8 - Uptake Measurements of Thyroid Hormones and Amino Acids in Cultured Cells

Abstract

Thyroid hormones (T3 and T4) are iodine-substituted tyrosines, joined to an iodine-substituted phenol via an ether linkage. The chemical nature of T3 and T4 is the basis for the relationship between cellular uptake of these hormones and those of neutral amino acids.. Therefore, the transport systems that are responsible for the cellular uptake of thyroid hormones can be classified as amino acid transport systems.Although the receptors for thyroid hormones are located intracellularly, the receptors for the amino acids with endocrine/neuroendocrine functions are present in the plasma membrane. The transport systems available in cells for the uptake of thyroid hormones make it possible for these hormones to interact with the intracellular receptors. On the other hand, the transport systems available for other amino acids play a role in lowering the extracellular concentrations of these amino acids, and consequently in reducing their interaction with the respective receptors in the plasma membrane. Therefore, the transport systems that are responsible for the cellular uptake of thyroid hormones and amino acids with endocrine/neuroendocrine functions are important determinants in the modulation of cellular signaling mediated by these compounds. Cultured cells offer a convenient experimental system to study the uptake of thyroid hormones and amino acids. Various aspects of the uptake process, such as the identity, kinetics, and energetics of the transport systems mediating the uptake can be investigated using this approach. The intact cell system and the membrane vesicle system complement each other as extremely useful in the characterization of the uptake of thyroid hormones and amino acids. With the intact cell system, the regulation of the uptake process can be investigated at transcriptional, translational, and post-translational levels.