CAFFEINE-INDUCED OSCILLATIONS OF CYTOSOLIC CA2-CELLS ARE NOT DUE TO CA2+ RELEASE FROM INTRACELLULAR STORES BUT TO ENHANCED CA2+ INFLUX THROUGH VOLTAGE-GATED CA2+ CHANNELS( IN GH(3) PITUITARY)

Caffeine, a well known facilitator of Ca2+-induced Ca2+ release: induc ed oscillations of cytosolic free Ca2+ ([Ca2+](i)) in GH(3) pituitary cells. These oscillations were dependent on the presence of extracellu lar Ca2+ and blocked by dihydropyridines, suggesting that they are due to Ca2+ entry through L-type Ca2+ channels, rather than to Ca2+ relea se from the intracellular Ca2+ stores. Emptying the stores by treatmen t 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 (TR H) without modifying phase 1 (Ca2+ release from the intracellular stor es). Caffeine also inhibited the [Ca2+](i) increase induced by depolar ization with high-K+ solutions (56% at 20 mM), suggesting direct inhib ition of the Ca2+ entry through voltage-gated Ca2+ channels. We propos e that the [Ca2+](i) increase induced by caffeine ain GH(3) cells take s place by a mechanism similar to that of TRH, i.e. membrane depolariz ation that increases the firing frequency of action potentials. The in crease of the electrical activity overcomes the direct inhibitory effe ct on voltage-gated Ca2+ channels with the result of increased Ca2+ en try and a rise in [Ca2+](i). Consideration of this action cautions int erpretation of previous experiments in which caffeine was assumed to i ncrease [Ca2+](i) only by facilitating the release of Ca2+ from intrac ellular Ca2+ stores.