In nine artificially ventilated rats anaesthetized with Saffan, systemic hypoxia induced a tachycardia followed by a bradycardia, a fall in systemic arterial pressure and an increase in pulmonary artery pressure (PPA) indicating pulmonary vasoconstriction. This increase in PPA was abolished by the adenosine receptor antagonist 8-phenyltheophylline (8-PT, 10 mg kg-1 I.V.). Further, in seven rats the Pa,O2 achieved during hypoxia was greater after 8-PT than before (38 vs. 40 mmHg). We suggest that adenosine makes a major contribution to hypoxia-induced pulmonary vasoconstriction in the rat. The better maintenance of Pa,O2 during hypoxia may reflect improved perfusion of well-ventilated alveoli.

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