31P-NMR studies on membrane phospholipids in microsomes, rat liver slices and intact perfused rat liver

1. The 36.4 and 81 MHz 31P-NMR spectra of isolated rat liver microsomes, rat liver slices and perfused rat liver have been recorded in the 4-40 degree C temperature range. 2. In isolated microsomes at 37 degrees C the majority of the phospholipids undergo isotropic motion, whereas at 4 degrees C most of the phospholipids give rise to typical 'bilayer' spectra. 3. Isolated hydrated rat liver microsomal phosphatidylethanolamine is organised in the hexagonal HII phase above 7 degrees C. 4. The Mn2+ permeability of the microsomal membrane is strongly temperature dependent. At 37 degrees C Mn2+ addition eliminates the entire 31P-NMR spectrum, demonstrating that all phospholipids interact with Mn2+. At 4 degrees C a 43% reduction in signal intensity is observed, indicating that at this temperature 43% of the phospholipids are located in the outer monolayer of a lipid bilayer. 5. In liver slices incubated in oxygenated Krebs-Ringer buffer at 4 degrees C there is a rapid decrease in ATP level such that within 20 min almost all ATP is degraded. In these ATP-depleted liver slices at 4 degrees C, virtually all phospholipids in all membranes have 31P-NMR spectra indicating bilayer structure. At 37 degrees C approx. 14% of the phospholipids undergo isotropic motion. 6. Preliminary experiments on perfused rat liver show stable ATP levels for 4 h at 37 degrees C. The spectra, furthermore, suggest similar behavior for the membrane phospholipids to that observed in the liver slices. 7. The possible sources of teh observed isotropic motion of the membrane phospholipids are discussed.