Transmural coronary vasodilator reserve and flow distribution during maximal exercise in normal and splenectomized ponies.

1. Transmural distribution of myocardial blood flow was studied using 15 micron diameter radionuclide-labelled microspheres in six normal ponies and nine splenectomized ponies at rest, and during maximal exercise performed without as well as with adenosine infusion (3 microM kg-1 min-1). The splenectomized ponies were also studied during submaximal exercise performed at 75% of the workload. 2. Maximal exertion in normal ponies increased heart rate (348%), mean arterial blood pressure (40.9%), rate-pressure product (563%), arterial O2 content (43.2%), and mean pulmonary artery pressure (247%). Accompanying these changes, the left ventricular, septal and right ventricular myocardial blood flows increased 419, 500, and 921% above control values, respectively, and the perfusion in all regions became nearly homogeneous. 3. Adenosine infusion during maximal exercise in normal ponies caused further significant increments in transmural myocardial blood flow in all regions as coronary vascular resistance decreased, thereby demonstrating considerable unutilized coronary vasodilator capacity. 4. In splenectomized ponies, with maximal exercise heart rate rose to a similar value as in normal ponies but mean aortic pressure, rate pressure product, pulmonary artery pressure and arterial O2 content were significantly less than in normal ponies (P less than 0.01). 5. Transmural myocardial perfusion in the splenectomized ponies also increased markedly with both exercise intensities and no significant differences were observed. 6. In the left ventricle and the septum of splenectomized ponies, transmural blood flow levels during maximal exertion were significantly higher (P less than 0.05) than in normal ponies. Adenosine infusion during maximal exercise in splenectomized ponies failed to cause further increments in blood flow to the inner layers of the left ventricle and the septum. 7. It is concluded that marked augmentation of arterial O2 content in normal ponies helped limit the increment in left ventricular myocardial perfusion required during maximal exertion, and thereby helped preservation of considerable unutilized coronary vasodilator capacity.