Enhanced Stimulation of Diacylglycerol and Lipid Synthesis by Insulin in Denervated Muscle: Altered Protein Kinase C Activity and Possible Link to Insulin Resistance

Denervated muscle is generally regarded as insulin resistant because the ability of insulin to stimulate glucose transport and glycogen synthesis is impaired. Previous studies indicate that insulin resistance in these muscles is likely due to a defect at a postreceptor site in the signaling pathway. Because glucose transport into cells has been reported to be linked to changes in diacylglycerol (DAG) and protein kinase C (PKC), we investigated the effect of denervation on the content and synthesis of DAG and the activity and distribution of PKC in the soleus muscle. The DAG content in muscles denervated for 24 h was 40% > in control muscles. This was associated with a two- to threefold increase in the percentage of total PKC activity that was membrane associated, with no significant change in total PKC activity, suggesting an increase in PKC activity in vivo. Studies of glucose disposition confirmed that the stimulation of glycogen synthesis by insulin and, to a lesser extent, 2-deoxyglucose uptake were impaired by denervation. However, the stimulation by insulin of glucose incorporation into DAG and other lipids was two- to threefold greater in denervated than in control muscles, and conversion of glucose to lactate and pyruvate and glucose oxidation to CO₂ were unchanged. The results reveal a dichotomy in the effects of denervation on various actions of insulin, with both insulin resistance and hyperresponsiveness occurring in different pathways of glucose metabolism. They also reveal a potential mechanism for the elevation of muscle DAG after denervation. The results do not support a direct link between DAG-PKC and glucose transport. Based on these findings, a working hypothesis that links enhanced DAG-PKC signaling to insulin resistance is proposed.