+ near calcium channels to assess its consistency with observed secretion delays.
- To investigate how Ca2+ channel spacing and distribution affect the time course of intracellular calcium concentration.
- To evaluate the role of Ca2+ dynamics in modulating both the timing and yield of catecholamine release.
Main Methods:
- Simulated buffered diffusion of Ca2+ in the vicinity of a Ca2+ channel pore using computational models.
- Varied the distribution and interchannel distances of Ca2+ channels to match experimental observations.
- Analyzed the impact of Ca2+ buffering and depletion on the rise and decay of [Ca2+]i.
Main Results:
- Simulations supported a model where release-ready granules are distributed within a grid of Ca2+ channels (300-600 nm spacing).
- A small fraction of granules (~10%) likely experience higher [Ca2+]i due to proximity to Ca2+ channels.
- Model predictions align with observations of synchronous release by single action potentials and desynchronized release with increased yield by trains of action potentials.
Conclusions:
- Ca2+ redistribution and buffering are critical factors delaying catecholamine secretion.
- The spatial distribution of Ca2+ channels and granules significantly influences the kinetics of exocytosis.
- Ca2+ dynamics not only trigger but also modulate the secretory response, with prior stimulation affecting subsequent release.