Effect of rhythmic tetanic skeletal muscle contractions on peak muscle perfusion

The purpose of this investigation was to examine the effect of rhythmic tetanic skeletal muscle contractions on peak muscle perfusion by using spontaneously perfused canine gastrocnemii in situ. Simultaneous pulsatile blood pressures were measured by means of transducers placed in the popliteal artery and vein, and pulsatile flow was measured with a flow-through-type transit-time ultrasound probe placed in the venous return line. Two series of experiments were performed. In series 1, maximal vasodilation of the muscles' vascular beds was elicited by infusing a normal saline solution containing adenosine (29.3 mg/min) and sodium nitroprusside (180 μg/min) for 15 s and then simultaneously occluding both the popliteal artery and vein for 5 min. The release of occlusion initiated a maximal hyperemic response, during which time four tetanic contractions were induced with supramaximal voltage (6-8 V, 0.2-ms stimuli for 200-ms duration at 50 Hz, 1/s). In series 2, the muscles were stimulated for 3 min before the muscle contractions were stopped for a period of 3 s; stimulation was then resumed. The results of series 1 indicate that, although contractions lowered venous pressure, muscle blood flow was significantly reduced from 2,056 ± 246 to 1,738 ± 225 ml·kg^-1·min^-1 when contractions were initiated and then increased significantly to 1,925 ± 225 ml·kg^-1·min^-1 during the first 5 s after contractions were stopped. In series 2, blood flow after 3 min of contractions averaged 1,454 ± 149 ml·kg^-1·min^-1. Stopping the contractions for 3 s caused blood flow to increase significantly to 1,874 ± 172 ml·kg^-1·min^-1; blood flow declined significantly to 1,458 ± 139 ml·kg^-1·min^-1 when contractions were resumed. We conclude that the mechanical action of rhythmic, synchronous, maximal isometric tetanic skeletal muscle contractions inhibits peak muscle perfusion during maximal and near-maximal vasodilation of the muscle's vascular bed. This argues against a primary role for the muscle pump in achieving peak skeletal muscle blood flow.