Ks. Kits et al., Diffusion barriers limit the effect of mobile calcium buffers on exocytosis of large dense cored vesicles, BIOPHYS J, 76(3), 1999, pp. 1693-1705
Fast exocytosis in melanotropic cells, activated by calcium entry through v
oltage-gated calcium channels, is very sensitive to mobile calcium buffers
(complete block at 800 mu M ethylene glycol bis(beta-aminoethyl ether)-N,N,
N'N'-tetraacetic acid (EGTA)). This indicates that calcium diffuses a subst
antial distance from the channel to the vesicle. Surprisingly, 1,2-bis(2-am
inophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA), having a similar K-D
for calcium as EGTA but a similar to 100 times faster binding rate, blocked
exocytosis only twice as effectively as EGTA. Using computer simulations,
we demonstrate that this result cannot be explained by free diffusion and b
uffer binding rates. We hypothesized that local saturation of calcium buffe
rs is involved. A diffusion barrier for both calcium and buffer molecules,
located 50-300 nm from the membrane and reducing diffusion 1000 to 10,000 t
imes, generated similar calcium concentrations for specific concentrations
of EGTA and BAPTA. With such barriers, calcium rise phase kinetics upon sho
rt step depolarizations (2-20 ms) were faster for EGTA than for BAPTA, impl
ying that short depolarizations should allow exocytosis with 50 mu M EGTA b
ut not with 25 mu M BAPTA. This prediction was confirmed experimentally wit
h capacitance measurements. Coupling exocytosis to calcium dynamics in the
model, we found that a barrier with a similar to 3000 times reduced diffusi
on at similar to 130 nm beneath the membrane best explains the experimental
ly observed effects of EGTA and BAPTA on block and kinetics of release.