Haemodynamic responses to exercise, ATP infusion and thigh compression in humans: insight into the role of muscle mechanisms on cardiovascular function

Abstract

The muscle pump and muscle vasodilatory mechanism are thought to play important roles in increasing and maintaining muscle perfusion and cardiac output ((.)Q) during exercise, but their actual contributions remain uncertain. To evaluate the role of the skeletal muscle pump and vasodilatation on cardiovascular function during exercise, we determined leg and systemic haemodynamic responses in healthy men during (1) incremental one-legged knee-extensor exercise, (2) step-wise femoral artery ATP infusion at rest, (3) passive exercise (n=10), (4)femoral vein or artery ATP infusion (n=6), and (5) cyclic thigh compressions at rest and during passive and voluntary exercise (n=7). Incremental exercise resulted in progressive increases in leg blood flow (DeltaLBF 7.4 +/- 0.7 l min(-1)), cardiac output (Delta (.)Q 8.7 +/- 0.7 l min(-1)), mean arterial pressure (DeltaMAP 51 +/- 5 mmHg), and leg and systemic oxygen delivery and (.)VO2 . Arterial ATP infusion resulted in similar increases in (.)Q , LBF, and systemic and leg oxygen delivery, but central venous pressure and muscle metabolism remained unchanged and MAP was reduced. In contrast,femoral vein ATP infusion did not alter LBF, (.)Q or MAP. Passive exercise also increased blood flow (DeltaLBF 0.7 +/- 0.1 l min(-1)), yet the increase in muscle and systemic perfusion, unrelated to elevations in aerobic metabolism, accounted only for approximately 5% of peak exercise hyperaemia.Likewise, thigh compressions alone or in combination with passive exercise increased blood flow (DeltaLBF 0.5-0.7 l min(-1)) without altering (.)Q, MAP or (.)VO2. These findings suggest that the skeletal muscle pump is not obligatory for sustaining venous return, central venous pressure,stroke volume and (.)Q or maintaining muscle blood flow during one-legged exercise in humans.Further, its contribution to muscle and systemic peak exercise hyperaemia appears to be minimal in comparison to the effects of muscle vasodilatation.

Figure 1. Leg and systemic haemodynamics during incremental knee-extensor exercise

Cardiovascular variables plotted against increases in power output during one-legged knee-extensor exercise. Data are mean ±s.e.m. for 9–10 subjects.

Figure 2. Leg and systemic haemodynamics with incremental exercise and graded intrafemoral artery ATP infusion

Cardiovascular variables plotted against leg blood flow during incremental one-legged knee-extensor exercise and graded intrafemoral artery ATP infusion. Open circles depict systemic haemodynamics whereas filled circles depict leg haemodynamics. Data are mean ±s.e.m. for 9 subjects. *Different from exercise, P < 0.05.

Figure 3. Haemodynamic effects of exercise, ATP infusion and external compressions

Change in blood flow and arterial pressure from baseline values during peak and light exercise, intrafemoral artery or vein ATP infusion, and thigh compressions alone and in combination with passive exercise or light exercise. Note that leg vasodilatation with arterial ATP infusion matches exercise hyperaemia, but not the increase in arterial blood pressure. ATP acts locally as intrafemoral vein ATP infusion did not alter these responses. Further, effect of mechanical-induced vasodilatation via cyclic external compressions at the same rate as exercise induces significant increases in leg and systemic blood flow, but this increase only accounts for ∼10% of the peak exercise hyperaemia. Data are mean ±s.e.m.*Significant change from corresponding baseline values, P < 0.05.

Figure 4. Leg blood flow and blood pressure as a function of leg aerobic metabolism

Changes in blood flow and arterial pressure from baseline values during incremental knee-extensor exercise, step-wise intrafemoral artery ATP infusion, passive exercise and thigh compressions alone and in combination with passive exercise or light exercise. Noteworthy is the significant correlations between the increases in leg blood flow, cardiac output and mean arterial pressure and the increases in leg during the exercise and thigh compressions protocols, but the dissociation in these responses during arterial ATP infusion.