Extracellular Na can be accurately partitioned into free and bound components; the latter component is directly related to the hydration and stiffness of the vascular wall. Cell Na can be similarly partitioned: [Na+]i is about 10 mM and the transmembrane gradient about 15:1 in a physiological medium. The transmembrane gradient remains constant over a wide range of change in [Na+]0· Although active Na transport is largely responsible for developing the transmembrane ion distribution pattern, the membrane potential depends mainly on movements of K+ and Cl-No certain direct role can be assigned to Na+ in short-term vasoconstriction, but an indirect role through the regulation of catecholamine release and uptake is now established. There is evidence that the entry of Na+ into cells plays a fundamental role in the protein-synthesizing activity of the vascular smooth muscle cell and is thereby involved in the longer term determination of peripheral vascular resistance