The effect of calcium on the bilayer stability of lipids from bovine rod outer segment disk membranes

The phase behavior of bovine rod outer segment disk lipids has been investigated using freeze-fracture and 31P nuclear magnetic resonance (NMR) techniques. 31P-NMR spectra of isolated disk membranes were taken as a function of temperature between 25 degrees C and 45 degrees C. The 31P-NMR spectrum characteristic of phospholipid bilayers was observed at all temperatures both in the absence of Ca2+ and in the presence of 10 mM and 50 mM Ca2+. A similar study was performed on lipids isolated from the disk membranes. In the absence of Ca2+ only lamellar phase behavior was observed. In the presence of less than 10 mM Ca2+, however, there was a change in morphology to non-lamellar structures. Removal of the Ca2+ caused the system to reassume the lamellar form.