Lipid fluidity of hepatocyte plasma membrane subfractions and their differential regulation by calcium

Rat hepatocyte plasma membranes were subfractionated by several methods into canalicular, sinusoidal and mixed contiguous plus sinusoidal membranes. Assessment of lipid fluidity by steady-state fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene and 12-(9-anthroyloxy)stearate indicates that the canalicular fraction is less fluid than the other membranes. Incubation with calcium decreases the fluidity of the sinusoidal and contiguous membranes by altering the lipid composition, an action which is not reversed by subsequent chelation of the cation. This effect of calcium is not observed in canalicular membranes.