OBJECTIVE: The direct effects of adipokines on skeletal muscle metabolism have been well established. As the combinatorial effects of adipokine mixtures are likely to be of more physiological relevance, we used a coculture system of primary rat adipocytes and L6 skeletal muscle cells to examine the effects of adiponectin derived from primary rat adipocytes on rat skeletal muscle cells. RESULTS: We showed that coculture with adipocytes stimulated glucose uptake in L6 cells within 30 min and this correlated with an increase of glucose transporter isoform 4 (GLUT4) localization to the plasma membrane. These effects were dependent on the reorganization of the actin cytoskeleton, demonstrated by rhodamine-labeled phalloidin immunofluorescence, as cytochalasin D attenuated the glucose uptake induced by adipocyte-conditioned media. Temporal analysis revealed that enhanced glucose uptake was maintained after 24 h of coculture, and this was attributed to an increase in both GLUT1 expression and the cell surface content of GLUT4. We established a role for adiponectin in mediating these effects as antibody-mediated neutralization attenuated the metabolic effects of adipocyte-conditioned media. Furthermore, compound C blocked these effects, suggesting an important role for AMPK. Importantly, when we compared the effects of full-length recombinant adiponectin with adipocyte-conditioned media, we confirmed that recombinant adiponectin was unable to stimulate glucose uptake in L6 cells despite having an important role in adipocyte-conditioned media. CONCLUSIONS: Our results demonstrate the importance of examining the effects of adipokines in the context of physiologically relevant mixtures to accurately determine their metabolic effects on skeletal muscle.
OBJECTIVE: The direct effects of adipokines on skeletal muscle metabolism have been well established. As the combinatorial effects of adipokine mixtures are likely to be of more physiological relevance, we used a coculture system of primary rat adipocytes and L6 skeletal muscle cells to examine the effects of adiponectin derived from primary rat adipocytes on rat skeletal muscle cells. RESULTS: We showed that coculture with adipocytes stimulated glucose uptake in L6 cells within 30 min and this correlated with an increase of glucose transporter isoform 4 (GLUT4) localization to the plasma membrane. These effects were dependent on the reorganization of the actin cytoskeleton, demonstrated by rhodamine-labeled phalloidin immunofluorescence, as cytochalasin D attenuated the glucose uptake induced by adipocyte-conditioned media. Temporal analysis revealed that enhanced glucose uptake was maintained after 24 h of coculture, and this was attributed to an increase in both GLUT1 expression and the cell surface content of GLUT4. We established a role for adiponectin in mediating these effects as antibody-mediated neutralization attenuated the metabolic effects of adipocyte-conditioned media. Furthermore, compound C blocked these effects, suggesting an important role for AMPK. Importantly, when we compared the effects of full-length recombinant adiponectin with adipocyte-conditioned media, we confirmed that recombinant adiponectin was unable to stimulate glucose uptake in L6 cells despite having an important role in adipocyte-conditioned media. CONCLUSIONS: Our results demonstrate the importance of examining the effects of adipokines in the context of physiologically relevant mixtures to accurately determine their metabolic effects on skeletal muscle.