Iodothyronine Interactions with the System L1 Amino Acid Exchanger in 3T3-L1 Adipocytes

Figure 6

Triiodothyronine inhibits the trans stimulatory effect of leucine on phenylalanine efflux from 3T3-L1 adipocytes. (a) Effect of iodothyronines on leucine trans stimulation of phenylalanine efflux. Release of preloaded [³H]phenylalanine tracer from cells was measured in the presence or absence of 10 $\mu$M leucine $\pm$ either T₃, T₄, rT₃ or TRIAC (each at 10 $\mu$M). The rate constants for efflux were calculated as described for Figure 4 and the change in rate constant from control ($\mathrm{\Delta }$ rate constant) was calculated by subtracting the control efflux (without leucine) from that in the presence of leucine $\pm$ iodothyronine/TRIAC. Values are shown $\pm$ SEM for 12–18 measurements. *$P<.05$, **$P<.01$; value significantly different from control (leucine only) value. (b) Concentration dependence of T₃ inhibition of leucine trans stimulated phenylalanine efflux. Release of preloaded [³H]phenylalanine tracer from cells was measured in the presence of 10 $\mu$M leucine $\pm$ 5 different T₃ concentrations (between 1 and 5 $\mu$M) as indicated. The rate constants for efflux were calculated as described for Figure 4 and then as the percentage of efflux in the presence of leucine alone. Values are shown $\pm$ SEM for 6 measurements (data points obscure error bars). The dotted lines indicate that 50% inhibition of 10 $\mu$M leucine trans stimulated phenylalanine efflux is achieved with 3.2 $\mu$M T₃.

(a)

(b)