Vasoconstrictors can stimulate (Type A) or inhibit (Type B) oxygen uptake of the constant-flow perfused rat hindlimb (but not incubated muscle) (Clark et al., Am. J. Physiol. 268: E797-E812, 1995) and may do so because of redistribution of flow. Thus FluoSpheres® were used to assess the distribution of perfusate flow due to a representative Type B vasoconstrictor (serotonin, 5-HT, 0.35 µM) while maintaining total flow constant. Single hindlimbs were perfused and either vehicle or 5-HT infused with arteriovenous pO2, pressure and total flow monitored. The foot and contralateral leg were tied off to prevent extraneous flow. Serotonin-mediated changes relative to vehicle alone were -39.3% (oxygen uptake) and +305% (perfusion pressure). A bolus of 2.5x105 FluoSpheres® (15Ám) were infused at steady state. Flow to the gastrocnemius-plantarus-soleus muscle group (as measured by FluoSpheres®) was increased from 6.78 to 11.42% of total in the presence of 5-HT, relative to vehicle. Conversely, the ratio of creatine phosphate to creatine (as an indicator of relative oxygen availability) was decreased in the same muscle group. This suggests that despite the increased flow, Type B vasoconstrictors inhibit metabolism by redistribution of flow to the non-nutritive vessels within skeletal muscle.
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