Cytoplasmic lipid droplets provide a reservoir for triglyceride storage and are a central hub for fatty acid trafficking in cells. The protein perilipin 5 (PLIN5) is highly expressed in oxidative tissues such as skeletal muscle and regulates lipid metabolism by coordinating the trafficking and the reversible interactions of effector proteins at the lipid droplet. PLIN5 may also regulate mitochondrial function, although this remains unsubstantiated. Hence, the aims of this study were to examine the role of PLIN5 in the regulation of skeletal muscle substrate metabolism during acute exercise and to determine whether PLIN5 is required for the metabolic adaptations and enhancement in exercise tolerance following endurance exercise training. Using muscle-specific Plin5 knockout mice (Plin5(MKO)), we show that PLIN5 is dispensable for normal substrate metabolism during exercise, as reflected by levels of blood metabolites and rates of glycogen and triglyceride depletion that were indistinguishable from control (lox/lox) mice. Plin5(MKO) mice exhibited a functional impairment in their response to endurance exercise training, as reflected by reduced maximal running capacity (20%) and reduced time to fatigue during prolonged submaximal exercise (15%). The reduction in exercise performance was not accompanied by alterations in carbohydrate and fatty acid metabolism during submaximal exercise. Similarly, mitochondrial capacity (mtDNA, respiratory complex proteins, citrate synthase activity) and mitochondrial function (oxygen consumption rate in muscle fiber bundles) were not different between lox/lox and Plin5(MKO) mice. Thus, PLIN5 is dispensable for normal substrate metabolism during exercise and is not required to promote mitochondrial biogenesis or enhance the cellular adaptations to endurance exercise training.
Inhibition of nitric oxide synthase (NOS) significantly attenuates the increase in skeletal muscle glucose uptake during contraction/exercise, and a greater attenuation is observed in individuals with Type 2 diabetes compared with healthy individuals. Therefore, NO appears to play an important role in mediating muscle glucose uptake during contraction. In this study, we investigated the involvement of neuronal NOSμ (nNOSμ), the main NOS isoform activated during contraction, on skeletal muscle glucose uptake during ex vivo contraction. Extensor digitorum longus muscles were isolated from nNOSμ(-/-) and nNOSμ(+/+) mice. Muscles were contracted ex vivo in a temperature-controlled (30°C) organ bath with or without the presence of the NOS inhibitor N(G)-monomethyl-l-arginine (L-NMMA) and the NOS substrate L-arginine. Glucose uptake was determined by radioactive tracers. Skeletal muscle glucose uptake increased approximately fourfold during contraction in muscles from both nNOSμ(-/-) and nNOSμ(+/+) mice. L-NMMA significantly attenuated the increase in muscle glucose uptake during contraction in both genotypes. This attenuation was reversed by L-arginine, suggesting that L-NMMA attenuated the increase in muscle glucose uptake during contraction by inhibiting NOS and not via a nonspecific effect of the inhibitor. Low levels of NOS activity (~4%) were detected in muscles from nNOSμ(-/-) mice, and there was no evidence of compensation from other NOS isoform or AMP-activated protein kinase which is also involved in mediating muscle glucose uptake during contraction. These results indicate that NO regulates skeletal muscle glucose uptake during ex vivo contraction independently of nNOSμ.