Role of Ca2+-ATPase in spontaneous oscillations of cytosolic free Ca2+ in GH(3) rat pituitary cells

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 i s not well characterized owing to a lack of specific inhibitors for a novel thapsigargin (Tg)-insensitive Ca2+-ATPase expressed in these cells. We sho w that spontaneous [Ca2+](i) oscillations in GH(3) cells 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+]( i) oscillations were highly temperature dependent. Decreasing the temperatu re from 22 to 17 degrees C resulted in an increase in the frequency, a redu ction in the amplitude, and large inhibition of [Ca2+](i) oscillations. Fur thermore, the rate of ATP-dependent Ca-45(2+) uptake into GH(3)-derived mic rosomes was greatly reduced at 17 degrees C. The effect of decreased temper atures on extracellular Ca2+ influx was minor because the frequency and amp litude of spontaneous action potentials, which activate L-type Ca2+ channel s, was relatively unchanged at 17 degrees C. These results suggest that in GH(3) secretory cells, Ca2+ influx via L-type Ca2+ channels initiates spont aneous [Ca2+], oscillations, which are then maintained by the combined acti vity of Ca2+-ATPase and Ca2+-induced Ca2+ release from Tg/IP3-insensitive i ntracellular stores.