Myocardial blood flow responses to acute hypoxia and volume loading in physically trained rats

Rats trained by a swimming programme were studied to see whether physical training can promote changes in coronary blood flow patterns that are physiologically important. Radioactive microspheres were employed in an open-chest preparation to measure coronary blood flow and cardiac output during the control state, during hypoxia, and during dextran infusion to increase the preload. Hearts from the trained groups of rats had elevated actomyosin ATPase activities, indicating that the swimming caused a cardiac training effect. In each group hypoxia produced a fall in cardiac output and myocardial blood flow, and these functions more than doubled during volume loading. However, cardiac output, stroke volume, coronary blood flow, and myocardial distribution of blood flow were not significantly different between either group during the control state, hypoxia, or volume loading. Energy supply-demand factors (ejection pressure time and diastolic pressure time) were also similar in the two groups. Therefore, despite evidence from our previous studies of increased coronary vascular responses when hearts from trained animals were perfused with aqueous solutions, the present experiments failed to demonstrate increased coronary flow responses when the hearts were perfused with blood. This study casts doubts on the physiological importance of increased vascularity in hearts of rats subjected to exercise training when only mild to moderate stress is imposed.