Ep. Seward et Mc. Nowycky, KINETICS OF STIMULUS-COUPLED SECRETION IN DIALYZED BOVINE CHROMAFFIN CELLS IN RESPONSE TO TRAINS OF DEPOLARIZING PULSES, The Journal of neuroscience, 16(2), 1996, pp. 553-562
Stimulus-secretion coupling in bovine chromaffin cells was investigate
d with whole-cell patch-clamp recordings and capacitance detection tec
hniques to monitor exocytosis in response to trains of depolarizing pu
lses. Two kinetically discrete modes of exocytotic responses were obse
rved. In one mode, the first depolarization of a train elicited a larg
e increase in membrane capacitance (C-m; mean similar to 70 fF). This
secretory mode was characterized by small Ca2+ requirements, relative
insensitivity to the pipette Ca2+ chelator concentration, and rapid de
pletion of the secretory response. This mode of stimulus-secretion cou
pling was labile and was seen only in response to the first and, occas
ionally, the second stimulus train of whole-cell recordings. The secon
d type of exocytotic response persisted for the remainder of the whole
-cell recordings and consisted of two distinct phases. During the earl
iest pulses of a stimulus train, Ca2+ entry did not evoke C-m increase
s. Instead, C-m responses were elicited by later pulses, despite dimin
ished Ca2+ entry per pulse caused by Ca2+ channel inactivation. The se
cretory phase was initiated after a specific ''threshold'' amount of C
a2+ had entered the cell, which was determined by the concentration, b
ut not the binding kinetics, of the Ca2+ chelator in the pipette. In b
oth the early and the secretory phases, the response of the cell was p
roportional to cumulative Ca2+ entry, regardless of current amplitude,
pulse duration, or number of pulses. Threshold-type secretory kinetic
s has been described previously in peptide-secreting neurohypophysial
(NHP) nerve terminals (Seward et al., 1995). Secretory kinetics with m
inimal Ca2+ requirements has not been observed in that preparation. Ch
romaffin cells appear to possess a broader repertoire of stimulus-secr
etion coupling modes than NHP terminals.