TIMING OF DENSE-CORE VESICLE EXOCYTOSIS DEPENDS ON THE FACILITATION L-TYPE CA CHANNEL IN ADRENAL CHROMAFFIN CELLS

Secretion from dense-core vesicles is reputedly much slower than that from typical synaptic vesicles, possibly because of noncolocalization of Ca channels and release sites. We reinvestigated this question by m easuring the kinetics of catecholamine release in chromaffin cells fro m calf and adult bovines. Amperometric recording from calf chromaffin cells stimulated by action potentials exhibited two latencies of secre tion that depended on both the frequency of stimulation and the pathwa y of Ca entry. Short-latency responses (<25 msec delay; ''strongly cou pled'') appeared at low (0.25 and 1 Hz) and high (7 Hz) frequencies an d were entirely dependent on recruitment of ''facilitation'' L-type Ca channels as revealed by nisoldipine blockade. Long-latency responses (>25 msec delay; ''weakly coupled'') were more apparent at higher freq uencies (7 Hz) and were substantially reduced by toxins that blocked N - and P-type Ca channels. Ca current recordings revealed that adult bo vine chromaffin cells lack facilitation channels; virtually all secret ion was weakly coupled in these cells. The mean delay of the strongly coupled signal was similar to 3 msec after the peak of the action pote ntial (at 24 degrees C), indicating that dense-core vesicles can exhib it a rate of exocytosis approaching that occurring in neurons. Althoug h other explanations are possible, these results are consistent with t he idea that facilitation Ca channels are colocalized with release sit es in calf chromaffin cells. Calculations based on a model incorporati ng this assumption suggest that these channels must be within 13 nm of secretory sites to account for such rapid exocytosis.