Adrenergic blockade blunts adenosine concentration and coronary vasodilation during hypoxia.

Myocardial hypoxia is thought to be an important stimulus for increasing interstitial adenosine concentration. The adenosine hypothesis of coronary control was investigated during steady-state hypoxia by making measurements of coronary venous and epicardial well adenosine concentrations in adrenergically intact dogs and in animals with alpha- and beta-receptor blockade. In the adrenergically intact group, hypoxia sufficient to lower coronary venous oxygen tension to 8 mm Hg increased coronary blood flow 243% from normoxic values. Both coronary venous and epicardial well adenosine concentrations were increased throughout the hypoxic period. In the adrenergically blocked group, hypoxia to a similar level of coronary venous oxygen tension produced an increase in coronary blood flow of only 75%, which was significantly less than in the adrenergically intact group (p less than 0.01). Coronary venous adenosine was only transiently elevated, and epicardial well adenosine was unchanged from control levels. In a separate group of alpha- and beta-receptor-blocked animals that received an infusion of L-homocysteine thiolactone during hypoxia, there was no difference in tissue S-adenosylhomocysteine levels compared with those of normoxic controls. It is concluded that much of the coronary vasodilation associated with systemic hypoxia is dependent on adrenergic activation and that adenosine may only play a role in sustained hypoxic vasodilation when adrenergic receptors are intact.