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Influence of Quantal Size and cAMP on the Kinetics of Quantal Catecholamine Release from Rat Chromaffin Cells

Kim San Tang ^*, Nan Wang ^* , Amy Tse ^*  and Frederick W. Tse ^* 

^* Department of Pharmacology and Centre for Neuroscience, University of Alberta, Edmonton, Alberta, Canada

Correspondence: Address reprint requests to Frederick W. Tse, 9-70 Medical Sciences Building, Dept. of Pharmacology, University of Alberta, Edmonton, Alberta, Canada T6G 2H7. E-mail: fred.tse{at}ualberta.ca.

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.