Effects of propranolol on rat myocardial l-ornithine decarboxylase activity during exercise and hypoxia.

Ornithine decarboxylase activity was higher in the left ventricle than in the right ventricle of the rat. Swimming exercise for 2 hours caused an increase in left ventricular but not right ventricular enzyme activity. This effect was not influenced by the prior injection of propranolol, which also had no influence on the resting level of ornithine decarboxylase activity. After 4 hours' exposure to 400 mm Hg the decarboxylase activity became greater in the right ventricle. This was attributed to the increased work-load on the right ventricle caused by hypoxic pulmonary hypertension. In the left ventricle, ornithine decarboxylase showed an initial increase in activity followed by a transient decrease at 16 hours. Propranolol had no effect on the changes observed in either ventricle. It is concluded that beta-receptor stimulation is not involved in the mechanism of an increased myocardial ornithine decarboxylase activity during increased cardiac work.