Kinetics of the secretory response in bovine chromaffin cells following flash photolysis of caged Ca2+.
The kinetics of the secretory response in bovine chromaffin cells following flash photolysis of caged Ca2+ were studied by capacitance (Cm) measurements with millisecond time resolution. After elevation of the internal Ca2+ concentration ([Ca2+]i), Cm rises rapidly with one or more exponentials. The time constant of the fastest component decreases for higher [Ca2+]i (range 3-600 microM) over three orders of magnitude before it saturates at approximately 1 ms. The corresponding maximal rates of secretion can be as fast as 100,000 fF/s or 40,000 vesicles/s. There is a Ca(2+)-dependent delay before Cm rises, which may reflect the kinetics of multiple Ca2+ ions binding to the secretory apparatus. The initial rise in Cm is described by models containing a sequence of two to four single Ca(2+)-binding steps followed by a rate-limiting exocytosis step. The predicted Ca2+ dissociation constant (Kd) of a single Ca(2+)-binding site is between 7 and 21 microM. At [Ca2+]i > 30 microM clear indications of a fast endocytotic process complicate the analysis of the secretory response.
Heinemann
C
C
Department of Membrane Biophysics, Max-Planck-Institute for Biophysical Chemistry, Göttingen, Germany.
Chow
R H
RH
Neher
E
E
Zucker
R S
RS
D000085:Acetates; D000818:Animals; D055592:Biophysical Phenomena; D001703:Biophysics; D002118:Calcium / Q000737:chemistry / Q000378:metabolism* / Q000528:radiation effects; D002417:Cattle; D002614:Chelating Agents; D002838:Chromaffin System / Q000166:cytology / Q000378:metabolism*; D004563:Electrochemistry; D004705:Endocytosis; D005029:Ethylenediamines; D005089:Exocytosis; D066298:In Vitro Techniques; D007700:Kinetics; D008954:Models, Biological; D010782:Photolysis
D000085:Acetates; D002614:Chelating Agents; D005029:Ethylenediamines; C057014:DM-nitrophen; D002118:Calcium