**6. Conclusions**

108 Thyroid Hormone

increases 10 fold within the first 7 days after birth and remains at this elevated level until the 55th day of age in the rat. In contrast, the mRNA for the α1, α2 and β isoforms reach their

In general, thyroid hormone was found to up-regulate Na+/K+-ATPase activity and expression in many tissues: For instance, in rat cardiomyocytes, T3 was observed to increase the mRNA pattern of Na+/K+-ATPase α1 and β1 subunits 4fold after 48hrs and the α2 mRNA expression even 7fold after 72hrs of treatment (217). A similar effect of T3 was found in a rat liver cell line. Here, a non transformed continuous cell line derived from adult rat liver treated with T3 showed a 1.3 fold increased activity of Na+/K+-ATPase. More specifically, the mRNA expression of the α1 and β1 isoforms of the Na+/K+-ATPase increased 1.5 and 2.9 fold respectively compared with controls maintained in T3 free (hypothyroid) media (218).

In rat brains thyroid hormone has been shown to up-regulate Na+/K+-ATPase activity and protein expression in synaptosomes only in the first two postnatal weeks (219). In addition Schmitt *et al.,* in 1988 showed that the hypothyroid condition reduces the expression of the mRNA for Na+/K+-ATPase α isoforms in rat brain (220). However, observing thyroid hormone effects in identified brain regions in the adult rat indicated, that hypothyroidism could down-regulate Na+/K+-ATPase activity in specific brain regions, such as the adult hippocampus (221, 222). Further experiments showed that the predominant brain cell specific α3 isoform of the Na+/K+-ATPase decreased in hypothyroid rat brain as well and that the relative sensitivity of the different Na+/K+-ATPase α subunits in brain cells for thyroid hormone is α3>α1>α2 (223). The expression of all Na+/K+-ATPase isoforms and their regulation by T3 was also observed in primary neuronal cell cultures of rat brain at the mRNA and protein level using northern and western blot techniques (224). In contrast to neurons, glia cells express α1, α 2 and β1, 2 not α3. The mRNAs as well as the proteins of the four subunits expressed in glia cells showed an upregulation when the cells were grown

Although a T3-responsive element has been found in the promotor region of the α3 subunit (226) two reports on muscle cells indicate, that the regulation of the Na+/K+-ATPase by thyroid hormone might be at least to some extent secondary to an enhanced sodium influx. Thus Brodie and Sampsom (227) observed that a blockage of Na+-influx by tetrodotoxin to block the voltage-gated Na+currents or by amiloride to block further Na+transport routes both reduced the T3-induced increase of 3[H]-ouabain binding sites, which represent membrane inserted Na+/K+-ATPase in cultured myotubes. These results were confirmed by Harrison and Clausen (228) in skeletal muscle, who showed that an increase in saxitoxin binding (reflecting Na+channel density) preceded an increase in 3[H]-ouabain binding (reflecting membrane inserted Na+/K+-ATPases). These experiments indicate a link between Na+ current regulation and the regulation of the Na+/K+-ATPase by thyroid hormone. This is in agreement with other experiments in chick skeletal muscle that suggested that the activation of voltage-gated Na+channels by veratridine leads to an increased biosynthesis of Na+/K+-ATPase in chick myogenic cultures (229). Whether these findings also apply to neurons, or whether some subunits are regulated directly by thyroid hormone receptors and

others are regulated by the sodium load of the cells remains, however, to be clarified.

maximal expression levels only after the rats are at 25 days old (215).

with the supplement of T3 for 5 and 10 days respectively (225).

Thyroid hormone deficiency leads to a general slowing of many body functions, including a slowing of heart rate, a slowing of intestinal movements as well as of thoughts and movements. As demonstrated here in an exemplary fashion on a small sample of patients the most conspicuous symptom to develop during a short period of severe hypothyroidism is a gradual, quantifiable slowing of speech and of critical flicker fusion frequency. Although several explanations at the cellular and molecular level are feasible an intriguing hypothesis is, that a central aspect of the origin of many of these symptoms might be a regulation of the sodium current density that is a key player of neuronal and cellular excitability. In fact, some effects of thyroid hormone can to some extent be blocked by the sodium channel blocker TTX: Thus the upregulation of the membrane Na+/K+ATPase expression in myotubes (227) and skeletal muscle (228) as well as of soma growth in L-GABAergic neurons (230) by thyroid hormone were all to some extent blockable by TTX, suggesting that some effects of thyroid hormone occur downstream of sodium channel regulation. In future it will be exciting to elucidate the full signal cascade involved in the regulation of the different sodium channel subunits as well as to conclusively sort out the primary and the secondary targets of thyroid hormone action. It will be interesting to study whether some of these thyroid hormone actions decline in the aging brain.
