Abstract


Triacylglycerol (TG) is stored in lipid droplets in the cytoplasm of skeletal muscle. The energy content of the TG depot is higher than the energy content of the muscle glycogen depot. The enzymatic regulation of intracellular TG hydrolysis in skeletal muscle has not been elucidated. Therefore, we investigated the expression and the regulation of hormone-sensitive lipase (HSL) in skeletal muscle. This enzyme is a neutral lipase and known as the rate-limiting enzyme of intracellular TG hydrolysis in adipose tissue. The total and the activated form of the neutral lipase are referred to as MOME and TO, respectively. In isolated rat skeletal muscle fibers, the presence of HSL was demonstrated by Western blotting. The expression of HSL was correlated to fiber type, being higher in oxidative than in glycolytic fibers. In incubated soleus and extensor digitorum longus (EDL) muscles stimulation with epinephrine or electrically induced contractions increased neutral lipase activity against triolein (TO), but not against a diacylglycerol analogue (MOME). Glycogen phosphorylase activity increased in parallel with TO activity. No measurable increase in muscle homogenate TO activity existed in the presence of an anti-HSL antibody. The effect of epinephrine could be blocked by propanolol and mimicked by incubation of a crude supernatant from control muscle with the catalytic subunit of cAMP-dependent protein kinase. The effect of contractions was transient as TO activity declined to basal levels after 10 min of electrical stimulation. Indicating involvement of protein kinase C the effect of contractions was abolished by Calphostin C. Okadaic acid doubled the contraction-mediated increase in TO activity, whereas the increase was reversed by phosphatase treatment. The effects of epinephrine and contractions were partially additive. In rats training increased epinephrine-stimulated TO activity and HSL concentration in adipose tissue but not in muscle. In humans, at the end of 60 min of exercise muscle, TO activity was increased in healthy, but not in adrenalectomized, subjects. In conclusion, HSL is present in skeletal muscle and can be activated by phosphorylation by both epinephrine and muscle contractions. In addition, HSL and glycogen phosphorylase are stimulated in parallel in muscle indicating simultaneous activation of triacylglycerol and glycogen breakdown.