Comparison of the effects of minoxidil and nifedipine on hypoxic pulmonary vasoconstriction in dogs.

We studied the effects of hypoxia on pulmonary hemodynamics and gas exchange before and after minoxidil or nifedipine administration. The studies were carried out in eight normal dogs to determine the relative effects of these two drugs in inhibiting hypoxic pulmonary vasoconstriction. Each dog was studied twice--once with each drug--and exposed repeatedly to hypoxia on each day. Minoxidil raised cardiac output from 2.84 +/- 0.19 to 4.67 +/- 0.29 L/min (mean +/- SE), whereas nifedipine raised it from 3.04 +/- 0.25 to 4.38 +/- 0.38 L/min. Prior to the drugs, hypoxia raised pulmonary vascular resistance 2.4-fold. Minoxidil abolished this pressor response, while nifedipine reduced this response to a 1.6-fold increase. Minoxidil raised venous admixture during normoxia from 6 +/- 1 to 16 +/- 4%, while nifedipine raised it from 7 +/- 1 to 11 +/- 2%. Both drugs are inhibitors of the vasoconstrictive response to hypoxia. At the doses used, minoxidil was a more potent inhibitor of the pressor response to hypoxia than was nifedipine.