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

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Abstract
The entry of Li into the vascular smooth muscle cells of the rat tail artery follows 1st-order kinetics with a rate constant of approximately 1.3 h-1 at 10 mM-[Li]o (initial concentration). The rate constant decreases gradually to about 0.5 h-1 when the [Li]o/[Na]o ratio is increased. Replacement of Na with Li over the range of [Li]o from 1-115 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 cell K and Na such that the ratio of cell K to cell Na remains constant at about 10:1 throughout. The changes in the ionic contents, induced by equilibration of the tissue with a Na-free, Li-substituted solution, are reversible. 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, about 10 mmol/kg dry wt, does not participate in this exchange. 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.