Ep. Seward et al., BA2-TERMINALS( IONS EVOKE 2 KINETICALLY DISTINCT PATTERNS OF EXOCYTOSIS IN CHROMAFFIN CELLS, BUT NOT IN NEUROHYPOPHYSEAL NERVE), The Journal of neuroscience, 16(4), 1996, pp. 1370-1379
The coupling between divalent cations and exocytosis of large dense-co
red vesicles (LDCV) was studied with capacitance-detection techniques
in nerve terminals of the rat neurohypophysis (NHP) and bovine chromaf
fin cells, Ba2+ substitution for Ca2+ produced kinetically distinct re
sponses in the two preparations.In NHP terminals, Ba2+ ions behave as
weak substitutes for Ca2+, Exocytotic events occur principally during
depolarizing pulses, i.e., events are ''stimulus-coupled'' to Ba2+ ent
ry through voltage-gated Ca2+ channels, Stimulus-coupled exocytosis ap
parently requires elevated submembrane cation concentrations that diss
ipate rapidly on hyperpolarization-induced Ca2+-channel closure. Intra
cellular dialysis of NHP terminals with Ba2+ does not evoke exocytosis
, nor does it interfere with depolarization-evoked Ca2+ influx and exo
cytosis. In chromaffin cells, Ba2+ ions evoke a small quantity of stim
ulus-coupled secretion, but the dominant response is an additional pro
nounced poststimulus capacitance increase that outlasts channel closur
es by 20-50 sec, ''Stimulus-decoupled'' exocytosis is slow (similar to
25-40 fF/sec) compared with Ca2+-evoked stimulus-coupled exocytosis (
similar to 1000 fF/sec), Decoupled secretion is not attributable to Ba
2+ displacement of intracellular Ca2+ ions, because it is insensitive
to 10 mM EGTA or thapsigargin, Slow exocytosis is initiated by inclusi
on of Ba2+ ions in the recording pipette and continues steadily for 5-
12 min, producing a total increase of several thousand fF, which ultim
ately doubles or triples the original cell-surface area, We propose th
at two pathways of regulated exocytosis with distinct kinetics and div
alent cation sensitivity exist in chromaffin cells, Only a single kine
tic pattern is detected in NHP terminals, suggesting that mechanisms f
or secretion are not universally distributed in excitable cells.