Spin label studies on rat liver plasma membrane: calcium effects on membrane fluidity.

Spin-labeled stearic acid probes were incorporated into rat liver plasma membrane purified by a zonal centrifugation procedure. Temperature effects on the order parameter calculated from the spectra indicated the possible presence of a membrane phase transition at 20 degrees, in agreement with earlier studies in other systems. Addition of Ca2+ (1.96 mM) at 37 degrees increased the order parameter by 6.0 +/- 0.7%, suggesting a Ca2+-mediated decrease in membrane fluidity.