Effect of triiodothyronine on system A amino acid transport in cells of rat submandibular gland.

The effect of L-3,5,3'-triiodothyronine (T3) on alpha-aminoisobutyric acid (AIB) transport in isolated cell suspensions of rat submandibular gland was investigated. The uptake of ATB by these cells appeared to require extracellular Na+ and was inhibited by ouabain (10(-3) M). Cell suspensions from thyroidectomized rats which have been given three successive doses of T3 on alternate days (50 micrograms/100 g BW) showed a significant increase in AIB uptake compared with cells isolated from thyroidectomized rats treated with diluent. Efflux of AIB from the cell suspension was not affected by T3. There was no significant changes in AIB uptake 12 h after a single injection of T3 (50 micrograms/100 g BW). However, there was a significant 49% and 65% increase in AIB net uptake at 24 and 48 h, respectively, after T3 treatment. Under similar conditions, the cell suspension showed a 48% increase in NaK-ATPase activity at 12 h and to a peak of 61% at 24 h. Therefore, changes in NaK-ATPase activity preceded the changes in AIB net uptake upon treatment with T3, implying that AIB uptake is probably mediated, at least in part, by the activity of NaK-ATPase.