Caffeine-induced oscillations of cytosolic Ca2+ in GH3 pituitary cells are not due to Ca2+ release from intracellular stores but to enhanced Ca2+ influx through voltage-gated Ca2+ channels.
Caffeine, a well known facilitator of Ca2+-induced Ca2+ release, induced oscillations of cytosolic free Ca2+ ([Ca2+]i) in GH3 pituitary cells. These oscillations were dependent on the presence of extracellular Ca2+ and blocked by dihydropyridines, suggesting that they are due to Ca2+ entry through L-type Ca2+ channels, rather than to Ca2+ release from the intracellular Ca2+ stores. Emptying the stores by treatment with ionomycin or thapsigargin did not prevent the caffeine-induced [Ca2+]i oscillations. Treatment with caffeine occluded phase 2 ([Ca2+]i oscillations) of the action of thyrotropin-releasing hormone (TRH) without modifying phase 1 (Ca2+ release from the intracellular stores). Caffeine also inhibited the [Ca2+]i increase induced by depolarization with high-K+ solutions (56% at 20 mM), suggesting direct inhibition of the Ca2+ entry through voltage-gated Ca2+ channels. We propose that the [Ca2+]i increase induced by caffeine in GH3 cells takes place by a mechanism similar to that of TRH, i.e. membrane depolarization that increases the firing frequency of action potentials. The increase of the electrical activity overcomes the direct inhibitory effect on voltage-gated Ca2+ channels with the result of increased Ca2+ entry and a rise in [Ca2+]i. Consideration of this action cautions interpretation of previous experiments in which caffeine was assumed to increase [Ca2+]i only by facilitating the release of Ca2+ from intracellular Ca2+ stores.
Villalobos
C
C
Instituto de Biología y Genética Molecular, Universidad de Valladolid and Consejo Superior de Investigaciones Científicas, Departamento de Bioquímica y Biología Molecular y Fisiología, Facultad de Medicina, E-47005 Valladolid, Spain.
García-Sancho
J
J
D000818:Animals; D002110:Caffeine / Q000494:pharmacology*; D002118:Calcium / Q000032:analysis / Q000378:metabolism* / Q000502:physiology*; D015220:Calcium Channels / Q000502:physiology*; D002460:Cell Line; D003600:Cytosol / Q000737:chemistry*; D004791:Enzyme Inhibitors / Q000494:pharmacology; D007424:Intracellular Fluid / Q000737:chemistry; D015640:Ion Channel Gating / Q000502:physiology; D015759:Ionomycin / Q000494:pharmacology; D010726:Phosphodiesterase Inhibitors / Q000494:pharmacology; D010903:Pituitary Gland, Anterior / Q000737:chemistry / Q000166:cytology* / Q000648:ultrastructure; D051381:Rats; D013729:Terpenes / Q000494:pharmacology; D019284:Thapsigargin; D013806:Theophylline / Q000494:pharmacology
D015220:Calcium Channels; D004791:Enzyme Inhibitors; D010726:Phosphodiesterase Inhibitors; D013729:Terpenes; D002110:Caffeine; D015759:Ionomycin; D019284:Thapsigargin; D013806:Theophylline; D002118:Calcium