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

Abstract

1. 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°C temperature range.

2. 2. In isolated microsomes at 37°C the majority of the phospholipids undergo isotropic motion, whereas at 4°C most of the phospholipids give rise to typical ‘bilayer’ spectra.

3. 3. Isolated hydrated rat liver microsomal phosphatidylethanolamine is organised in the hexagonal HII phase above 7°C.

4. 4. The Mn2+ permeability of the microsomal membrane is strongly temperature dependent. At 37°C Mn2+ addition eliminates the entire 31P-NMR spectrum, demonstrating that all phospholipids interact with Mn2+. At 4°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. 5. In liver slices incubated in oxygenated Krebs-Ringer buffer at 4°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°C, virtually all phospholipids in all membranes have 31P-NMR spectra indicating bilayer structure. At 37°C approx. 14% of the phospholipids undergo isotropic motion.

6. 6. Preliminary experiments on perfused rat liver show stable ATP levels for 4 h at 37°C. The spectra, furthermore, suggest similar behaviour for the membrane phospholipids to that observed in the liver slices.