title: Uptake of thyroid hormones in neonatal rat cardiac myocytes.
abstract: The uptake and metabolism of T3 and T4 were investigated in cardiomyocytes isolated from 2-day-old rats. Myocytes (2-5 x 10(5) cells/well) were cultured for 1 day in medium with 5% horse serum-5% FCS and subsequently for 4 days without serum; in some cases myocytes were cultured with serum throughout the culture period. Experiments were performed at 37 C in medium with 0.5% BSA for measurement of [125I]T3 (200,000 cpm; 200 pM) uptake and with 0.1% BSA for measurement of [125I]T4 (200,000 cpm; 350 pM) uptake. Uptake of [125I]T3, expressed as femtomoles per picomolar concentration of free hormone, with any incubation time between 15 min and 24 h was at least 2-fold higher than that of [125I]T4. Neither T3 nor T4 was deiodinated within 24 h. This was observed in cells cultured in the absence or presence of serum. After 15 min of incubation, [125I]T3 uptake was 0.048 +/- 0.002 fmol/pM free T3 (n = 9), and [125I]T4 uptake was 0.018 +/- 0.003 fmol/pM free T4 (n = 9). Although [125I]T3 uptake was reduced by 31-40% (P < 0.05) by coincubation with 100 nM to 10 microM unlabeled T3, that of [125I]T4 was not affected by 1 nM to 10 microM unlabeled T4, nor was [125I]T3 uptake reduced by 10 microM unlabeled T4. Preincubation (30 min) and incubation (15 min) with 10 microM oligomycin reduced cellular ATP by 56% (P < 0.05) and [125I]T3 uptake by 73% (P < 0.05), but had no effect on [125I]T4 uptake. Similarly, [125I]T3 uptake, but not [125I]T4 uptake, was dependent on temperature and partly dependent on the Na+ gradient, as shown by the inhibitory effect of 10 microM monensin (27%; P < 0.05). The effect of aromatic amino acids (2 mM) on [125I]T3 uptake increased in the order phenylalanine < tyrosine < tryptophan. It is concluded that T3 is taken up in neonatal cardiomyocytes by an energy-dependent carrier-mediated mechanism that is also partly dependent on the Na+ gradient. Such a transport mechanism for T4 is not present in the neonatal heart, but it may appear later during development.
authors.affiliation: Department of Internal Medicine III, Erasmus University Medical School, Rotterdam, The Netherlands.
authors.forename: M E
authors.initials: ME
authors.lastname: Everts
authors.forename: F A
authors.initials: FA
authors.lastname: Verhoeven
authors.forename: K
authors.initials: K
authors.lastname: Bezstarosti
authors.forename: E P
authors.initials: EP
authors.lastname: Moerings
authors.forename: G
authors.initials: G
authors.lastname: Hennemann
authors.forename: T J
authors.initials: TJ
authors.lastname: Visser
authors.forename: J M
authors.initials: JM
authors.lastname: Lamers
mesh_terms: D000818:Animals; D000831:Animals, Newborn / Q000378:metabolism*; D002462:Cell Membrane / Q000378:metabolism; D002478:Cells, Cultured; D003470:Culture Media; D016895:Culture Media, Serum-Free; D009206:Myocardium / Q000166:cytology / Q000378:metabolism*; D051381:Rats; D017208:Rats, Wistar; D013974:Thyroxine / Q000493:pharmacokinetics*; D013997:Time Factors; D014284:Triiodothyronine / Q000493:pharmacokinetics*
chemical_list: D003470:Culture Media; D016895:Culture Media, Serum-Free; D014284:Triiodothyronine; D013974:Thyroxine