The effects of external sodium substitution on cell sodium and potassium in vascular smooth muscle.

1. The entry of Li into the vascular smooth muscle cells of the rat tail artery follows first-order kinetics with a rate constant of approximately 1.3 hr(-1) at 10 mM-[Li](o). The rate constant decreases gradually to ca. 0.5 hr(-1) when the [Li](o)/[Na](o) ratio is increased.2. Replacement of Na with Li over the range of [Li](o) from 1 to 115 mM, accomplished at constant ionic strength and osmolarity of the bathing solution, produces changes in cell Na and K without apparent change in cell water. At equilibrium, cell Li increases in linear proportion to [Li](o), at a ratio of 2:1 throughout the range. The increase in cell Li is associated with inverse falls in both cell K and Na such that the ratio of cell K to cell Na remains constant at ca. 10:1 throughout.3. The changes in the ionic contents, induced by equilibration of the tissue with a Na-free, Li-substituted solution, are reversible.4. Replacement of Na with sucrose over the range of 40-115 mM results, at equilibrium, in a linear fall in cell Na without conspicuous change in cell K. A constant portion of the cell Na, ca. 10 m-mole/kg dry wt., does not participate in this exchange.5. At equilibrium, reductions in [Na](o) are reflected in corresponding reductions in apparent [Na](i) such that the [Na](o)/[Na](i) ratio remains constant.