The mechanical effects of contractions on blood flow to the muscle

To determine whether muscle contractions can increase muscle blood flow independently from metabolic factors, we isolated the vasculature of the left diaphragm or gastrocnemius muscle of anesthetized and mechanically ventilated dogs. Arterial blood flow was controlled with a constant pressure source and the arterial pressure (Pa) was decreased in steps to obtain pressure-flow relationships (P-Q). The local vasculatures were maximally dilated with nitroprusside [mean (SD) 114.0 (32.0) micrograms.min-1], adenosine [1.43 (0.41) mmol.l-1.min-1], and acetylcholine [1.43 (0.41) mmol.l-1.min-1] and the P-Q with and without spontaneous contractions (n = 6), stimulated twitches (n = 12, 2-4 Hz), or tetanic trains (n = 7, 25 Hz) in the diaphragm and stimulated twitches (n = 6, 2-4 Hz), or tetanic contractions (n = 6, 12-16 trains) in the gastrocnemius were compared. The pressure axis intercept decreased (P < 0.5) with spontaneous contractions in the diaphragm and the slope did not change. At Pa of 13.3 kPa, flow increased from 36.2 (34.9) to 43.9 (38.2) ml.min-1.100 g-1 (P < 0.05). During twitch contractions, the slope and intercept of the P-Q were not significantly different from vasodilatation alone, but the flow at a pressure of 13.3 kPa increased slightly. In the gastrocnemius (n = 6), continuous and intermittent tetanic contractions did not affect P-Q or flow at Pa of 100 mmHg (n = 6). Furthermore, increasing venous pressure to 6.7 kPa did not affect flow in this muscle. We conclude that the muscle pump has only a small direct effect on muscle blood flow and its main effect is to reduce venous pressures.