Attenuation of hypoxic pulmonary vasoconstriction by verapamil in intact dogs

The hypothesis that hypoxic pulmonary vasoconstriction is mediated directly by depolarization of the vascular smooth muscle was tested in anesthetized dogs. Pulmonary vascular responses to hypoxia were first determined in eight dogs during 20-min exposures to 10% O2. Each animal was then treated with verapamil (0.5 mg/kg, iv), to block transmembrane Ca2+ influx in an attempt to abolish the vasoconstrictor responses to hypoxia. The hypoxic exposures were then repeated, and the pulmonary vascular responses were compared to the control responses. Verapamil administration attenuated hypoxic pulmonary vasoconstriction, but did not abolish the responses to hypoxia. Pulmonary vascular resistance increased 87% during the control hypoxic exposure, but increased only 38% during hypoxia after verapamil. The response to another vasoconstrictor, prostaglandin F2alpha, was not reduced by verapamil indicating a different mechanism of mediation. These results suggest that the pulmonary vasoconstrictor response to alveolar hypoxia, in the intact dog, involves transmembrane Ca2+ influx, and are consistent with the idea that hypoxia acts primarily by directly depolarizing vascular smooth muscle, rather than acting indirectly through a chemical mediator.