Sarcoplasmic Reticulum Phospholipid Fatty Acid Composition and Sarcolipin Content in Rat Skeletal Muscle.

In a previous study, we reported lower sarcoplasmic reticulum (SR) Ca(2+) pump ionophore ratios in rat soleus compared to red and white gastrocnemius (RG, WG) muscles which may be indicative of greater SR Ca(2+) permeability in soleus. Here we assessed the lipid composition of the SR membranes obtained from these muscles to determine if SR docosahexaenoic acid (DHA) content and fatty acid unsaturation could help to explain the previously observed differences in SR Ca(2+) permeability. Since we have shown previously that sarcolipin may also influence SR Ca(2+) permeability, we also examined the levels of sarcolipin in rat muscle. We found that SR membrane DHA content was significantly higher in soleus (5.3 ± 0.2 %) compared to RG (4.2 ± 0.2 %) and WG (3.3 ± 0.2 %). Likewise, total SR membrane unsaturation and unsaturation index (UI) were significantly higher in soleus (% unsaturation: 59.1 ± 2.4; UI: 362.9 ± 0.8) compared to RG (% unsaturation: 55.3 ± 1.0; UI: 320.9 ± 2.5) and WG (% unsaturation: 52.6 ± 1.1; UI: 310. ± 2.2). Sarcolipin protein was 17-fold more abundant in rat soleus compared to RG and was not detected in WG; however, comparisons between soleus, RG, and WG in sarcolipin-null mice revealed that, in the absence of sarcolipin, ionophore ratios are still lowest in soleus and highest in WG. Overall, our results suggest that SR membrane DHA content and unsaturation, and, in part, sarcolipin expression may contribute to SR Ca(2+) permeability and, in turn, may have implications in muscle-based metabolism and diet-induced obesity.