Abstract
Several ion transport enzymes are positively regulated by genomic mechanisms, through the thyroid hormones T3 and T4. However, there is evidence indicating that these hormones might also exert non genomic effects on different transporting enzymes. A normal activity of the Na, K-ATPase of syncytiotrophoblast is essential for ion homeostasis in this tissue, as well as for the growth and development of the fetus. In the current work it was studied the non genomic effect of thyroid hormones on the Na, K-ATPase activity of syncytiotrophoblast. The study was carried-out by incubating human term placental explants in the absence and presence of 10nMT3 or 350nMT4 and then isolating the syncytiotrophoblast plasma membranes and assaying them for Na, K-ATPase activity. The enzyme activity was also assayed for freshly prepared plasma membranes in the presence of (0–15)nMT3 or (0–500)nMT4.
Highlights
Several ion transporting mechanisms, such as the plasma and sarcoplasmic membrane Ca-ATPase, the Na,K-ATPase, the Na/Hex changer and the Na/Pico transporter can be modulated by thyroid hormones, i.e.3,5,3′-triiodo-L-thyronine (T3) and 3,5,3′,5′-tetraiodothyronine (T4) [1]
Considering the fact that T3 and T4 are both present in the blood plasma, there is a possibility that T4 could interfere with the effect of T3 on the Na,K-ATPase activity of microvillous membrane (MVM)
It can be seen that T4 does not interfere with the inhibitory effect of T3 on the Na,K-ATPase activity of MVM
Summary
Several ion transporting mechanisms, such as the plasma and sarcoplasmic membrane Ca-ATPase, the Na,K-ATPase, the Na/Hex changer and the Na/Pico transporter can be modulated by thyroid hormones, i.e.3,5,3′-triiodo-L-thyronine (T3) and 3,5,3′,5′-tetraiodothyronine (T4) [1] These transporters are positively modulated by thyroid hormones through genomic mechanisms [2] some of them have been found to be affected by non genomic mechanisms [2,3,4,5], with short time courses and probably mediated by specific receptors of the plasma membranes. Na,K-ATPase is an important primary ion transport system that is essential to keep the ion homeostasis and cell volume as well as the resting membrane potential. This enzyme is widely distributed in practically all animal tissues and it is regulated by different hormonal and non-hormonal factors. The Na+ gradient between the cell and its medium, maintained by the enzyme, activates Na+ dependent amino acid transporters, driving toward the fetus amino acids and nutrients essentials for his normal growth
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