Blood flow in “red” and “white” calf muscles in cats during isometric and isotonic exercise

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Abstract
Muscle blood flow was measured with the radioactive microspheres technique in plantar flexors of seven cats. Right leg performed rhythmic isotonic and left leg sustained isometric contractions. Flows were determined at rest, during stimulation periods, and immediately after 90 sec of high frequency stimulation. During isotonic contraction, flow to m. soleus (a “red muscle”) increased progressively as stimulation increased: 0.06±0.02, 0.21±0.05, 0.41±0.12, and 0.26±0.06 ml/min/g during rest, 10 Hz, 50 Hz, and after 50 Hz stimulation frequencies, respectively. Corresponding values for gastrocnemius medialis were 0.06±0.01, 0.38±0.05, 0.27±0.03, and 0.28±0.06 (x± S.E.). These last values were characteristic also for the other “white” muscles investigated: gastrocnemius lateralis, and plantaris. During sustained isometric exercise at 5 and 25 Hz stimulation frequency the patterns were similar, except that higher flows were observed, and during recovery flows actually increased again in the “white” muscles. Blood pressure did not change. Resting flows were similar in innervated and denervated muscles. These data demonstrate that during isotonic as well as isometric exercise there is an increased resistance to flow with increasing muscle tension in “white” but not in “red” muscles. This supports the notion that a condition of relative ischaemia exists in “white” muscles during both isometric and isotonic contractions which is more pronounced during isometric exercise. The results corroborate previous studies on isometric exercise and suggest that the pattern during isotonic exercise is similar.