MODULATION OF ARACHIDONIC-ACID RELEASE AND MEMBRANE FLUIDITY BY ALBUMIN IN VASCULAR SMOOTH-MUSCLE AND ENDOTHELIAL-CELLS

Albumin is the major plasma protein circulating in blood. Albumin pote ntly decreases capillary permeability, although the mechanisms are not understood completely. Albumin also effectively binds arachidonic aci d (AA), which increases capillary permeability. To investigate the int eractions of BSA and AA with the cell membrane, the effect of these su bstances on [H-3]AA release and membrane fluidity was studied in vascu lar myocytes and endothelial cells. BSA (0.2 and 1 mg.mL(-1)) stimulat ed a significant release of [H-3]AA from both intact rat aorta and cul tured smooth muscle cells. This effect was not mimicked by gamma-globu lin or myoglobin (both 1 mg.mL(-1)) in intact tissue. BSA, but not gam ma-globulin and myoglobin, decreased the membrane fluidity (assessed a s changes in the steady-state fluorescence anisotropy of 1,6-diphenyl- 1,3,5-hexatriene) in a concentration-dependent manner with a half-maxi mum concentration between 0.007 and 0.4 mg.mL(-1) in both freshly isol ated and cultured rat aortic myocytes and human umbilical vein endothe lial cells. AA (1 to 200 mu mol/L) caused the opposite effect, increas ing membrane fluidity and antagonizing the effect of BSA. BSA modified at its arginine residues, which are thought to be important in AA bin ding, did not stimulate [H-3]AA release and was significantly less pot ent than native BSA in altering the membrane fluidity. The effect of B SA can be explained by a high-affinity binding of AA to the protein an d extraction of AA from the cell membrane. The interaction between BSA and AA could play a role in the regulation of vascular permeability.