ADAPTIVE-CHANGES IN LIPID-COMPOSITION OF SKELETAL SARCOPLASMIC-RETICULUM MEMBRANES ASSOCIATED WITH AGING

We have undertaken a detailed examination of changes associated with a ging in lipid composition and corresponding physical properties of hin dlimb skeletal sarcoplasmic reticulum (SR) membranes isolated from you ng (5 months), middle-aged (16 months), and old (28 months) Fischer st rain 344 rats. Silica gel HPLC chromatography was used to separate pho spholipid headgroup species. Subsequent reversed-phase HPLC was used t o resolve fatty acid chain compositions of phosphatidylcholine, phosph atidylethanolamine, and phosphatidylinositol species. For all three ph ospholipid pools, significant age-related variations are observed in t he abundance of multiple molecular species, particularly those having polyunsaturated fatty acid chains. Using mass spectrometry (fast atom bombardment and tandem techniques) to distinguish ester- from ether-li nked phosphatidylethanolamine species, we demonstrate that overall pla smenylethanolamine content is substantially increased with age, from 4 8 mol% to 62 mol%. A substantial increase is also observed in the sing le molecular species 18:0-20:4 phosphatidylinositol suggesting implica tions for signalling pathways. In addition, associated with senescence we find a significant increase in the rigidifying lipid, cholesterol. Despite these changes in lipid composition of different aged animals, the average bilayer fluidity examined at several bilayer depths with stearic acid spin labels, is not altered. Neither do we find differenc es in the rotational mobility of maleimide spin-labeled Ca2+-ATPase, a s determined from saturation-transfer electron paramagnetic resonance, which is sensitive to both the fluidity of lipids directly associated with the Ca2+-ATPase and to its association with proteins.