Decreased contraction-stimulated glucose transport in isolated epitrochlearis muscles of pregnant rats

Fiber type effects on contraction-stimulated glucose uptake and GLUT4 abundance in single fibers from rat skeletal muscle

To fully understand skeletal muscle at the cellular level, it is essential to evaluate single muscle fibers. Accordingly, the major goals of this study were to determine if there are fiber type-related differences in single fibers from rat skeletal muscle for: 1) contraction-stimulated glucose uptake and/or 2) the abundance of GLUT4 and other metabolically relevant proteins. Paired epitrochlearis muscles isolated from Wistar rats were either electrically stimulated to contract (E-Stim) or remained resting (No E-Stim). Single fibers isolated from muscles incubated with 2-deoxy-d-[(3)H]glucose (2-DG) were used to determine fiber type [myosin heavy chain (MHC) isoform protein expression], 2-DG uptake, and abundance of metabolically relevant proteins, including the GLUT4 glucose transporter. E-Stim, relative to No E-Stim, fibers had greater (P < 0.05) 2-DG uptake for each of the isolated fiber types (MHC-IIa, MHC-IIax, MHC-IIx, MHC-IIxb, and MHC-IIb). However, 2-DG uptake for E-Stim fibers was not significantly different among these five fiber types. GLUT4, tethering protein containing a UBX domain for GLUT4 (TUG), cytochrome c oxidase IV (COX IV), and filamin C protein levels were significantly greater (P < 0.05) in MHC-IIa vs. MHC-IIx, MHC-IIxb, or MHC-IIb fibers. TUG and COX IV in either MHC-IIax or MHC-IIx fibers exceeded values for MHC-IIxb or MHC-IIb fibers. GLUT4 levels for MHC-IIax fibers exceeded MHC-IIxb fibers. GLUT4, COX IV, filamin C, and TUG abundance in single fibers was significantly (P < 0.05) correlated with each other. Differences in GLUT4 abundance among the fiber types were not accompanied by significant differences in contraction-stimulated glucose uptake.

Postpartum glycemic homeostasis in early lactating rats is accompanied by transient and specific increase of soleus insulin response through IRS2/AKT pathway

It is known that at the moment of delivery immediate lost of conceptus (main site of glucose disposal in late pregnancy) is not able to disturb glucose homeostasis in early lactating mothers. However, the mechanism by which this adaptation takes place in early lactation is still unknown. Most studies concerning insulin sensitivity in lactating rats were carried out at 11–13 days postpartum and did not describe functional changes in insulin response in early lactation. Here we show that lactation hypersensitivity to insulin is observed as early as 3 days after delivery (L3). We show that the oxidative soleus muscle displays a transient increased maximal insulin-induced glucose uptake and CO2production, which is temporally limited to L3. Response of soleus muscle was accompanied by an increase in glucose transporter 4 (GLUT4) content at L3. This adaptive response was not detected in the glycolytic plantaris muscle, which displayed lower content of GLUT4. We also found that soleus muscle from early lactating rats have higher insulin receptor expression and tyrosine phosphorylation. Downstream steps of insulin signaling pathway; e.g., insulin receptor substrate 2 tyrosine phosphorylation and its association with phosphatidylinositol 3-kinase were also upregulated in soleus muscle. In parallel, protein tyrosine phosphatase 1B expression, a negative regulator of insulin signal, was reduced. Importantly, all of these molecular alterations were time limited to L3 and were not observed in plantaris muscle. These results suggest that improved insulin action in oxidative, but not in glycolytic muscle might contribute to achievement of glucose homeostasis postpartum.