MYOCARDIAL BLOOD-FLOW AND HEMODYNAMIC-RESPONSES TO EXERCISE TRAINING IN DOGS

Left ventricular (LV) hemodynamics and myocardial blood flow (MBF) were examined under acute pressure (P) and volume (V) load states in six exercise trained (T) and six nontrained (NT) dogs. MBF was measured with 9-μm radioactive microspheres. Training was achieved by 8 wk of treadmill running and was verified by increased gastrocnemius cytochrome oxidase (T, 282 ± 20 μl O2/min.g; NT, 209 ± 18; P < 0.025), and a reduced heart response to submaximal exercise. Phenylephrine infusion doubled stroke work in both T and NT groups, and caused a significant rise in left atrial pressure (Pla) in the T group. Dextran and saline infusion produced a 50% rise in Pla in both groups, and increased stroke work in T. The volume needed to raise Pla by 50% was higher in the T group (NT, 12.9 ± 1.9; T, 17.7 ± 3.1 ml/kg; P < 0.05). Total LVMBF increased with pressure and volume loads in the T group only. Resting (R) LVMBF (ml/min per 100 g) was lower in T (NT, 114 ±57; T, 83 ±26; P < 0.10). Endocardial to epicardial flow ratio was unchanged in the T group with loading (R, 1.16 ± 0.04; P, 1.13 ± 0.12; NS; V, 1.26 ± 0.03; NS), whereas in the NT group this ratio was increased (R, 1.15 ± 0.03; P, 1.26 ± 0.07; P < 0.05; V, 1.26 ± 0.03; P < 0.01). Heart-to-body weight ratio was increased in the T group (NT, 6.31 ± 0.22 g/kg; T, 6.97 ± 0.23; P < 0.05). Exercise training results in an altered left atrial pressure response to acute volume loading and unchanged MBF under load, when compared to the untrained state.