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 measuring the kinetics of catecholamine release in chromaffin cells from calf and adult bovines. Amperometric recording from calf chromaffin cells stimulated by action potentials exhibited two latencies of secretion that depended on both the frequency of stimulation and the pathway of Ca entry. Short-latency responses (<25 msec delay; "strongly coupled") appeared at low (0.25 and 1 Hz) and high (7 Hz) frequencies and 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 frequencies (7 Hz) and were substantially reduced by toxins that blocked N- and P-type Ca channels. Ca current recordings revealed that adult bovine chromaffin cells lack facilitation channels; virtually all secretion was weakly coupled in these cells. The mean delay of the strongly coupled signal was approximately 3 msec after the peak of the action potential (at 24 degreesC), indicating that dense-core vesicles can exhibit a rate of exocytosis approaching that occurring in neurons. Although other explanations are possible, these results are consistent with the idea that facilitation Ca channels are colocalized with release sites in calf chromaffin cells. Calculations based on a model incorporating this assumption suggest that these channels must be within 13 nm of secretory sites to account for such rapid exocytosis.