Oxygen-induced fetal pulmonary vasodilation is mediated by intracellular calcium activation of K_Ca channels

O₂ sensing in fetal pulmonary artery smooth muscle is critically important in the successful transition to air breathing at birth. However, the mechanism by which the fetal pulmonary vasculature senses and responds to an acute increase in O₂ tension is not known. Isolated fetal pulmonary artery smooth muscle cells were kept in primary culture for 5-14 days in a hypoxic environment (20-30 mmHg). These cells showed a 25.1 ± 1.7% decrease in intracellular calcium in response to an acute increase in O₂ tension. Low concentrations of caffeine (0.5 mM) and diltiazem also decreased intracellular calcium. The decrease in intracellular calcium concentration in response to increasing O₂ was inhibited by iberiotoxin and ryanodine. Freshly isolated fetal pulmonary artery smooth muscle cells exhibited "spontaneous transient outward currents," indicative of intracellular calcium spark activation of calciumsensitive potassium channels. The frequency of spontaneous transient outward currents increased when O₂ tension was increased to normoxic levels. Increasing fetal pulmonary O₂ tension in acutely instrumented fetal sheep increased fetal pulmonary blood flow. Ryanodine attenuated O₂-induced pulmonary vasodilation. This study demonstrates that fetal pulmonary vascular smooth muscle cells are capable of responding to an acute increase in O₂ tension and that this O₂ response is mediated by intracellular calcium activation of calcium-sensitive potassium channels.