Important differences exist between perfused and incubated (or perifused) skeletal muscle preparations with regard to their metabolism and control. A growing body of evidence suggests that the differences may be due to the role played by the vascular system. In the constant-flow perfused rat hindlimb preparation, a group of vasoconstrictors has been identified that enhance muscle metabolism and aerobic contractility. Another group of vasoconstrictors decrease muscle metabolism and aerobic contractility even though perfusate flow remains constant. All effects of both groups of vasoconstrictors are opposed by vasodilators. Because none of the vasoconstrictor effects is evident when isolated muscles are incubated or perifused, involvement of an active vascular system is indicated. Although some hormones may act directly on muscle by purely endocrine effects, a vascular component of their actions is now emerging. Mechanisms to account for vascular control of perfused skeletal muscle metabolism may involve 1) functional vascular shunts where the proportion of flow processed by these is regulated by site-specific vasomodulators, 2) a direct response to a change in the rate of supply of nutrients and removal of products, and 3) a signal substance released by vascular tissue in association with vasoconstriction that interacts with surrounding skeletal muscle cells. Impaired control at the level of the vascular system may have implications for long-term access of nutrients and hormones and therefore the control of skeletal muscle metabolism and contractile performance.

The Tasmanian bettong (Bettongia gaimardi, a marsupial) is a rat-kangaroo that increases nonshivering thermogenesis (NST) in response to norepinephrine (NE). This study attempted to assess whether brown adipose tissue (BAT), a specialized thermogenic effector, is involved in NST in the bettong. Regulatory NST, indicated by resting oxygen consumption (Vo2) of the whole body, was measured under conscious conditions at 20 degrees C with various stimuli: cold (4 degrees -5 degrees C) or warm (25 degrees C) acclimation, NE injection, and the beta3-adrenoceptor agonist (BRL) 37344. In line with the functional studies in vivo, the presence of BAT was evaluated by examining the expression of the uncoupling protein 1 (UCP1) with both rat cDNA and oligonucleotide probes. Both NE and BRL 37344 significantly stimulated NST in the bettong. After cold acclimation of the animals (at 4 degrees -5 degrees C for 2 wk), the resting Vo2 was increased by 15% and the thermogenic effect of NE was enhanced; warm-acclimated animals showed a slightly depressed response. However, no expression of UCP1 was detected in bettongs either before or after cold exposure (2 wk). These data suggest that the observed NST in the marsupial bettong is not attributable to BAT.

The vasoconstrictors, norepinephrine at low dose ( < or = 0.1 microM; LDNE) and serotonin (5-HT), produce opposing metabolic effects in the constant-flow perfused rat hindlimb characterized by increased and decreased oxygen uptake, respectively. In the present study, the effects of each vasoconstrictor are compared in the red blood cell-free buffer-perfused hindlimb on postequilibration endogenous red blood cell efflux, vascular entrapment of fluorescein-labeled dextran (Fx), and vascular corrosion casting by use of 30-micron spheres of methyl methacrylate (MM). A marked transient washout of red blood cells occurred immediately in association with vasoconstriction induced by LDNE that was not apparent when a similar extent of vasoconstriction was induced by 5-HT. Fx perfusions indicated that LDNE recruited a new vascular space that was reaccessed by a second exposure to the vasoconstrictor. 5-HT closed off a previously perfused vascular space that was reaccessed when the vasoconstrictor was removed. Corrosion casting of the arterial tree with MM showed no increase in cast weight, but more vessels filled because of LDNE. Higher doses of NE (2.5 microM) or 5-HT caused a marked decrease in cast weight with fewer vessels filled. The data suggest that LDNE and 5-HT, in association with vasoconstriction at different sites, control different capillary flow routes in the hindlimb that in turn may influence metabolism by increasing or decreasing nutrient access, respectively.

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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