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% NaCIO₄added to the drinking water, and for the last 2 weeks of this period received daily sc injections of either T₃ (1 μg/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 < 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.