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Abstract Aim: The enzymatic regulation of intramuscular triacylglycerol (TG) breakdown has until recently not been well understood. Our aim was to elucidate the role of hormone-sensitive lipase (HSL), which controls TG breakdown in adipose tissue. Methods: Isolated rat muscle as well as exercising humans were studied. Results: The presence of HSL was demonstrated in all muscle fibre types by Western blotting of muscle fibres isolated by collagenase treatment or after freeze-drying. The content of HSL varies between fibre types, being higher in oxidative than in glycolytic fibres. Analysed under conditions optimal for HSL, neutral lipase activity in muscle can be stimulated by adrenaline as well as by contractions. These increases are abolished by presence of anti-HSL antibody during analysis. Moreover, immunoprecipitation with affinity-purified anti-HSL antibody causes similar reductions in muscle HSL protein concentration and in measured neutral lipase responses to contractions. The immunoreactive HSL in muscle is stimulated by adrenaline via beta-adrenergic activation of protein kinase A (PKA). From findings in adipocytes it is likely that PKA phosphorylates HSL at residues Ser563 , Ser659 and Ser660 . Contraction probably also enhances muscle-HSL activity by phosphorylation, because the contraction-induced increase in HSL activity is increased by the protein phosphatase inhibitor okadaic acid and reversed by alkaline phosphatase. A novel signalling pathway in muscle by which HSL activity may be stimulated by protein kinase C (PKC) via extracellular signal regulated kinase (ERK) has been demonstrated. In contrast to previous findings in adipocytes, in muscle activation of ERK is not necessary for stimulation of HSL by adrenaline. However, contraction-induced HSL activation is mediated by PKC, at least partly via the ERK pathway. In fat cells ERK is known to phosphorylate HSL at Ser600 . So, phosphorylation of... [ABSTRACT FROM AUTHOR]

Endurance training elicits profound adaptations of skeletal muscle, including increased expression of several proteins. The 5′-AMP activated protein kinase (AMPK) may be one of these, considering the fact that acute exercise increases AMPK activity. Eight young (26 ± 1 year) lean, healthy males endurance trained one leg (while the other leg remained resting) on an ergometer bicycle for 30 min/day for four weeks (workload corresponding to ∼ 70% of maximal oxygen uptake). Muscle biopsies were obtained ∼18 h after the previous training session. On day eight GLUT4 protein expression was 36% higher in trained (T) compared with untrained (UT) (P < 0.05), but no further increase was seen at day 14 and 30 despite continuously increasing absolute workloads. Expression of AMPKα 2 and actin did not change with training. In contrast, expression of AMPKα 1 was 27% higher in T vs. UT muscle (P < 0.05) (measured only on day 30). Conclusions: GLUT4 protein expression increases substantially after seven days of endurance training with no further increase with prolonged training at progressively increasing workloads. AMPKα 1 and α 2 behave differently in their expression in response to endurance training. AMPKα 1 protein content is increased after one month of training, while no change in AMPKα 2 and actin expression was detected over the time course of the training period . [ABSTRACT FROM AUTHOR]

Abstract Aim: Hormone-sensitive lipase (HSL) has been proposed to regulate triacylglycerol (TG) breakdown in skeletal muscle. In muscles with different fibre type compositions the influence on HSL of two major stimuli causing TG mobilization was studied. Methods: Incubated soleus and extensor digitorum longus (EDL) muscles from 70 g rats were stimulated by adrenaline (5.5 μ m, 6 min) or contractions (200 ms tetani, 1 Hz, 1 min) in maximally effective doses or by both adrenaline and contractions. Results: Hormone-sensitive lipase activity was increased significantly by adrenaline as well as contractions, and the highest activity (P < 0.05) was seen with combined stimulation [Soleus: 0.40 ± 0.03 (SE) m-unit mg protein-1 (basal), 0.65 ± 0.02 (adrenaline), 0.65 ± 0.03 (contractions), 0.78 ± 0.03 (adrenaline and contractions); EDL: 0.18 ± 0.01, 0.30 ± 0.02, 0.26 ± 0.02, 0.32 ± 0.01]. Glycogen phosphorylase activity was always increased more by adrenaline compared with contractions [Soleus: 60 ± 4 (a /a + b )% vs. 46 ± 3 (P < 0.05); EDL: 60 ± 5 vs. 39 ± 6 (P < 0.05)]. After combined stimulation glycogen phosphorylase activity in soleus [59 ± 3 (a /a + b )%] was identical to and in EDL [45 ± 4 (a /a + b )%] smaller (P < 0.05) than the activity after adrenaline only. Conclusions: In slow-twitch oxidative as well as in fast-twitch glycolytic muscle HSL is activated by both adrenaline and contractions. These stimuli are partially additive indicating at least partly different mechanisms of action. Contractions may impair the enhancing effect of adrenaline on glycogen phosphorylase activity in muscle. [ABSTRACT FROM AUTHOR]