title: A caffeine-sensitive Ca2+ store modulates K+-evoked secretion in chromaffin cells.
abstract: Catecholamine release from bovine adrenal medulla chromaffin cells superfused with a Krebs-N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic 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 Ca2+ (70 K+/2 Ca2+) in cells superfused with a Krebs-N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid solution containing 0 mM Ca2+ + 0.5 mM EGTA; blockade reached 80% at the third 70 K+/2 Ca2+ 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 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 an intracellular Ca2+ 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 Ca2+ 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 a store might act as a sink for the external Ca2+ entering through Ca2+ channels during cell depolarization, thus decreasing the intracellular Ca2+ concentration available for exocytosis.
authors.affiliation: Departamento de Farmacología y Terapéutica, Facultad de Medicina, Universidad Autónoma de Madrid, Spain.
authors.forename: B
authors.initials: B
authors.lastname: Lara
authors.forename: M G
authors.initials: MG
authors.lastname: López
authors.forename: M
authors.initials: M
authors.lastname: Villarroya
authors.forename: L
authors.initials: L
authors.lastname: Gandía
authors.forename: L
authors.initials: L
authors.lastname: Cleeman
authors.forename: M
authors.initials: M
authors.lastname: Morad
authors.forename: A G
authors.initials: AG
authors.lastname: García
mesh_terms: D000818:Animals; D002110:Caffeine / Q000494:pharmacology*; D002118:Calcium / Q000378:metabolism / Q000502:physiology*; D015220:Calcium Channels / Q000378:metabolism / Q000502:physiology; D002395:Catecholamines / Q000378:metabolism; D002417:Cattle; D002478:Cells, Cultured; D019439:Chromaffin Cells / Q000187:drug effects* / Q000378:metabolism* / Q000502:physiology; D007425:Intracellular Membranes / Q000378:metabolism; D018408:Patch-Clamp Techniques; D011188:Potassium / Q000494:pharmacology*; D013997:Time Factors
chemical_list: D015220:Calcium Channels; D002395:Catecholamines; D002110:Caffeine; D011188:Potassium; D002118:Calcium