Ca2+ responses of pulmonary arterial myocytes to acute hypoxia require release from ryanodine and inositol trisphosphate receptors in sarcoplasmic reticulum

Wang J, Shimoda LA, Sylvester JT. Ca2+ responses of pulmonary arterial myocytes to acute hypoxia require release from ryanodine and inositol trisphosphate receptors in sarcoplasmic reticulum. Am J Physiol Lung Cell Mol Physiol 303: L161-L168, 2012. First published May 11, 2012; doi:10.1152/ajplung.00348.2011.-In pulmonary arterial smooth muscle cells (PASMC), acute hypoxia increases intracellular Ca2+ concentration ([Ca2+](i)) by inducing Ca2+ release from the sarcoplasmic reticulum (SR) and Ca2+ influx through store-and voltage-operated Ca2+ channels in sarcolemma. To evaluate the mechanisms of hypoxic Ca2+ release, we measured [Ca2+](i) with fluorescent microscopy in primary cultures of rat distal PASMC. In cells perfused with Ca2+-free Krebs Ringer bicarbonate solution (KRBS), brief exposures to caffeine (30 mM) and norepinephrine (300 mu M), which activate SR ryanodine and inositol trisphosphate receptors (RyR, IP3R), respectively, or 4% O-2 caused rapid transient increases in [Ca2+](i), indicating intracellular Ca2+ release. Preexposure of these cells to caffeine, norepinephrine, or the SR Ca2+-ATPase inhibitor cyclopiazonic acid (CPA; 10 mu M) blocked subsequent Ca2+ release to caffeine, norepinephrine, and hypoxia. The RyR antagonist ryanodine (10 mu M) blocked Ca2+ release to caffeine and hypoxia but not norepinephrine. The IP3R antagonist xestospongin C (XeC, 0.1 mu M) blocked Ca2+ release to norepinephrine and hypoxia but not caffeine. In PASMC perfused with normal KRBS, acute hypoxia caused a sustained increase in [Ca2+](i) that was abolished by ryanodine or XeC. These results suggest that in rat distal PASMC 1) the initial increase in [Ca2+](i) induced by hypoxia, as well as the subsequent Ca2+ influx that sustained this increase, required release of Ca2+ from both RyR and IP3R, and 2) the SR Ca2+ stores accessed by RyR, IP3R, and hypoxia functioned as a common store, which was replenished by a CPA-inhibitable Ca2+-ATPase.