Hypoxic pulmonary vasoconstriction (HPV) requires influx of extracellular Ca2+ in pulmonary arterial smooth muscle cells (PASMCs). To determine whether capacitative Ca2+ entry (CCE) through store-operated Ca2+ channels (SOCCs) contributes to this influx, we used fluorescent microscopy and the Ca2+-sensitive
dye fura-2 to measure effects of 4% O2 on intracellular [Ca2+] ([Ca2+]i) and CCE in primary cultures of PASMCs from rat distal pulmonary arteries. In PASMCs perfused with Ca2+-free Krebs Ringer
bicarbonate solution (KRBS) containing
cyclopiazonic acid to deplete Ca2+ stores in sarcoplasmic reticulum and
nifedipine to prevent Ca2+ entry through L-type voltage-operated Ca2+ channels (VOCCs),
hypoxia markedly enhanced both the increase in [Ca2+]i caused by restoration of extracellular [Ca2+] and the rate at which extracellular Mn2+ quenched
fura-2 fluorescence. These effects, as well as the increased [Ca2+]i caused by
hypoxia in PASMCs perfused with normal
salt solutions, were blocked by the SOCC antagonists
SKF-96365, NiCl2, and LaCl3 at concentrations that inhibited CCE >80% but did not alter [Ca2+]i responses to 60 mM KCl. In contrast, the VOCC antagonist
nifedipine inhibited [Ca2+]i responses to
hypoxia by only 50% at concentrations that completely blocked responses to KCl. The increased [Ca2+]i caused by
hypoxia was completely reversed by perfusion with Ca2+-free KRBS. LaCl3 increased basal [Ca2+]i during normoxia, indicating effects other than inhibition of SOCCs. Our results suggest that acute
hypoxia enhances CCE through SOCCs in distal PASMCs, leading to depolarization, secondary activation of VOCCs, and increased [Ca2+]i. SOCCs and CCE may play important roles in HPV.