Ca²⁺ release from intracellular stores is an initial step in hypoxic pulmonary vasoconstriction of rat pulmonary artery resistance vessels

Background: A reduction in oxygen tension in the lungs is believed to inhibit a voltage-dependent K⁺ (Kv) current, which is thought to result in membrane depolarization leading to hypoxic pulmonary vasoconstriction (HPV). However, the direct mechanism by which hypoxia inhibits Kv current is not understood. Methods and Results: Experiments were performed on rat pulmonary artery resistance vessels and single smooth muscle cells isolated from these vessels to examine the role of Ca²⁺ release from intracellular stores in initiating HPV. In contractile experiments, hypoxic challenge of endothelium- denuded rat pulmonary artery resistance vessels caused either a sustained or transient contraction in Ca²⁺ -containing or Ca²⁺ -free solution, respectively (n=44 vessels from 11 animals). When the ring segments were treated with either thapsigargin (5 μmol/L), ryanodine (5 μmol/L), or cyclopiazonic acid (5 μmol/L) in Ca²⁺ -containing or Ca²⁺ -free solution, a significant increase in pulmonary arterial tone was observed (n=44 vessels from 11 animals). Subsequent hypoxic challenge in the presence of each agent produced no further increase in tone (n=44 vessels from 11 animals). In isolated pulmonary resistance artery cells loaded with fura 2, hypoxic challenge, thapsigargin, ryanodine, and cyclopiazonic acid resulted in a significant increase in [Ca²⁺](i) (n=18 cells from 6 animals) and depolarization of the resting membrane potential (n=22 cells from 6 animals). However, with prior application of thapsigargin, ryanodine, or cyclopiazonic acid, a hypoxic challenge produced no further change in [Ca²⁺](i) (n=18 from 6 animals) or membrane potential (h=22 from 6 animals). Finally, application of an anti-Kv1.5 antibody increased [Ca²⁺](i) and caused membrane depolarization. Subsequent hypoxic challenge resulted in a further increase in [Ca²⁺](i) with no effect on membrane potential (n=16 cells from 4 animals). Conclusions: In rat pulmonary artery resistance vessels, an initial event in HPV is a release of Ca²⁺ from intracellular stores. This rise in [Ca²⁺](i) causes inhibition of voltage-dependent K⁺ channels (possibly Kv1.5), membrane depolarization, and an increase in pulmonary artery tone.