Role of Ca2+-ATPase in spontaneous oscillations of cytosolic free Ca2+in GH3rat pituitary cells

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The relative contribution of voltage-sensitive Ca2+channels, Ca2+-ATPases, and Ca2+release from intracellular stores to spontaneous oscillations in cytosolic free Ca2+concentration ([Ca2+]i) observed in secretory cells is not well characterized owing to a lack of specific inhibitors for a novel thapsigargin (Tg)-insensitive Ca2+-ATPase expressed in these cells. We show that spontaneous [Ca2+]ioscillations in GH3cells were unaffected by Ca2+depletion in inositol-1,4,5-trisphosphate (IP3)-sensitive Ca2+stores by the treatment of Tg, but could be initiated by application of caffeine. Moreover, we demonstrate for the first time that these spontaneous [Ca2+]ioscillations were highly temperature dependent. Decreasing the temperature from 22 to 17°C resulted in an increase in the frequency, a reduction in the amplitude, and large inhibition of [Ca2+]ioscillations. Furthermore, the rate of ATP-dependent45Ca2+uptake into GH3-derived microsomes was greatly reduced at 17°C. The effect of decreased temperatures on extracellular Ca2+influx was minor because the frequency and amplitude of spontaneous action potentials, which activate L-type Ca2+channels, was relatively unchanged at 17°C. These results suggest that in GH3secretory cells, Ca2+influx via L-type Ca2+channels initiates spontaneous [Ca2+]ioscillations, which are then maintained by the combined activity of Ca2+-ATPase and Ca2+-induced Ca2+release from Tg/IP3-insensitive intracellular stores.