Caffeine induced oscillations of cytosolic Ca2+ in GH(3) 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 GH(3) 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 ain GH(3) 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.