A caffeine-sensitive ca2& store modulates &-evoked
secretion in chromaffin cells.
Lara, Baldomero, Manuela G. L[acute]opez, Mercedes Villarroya, Luis
Gand[acute]ia, Lars Cleeman, Martin Morad, and Antonio G.
Garc[acute]ia.
Departamento de Farmacolog[acute]ia y Terap[acute]eutica, Facultad
de Medicina, Universidad Aut[acute]onoma de Madrid, Arzobispo
Morcillo, 4, 28029 Madrid, Spain
APStracts 3:0378C, 1996.
Catecholamine release from bovine adrenal medulla chromaffin cells
superfused with a Krebs-HEPES solution was monitored on-line with an
electrochemical detector. Caffeine (10 mM) progressively depressed
the magnitude of secretory responses to depolarizing pulses of 70 mM
K& and 2 mM Ca2& ( 70K&/2Ca2&), in cells superfused
with a 0Ca2&/EGTA Krebs-HEPES solution; blockade reached 80% at
the third 70K&/2Ca2& challenge given in the presence of
caffeine. A similar effect was obtained when instead of continuous
superfusion, prepulses of caffeine were applied (10 mM for 60 s). The
blocking effects of caffeine on K&-induced secretion depended on
the time of exposure to the drug; the longer the exposure time the
greater the blockade. The recovery of the K& secretory responses
previously impaired by caffeine was always gradual and followed a
staircase mode. This contrasts with the behaviour of caffeine on
various parameters measuring Ca2& entry through Ca2&
channels, which did not parallel its effects on K&-evoked
secretion. The secretion data, however, are compatible with the
disappearance and recovery of a [Ca2&]i signal triggered by
K& in single chromaffin cells loaded with fura-2, and treated
with 10 mM caffeine. Thus, contrary to previous views, the depression
of secretion by caffeine does not seem to be associated to inhibition
of Cao2& entry through Ca2& channels. These functional data
are rather compatible with the view that the degree of filling of a
caffeine-sensitive intracellular Ca2& store might regulate the
extent of exocytosis. When emptied, such store might act as a sink
for the external Ca2& entering through Ca2& channels during
cell depolarization, thus decreasing the [Ca2&]i available for
exocytosis.
Received 13 June 1996; accepted in final form 11 November 1996.
APS Manuscript Number C340-6.
Article publication pending Am. J. Physiol. (Cell Physiology).
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 31 December 1996