THE EFFECTS OF HYPOXIA AND HYPERCARBIA ON MYOCARDIAL CATECHOLAMINES

¹ Cardiology Division, Stanford University School of Medicine, Palo Alto, California

The responses of the cardiac sympathetic nervous system to hypoxia and hypercarbia were studied in adrenalectomized and nonadrenalectomized rats. Myocardial catecholamine concentrations were determined at varying times after the rate were first given -methyltyrosine, then made to breathe various concentrations of oxygen and carbon dioxide. In adrenalectomized rats, 20% CO₂ and 6% oxygen caused increased loss of cardiac norepinephrine, whereas 10% CO1 and 8% oxygen caused no increased norepinephrine loss when compared with controls breathing room air. In nonadrenalectomized rats, 6% oxygen produced decreased cardiac norepinehrine levels, whereas 20% CO₂ had no effect on total myocardial catecholamines or norepinephrine concentration. However, myocardial epinephrine was significantly increased in animals breathing 20% CO₂. The effect of 6% oxygen on cardiac norepinephrine loss was blocked by hexamethonium chloride, indicating that the effects of severe hypoxia on myocardial norepinephrine loss are refiexly mediated. The effect of 20% CO₂ on cardiac norepinephrine loss was only partially blocked by hexmethonium chloride, indicating that severe hypercarbia has both a direct and a reflexly mediated effect on myocardial norepinephrine stores. With varying degrees of hypercarbia, graded responses of the heart's sympathetic nervous system were found, whereas with hypoxia no such graded responses could be demonstrated.