INCREASED LIPID FLUIDITY IN SYNAPTOSOMES FROM BRAINS OF HYPEROSMOLAL RATS

Taurine, a product of sulfur amino acid metabolism is important in cer ebral osmoregulation. To understand the adaptive changes in transport which accompany different hyperosmolal states, we determined lipid com position and fluorescence anisotropy of synaptosomal liposomes from ra ts with chronic hypernatremic dehydration (CHD), streptozocin-induced (STZ) diabetes, and insulin treated diabetes. Induction of CHD increas ed serum osmolality, and enhanced in vitro synaptosomal taurine uptake (P < 0.01, n = 3, vs. control). Fluorescence anisotropy studies showe d that the fluidity of lipids from CHD synaptosomes was higher than co ntrol (P < 0.05, n = 3). STZ-diabetes resulted in hyperglycemia, incre ased serum osmolality, and stimulated synaptosomal taurine uptake (P < 0.01, n = 3, vs. control). Insulin treatment of diabetic rats restore d serum osmolality and taurine transport to control values. The fluidi ty of diabetic rat brain synaptosomal lipids was significantly higher than control (P < 0.05, n = 3); fluidity was normalized by insulin adm inistration to diabetic rats. Total fatty acid, cholesterol, and chole sterol/phospholipid molar ratio of CHD, STZ, and insulin treated diabe tic rats were similar to control. However, the ratio of saturated to u nsaturated fatty acids was decreased in hyperosmolal states. This sugg ests that adaptive increases in cerebral taurine transport during hype rosmolality may result from a direct effect on membrane composition th at alters fluidity and permits enhanced transmembrane flux of osmoprot ective molecules.