Mechanism of spontaneous intracellular calcium fluctuations in single GH sub(4)C sub(1) rat pituitary cells

Individual unstimulated GH sub(4)C sub(1) cells exhibited spontaneous dynamic fluctuations in cytosolic free Ca super(2+) concentration ([Ca super(2+)] sub(i)). Either chelation of extracellular Ca super(2+) with EGTA or treatment with nifedipine inhibited spontaneous [Ca super(2+)] sub(i) fluctuations, indicating that the [Ca super(2+)] sub(i) profile was dependent on the entry of extracellular Ca super(2+) via voltage-operated Ca super(2+) channels (VOCC). Spontaneous [Ca super(2+)] sub(i) fluctuations did not resume immediately after exposure of EGTA-pretreated cells to extracellular Ca super(2+), supporting the hypothesis that the complex [Ca super(2+)] sub(i) profiles observed in unstimulated cells required filling of an intracellular Ca super(2+) pool. BAY K 8644 elicited large rapid oscillations in [Ca super(2+)] sub(i). After chelation of extracellular Ca super(2+), however, re-addition of Ca super(2+) plus BAY K 8644 did not result in [Ca super(2+)] sub(i) oscillations. The intracellular Ca super(2+) pool necessary for BAY K-induced oscillations was not the same Ins(1,4,5)P sub(3)-sensitive pool stimulated by thyrotropin-releasing hormone (TRH), because the TRH-stimulated Ins(1,4,5)P sub(3)-induced [Ca super(2+)] sub(i) spike and the BAY K 8644-induced oscillations were differentially sensitive to chelation of extracellular Ca super(2+) and thapsigargin. Caffeine caused an increase in [Ca super(2+)] sub(i) fluctuations in quiescent cells, supporting a role for Ca super(2+)-induced Ca super(2+) release (CICR) in the generation of spontaneous [Ca super(2+)] sub(i) fluctuations.