1. The effect of infusion of noradrenaline (0.42 μmol min −1 kg −1 ) on the exchange of non-esterified fatty acids, glycerol and other metabolites across subcutaneous abdominal adipose tissue was investigated in five healthy subjects using an arteriovenous catheterization technique and measurement of adipose tissue blood flow using the 133 Xe clearance technique. At the same time, the net rate of fat oxidation in the whole body was assessed by indirect calorimetry, and the turnover of glycerol in the whole body and in subcutaneous adipose tissue was estimated using [5- 2 H]glycerol, which was administered as a primed constant infusion for 1 h before (basal turnover) noradrenaline administration and continued during the 1 h of noradrenaline infusion. 2. The noradrenaline infusion increased the plasma noradrenaline concentration from a basal value of 0.9 ± 0.1 to 12.6 ± 1.2 nmol/(mean ± SEM) at 60 min. It also increased the arterialized concentration of glycerol by 50% (basal value 81 ± 11/μmol/l −1 ) and that of plasma non-esterified fatty acids three-fold (basal value 357 ± 86 μmol/l). 3. Noradrenaline increased the net release of glycerol by adipose tissue three-fold and that of non-esterified fatty acids three- to four-fold. Although the ratio of non-esterified fatty acid to glycerol release by adipose tissue increased in all subjects from a mean value of 2.7 in the basal period to 3.6 and 3.9 at 50 and 60 min of the noradrenaline infusion, respectively ( P < 0.02), at no time point did the ratio differ significantly from 3.0 4. Noradrenaline increased the estimated rate of appearance of glycerol in the whole body from a basal value of 1.5 ± 0.3 to 2.6 ± 0.3 μmol min −1 kg −1 body weight, and the net rate of triacylglycerol oxidation from 1.2 ± 0.1 to 1.7 ± 0.13 μmol min −1 kg −1 . The enrichment of glycerol in venous blood draining adipose tissue was two-fold lower than that predicted from the net addition of glycerol to the blood in the basal period ( P < 0.02). 5. This study provides a direct demonstration of a ‘hormone’ stimulating lipolysis in human adipose tissue in viva The effect of noradrenaline in significantly increasing the ratio of non-esterified fatty acid to glycerol release by adipose tissue may be partly explained by accumulation in adipose tissue of diacylglycerol, which is associated with release of non-esterified fatty acids but not glycerol. Finally, since the low enrichment of glycerol in venous blood draining adipose tissue cannot be entirely explained by the net addition of glycerol in adipose tissue, there must be exchange between enriched glycerol in blood and unenriched glycerol in adipose tissue. This raises questions about the accuracy of glycerol turnover studies, which are typically carried out over 1 h.
1. An investigation was carried out in five healthy lean adults to assess whether forearm and calf plethysmography largely reflect muscle blood flow as measured by 133 Xe and whether there is substantial variability in the blood flow to muscles located at different sites in the body. 2. Blood flow to forearm and calf flexors and extensors, biceps, triceps and quadriceps was assessed using the 133 Xe clearance technique. Blood flow to forearm skin and subcutaneous adipose tissue was also measured using the 133 Xe clearance technique, whereas blood flow to the forearm and calf was measured using strain gauge plethysmography. 3. The mean blood flow to different muscles ranged from 1.4 ± 0.6 (gastrocnemius) to 1.8 ± 0.7 (forearm extensor) ml min −1 100 g −1 muscle (1.4 ± 0.6 and 1.9 ± 0.8 ml min −1 100 ml −1 muscle, respectively) but there were no significant differences between them. Forearm and calf blood flows (2.7 ± 0.3 and 3.0 ± 0.7 ml min −1 100 ml −1 limb tissue, respectively) were about 50% to more than 100% greater ( P <0.025) than blood flow to the muscles within them (1.7 ± 0.5 and 1.4 ± 0.5ml min −1 100g −1 muscle, respectively, or 1.8 ± 0.6 and 1.5 ± 0.5 ml min −1 100 ml −1 muscle, respectively). In contrast, the blood flows to 100 g of forearm skin (9.1 ± 2.6 ml min −1 100 g −1 ) and adipose tissue (3.8 ± 1.1 ml min −1 100 g −1 ) were higher than the blood flow to 100 g of forearm ( P <0.01 and not significant, respectively). 4. Although several possibilities can explain the discrepancy between muscle blood flow measured by 133 Xe and blood flow to the distal limbs measured by plethysmography, the results suggest that non-muscular blood flow, especially that to skin, is substantially greater than muscular blood flow. Indeed, the overall blood flow to the forearm could be accounted for by summation of blood flows to individual constituent tissues, which were assumed to be present in proportions typical of lean subjects. The results have important implications in the use of arteriovenous catheterization studies for assessing flux of oxygen, carbon dioxide and metabolites across muscle.
1. Whole-body kinetics and regional exchange of glycerol across forearm muscle were assessed in eight lean subjects by a combination of a tracer method (infusion of [ 2 H 5 ]glycerol) and arteriovenous catheterization. 2. During an apparent steady state, the enrichment of glycerol in deep venous blood from the muscle bed of the forearm was about half (4.40 ± 1.72 atom per cent excess) that observed in arterialized blood (8.41 ± 4.30 atom per cent excess). Under the same conditions, the circulating concentrations of glycerol in arterialized (91 ± 24 μmol/l) and venous (87 ± 32 μmol/l) blood were similar. 3. In a further group of 37 subjects it was found that about half had a positive arteriovenous concentration difference and the other had half a negative arteriovenous concentration difference (mean −1.6 ± 11.9 μmol/l; range −25 to +22 μmol/l). 4. These results suggest: (a) that human muscle does not always release glycerol and may take it up; (b) that there is substantial isotopic exchange of glycerol across forearm muscle tissue, which is not reflected by the net exchange of glycerol; this could be due to slow equilibrium of enriched glycerol from the circulation, with unenriched free glycerol in the muscle pool, or due to the simultaneous metabolic utilization of enriched glycerol and metabolic production of unenriched glycerol; (c) that the estimation of glycerol flux rates is strongly dependent on whether the blood is arterialized or deep venous.