Syntillas release Ca2+ at a site different from the microdomain where exocytosis occurs in mouse chromaffin cells.
Spontaneous, short-lived, focal cytosolic Ca2+ transients were found for the first time and characterized in freshly dissociated chromaffin cells from mouse. Produced by release of Ca2+ from intracellular stores and mediated by type 2 and perhaps type 3 ryanodine receptors (RyRs), these transients are quantitatively similar in magnitude and duration to Ca2+ syntillas in terminals of hypothalamic neurons, suggesting that Ca2+ syntillas are found in a variety of excitable, exocytotic cells. However, unlike hypothalamic nerve terminals, chromaffin cells do not display syntilla activation by depolarization of the plasma membrane, nor do they have type 1 RyRs. It is widely thought that focal Ca2+ transients cause "spontaneous" exocytosis, although there is no direct evidence for this view. Hence, we monitored catecholamine release amperometrically while simultaneously imaging Ca2+ syntillas, the first such simultaneous measurements. Syntillas failed to produce exocytotic events; and, conversely, spontaneous exocytotic events were not preceded by syntillas. Therefore, we suggest that a spontaneous syntilla, at least in chromaffin cells, releases Ca2+ into a cytosolic microdomain distinct from the microdomains containing docked, primed vesicles. Ryanodine (100 microM) reduced the frequency of Ca2+ syntillas by an order of magnitude but did not alter the frequency of spontaneous amperometric events, suggesting that syntillas are not involved in steps preparatory to spontaneous exocytosis. Surprisingly, ryanodine also increased the total charge of individual amperometric events by 27%, indicating that intracellular Ca2+ stores can regulate quantal size.
ZhuGe
Ronghua
R
Department of Physiology, and Biomedical Imaging Group, University of Massachusetts Medical School, Worcester, Massachusetts 01655, USA.
DeCrescenzo
Valerie
V
Sorrentino
Vincenzo
V
Lai
F Anthony
FA
Tuft
Richard A
RA
Lifshitz
Lawrence M
LM
Lemos
Jose R
JR
Smith
Corey
C
Fogarty
Kevin E
KE
Walsh
John V
JV
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D020013:Calcium Signaling / Q000502:physiology*; D002478:Cells, Cultured; D019439:Chromaffin Cells / Q000166:cytology / Q000502:physiology*; D005089:Exocytosis / Q000502:physiology*; D021962:Membrane Microdomains / Q000502:physiology* / Q000648:ultrastructure; D051379:Mice; D019837:Ryanodine Receptor Calcium Release Channel / Q000378:metabolism*; D013572:Synaptic Vesicles / Q000502:physiology* / Q000648:ultrastructure
D019837:Ryanodine Receptor Calcium Release Channel; D002118:Calcium