Many nuclear hormones have physiological effects that are too rapid to be explained by changes in gene expression and are often attributed to unidentified or novel G protein-coupled receptors. Thyroid hormone is essential for normal human brain development, but the molecular mechanisms responsible for its effects remain to be identified. Here, we present direct molecular evidence for potassium channel stimulation in a rat pituitary cell line (GH 4C1) by a nuclear receptor for thyroid hormone, TR␤, acting rapidly at the plasma membrane through phosphatidylinositol 3-kinase (PI3K) to slow the deactivation of KCNH2 channels already in the membrane. Signaling was disrupted by heterologous expression of TR␤ receptors with mutations in the ligand-binding domain that are associated with neurological disorders in humans, but not by mutations that disrupt DNA binding. More importantly, PI3K-dependent signaling was reconstituted in cell-free patches of membrane from CHO cells by heterologous expression of human KCNH2 channels and TR␤, but not TR␣, receptors. TR␤ signaling through PI3K provides a molecular explanation for the essential role of thyroid hormone in human brain development and adult lipid metabolism.neuronal development ͉ phosphatidylinositol 3-kinase ͉ potassium channels ͉ Rac ͉ KCNH2 T he thyroid hormone, L-3,5,3Јtriiodothyronine (T 3 ), plays an essential role in the development and metabolism of many tissues and organs (1). In particular, mammalian brain development is disrupted when T 3 signaling is reduced by inherited mutations in the T 3 receptor gene, THR␤, or when the synthesis or transport of the T 3 prohormone thyroxine (tetraiodothyronine) is reduced by dietary iodine deficiency or environmental toxicants (2-4). T 3 also regulates its own circulating levels through negative feedback on pituitary thyrotropes, which secrete thyroid stimulating hormone (1). Two genes, THR␣ and THR␤, encode four alternately spliced ligand-binding zincfinger receptor proteins from the c-erbA family, which mediate T 3 actions on gene expression (5), but specific T 3 -regulated genes involved directly in neural development remain to be identified. Recently, however, attention has shifted to the rapid, nongenomic signaling by T 3 (6) and other thyroxine metabolites (7).For example, we previously reported a Rac-dependent effect of T 3 on voltage-activated potassium channels encoded by the ether-a-go-go-related gene KCNH2 in a rat pituitary cell line (8). KCNH2 proteins are voltage-activated potassium channels that regulate spike frequency in electrically excitable cells, such as the endocrine cells of the pituitary, through their unique kinetics of inactivation and recovery (9). Because they inactivate almost immediately after activation at positive voltages during the peak of each spike, and then reactivate before closing when the membrane repolarizes after the spike, they contribute to determining the interspike interval. Thus, increasing KCNH2 activity decreases excitability and secretion. Rapid effects of thyroid hormo...

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As a function of the urinary concentrating mechanism, the cells of the renal medulla are exposed to elevated and constantly varying osmolalities and adapt to this environment by selectively expressing certain mRNAs. We evaluated the expression and regulation of two RNAs that may be important in adaptation of rental medullary cells to hyperosmolality. We demonstrate selective, modulated expression in the renal medulla of heat shock protein HSP-70 mRNA and prostaglandin synthase-1 mRNA, with the abundance of these two mRNAs regulated in vivo in concert with changes in medullary sodium and urea. We also determined the abundance of these mRNAs in cultured kidney cells (MDCK) in response to an increase in extracellular osmolality due to selected osmotic agents. HSP-70 and prostaglandin synthase-2 mRNA levels increased when extracellular osmolality was increased to 400-600 mosmol/kg by the addition of NaCl. At 500 mosmol/kg this response was evident at 6 h, was maximal near 24 h, and persisted for a total of 90 days. Prostaglandin synthase-1 mRNA levels in MDCK cells were also increased after chronic exposure to extracellular osmolality. Increased extracellular osmolality caused by agents to which cells are impermeable caused increased levels of HSP-70 and prostaglandin synthase-2 mRNAs, whereas increased extracellular osmolality caused by agents to which cells are permeable did not; thus osmotic regulation involved osmotic water movement. We conclude that the abundance of HSP-70 and prostaglandin synthase-1 mRNAs in the renal medulla is regulated in response to renal medullary osmolality and suggest that this may also be true for other medullary mRNAs yet to be described.

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