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 X 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.