Abstract
The action of L-triiodothyronine (T3) on amino acid transport in the GC clonal strain of rat pituitary cells was investigated by measurement of the uptake of the nonmetabolizable amino acid, alpha-aminoisobutyric acid (AIB). The uptake of AIB by GC cells appeared to require energy and Na+ and displayed Michaelis-Menten kinetics. In comparison to cultures maintained in the absence of T3, T3 addition resulted in an increase in AIB uptake which seemed due to an increase in the initial rate of AIB transport. T3 addition resulted in increased AIB accumulation at later time points as well. T3 induction of AIB transport did not occur until 3.5 h after addition of T3, and this effect was blocked by cycloheximide. Maximal induction occurred 48 to 72 h later. One-half maximal induction occurred 24 to 48 h after addition of T3. No detectable changes either in AIB uptake or intracellular water space, measured by uptake of the nonmetabolizable sugar, 3-O-methyl-D-glucose, were noted for the first 120 min after addition of T3. Induction of AIB transport occurred at 0.05 nM T3 (total medium concentration) and one-half maximal induction occurred at 0.17 nM T3. The relative potencies of four iodothyronine analogues for AIB transport were in accord with their reported activities in nuclear T3 receptor binding assays. These data suggest that induction of AIB transport by T3 may be mediated by the nuclear T3 receptor and may reflect the pleiotrophic response of GC cells to thyroid hormone.
Highlights
RESULTSMedium was removed and replaced with 2.5 to 3.0 ml of the same experimental medium until the experiment was performed
In comparison to cultures maintained in the absence of T3,T3 addition resulted in an increase inAIB uptake which seemed due to an increase in the initial rate of AIB transport
Our present findings clearly demonstrate that T3 stimulates the influx of AIB into cultured GC cells
Summary
Medium was removed and replaced with 2.5 to 3.0 ml of the same experimental medium until the experiment was performed. The medium was removed, and the cells were rinsed and incubated in DBSS, 20 mM Hepes, pH 7.4, with different concentrations of 3-O-[methyl-“C]-~glucose from 0.1 to 10 mM for 30 min at 37 “C.The uptake of 3methylglucosewas measured as described under “Experimental Procedures.”Each point represents the mean value for three observations with variation of less than 10%.The lines were calculated by least mean squares. GC cell cultures maintained maintained in the absence of Tarthe shape of the curve in the absence of T3 had a 30%decrease in V,, forAIB describing AIB uptake with time was similar to DMEM +T3 transport The intracellular water space was unchanged when measured during the first 60 min after T3 was added; 0.55 & 0.03 p1/106 cells for -T3 conditions and0.57 f 0.03 @I/ IO6cells for +T3conditions
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