The effect of serotonin (5-HT) on the metabolism of infused 1-methylxanthine (1-MX), a putative substrate of capillary endothelial xanthine oxidase (XO), and on the distribution of infused fluorescent microspheres (15 microns) by the artificially constant-flow perfused rat hindlimb preparation was investigated. 1-MX (5-100 microM) caused a slight inhibition of oxygen uptake (Vo2) but was not vasoactive, either alone or with 5-HT. 1-MX was converted to 1-methylurate (1-MU) and this conversion was inhibited by allopurinol and xanthine. 5-HT (0.35 microM), which caused vasoconstriction and decreased Vo2, also inhibited the conversion of 1-MX, indicated by a lowered venous perfusate steady-state 1-MU:1-MX ratio from 1.14 +/- 0.02 to 0.71 +/- 0.02 (P < 0.001), which is equivalent to the rate of conversion decreasing from 0.83 +/- 0.03 to 0.63 +/- 0.05 nmol min-1 g-1. This change closely followed the time course for changes in Vo2 and perfusion pressure and all three changes reversed in parallel when 5-HT was removed. Recoveries of 1-MU plus 1-MX at all times were high (100 +/- 5%). 5-HT did not act to inhibit XO. When compared with vehicle alone, 5-HT had either no effect (plantairs, gastrocnemius white, tibialis, extensor digitorum longus, vastus and thigh), or increased microsphere content (soleus and gastrocnemius red, P < 0.05) of muscles with only bone showing a significant decrease (P < 0.05). Since 5-HT did not inhibit XO or alter the net flow to individual muscles in this constant-flow model, the inhibition of conversion of 1-MX to 1-MU is concluded to be the result of a 5-HT-mediated decrease in the access of 1-MX to capillary XO within individual muscles. Possibilities include the redirection of flow to capillaries either in muscle or in connective tissue closely associated with muscle, where resistance is low and effective surface area is less. 1-MX has potential as a marker for muscle nutritive flow.

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Exogenous substrates for capillary endothelial enzymes have potential as markers for changes in capillary recruitment (albeit nutritive flow). The metabolism of infused 1-methylxanthine (1-MX) to 1-methylurate (1-MU) by capillary endothelial xanthine oxidase of the constant-flow perfused rat hindlimb was shown previously to decrease with oxygen uptake (VO2) when nutritive flow was decreased. In the present study, the metabolism of 1-MX was investigated under conditions when VO2 and nutritive flow are known to increase during muscle contraction. The constant-flow red blood cell-perfused rat hindlimb at 37 degrees C was used with sciatic nerve stimulation, and perfusate samples from whole hindlimb and working muscles taken for analysis of oxygen, lactate, 1-MX and 1-MU. Flow to muscle was assessed separately using fluorescent microspheres and was found to increase 2.3-fold to the working muscles while flow to the non-working leg muscles decreased to compensate. The activity of xanthine oxidase of whole muscle extracts was not altered by contraction. Samples from the vein draining the working muscles, and microsphere measurements of flow, indicated increased VO2 (5.5-fold to 249.2 +/- 43.1 micromol h-1 g-1, P < 0.001), and 1-MX conversion (2.5-fold to 1.87 +/- 0.25 micromol h-1 g-1, P < 0.01) (SEM are shown). It is concluded that as 1-MX metabolism parallels VO2, this substrate may be a useful indicator of changes in capillary (nutritive) surface area in muscle.

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