Time course of Ca2+ concentration triggering exocytosis in neuroendocrine cells.
We have used the secretory response of chromaffin cells to estimate the submembrane intracellular Ca2+ concentration ([Ca2+]i) "seen" by secretory granules during short depolarizations. The rate of secretion during a depolarization was assessed by combining the electrochemical method of amperometry and electrical capacitance measurements. The rate was then related to [Ca2+]i based on a previous characterization of how Ca2+ affects the dynamics of vesicle priming and fusion in chromaffin cells [Heinemann, C., Chow, R. H., Neher, E. & Zucker, R. S. (1994) Biophys. J. 67, in press]. Calculated [Ca2+]i rose during the depolarization to a peak of < 10 microM, then decayed over tens of milliseconds. In synapses, vesicles are presumed to be located within nanometers of Ca2+ channels where [Ca2+]i is believed to rise in only microseconds to near steady-state levels of hundreds of micromolar. Channel closure should lead to a decrease in [Ca2+]i also in microseconds. Our findings of the slower time course and the lower peak [Ca2+]i suggest that in chromaffin cells, unlike synapses, Ca2+ channels and vesicles are not strictly colocalized. This idea is consistent with previously published data on dense-core vesicle secretion from diverse cell types.
Chow
R H
RH
Department of Membrane Biophysics, Max Planck Institute for Biophysical Chemistry, Göttingen, Germany.
Klingauf
J
J
Neher
E
E
D000313:Adrenal Medulla / Q000378:metabolism*; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002417:Cattle; D005089:Exocytosis; D066298:In Vitro Techniques; D008561:Membrane Fusion; D008564:Membrane Potentials; D013997:Time Factors
D002118:Calcium