The action of hypercapnia during hypoxia on pulmonary vessels

Two opposing actions of CO2 on pulmonary vessels, vasoconstriction and vasodilatation, were studied in dogs, cats, ferrets and rats using isolated lungs or a lobe of lung in vivo. Both preparations were perfused at a constant flow rate so that changes in inflow pressure at constant outflow pressure represented changes in vascular resistance. Lungs were ventilated with 5-15 percent CO2 during hypoxic vasoconstriction in order to permit demonstration of dilatation and because the two stimuli frequently concur. During hypoxia CO2 caused further constriction, dilatation or a biphasic response. Only constriction occurred in dogs and only dilatation in rats. Constriction predominated in cats but dilatation sometimes occurred in later tests in isolated lungs and after a carbonic anhydrase inhibitor. All three responses were seen in ferrets. The type of response did not depend on CO2 concentration. In rats dilatation turned to constriction after beta-adrenoreceptor blocking drugs. The vessels affected by dilatation could not be determined. Thus CO2 can enhance or diminish the action of hypoxia but the circumstances determining one or other action are not resolved.