The failure of oxygen breathing to decrease the myocardial contractile force in denervated dogs.

This study was done to discover whether or not the oxygen-induced depression of sympathoadrenal activity contributes to a reduction of myocardial contractile force during oxygen breathing. In 10 open-chest dogs, myocardial contractile force was measured using a myocardial strain gauge arch during air and oxygen breathing before denervation (intact heart) and after bilateral vagotomies, sympathectomies and adrenalectomies with the intravenous administration of propranolol, phenoxybenzamine and atropin (denervated heart). One hundred percent oxygen breathing caused similar increases in arterial pO2 in both the intact (from 94 +/- 10 to 442 +/- 25 mmHg) and the denervated dogs (from 113 +/- 11 to 456 +/- 15 mmHg). Coronary blood flow measured at the left anterior descending coronary artery was reduced by oxygen breathing from 28.4 +/- 3.4 to 21.7 +/- 2.3 ml/min in the intact dogs, and from 19.4 +/- 3.4 to 14.9 +/- 2.6 ml/min in the denervated dogs. Myocardial contractile force was significantly reduced by oxygen breathing in the intact dogs (a reduction of 5.8 +/- 1.4%). In the denervated dogs, on the other hand, no significant changes in myocardial contractile force was seen. This study suggests that the reduction in myocardial contractile force is mediated through sympathoadrenal activity, and thus, is abolished by sympathoadrenal blockade.