Additivity of adrenaline and contractions on hormone-sensitive lipase, but not on glycogen phosphorylase, in rat muscle

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 mu 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.