Effect of triiodo-L-thyronine on amino acid transport by Xenopus laevis embryos

Tissue uptake of thyroid hormone by amino acid transporters

Thyroid hormones (THs) -- thyroxine (T4) and tri-iodothyronine (T3) -- are iodinated derivatives of the amino acid tyrosine, which regulates growth, development and critical metabolic functions. THs are taken up by target cells and act at the genomic level via nuclear thyroid receptors. Saturable transport mechanisms mediate the greater part of TH movement across the plasma membrane. System L1 permease is a transporter of THs and amino acids in mammalian adipose tissue, placenta and brain. T(3) is also a substrate of a putative System T transporter, which is selective for aromatic amino acids. The activity and functional mechanisms of these transporters can be crucial to cells in determining both their hormone sensitivity and their responses to change in circulating hormone concentrations or availability of competing substrates (e.g. amino acids). TH transporters are potentially important pharmacological targets in the design of novel or improved therapies for thyroid-related disorders.

The role of the jelly coat in amino acid transport in embryos of Bombina orientalis

The obligatory role of the jelly coat for maximal transport of all amino acids, including those found to be jelly coat-independent in Xenopus laevis embryos, has been shown in Bombina orientalis embryos. Amino acid transport in dejellied embryos (without fertilization membrane and jelly coats) of Bombina, reconstituted with either intact or homogenized jelly coats, was similar to the values in normal embryos. Amino acid transport in totally dejellied embryos, and those surrounded with fertilization membrane only, was similar. Reconstitution of dejellied embryos with physically denatured jelly coats did not restore full amino acid transport. Amino acid transport values using heterologous combinations of dejellied embryos and jelly coats of Bombina orientalis and Xenopus laevis were equivalent to those in homologous combinations.

Effectors of amino acid transport processes in animal cell membranes

Various effectors, which act upon ion gradients, protein synthesis, membrane components or cellular functional groups, have been employed to provide insights into the nature of amino acid-membrane transport processes in animal cells. Such effectors, for example, include ions, hormones, metabolites and various organic reagents and their judicious use has allowed the following list of conclusions. Sodium ion has been found to stimulate amino acid transport in a wide variety of cell systems, although depending on the tissue and/or substrate, this ion may have no effect on such transport, or even inhibit it. Amino acid transport can be stimulated in some cell systems by other ions such as K+, Li+, H+ or Cl-. Both H+ and K+ have been found to be inhibitory in other systems. Amino acid transport is dependent in many cell systems upon an inwardly directed Na+ gradient and is stimulated by a membrane potential (negative cell interior). In some cell systems an inwardly directed Cl- and H+ gradient or an outwardly directed K+ gradient can energize transport. Structurally dissimilar effectors such as ouabain, Clostridium enterotoxin, aspirin and amiloride inhibit amino acid transport presumably through dissipation of the Na+ gradient. Inhibition by certain sugars or metabolic intermediates of the tricarboxylic acid cycle may compete with the substrate for the energy of the Na+ gradient or interact with the substrate at the carrier level either allosterically or at a common site. Stimulation of transport by other sugars or intermediates may result from their catabolism to furnish energy for transport. Insulin and glucagon stimulate transport of amino acids in a variety of cell systems by a mechanism which involves protein synthesis. Microtubules may be involved in the regulation of transport by insulin or glucagon. Some reports also suggest that insulin has a direct effect on membranes. In addition, a number of growth hormones and factors have stimulatory effects on amino acid transport which are also mediated by protein synthesis. Steroid hormones have been noted to enhance or diminish transport of amino acids depending on the nature of the hormone. These agents appear to function at the level of protein synthesis. While stimulation may involve increased carrier synthesis, inhibition probably involves synthesis of a labile protein which either decreases the rate of synthesis or increases the rate of degradation of a component of the transport system.(ABSTRACT TRUNCATED AT 400 WORDS)