Mechanisms of fatty acid effects on sarcoplasmic reticulum. II. Structural changes induced by oleic and palmitic acids.

The interaction of micromolar concentrations of palmitic and oleic acids with the sarcoplasmic reticulum membrane was studied by electron microscopic techniques in an attempt to define their different effects on ATP-induced calcium sequestration in sarcoplasmic reticulum vesicles. Oleic acid had a concentration-dependent effect on the morphology of sarcoplasmic reticulum vesicles, promoting vesicle fusion and eventual solubilization. Palmitic acid did not alter the morphology of sarcoplasmic reticulum, but its probable site(s) of interaction could be determined. In the presence of palmitic acid, large lamellar structures that formed external to sarcoplasmic reticulum vesicles are probably composed of pure palmitic acid and/or palmitic acid/phospholipid mixed "micelles," but internalization of palmitic acid into sarcoplasmic reticulum vesicles was not detected. Palmitic acid reduced the phospholipid content of sarcoplasmic reticulum membranes with a preservation of the average interparticle protein spacing as observed in freeze-fracture electron micrographs. Thus, palmitic acid appears to be incorporated into the sarcoplasmic reticulum lipid bi-layer. Oleic acid inhibition of ATP-induced calcium sequestration by sarcoplasmic reticulum vesicles is probably caused by net permeability changes of the membrane. A structural mechanism for palmitic acid stimulation of ATP-induced calcium sequestration is proposed in light of the probable insertion of palmitic acid into the sarcoplasmic reticulum lipid bilayer.