Activation of the dopamine 1 and dopamine 5 receptors increase skeletal muscle mass and force production under non-atrophying and atrophying conditions

<p>Abstract</p> <p>Background</p> <p>Control of skeletal muscle mass and force production is a complex physiological process involving numerous regulatory systems. Agents that increase skeletal muscle cAMP levels have been shown to modulate skeletal muscle mass and force production. The dopamine 1 receptor and its closely related homolog, the dopamine 5 receptor, are G-protein coupled receptors that are expressed in skeletal muscle and increase cAMP levels when activated. Thus we hypothesize that activation of the dopamine 1 and/or 5 receptor will increase skeletal muscle cAMP levels thereby modulating skeletal muscle mass and force production.</p> <p>Methods</p> <p>We treated isolated mouse tibialis anterior (TA) and medial gastrocnemius (MG) muscles in tissue bath with the selective dopamine 1 receptor and dopamine 5 receptor agonist SKF 81297 to determine if activation of skeletal muscle dopamine 1 and dopamine 5 receptors will increase cAMP. We dosed wild-type mice, dopamine 1 receptor knockout mice and dopamine 5 receptor knockout mice undergoing casting-induced disuse atrophy with SKF 81297 to determine if activation of the dopamine 1 and dopamine 5 receptors results in hypertrophy of non-atrophying skeletal muscle and preservation of atrophying skeletal muscle mass and force production.</p> <p>Results</p> <p>In tissue bath, isolated mouse TA and MG muscles responded to SKF 81297 treatment with increased cAMP levels. Treating wild-type mice with SKF 81297 reduced casting-induced TA and MG muscle mass loss in addition to increasing the mass of non-atrophying TA and MG muscles. In dopamine 1 receptor knockout mice, extensor digitorum longus (EDL) and soleus muscle mass and force was not preserved during casting with SKF 81297 treatment, in contrast to significant preservation of casted wild-type mouse EDL and soleus mass and EDL force with SKF 81297 treatment. Dosing dopamine 5 receptor knockout mice with SKF 81297 did not significantly preserve EDL and soleus muscle mass and force although wild-type mouse EDL mass and force was significantly preserved SKF 81297 treatment.</p> <p>Conclusions</p> <p>These data demonstrate for the first time that treatment with a dopamine 1/5 receptor agonist results in (1) significant preservation of EDL, TA, MG and soleus muscle mass and EDL muscle force production during periods of atrophy and (2) hypertrophy of TA and MG muscle. These effects appear to be mainly mediated by both the dopamine 1 and dopamine 5 receptors.</p

Methionine metabolism in an animal model of sepsis

Background: Sepsis is a disease with high incidence and lethality and is accompanied by profound metabolic disturbances. In mammalian methionine metabolism, S-adenosylmethionine (SAM) is produced, which is important in the synthesis of neurotransmitters and glutathione and as an anti-inflammatory agent. The degradation product and antagonist of SAM is S-adenosylhomocysteine (SAH). In this study, we investigated changes in methionine metabolism in a rodent model of sepsis. Methods: Sepsis was induced in male Wistar rats (n=21) by intraperitoneal injection of bacterial lipopolysaccharide (10 mg/kg). Controls (n=18) received vehicle only. Blood was collected by cardiac puncture 24 h later. Puncture of the suboccipital fossa was performed to collect cerebrospinal fluid (CSF). Methionine metabolites were measured using stable isotope dilution tandem mass spectrometry. Plasma total homocysteine and cysteine were measured by HPLC using fluorescence detection. Glutathione was assayed using a modified enzymatic microtiter plate assay. Results: We observed significantly higher plasma levels of SAM (p<0.001) and SAM/SAH ratio (p=0.004) in septic animals. In CSF, there was also a trend for higher levels of SAM in septic animals (p=0.067). Oxidative stress was reflected by an increase in the ratio of oxidized/reduced glutathione in septic animals (p=0.001). Conclusions: Sepsis is associated with an increase in SAM/SAH ratio in plasma and CSF in rodents. This indicates an altered methylation potential during sepsis, which may be relevant for sepsis-associated impairment of transmethylation reactions, circulation and defense against oxidative stress. If verified in humans, such findings could lead to novel strategies for supportive treatment of sepsis, as methionine metabolism can easily be manipulated by dietary strategies

Skeletal muscle protein balance in mTOR heterozygous mice in response to inflammation and leucine

Sepsis and lipopolysaccharide (LPS) may decrease skeletal muscle protein synthesis by impairing mTOR (mammalian target of rapamycin) activity. The role of mTOR in regulating muscle protein synthesis was assessed in wild-type (WT) and mTOR heterozygous (+/−) mice under basal conditions and in response to LPS and/or leucine stimulation. No difference in body weight of mTOR+/− mice was observed compared with WT mice; whereas whole body lean body mass was reduced. Gastrocnemius weight was decreased in mTOR+/− mice, which was attributable in part to a reduced rate of basal protein synthesis. LPS decreased muscle protein synthesis in WT and mTOR+/− mice to the same extent. Reduced muscle protein synthesis in mTOR+/− mice under basal and LPS-stimulated conditions was associated with lower 4E-BP1 and S6K1 phosphorylation. LPS also decreased PRAS40 phosphorylation and increased phosphorylation of raptor and IRS-1 (Ser307) to the same extent in WT and mTOR+/− mice. Muscle atrogin-1 and MuRF1 mRNA content was elevated in mTOR+/− mice under basal conditions, implying increased ubiquitin-proteasome-mediated proteolysis, but the LPS-induced increase in these atrogenes was comparable between groups. Plasma insulin and IGF-I as well as tissue expression of TNFα, IL-6, or NOS2 did not differ between WT and mTOR+/− mice. Finally, whereas LPS impaired the ability of leucine to stimulate muscle protein synthesis and 4E-BP1 phosphorylation in WT mice, this inflammatory state rendered mTOR+/− mice leucine unresponsive. These data support the idea that the LPS-induced reduction in mTOR activity is relatively more important in regulating skeletal muscle mass in response to nutrient stimulation than under basal conditions