| Abstract: | Calcium ion decreases the motional freedom of lipid molecules in isolated rat hepatocyte plasma membranes and in sonicated dispersions (liposomes) of the membrane lipid. The decrease in lipid fluidity was monitored by estimation of the fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene. At least two processes are involved in the mode of action of the cation. The first is direct, i.e., observed on addition of calcium to the liposomes, relatively rapid, with a half-time of 10-15 at 37 degrees C, proportional to the calcium concentration in the range 0-4 mM, and readily reversed on addition of excess EDTA. The second mechanism is indirected and requires the presence of the membrane proteins. It occurs relatively slowly, with a half-time of 75 min at 37 degrees C, tends to plateau with a calcium half-saturation concentration of approximately 1 mM, is of greater magnitude than the direct effect, and cannot be reversed on chelation of calcium by EDTA. Moreover, the indirect effect is specific for Ca2+ as compared to other divalent cations and it results in changes in the lipid composition. Stimulation of phospholipase A activity is likely but does not account for the change in fluidity. The direct action of calcium is ascribed to binding to the lipid bilayer, whereas the indirect action probably results from modulation of membrane-bound enzymes which can alter the lipid composition. The effects of calcium on the membrane lipid fluidity may underly certain of its regulatory actions on membrane functions. |
