Propranolol-induced alterations of membrane structure were studied in rat erythrocytes using electron spin resonance techniques. Propranolol produced a concentration-dependent change in membrane fluidity in hydrophobic membrane regions, while producing virtually no change in hydrophilic membrane regions. The changes were associated with depth-dependent alterations in "apparent" phase-transition profiles and transition temperatures. The effects of propranolol on these membrane characteristics were similar to those produced by cholesterol. Propranolol fluidized erythrocyte membranes in a depth-specific fashion, by virtue of its association with the rigid phospholipid acyl chains and cholesterol sterol rings in the hydrophilic regions of the membrane, which produced distant perturbations within the hydrophilic regions of the membrane.
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