Transport of thyroxine (T(4)) into the liver is inhibited in fasting and by bilirubin, a compound often accumulating in the serum of critically ill patients. We tested the effects of chronic and acute energy deprivation, bilirubin and its precursor biliverdin on the 15-min uptake of [(125)I]tri-iodothyronine ([(125)I]T(3)) and [(125)I]T(4) and on TSH release in rat anterior pituitary cells maintained in primary culture for 3 days. When cells were cultured and incubated in medium without glucose and glutamine to induce chronic energy deprivation, the ATP content was reduced by 45% (P<0. 05) and [(125)I]T(3) uptake by 13% (NS), but TSH release was unaltered. Preincubation (30 min) and incubation (15 min) with 10 microM oligomycin reduced ATP content by 51% (P<0.05) and 53% (P<0. 05) under energy-rich and energy-poor culture conditions respectively; [(125)I]T(3) uptake was reduced by 66% (P<0.05) and 64% (P<0.05). Neither bilirubin nor biliverdin (both 1-200 microM) affected uptake of [(125)I]T(3) or [(125)I]T(4). Bilirubin (1-50 microM) did not alter basal or TRH-induced TSH release. In conclusion, the absence of inhibitory effects of chronic energy deprivation and bilirubin on thyroid hormone uptake by pituitary cells supports the view that the transport is regulated differently than that in the liver.

We have investigated the potential role of fibroblasts in local thyroid hormone metabolism in neonatal rat heart. Incubation of cardiac fibroblasts with thyroxine (T4) or 3,5,3'-tri-iodothyronine (T3) resulted in the appearance of water-soluble metabolites, whereas incubation of cardiomyocytes under the same conditions did not or did so to a much lesser extent. Time-course studies showed that production is already evident after 1-5 h of exposure and that the process equilibrates after 24-48 h. Analysis of the products revealed both the T4 and the T3 metabolites to be glucuronides. These results were corroborated by the detection of uridine diphosphate (UDP)-glucuronyltransferase activity in cardiac fibroblasts. We found no indication for outer ring deiodination in fibroblasts, cardiomyocytes or heart homogenates. From these results we have concluded that cardiac fibroblasts, but not cardiomyocytes, are able to glucuronidate T4 and T3 and secrete the conjugates. This could play a role in local metabolism, e.g. to protect the heart tissue from high levels of thyroid hormones.

We determined the effects of food supply and low-intensity training on growth, serum thyroid hormone levels and the Na(+),K(+)-pump concentration in equine skeletal muscle. Twenty-two Shetland ponies were subjected to two different feeding regimes for 2(1/2) years (11 ponies per group): food restriction (body condition score kept at 2) or ad libitum fed (body condition score kept at 8). Five ponies in each group underwent low-intensity training. Gluteus medius muscle and serum samples were obtained in April 1998. Subsequently, all ponies were fed ad libitum and the training programme was stopped. Muscle biopsies and serum samples were collected again in November 1998. Food restriction was associated with a 30-50% reduction of body weight gain. While the total thyroxine (T(4)) level was increased, the free T(4) remained at the control level. The serum total tri-iodothyronine (T(3)) and free T(3) were reduced by 30% and 49% respectively. After 6 months of refeeding there were no differences in any of the hormone levels between the ad libitum fed and the food-restricted groups. Food restriction produced a minor, but not significant, decrease in the Na(+),K(+)-pump concentration in the gluteus medius muscle of the Shetland ponies. Low-intensity training reduced weight gain of the ad libitum fed group by 25%, but had no detectable effect on the concentration of the Na(+), K(+)-pumps. We conclude that prolonged food restriction in Shetland ponies results in a weight gain reduction of 30-50%, and is associated with similar decreases in serum total and free T(3). The reduction in serum T(3) only slightly influenced the Na(+), K(+)-ATPase concentration in skeletal muscle, indicating that muscle tissue of different species may respond differently to changes in circulating thyroid hormones.

Uptake of tri-iodothyronine (T(3)) was compared with that of thyroxine (T(4)) in the embryonic heart cell line H9c2 (2-1). These cells propagate as myoblasts and form differentiated myotubes upon reduction of the serum concentration, as indicated by a 31-fold increase in creatine kinase activity. Protein and DNA content per well were around 2-fold higher in myotubes than in myoblasts. When expressed per well, T(3) and T(4) uptake were, compared with myoblasts, 1.9- to 2-fold and 3.1- to 4-fold higher in myotubes respectively. On the other hand, the characteristics of T(3) and T(4) uptake were similar in myoblasts and myotubes. At any time-point, T(4) uptake was 2-fold higher than that of T(3), and both uptakes were energy but not Na(+) dependent. T(3) and T(4) uptake exhibited mutual inhibition in myoblasts and myotubes: 10 microM unlabeled T(3) reduced T(4) uptake by 51-60% (P<0.001), while 10 microM T(4) inhibited T(3) uptake by 48-51% (P<0.001). Furthermore, T(3) and T(4) uptake in myoblasts was dose-dependently inhibited by tryptophan (maximum inhibition around 70%; P<0.001). Exposure of the cells to T(3) or T(4) during differentiation significantly increased the fusion index (35 and 40%; P < 0.01). Finally, both myoblasts and myotubes showed a small deiodinase type I activity, while deiodinase type II activity was undetectable. In conclusion, T(3) and T(4) share a common energy-dependent transport system in H9c2(2-1) cells, that may be important for the availability of thyroid hormone during differentiation.

We have investigated the hypothesis that uridine 5'-diphosphate (UDP)-glucuronyltransferases (UGTs) and beta-glucuronidase are jointly involved in a mechanism for the storage and mobilization of iodothyronine metabolites in liver, kidney, heart and brain. Specifically, we predicted UGT activities to decrease and increase respectively, and beta-glucuronidase activity to increase and decrease respectively in hypo- and hyperthyroidism. To this end we have studied the effects of thyroid status on the activities of different enzymes involved in thyroid hormone metabolism in liver, kidney, heart and brain from adult rats with experimentally induced hypo- and hyperthyroidism. We used whole organ homogenates to determine the specific enzyme activities of phenol- and androsteron-UGT, beta-glucuronidase, as well as iodothyronine deiodinase types I and II. Deiodinase type I activities in liver and kidney were decreased in hypothyroid animals and, in liver only, increased in hyperthyroidism. Deiodinase type II activity was increased in hyperthyroid rat kidney only. Interestingly, in the heart, deiodinase type I-specific activity was increased fourfold, although the increase was not statistically significant. Cardiac deiodinase type I activity was detectable but not sensitive to thyroid status. Hepatic phenol-UGT as well as androsteron-UGT activities were decreased in hypothyroid rats, with specific androsteron-UGT activities two to three orders of magnitude lower than phenol-UGT activities. Both UGT isozymes were well above detection limits in heart, but appeared to be insensitive to thyroid status. In contrast, cardiac beta-glucuronidase activity decreased in hypothyroid tissue, whereas the activity of this enzyme in the other organs investigated did not change significantly.In summary, cardiac beta-glucuronidase, albeit in low levels, and hepatic phenol-UGT activities were responsive only to experimental hypothyroidism. Although a high basal activity of the pleiotropic beta-glucuronidase masking subtle activity changes in response to thyroid status cannot be ruled out, we conclude that hepatic, renal and cardiac UGT and beta-glucuronidase activities are not regulated reciprocally with thyroid status.

The uptake of tri-iodothyronine (T(3)) in cultured neonatal rat cardiomyocytes was investigated and compared with the uptake of reverse T(3 )(rT(3)) and thyroxine (T(4)). Cellular compartmentalization of T(3) was studied by distinguishing T(3) activity associated with the plasma membrane from that in the cytosol or incorporated in the cell nucleus. T(3) and T(4) uptake displayed similar temperature dependencies which, in magnitude, differed from that of rT(3) uptake. T(3) uptake was Na(+ )independent, and sensitive to oligomycin and monodansylcadaverine (42-49% and 25% inhibition of 15-min cellular uptake respectively). Furthermore, T(3) uptake could be inhibited by tryptophan (20%) and tyrosine (12%), while 2-aminobicyclo[2,2,1]heptane-carboxylic acid had no effect. Co-incubation with tryptophan and oligomycin resulted in an additive inhibition of T(3) uptake (77%). We therefore conclude that (i) T(3) uptake is energy dependent, (ii) receptor-mediated endocytosis may be involved and (iii) the aromatic amino acid transport system T may play a role, while system L is not involved in T(3) transport in cardiomyocytes. Co-incubation with unlabeled iodothyronines showed that 3,3'-di-iodothyronine and T(3) itself were the most effective inhibitors of T(3) uptake (30% and 36% inhibition of 15-min cellular uptake respectively). At 15-min incubation time, 38% of the total cell-associated T(3) was present in the cytosol and nucleus, and 62% remained associated to the plasma membrane. Unidirectional uptake rates did not saturate over a free T(3) concentration range up to 3.9 microM. We have concluded that T(3) uptake in neonatal rat cardiomyocytes occurs by an energy- and temperature-dependent mechanism that may include endocytosis and amino acid transport system T, and is not sensitive to the Na(+) gradient. Elucidation of the molecular basis for the T(3) transporter is the subject of current investigation.

Cellular and nuclear uptake of [125I]tri-iodothyronine (T3) and [125I]triiodothyroacetic acid (Triac) were compared in cardiomyocytes of 2-3 day old rats, and the effect of thyroid hormone analogs on cellular T(3) uptake was measured. Cells (5-10 x 10(5) per well) were cultured in DMEM-M199 with 5% horse serum and 5% FCS. Incubations were performed for from 15 min to 24 h at 37 degrees C in the same medium, 0.5% BSA and [125I]T3 (100 pM), or [125I]Triac (240 pM). Expressed as % dose, T(3) uptake was five times Triac uptake, but expressed as fmol/pM free hormone, Triac uptake was at least 30% (P<0.001) greater than T3 uptake, whereas the relative nuclear binding of the two tracers was comparable. The 15 min uptake of [125I]T3 was competitively inhibited by 10 microM unlabeled T3 (45-52%; P<0.001) or 3,3'- diiodothyronine (T2) (52%; P<0.001), and to a smaller extent by thyroxine (T(4)) (27%; 0.05<0.1). In contrast, 10 microM 3,5-T2, Triac, or tetraiodothyroacetic acid (Tetrac) did not affect T3 uptake after 15 min or after 24 h. Diiodothyropropionic acid (DITPA) (10 microM) reduced 15-min T3 uptake by about 24% (P<0.05), but it had a greater effect after 4 h (56%; P<0.001). Exposure to 10 nM DITPA during culture reduced cellular T3 uptake, as did 10 nM T3, suggesting down-regulation of the plasma membrane T3 transporters. We conclude that i) Triac is taken up by cardiomyocytes; ii) 3,3'-T2 and, to a lesser extent, DITPA and T4 interfere with plasma membrane transport of T3, whereas 3,5-T2, Triac, or Tetrac do not; iii) the transport mechanism for Triac is probably different from that for T3.