Acute normoxia increases fetal pulmonary artery endothelial cell cytosolic Ca²⁺ via Ca²⁺-induced Ca²⁺ release

To test the hypothesis that an acute increase in O2 tension increases cytosolic calcium ([Ca]i) in fetal pulmonary artery endothelial cells (PAECs) via entry of extracellular calcium and subsequent calcium-induced calcium release (CICR) and nitric oxide release, low-passage PAECs (<10 passages) were isolated from the intralobar pulmonary artery (PA) of fetal sheep and maintained under hypoxic conditions (Po2, 25 Torr). Using the calcium-sensitive dye fura-2, we demonstrated that acute normoxia (Po2 = 120 Torr) increased PAECs [Ca]i by increasing the rate of entry of extracellular calcium. In the presence of either ryanodine or 2-aminoethoxy-diphenylborate (2APB), normoxia did not lead to a sustained increase in PAECs [Ca]i Whole-cell patch clamp studies demonstrated that acute normoxia causes PAEC membrane depolarization. When loaded with the nitric oxide (NO)-sensitive dye, DAF - FM, acute normoxia increased PAEC fluorescence. In PAECs derived from fetal lambs with pulmonary hypertension, an acute increase in O2 tension had no effect on either [Ca]i or NO production. Hypoxia increases loading of acetylcholine-sensitive calcium stores, as hypoxia potentiated the response to acetylcholine We conclude that acute normoxia increases [Ca]i and NO production in normotensive but not hypertensive fetal PAECs via extracellular calcium entry and calcium release from calcium-sensitive intracellular stores.