Substrate utilization across the leg during 90 min of bicycle exercise at 58% of peak oxygen uptake (VO(2 peak)) was studied in seven endurance-trained males and seven endurance-trained, eumenorrheic females by applying arteriovenous catheterization, stable isotopes, and muscle biopsies. The female and male groups were matched according to VO(2 peak) per kilogram of lean body mass, physical activity level, and training history of the subjects. All subjects consumed the same diet, well controlled in terms of nutrient composition as well as energy content, for 8 days preceding the experiment, and all females were tested in the midfollicular phase of the menstrual cycle. During exercise, respiratory exchange ratio (RER) and leg respiratory quotient (RQ) were similar in females and males. Myocellular triacylglycerol (TG) degradation was negligible in males but amounted to 12.4 +/- 3.2 mmol/kg dry wt in females and corresponded to 25.0 +/- 6.0 and 5.0 +/- 7.3% of total oxygen uptake in females and males, respectively (P < 0.05). Utilization of plasma fatty acids (12.0 +/- 2.5 and 9.6 +/- 1.5%), blood glucose (13.6 +/- 1.5 and 14.3 +/- 1.5%), and glycogen (48.5 +/- 4.9 and 42.8 +/- 2.1%) were similar in females and males. Thus, in females, measured substrate oxidation accounted for 99% of the leg oxygen uptake, whereas in males 28% of leg oxygen uptake was unaccounted for in terms of measured oxidized lipid substrates. These findings may indicate that males utilized additional lipid sources, presumably very low density lipoprotein-TG or TG located between muscle fibers. On the basis of RER and leg RQ, it is concluded that no gender difference existed in the relative contribution from carbohydrate and lipids to the oxidative metabolism across the leg during submaximal exercise at the same relative workload. However, an effect of gender appears to occur in the utilization of the different lipid sources.

.-The resting content and use of myocellular triacylglycerol (MCTG) during 90 min of submaximal exercise [60% of peak oxygen uptake (V O2 peak)] were studied in 21 eumenorrheic female and 21 male subjects at different training levels [untrained (UT), moderately trained (MT), and endurance trained (END)]. Males and females were matched according to their V O2 peak expressed relative to lean body mass, physical activity level, and training history. All subjects ingested the same controlled diet for 8 days, and all females were tested in the midfollicular phase of the menstrual cycle. Resting MCTG, measured with the muscle biopsy technique, averaged 48.4 Ϯ 4.2, 48.5 Ϯ 8.4, and 52.2 Ϯ 5.8 mmol/kg dry wt in UT, MT, and END females, respectively, and 34.1 Ϯ 4.9, 31.6 Ϯ 3.3, and 38.4 Ϯ 3.0 mmol/kg dry wt in UT, MT, and END males, respectively (P Ͻ 0.001, females vs. males in all groups). Exercise decreased MCTG content in the female subjects by an average of 25%, regardless of training status, whereas in the male groups MCTG content was unaffected by exercise. The arterial plasma insulin concentration was higher (P Ͻ 0.05) and the arterial plasma epinephrine concentration was lower (P Ͻ 0.05) in the females than in the males at rest and during exercise. MCTG use was correlated to the resting concentration of MCTG (P Ͻ 0.001). We conclude that resting content and use of MCTG during exercise are related to gender and furthermore are independent of training status. muscle substrate; training; triglycerides IT HAS PREVIOUSLY BEEN SHOWN (25) that myocellular triacylglycerol (MCTG) is utilized during the postexercise period. MCTG stores also represent a potentially large energy source during exercise. However, the extent to which MCTG is used during exercise and the possible existence of differences in its use between trained and untrained (UT) subjects are still under debate. In studies (27, 37) using stable isotope techniques combined with indirect calorimetry, it was estimated that MCTG accounted for 20-25% of the oxidative metabolism during submaximal exercise. However, when direct measurements of MCTG concentration in muscle biopsies have been used, some studies (4, 19, 34) have found a decrease in MCTG concentration during submaximal exercise, whereas others (1, 21, 22, 25, 44) have observed no change. In all of the above-mentioned studies, only male subjects have participated. Thus it is unknown whether gender differences exist in the utilization of MCTG during exercise. Some studies (18,45,46) have shown that females utilize lipids to a greater extent than males during submaximal exercise, but to our knowledge it has not been investigated whether this increased lipid utilization in females is primarily from MCTG or other lipid sources. Other studies (3, 6, 32) have not been able to find gender differences in lipid utilization during exercise. This could be due to differences in training status and exercise mode in the experimental designs. The aim of the present study was therefore to evaluate the contribution of MCT...

5 AMP-activated protein kinase (AMPK) is an energy sensor activated by perturbed cellular energy status such as during muscle contraction. Activated AMPK is thought to regulate several key metabolic pathways. We used sex comparison to investigate whether AMPK signalling in skeletal muscle regulates fat oxidation during exercise. Moderately trained women and men completed 90 min bicycle exercise at 60%V O 2 peak . Both AMPK Thr 172 phosphorylation and α 2 AMPK activity were increased by exercise in men (∼200%, P < 0.001) but not significantly in women. The sex difference in muscle AMPK activation with exercise was accompanied by an increase in muscle free AMP (∼164%, P < 0.01), free AMP/ATP ratio (159%, P < 0.05), and creatine (∼42%, P < 0.001) in men but not in women (NS), suggesting that lack of AMPK activation in women was due to better maintenance of muscle cellular energy balance compared with men. During exercise, fat oxidation per kg lean body mass was higher in women than in men (P < 0.05). Regression analysis revealed that a higher proportion of type 1 muscle fibres (∼23%, P < 0.01) and a higher capillarization (∼23%, P < 0.05) in women than in men could partly explain the sex difference in α 2 AMPK activity (r = −0.54, P < 0.05) and fat oxidation (r = 0.64, P < 0.05) during exercise. On the other hand, fat oxidation appeared not to be regulated via AMPK. In conclusion, during prolonged submaximal exercise at 60%V O 2 peak , higher fat oxidation in women cannot be explained by higher AMPK signalling but is accompanied by improved muscle cellular energy balance in women probably due to sex specific muscle morphology.

The protein and mRNA levels of several muscle lipid-binding proteins and the activity and mRNA level of muscle lipoprotein lipase (mLPL) were investigated in healthy, nonobese, nontrained (NT), moderately trained, and endurance-trained (ET) women and men. FAT/CD36 protein level was 49% higher (P < 0.05) in women than in men, irrespective of training status, whereas FAT/CD36 mRNA was only higher (P < 0.05) in women than in men in NT subjects (85%). Plasma membrane-bound fatty acid binding protein (FABPpm) content was higher in ET men compared with all other groups, whereas training status did not affect FABPpm content in women. FABPpm mRNA was higher (P < 0.05) in NT women than in ET women and NT men. mLPL activity was not different between gender, but mLPL mRNA was 160% higher (P < 0.001) in women than in men. mLPL activity was 48% higher (P < 0.05) in ET than in NT subjects, irrespective of gender, in accordance with 49% higher (P < 0.05) mLPL mRNA in ET than in NT subjects. A 90-min exercise bout induced an increase (P < 0.05) in FAT/CD36 mRNA (approximately 25%) and FABPpm mRNA (approximately 15%) levels in all groups. The present study demonstrated that, in the NT state, women had higher muscle mRNA levels of several proteins related to muscle lipid metabolism compared with men. In the ET state, only the gender difference in mLPL mRNA persisted. FAT/CD36 protein in muscle was higher in women than in men, irrespective of training status. These findings may help explain gender differences in lipid metabolism and, furthermore, suggest that the balance between gene transcription, translation, and possibly breakdown of several proteins in muscle lipid metabolism depend on gender.

. Malonyl-CoA and carnitine in regulation of fat oxidation in human skeletal muscle during exercise.