Naloxone: effects on hypoxic pulmonary vasoconstriction

To determine the effect of naloxone on the hypoxic pulmonary vasoconstrictive response, six mongrel dogs were rendered hypoxic with 10% oxygen and were given either saline or naloxone. Following hypoxia all dogs had significant increases in mean pulmonary artery pressure (PAP) and pulmonary arterial resistance index (PARI) without changes in cardiac output or systemic blood pressure. Beta endorphins did not change at any time following hypoxia. Dogs receiving naloxone had significant lowering of PAP and PARI without changes in plasma beta endorphin levels. We conclude that naloxone attenuates hypoxic pulmonary vasoconstriction without measurable alterations of plasma beta endorphin levels.