Stimulation of active na⁺ and k⁺ transport by thyroid hormone in a rat liver cell line: Role of enhanced na⁺ entry

A continuous cell line derived from rat liver (ARL 15) has been identified that responds to thyroid hormone with a stimulation of active Na, K transport. Stimulation of ouabain-inhibitable K⁺ uptake, which is half-maximal at a T³ concentration of 1.4 × 10^-10 M, is accompanied by corresponding increases in passive Na⁺ influx and in passive K⁺ efflux. The enhancement of Na⁺ and K⁺ fluxes by T³ is shown to be accompanied by smaller and equivalent increases both in enzymatically measured Na, K-ATPase activity and in maximal ouabain- sensitive Na, K transport in the presence of the Na⁺ ionophore monensin. The demonstration that both passive Na⁺ influx and passive fractional K⁺ efflux are simultaneously increased by T³ supports the earlier suggestion that the stimulation of active Na, K transport by thyroid hormone is attributable, at least in part, to an enhancement of rate-limiting passive Na⁺ and K⁺ fluxes by an increase in membrane permeability.