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 (C-m) measurements with millisecond time resolution. After elevation of the internal Ca2+ concentration ([Ca2+](i)), C-m rises rapidly with one or more exponentials. The time constant of the fastest component decreases for higher [Ca2+](i) (range 3-600 mu M) over three orders of magnitude before it saturates at similar to 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 Ca2+-dependent delay before C-m rises, which may reflect the kinetics of multiple Ca2+ ions binding to the secretory apparatus. The initial rise in C-m is described by models containing a sequence of two to four single Ca2+-binding steps followed by a rate-limiting exocytosis step. The predicted Ca2+ dissociation constant (K-d) of a single Ca2+-binding site is between 7 and 21 mu M. At [Ca2+](i) > 30 mu M clear indications of a fast endocytotic process complicate the analysis of the secretory response.