Influence of quantal size and cAMP on the kinetics of quantal catecholamine release from rat chromaffin cells

Using carbon fiber amperometry, we exploited the natural variation in quantal size (Q) among individual granules in rat chromaffin cells to examine the influence of Q on quantal release kinetics. Although it is generally accepted that granules with larger Q have slower kinetics of release, we found that this trend was applicable only to granules with Q(1/3) < 0.6 pC(1/3). Granules with larger Q adapted specific mechanisms to maintain a rapid kinetic of release. The semistable fusion pores in the large-Q granules persisted for a longer duration and could reach a bigger size before the onset of very rapid dilation to allow a longer and larger foot signal. Most importantly, a large proportion of large-Q granules maintained a relatively short half-width in the main spike. This suggests that the most rapid phase of fusion pore dilation in many large-Q granules may be faster than that in small-Q granules. Moreover, cAMP selectively advanced the onset of the rapid dilation of the fusion pore in the large- but not the small-Q granules. Thus, our finding raises the possibility that fusion pore and/or granule matrix in small- and large-Q granules may have different molecular structures.