Uptake of thyroid hormones in neonatal rat cardiac myocytes

The uptake and metabolism of T-3 and T-4 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 [I-125]T-3 (200,000 cpm; 200 pM) uptake and with 0.1% BSA for measurement of [I-125]T-4 (200,000 cpm; 350 pM) uptake. Uptake of [I-125]T-3, 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 [I-125]T-4. Neither T-3 nor T-4 was deiodinated within 24 h. This was observed in cells cultured in the absence or presence of serum. After 15 min of incubation, [I-125]T-3 uptake was 0.048 +/- 0.002 fmol/pM free T-3 (n = 9), and was 0.018 +/- 0.003 fmol/pM free T-4 (n = 9). Although [I-125]T-3 uptake was reduced by 31-40%; (P < 0.05) by coincubation with 100 nM to 10 mu M unlabeled T-3, that of [I-125]T-4 was not affected by 1 nM to 10 mu M unlabeled T-4, nor was [I-125]T-3 uptake reduced by 10 mu M unlabeled T-4. Preincubation (30 min) and incubation (15 min) with 10 mu M oligomycin reduced cellular ATP by 56% (P < 0.05) and [I-125]T-3 uptake by 73% (P < 0.05), but had no effect on [I-125]T-4 uptake. Similarly, [I-125]T-3 uptake, but not [I-125]T-4 uptake, was dependent on temperature and partly dependent on the Na+ gradient, as shown by the inhibitory effect of 10 mu M monensin (27%; P < 0.05). The effect of aromatic amino acids (2 mM) on [I-125] T-3 uptake increased in the order phenylalanine < tyrosine < tryptophan. It is concluded that T-3 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 T-4 is not present in the neonatal heart, but it may appear later during development.