OBJECTIVE: We have studied the effect of tryptophan on cellular [(125)I]tri-iodothyronine (T3) uptake by mouse thymocytes. MATERIALS AND METHODS: Mouse thymocytes (20 x 10(6 )cells/ml) were suspended in Krebs-Ringer solution buffered by Tris-HCl and incubation (23 degrees C at pH7.45+/-0.6), in the presence or absence of 1mM tryptophan, was started by adding 25 pM [(125)I]T3. At the end of incubation, samples were cooled in ice, centrifuged over a 30% sucrose cushion and the cell-associated radioactivity was measured in the pellet. RESULTS: Tryptophan reduced both the total and the saturable fraction of [(125)I]T3 uptake by 44% (P=0.0009) and 60% (P=0.0006) respectively, following 1 min of incubation. This effect was specific and dose-dependent, being maximal at 5mM concentration (-82%). In contrast, the pre-exposure of cells to tryptophan for up to 2h had no effect on the subsequent uptake of [(125)I]T3, in the absence of tryptophan. The effect of D-tryptophan on saturable T3 uptake was not different from that obtained using the L-stereoisomer. Tryptophan reduced the V(max) of the initial rate of saturable [(125)I]T3 uptake by two-thirds without affecting the apparent K(m) (2.2 nM) of the process, thus indicating the non-competitive nature of the inhibition. In sodium-free medium the saturable [(125)I]T3 uptake was reduced by 43%. The inhibitory effect of tryptophan on [(125)I]T3 uptake was exerted in both the presence and the absence of sodium. In fact, the inhibitory effect of tryptophan on T3 transport was greater and significantly different (P=0.0046) from that obtained by sodium depletion alone. CONCLUSIONS: Tryptophan interferes with both the sodium-dependent and -independent components of [(125)I]T3 uptake by a dose-dependent, non-competitive mechanism which operates in cis-modality at the plasma membrane level of mouse thymocytes.
M Centanni, G Canettieri, N Viceconti, R Sibilla, A Bei and M Andreoli
M Centanni, R Cesareo, O Verallo, M Brinelli, G Canettieri, N Viceconti and M Andreoli
Objective: The aim of the study was to analyse the relationship between the ocular parameters, namely intraocular pressure (IOP), and the early forms of subclinical hypothyroidism.
Design: Fifty-three subjects (9 male and 44 female) aged from 18 to 45 years (mean 32±7 years) were selected for this study. Twenty-nine met the criteria of subclinical hypothyroidism and 24 euthyroid subjects, age- and sex-matched, were used as controls.
Methods: All individuals underwent a complete ocular examination, including visual field examination and serial measurement of IOP by means of a Goldmann tonometer. A tonographic examination was also performed.
Results: The hypothyroid patients showed a substantially higher pressure in both eyes compared with control subjects (right eye=17·52±4·74 vs 13·42±1·95 mmHg, p<0.0001; left eye=17·55±3·99 vs 13·71±1·55 mmHg, p<0.0001). Indeed, the tonometric pressure exceeded 18 mmHg in 11 out of the 29 (38%) patients in the right eye and in 8 out of 29 (27%) patients in the left eye. The outflow index was normal in all subjects except in two hypothyroid patients. After two months of L-thyroxine (L-T4) replacement therapy, only one patient continued to show tonometric values above 18 mmHg and the hypothyroid patients showed a significant reduction in mean IOP in both eyes compared with pretreatment values (right eye=14·96± 1·32 mmHg. p<0.0097; left eye=15·03± 1·38 mmHg, p<0.0018). Treatment did not lead to any change in the outflow indices; however, the C value (outflow coefficient at the sclerocorneal corner) returned to normal in the two patients with increased pre-treatment tonographic values.
Conclusions: These findings indicate that the intraocular pressure is increased even in subclinical hypothyroid patients and that, at this early stage, the impairment is fully reversible with L-T4 therapy.
European Journal of Endocrinology 136 595–598