Thyroid hormone stimulates the renal Na/H exchanger NHE3 by transcriptional activation

-The purpose of this study was to examine protein expression of renal aquaporins (AQP) and ion transporters in hypothyroid (HT) rats in response to an oral water load compared with controls (CTL) and HT rats replaced with L-thyroxine (HTϩT). Hypothyroidism was induced by aminotriazole administration for 10 wk. Body weight, water intake, urine output, solute and urea excretion, and serum and urine osmolality were comparable among the three groups at the conclusion of the 10-wk treatment period. One hour after oral gavage of water (50 ml/kg body wt), HT rats demonstrated significantly less water excretion, higher minimal urinary osmolality, and decreased serum osmolality compared with CTL and HTϩT rats. Despite the hyposmolality, plasma vasopressin concentration was elevated in HT rats. These findings in HT rats were associated with an increase in protein abundance of renal cortex AQP1 and inner medulla AQP2. AQP3, AQP4, and the Na-K-2Cl cotransporter were also increased. Moreover, 1 h following the oral water load, HT rats demonstrated a significant increase in the membrane-to-vesicle fraction of AQP2 by Western blot analysis. The defect in urinary dilution in HT rats was reversed by the V 2 vasopressin antagonist OPC-31260. In conclusion, impaired urinary dilution in HT rats is primarily compatible with the nonosmotic release of vasopressin and increased protein expression of renal AQP2. The impairment of maximal solute-free water excretion in HT rats, however, appears also to involve diminished distal fluid delivery.water transport; solute-free water excretion; thyroid disease HYPOTHYROIDISM IS A COMMON clinical disorder that is associated with significant abnormalities in systemic and renal hemodynamics and renal handling of salt and water. Patients and experimental animals with hypothyroidism demonstrate impaired water excretion, failure to achieve maximal urinary dilution, and hyponatremia (4,6,15,20). Maximal solute-free water excretion is dependent on factors including 1) appropriate delivery of fluid to the diluting segments of the distal nephron, determined by glomerular filtration rate and proximal tubular fluid and sodium reabsorption; 2) ion reabsorption in water-impermeable segments of the nephron; 3) decreased secretion of arginine vasopressin (AVP) in response to serum hyposmolality; and 4) decreased collecting duct solute-free water reabsorption via aquaporin (AQP)-2 water channels in response to suppressed plasma AVP concentrations. Previous studies have suggested that dysregulation of each of these factors contributes to impaired urinary dilution in hypothyroidism (4,6,15,20). However, a comprehensive molecular analysis has not been undertaken. The goal of this study was to examine the molecular mechanisms contributing to impaired urinary dilution in hypothyroidism. METHODSAnimal model. The study protocol was approved by the University of Colorado Institutional Animal Care and Use Committee. Animals were housed individually in metabolic cages (Nalgene metabolic cages; Nalge, Rochester, NY) and w...

Section: Discussionsupporting

confidence: 92%

Nonosmotic release of vasopressin and renal aquaporins in impaired urinary dilution in hypothyroidism

Chen

Cadnapaphornchai²,

Yang³

et al. 2005

“…The slightly higher increase in surface NHE3 compared with total NHE3 may reflect an additional step, whereas the increased cellular pool of NHE3 is preferentially targeted to the cell membrane. Moreover, an increase in NHE3 protein is associated with an increase in NHE3 mRNA at 24 h. This pattern of coordinated upregulation at the levels of activity, surface protein, total protein, and mRNA is reminiscent of the effects of thyroid hormone on NHE3 (18).…”

Section: Discussionmentioning

confidence: 88%

“…Na ϩ /H ϩ exchangers are regulated by a wide variety of agonists through vastly different mechanisms. Regulation at the level of transcription (4,5,7,11,17,18,37), translation (67) (12) or lipid regulators (1) has been implicated or proven. A single condition or agonist can regulate NHE3 at more than one step.…”

Section: Discussionmentioning

confidence: 99%

Insulin activates Na⁺/H⁺exchanger 3: biphasic response and glucocorticoid dependence

Klisic

Hu²,

Nief

et al. 2002

Self Cite

166

123

Insulin is an important regulator of renal salt and water excretion, and hyperinsulinemia has been implicated to play a role in hypertension. One of the target proteins of insulin action in the kidney is Na+/H+ exchanger 3 (NHE3), a principal Na+ transporter responsible for salt absorption in the mammalian proximal tubule. The molecular mechanisms involved in activation of NHE3 by insulin have not been studied so far. In opossum kidney (OK) cells, insulin increased Na+/H+exchange activity in a time- and concentration-dependent manner. This effect is due to activation of NHE3 as it persisted after pharmacological inhibition of NHE1 and NHE2. In the early phase of stimulation (2–12 h), NHE3 activity was increased without changes in NHE3 protein and mRNA. At 24 h, enhanced NHE3 activity was accompanied by an increase in total and cell surface NHE3 protein and NHE3 mRNA abundance. All the effects of insulin on NHE3 activity, protein, and mRNA were amplified in the presence of hydrocortisone. These results suggest that insulin stimulates renal tubular NHE3 activity via a biphasic mechanism involving posttranslational factors and an increase in NHE3 gene expression and the effects are dependent on the permissive action of hydrocortisone.

“…Neither thyroid hormone nor dexamethasone affected NHE3 mRNA stability (6,14). In addition, triiodothyronine did not affect rat NHE3 protein stability (14).…”

Section: Discussionmentioning

confidence: 99%

Maturation of the Na⁺/H⁺antiporter (NHE3) in the proximal tubule of the hypothyroid adrenalectomized rat

Gupta¹,

Dwarakanath²,

Baum³

2004

Self Cite

In previous studies examining the role of glucocorticoids and thyroid hormone on the maturation of the Na + /H + antiporter (NHE3), we found attenuation in the maturational increase in proximal tubule apical Na + /H + antiporter activity but no change in NHE3 mRNA abundance in either glucocorticoid-deficient or hypothyroid rats. In addition, prevention of the maturational increase in either hormone failed to totally prevent the maturational increase in Na + /H + antiporter activity. We hypothesized that one hormone played a compensatory role when the other was deficient. The present study examined whether combined deficiency of thyroid and glucocorticoid hormones would completely prevent the maturation of the Na + /H + antiporter. Adrenalectomy was performed in 9-day-old hypothyroid Sprague-Dawley rats, a time before the normal postnatal maturational increase in these hormones occurs. Nine-and 30-day-old adrenalectomized (ADX), hypothyroid rats had comparable NHE3 mRNA abundance, which was 5-to 10-fold less than 30-day-old ADX, hypothyroid rats that received corticosterone-thyroxine replacement and 30-day-old sham control rats (P < 0.05). Brush-border membrane NHE3 protein abundance was comparable in 9-and 30-day-old ADX, hypothyroid groups and ~20-fold lower than both the 30-day replacement and 30-day sham groups (P < 0.05). Similarly, the replacement and sham groups had higher sodium-dependent proton secretion than 9-and 30-day-old ADX, hypothyroid groups (P < 0.05). We conclude that combined deficiency of both hormones totally prevents the maturational increase in NHE3 mRNA and protein abundance and Na + /H + antiporter activity. Keywords NHE3; adrenalectomy; renal development; microperfusion; cell pHNeonates have a lower bicarbonate threshold than adults (21), which is due in large part to immaturity of neonatal proximal tubule acidification (10,11,35). There is a fourfold increase in rat apical membrane proximal convoluted tubule Na + /H + antiporter activity The levels of both glucocorticoids and thyroid hormone are lower in the first 1-2 wk of life compared with those of adults (24,39). The maturational increase in these hormones parallels the maturational increase in NHE3 (36), the predominant Na + /H + antiporter isoform on the apical membrane of the proximal tubule (15, 40, 41), and Na + /H + antiporter activity (36). We examined previously the role of glucocorticoids and thyroid hormone in mediating the increase in Na + /H + antiporter activity (8,23). In these studies, we either prevented the maturational increase in glucocorticoids or thyroid hormone at a time well before the normal increase during postnatal development. We found that preventing the maturational increase in either thyroid hormone or glucocorticoids suppressed the maturational increase in Na + /H + antiporter activity and brush-border membrane NHE3 protein abundance without any effect on the basal level of NHE3 mRNA abundance (8,23). Furthermore, the rate of Na + /H + antiporter activity in both adrenalectomized and hypothyro...