Stimulation of Potassium Efflux in Rat Liver by a Low Dose of Thyroid Hormone: Evidence for Enhanced Cation Permeability in the Absence of Na,K-ATPase Induction*

The effects of a low dose of T3 on passive cellular K+ efflux and Na,K-ATPase activity were studied in hypothyroid rat liver. Male Sprague-Dawley rats were rendered hypothyroid by 4 weeks of a low iodine diet with 0.5% NaClO4 added to the drinking water, and for the last 2 weeks of this period received daily sc injections of either T3 (1 microgram/100 g BW) or diluent alone. At the end of this time, both the passive efflux of 86Rb+ (a K+ analog) from liver slices isotopically prelabeled in vitro and Na,K-ATPase activity in liver homogenates were determined. The T3 treatment regimen resulted in a 55% increase in the 86Rb+ efflux rate constant (P less than 0.003), while, in contrast, Na,K-ATPase activity remained unchanged. These results show that T3, even at a low dose, enhances passive K+ efflux from liver slices and that, consistent with previous observations, this enhancement can occur in the absence of any detectable change in the number of Na,K pumps. Since the rate of Na,K pump function appears in general to be limited by the rate of passive cation permeation, rather than by Na,K pump number per se, these observations provide additional evidence that increased cation permeability may play a role in the stimulation of active cation transport by thyroid hormone.