A caffeine-sensitive Ca2+ store modulates K+-evoked secretion in chromaffin cells
Lara, B.; López, M.G.; Villarroya, M.; Gandía, L.; Cleeman, L.; Morad, M.; García, A.G.
American Journal of Physiology 272(4 Pt 1): C1211-C1221
1997
ISSN/ISBN: 0002-9513 PMID: 9142846 Document Number: 481166
Catecholamine release from bovine adrenal medulla chromaffin cells superfused with a Krebs-N-2-hydroxyethylpiperazine-N'-2-ethane-sulfonic acid 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 Ca-2+ (70 K+/2 Ca-2+) in cells superfused with a Krebs-N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid solution containing 0 mM Ca-2+ + 0.5 mM EGTA; blockade reached 80% at the third 70 K+/2 Ca-2+ 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 effects of caffeine on various parameters measuring Ca-2+ entry through Ca-2+ channels, which did not parallel its effects on K+-evoked secretion. The secretion data, however, are compatible with the disappearance and recovery of an intracellular Ca-2+ concentration 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 with inhibition of extracellular Ca-2+ entry through Ca-2+ channels. These functional data are, rather, compatible with the view that the degree of filling of a caffeine-sensitive intracellular Ca-2+ store might regulate the extent of exocytosis. When emptied, such a store might act as a sink for the external Ca-2+ entering through Ca-2+ channels during cell depolarization, thus decreasing the intracellular Ca-2+ concentration available for exocytosis.