Viscoelastic and polyelectrolytic heparan sulfate proteoglycan (syndecan) integrated into the membrane of vascular endothelial cells may serve as a flow sensor.This biosensor macromolecule responds to shear stress by a conformational change. Cations function as a first messenger in the signal transduction chain for a dilatory vessel reaction with increasing blood flow. Application of 23Na + NMR techniques proved useful in the characterization of shear stress-dependent conformational changes and reversible Na+ binding of proteoheparan sulfate, a strongly negatively charged proteoglycan. Ca2+ ions interfere with Na+ uptake and release in a competitive or cooperative manner. The adsorption of heparan sulfate proteoglycan at hydrophobic silica surfaces, as measured by in situ ellipsometry, was shown to be dependent on both general electrostatic and cation-specific ion binding actions. The interfacial behaviour of this macromolecule is characterized by an increase in the adsorbed amount upon addition of Ca2+, while Mg2+ induces the opposite effect of much smaller magnitude.