Renal handling of sodium and water in the hypothyroid rat. Clearance and micropuncture studies

Salt-losing nephropathy associated with severe hypothyroidism

Here, we present a case of an older man presenting with worsening confusion. Laboratory tests showed serum sodium of 120 mmol/L with severe hypothyroidism and renal salt wasting that improved with treatment of hypothyroidism, normalising the serum sodium.

Protocol and Methods: Role of Levothyroxine on the Progression of Chronic Kidney Disease in Subclinical Hypothyroid Populations (LP-CKD) - A Multicenter Randomized Controlled Trial

Introduction

Subclinical hypothyroidism (SCH) is highly prevalent and associated with chronic kidney disease (CKD). However, it is still unanswered whether the restoration of euthyroid status in these patients will be beneficial in retarding a decline in glomerular filtration rate in early CKD patients. We aim to evaluate the efficacy of levothyroxine therapy versus placebo in slowing estimated glomerular filtration rate (eGFR) decline among CKD patients (stage 2-4) with SCH.

Methods

This study will be a multicentric, double-blind, randomized, parallel-group, placebo-controlled study. A total of 500 CKD patients, 250 patients in the treatment group and 250 patients in the placebo group, will be randomized. The randomization between the treatment arm and placebo arm will be performed as per the computer-generated random number table in a 1:1 ratio. The sample size was calculated based on the assumed reduction in eGFR after 1-year follow-up in the treatment and placebo groups of 10% and 25%, respectively, at a minimum two-sided 99% confidence interval and 90% power of the study and considering 20% loss on follow-up. Each patient will be followed every 3 months for at least 1 year after randomization. Individuals completing 1-year follow-up visits will be considered for analysis. The baseline and follow-up data will be compared between the treatment and placebo groups. The study will evaluate the efficacy and safety of levothyroxine therapy versus placebo in slowing eGFR decline among CKD patients (stage 2-4) with SCH. The primary endpoint will be the end of follow-up of the patients, reduction of eGFR by ≥50% from a baseline of that patient, or development of ESKD or death of the patients. The secondary endpoint will be any cardiovascular event or arrhythmia after the institution of the drug.

Identification of Protein Changes in the Urine of Hypothyroid Patients Treated with Thyroxine Using Proteomics Approach

The thyroid gland and thyroid hormones control a multitude of homeostatic functions including maintenance of fluid and electrolyte balance and normal functioning of the kidneys. Thyroid dysfunction alters the sytemic hemodynamic and metabolic balance, thereby affecting the kidney. In this study, we aimed to identify and characterize the urinary proteome of the patients with hypothyroidism. An untargeted proteomic approach with network analysis was used to identify changes in total urinary proteome in patients with newly diagnosed overt hypothyroidism. Urine samples were collected from nine age-matched patients' before and after l-thyroxine treatment. Differences in the abundance of urinary proteins between hypothyroid and euthyroid states were determined using a two-dimensional difference in gel electrophoresis (2D-DIGE) coupled to matrix-assisted laser desorption and ionization time-of-flight (MALDI TOF) mass spectrometry. Alterations in the abundance of urinary proteins, analyzed by Progenesis software, revealed statistically significant differential abundance in a total of 49 spots corresponding to 42 proteins, 28 up and 14 down (≥1.5-fold change, analysis of variance (ANOVA), p ≤ 0.05). The proteins identified in the study are known to regulate processes related to transport, acute phase response, oxidative stress, generation of reactive oxygen species, cellular proliferation, and endocytosis. Bioinformatic analysis using Ingenuity Pathway Analysis (IPA) identified dysregulation of pathways related to amino acid metabolism, molecular transport, and small-molecule biochemistry and involved the MAPK kinase, vascular endothelial growth factor (VEGF), PI3 kinase/Akt, protein kinase C (PKC), signaling pathways. The identified proteins were involved in the regulation of thyroglobulin (Tg) and thyrotropin (TSH) metabolism. Alterations in their levels indicate the presence of a compensatory mechanism aimed at increasing the regulation of Tg in the hypothyroid state.

Comparative Analysis and Refinement of Human PSC-Derived Kidney Organoid Differentiation with Single-Cell Transcriptomics

Kidney organoids derived from human pluripotent stem cells have great utility for investigating organogenesis and disease mechanisms and, potentially, as a replacement tissue source, but how closely organoids derived from current protocols replicate adult human kidney is undefined. We compared two directed differentiation protocols by single-cell transcriptomics of 83,130 cells from 65 organoids with single-cell transcriptomes of fetal and adult kidney cells. Both protocols generate a diverse range of kidney cells with differing ratios, but organoid-derived cell types are immature, and 10%-20% of cells are non-renal. Reconstructing lineage relationships by pseudotemporal ordering identified ligands, receptors, and transcription factor networks associated with fate decisions. Brain-derived neurotrophic factor (BDNF) and its cognate receptor NTRK2 were expressed in the neuronal lineage during organoid differentiation. Inhibiting this pathway improved organoid formation by reducing neurons by 90% without affecting kidney differentiation, highlighting the power of single-cell technologies to characterize and improve organoid differentiation.

Subclinical and overt hypothyroidism is associated with reduced glomerular filtration rate and proteinuria: a large cross-sectional population study

Subclinical hypothyroidism has been associated with dyslipidemia, hypertension, and increased risk of coronary artery disease. However, limited is known for its effect on renal function. Here we aimed to investigate whether subclinical hypothyroidism is associated with reduced estimated glomerular filtration rate (eGFR) and proteinuria in the general population. A cross-sectional cohort of 74,356 adults aged ≥20 year participating in voluntary health examinations without previous thyroid diseases were recruited in Taiwan. The mean eGFR of persons with euthyroidism, subclinical, and overt hypothyroidism are 87.99, 83.46, and 72.22 mL/min/1.73 m2, respectively (P-for- trend < 0.001). The proportion of proteinuria in persons with euthyroidism, subclinical and overt hypothyroidism is 1.29%, 2.2%, and 2.97%, respectively (P-for-trend: 0.001). The odds ratio of CKD for subclinical, clinical, and all hypothyroidism is 2.04 (95% confidence interval (CI): 1.67-2.50) and 7.61 (95% CI: 4.92-11.77), and 2.41 (95% CI: 2.01–2.89), respectively as compared to euthyroidism. These odd ratios remained significant after further adjustments. The odds ratios for proteinuria is 2.04 (95% CI: 1.67–2.50), 7.61 (95% CI: 4.92–11.77), and 2.41 (95% CI: 2.01–2.89) for subclinical, clinical, and total hypothyroidism, respectively, although the odds ratios were attenuated after further adjustment. Our results suggest subclinical hypothyroidism is a novel risk factor of reduced renal function but not proteinuria.

Thyroid dysfunction and kidney disease: An update

Thyroid hormones influence renal development, kidney hemodynamics, glomerular filtration rate and sodium and water homeostasis. Hypothyroidism and hyperthyroidism affect renal function by direct renal effects as well as systemic hemodynamic, metabolic and cardiovascular effects. Hypothyroidism has been associated with increased serum creatinine and decreased glomerular filtration rate. The reverse effects have been reported in thyrotoxicosis. Most of renal manifestations of thyroid dysfunction are reversible with treatment. Kidney disease may also cause thyroid dysfunction by several mechanisms. Nephrotic syndrome has been associated to changes in serum thyroid hormone concentrations. Different forms of glomerulonephritis and tubulointerstitial disease may be linked to thyroid derangements. A high prevalence of thyroid hormone alteration has been reported in acute kidney injury. Thyroid dysfunction is highly prevalent in chronic kidney disease patients. Subclinical hypothyroidism and low triiodothyronine syndrome are common features in patients with chronic kidney disease. Patients treated by both hemodialysis and peritoneal dialysis, and renal transplantation recipients, exhibit thyroid hormone alterations and thyroid disease with higher frequency than that found in the general population. Drugs used in the therapy of thyroid disease may lead to renal complications and, similarly, drugs used in kidney disorders may be associated to thyroid alterations. Lastly, low thyroid hormones, especially low triiodothyronine levels, in patients with chronic kidney disease have been related to a higher risk of cardiovascular disease and all-cause mortality. Interpretation of the interactions between thyroid and renal function is a challenge for clinicians involved in the treatment of patients with thyroid and kidney disease.

Unusual Presentation of Central Diabetes Insipidus in a Patient With Neurosarcoidosis

Hypernatremia is a frequent cause of intensive care unit admission. The patient presented in this article had hypernatremia refractory to D5W (dextrose 5% water) therapy, which led to a complex investigation. Workup revealed central diabetes insipidus most likely secondary to flare up of neurosarcoidosis. The challenge in terms of diagnosis was a presentation with low urine output in the setting of hypernatremia resistant to treatment with desmopressin. This case unfolded the role of hypothyroidism causing secondary renal dysfunction and hence needed continued treatment with thyroxine in addition to treatment for hypernatremia.

The relationships between cortisol levels, insulin levels, and thyroid hormones with 24-h urinary sodium excretion in never treated essential hypertensive patients

BACKGROUND

To study the relationship between cortisol, insulin, and thyroid hormone levels with 24-h urinary sodium (Na) excretion levels in essential hypertensive patients.

METHODS

All patients underwent history taking, physical examination, blood pressure (BP) measurement, 12 lead electocardiographic evaluation, routine urine analysis, biochemical analysis including measurement of cortisol, insulin, and thyroid hormone levels, 24-h urine collection to measure urinary Na and protein excretion and creatinine clearance.

RESULTS

In total, 68 newly diagnosed hypertensive patients were included. Spearman correlation analysis revealed that 24-h urinary Na excretion was correlated with insulin levels (ρ = -0.473, P < 0.0001), serum cortisol levels (ρ = -0.404, P = 0.0010) and creatinine clearance (ρ = 0.407, P = 0.0010). Linear regression of independent factors has revealed that systolic BP (B = 0.004, CI = 0.001-0.008, P = 0.0170), body mass index (B = 0.014, CI = 0.005-0.023, P = 0.0030), being male (B = 0.077, CI = 0.001-0.153, P = 0.0480), creatinine clearance (B = 0.003, CI = 0.001-0.006, P = 0.0120) and insulin levels (B = -0.008, CI = -0.014 to -0.002, P = 0.0070) were independently related with logarithmically converted 24-h Na excretion.

CONCLUSION

In conclusion, we found that insulin but not cortisol and thyroid hormone levels were independently related with 24-h urinary Na excretion in newly diagnosed essential hypertensive patients.

Hormonal regulation of salt and water excretion: a mathematical model of whole kidney function and pressure natriuresis

We present a lumped-nephron model that explicitly represents the main features of the underlying physiology, incorporating the major hormonal regulatory effects on both tubular and vascular function, and that accurately simulates hormonal regulation of renal salt and water excretion. This is the first model to explicitly couple glomerulovascular and medullary dynamics, and it is much more detailed in structure than existing whole organ models and renal portions of multiorgan models. In contrast to previous medullary models, which have only considered the antidiuretic state, our model is able to regulate water and sodium excretion over a variety of experimental conditions in good agreement with data from experimental studies of the rat. Since the properties of the vasculature and epithelia are explicitly represented, they can be altered to simulate pathophysiological conditions and pharmacological interventions. The model serves as an appropriate starting point for simulations of physiological, pathophysiological, and pharmacological renal conditions and for exploring the relationship between the extrarenal environment and renal excretory function in physiological and pathophysiological contexts.

A literature review on the adverse effects of hypothyroidism on kidney function

Thyroid produce two important hormone of thyroxine or tetraidothyronine (T4) and triidothyronine (T3), which are involved in whole aspect of metabolism. T4 and T3 play vital role in all biochemical function, growth and development in human body. The basic metabolic pathways in kidney and every organ in human controlled by these hormones. T4 and T3 are involved in kidney function in health and diseases condition therefore the pathophysiology of kidney can be directly influenced and regulated by thyroid hormones. Kidney growth, haemodynamic, blood circulation, tubular, electrolyte balance and glomerular filtration rate (GFR) are among such crucial process. Hypothyroidism which accompanied with reduced thyroid hormone production adversely affect the renal functions, development and eventually leading to reduced weight, kidney vascular disorders, electrolyte, tubular transport imbalances, lower filtration rate and other adverse consequences of hypothyroidism. On other hand kidney diseases can also disrupt the thyroid function metabolism resulting in the subsequent hypothyroidism. It is an interesting subject in how thyroid and kidney in health and diseases closely interacted. For the ideal clinical follow up of either of thyroid and renal diseases the two organs should be simultaneously examined for a proper patient management. Close correlation of thyroid and kidney clinical teams are essential to check the cross reactions and adverse interactions which might be produced between these two vital organs to avoid misdiagnosis either of thyroid or kidney abnormalities.

Interactions between thyroid disorders and kidney disease

There are several interactions between thyroid and kidney functions in each other organ's disease states. Thyroid hormones affect renal development and physiology. Thyroid hormones have pre-renal and intrinsic renal effects by which they increase the renal blood flow and the glomerular filtration rate (GFR). Hypothyroidism is associated with reduced GFR and hyperthyroidism results in increased GFR as well as increased renin - angiotensin - aldosterone activation. Chronic kidney disease (CKD) is characterized by a low T3 syndrome which is now considered a part of an atypical nonthyroidal illness. CKD patients also have increased incidence of primary hypothyroidism and subclinical hypothyroidism. The physiological benefits of a hypothyroid state in CKD, and the risk of CKD progression with hyperthyroidism emphasize on a conservative approach in the treatment of thyroid hormone abnormalities in CKD. Thyroid dysfunction is also associated with glomerulonephritis often by a common autoimmune etiology. Several drugs could affect both thyroid and kidney functions. There are few described interactions between thyroid and renal malignancies. A detailed knowledge of all these interactions is important for both the nephrologists and endocrinologists for optimal management of the patient.

Salt sensitivity in experimental thyroid disorders in rats

This study assessed salt sensitivity, analyzing the effects of an increased saline intake on hemodynamic, morphological, and oxidative stress and renal variables in experimental thyroid disorders. Six groups of male Wistar rats were used: control, hypothyroid, hyperthyroid, and the same groups treated with salt (8% via food intake). Body weight, blood pressure (BP), and heart rate (HR) were recorded weekly for 6 wk. Finally, BP and HR were recorded directly, and morphological, metabolic, plasma, and renal variables were measured. High-salt intake increased BP in thyroxine-treated rats but not in control or hypothyroid rats. High-salt intake increased cardiac mass in all groups, with a greater increase in hyperthyroid rats. Urinary isoprostanes and H(2)O(2) were higher in hyperthyroid rats and were augmented by high-salt intake in all groups, especially in hyperthyroid rats. High-salt intake reduced plasma thyroid hormone levels in hyperthyroid rats. Proteinuria was increased in hyperthyroid rats and aggravated by high-salt intake. Urinary levels of aminopeptidases (glutamyl-, alanyl-, aspartyl-, and cystinylaminopeptidase) were increased in hyperthyroid rats. All aminopeptidases were increased by salt intake in hyperthyroid rats but not in hypothyroid rats. In summary, hyperthyroid rats have enhanced salt sensitivity, and high-salt intake produces increased BP, cardiac hypertrophy, oxidative stress, and signs of renal injury. In contrast, hypothyroid rats are resistant to salt-induced BP elevation and renal injury signs. Urinary aminopeptidases are suitable biomarkers of renal injury.

Severe hyponatremia in association with I(131) therapy in a patient with metastatic thyroid cancer

Hyponatremia is a common clinical problem that results from various causes. Hypothyroidism is known to be one of the causes of this disorder. We report a case of metastatic thyroid cancer presenting with severe hyponatremia in association with hypothyroidism induced by pretreatment of I(131) therapy, such as a low-iodine diet and withdrawal of thyroid hormone. Serum arginine vasopressin (AVP) was elevated and urine osmolality was higher than that of serum. Saline infusion and thyroid hormone replacement normalized serum sodium and AVP. Inappropriate secretion of AVP in hypothyroid state was thought to be one of the causes of this hyponatremia.

Hypothyroidism and renal function in patients with systolic heart failure

The extent to which hypothyroidism affects renal function in patients with heart failure remains incompletely explored, despite the known adverse prognostic implications of renal dysfunction in these patients.In a pilot retrospective study, we evaluated 75 patients (age, >or=18 yr) with left ventricular ejection fractions <0.40. Forty-five patients had normal thyroid function (thyroid-stimulating hormone [TSH], 0.35-5.5 micro IU/mL) and 30 had hypothyroidism. The group with hypothyroidism was subdivided into 17 patients who had controlled hypothyroidism (TSH, 0.35-5.5 micro IU/mL) and 13 who had uncontrolled hypothyroidism (TSH, >5.5 micro IU/mL). Renal function, measured in terms of glomerular filtration rate, was analyzed once in each patient, and the populations were statistically compared, with P <0.05 conferring statistical significance.Baseline characteristics in all groups were similar. Mean glomerular filtration rate was better in patients with normal thyroid function than those with hypothyroidism (75.45 +/- 31.48 vs 63.95 +/- 21.43 mL/min/1.73 m2; P=0.032). There was no significant difference between patients with controlled hypothyroidism (66.89 +/- 24.18 mL/min/1.73 m2) and those with normal thyroid function (P=0.131). In patients with uncontrolled hypothyroidism, mean glomerular filtration rate (60.2 +/- 17.4 mL/min/1.73 m2) was significantly worse than in patients with normal thyroid function (P=0.015).We found that heart-failure patients with insufficiently treated hypothyroidism have worse renal function than do patients whose thyroid function is normal or whose hypothyroidism is effectively treated. Larger studies will be needed in order to evaluate this conclusion further. We recommend that hypothyroidism in heart-failure patients be strictly controlled, lest it affect prognosis adversely.

Thyroid dysfunction and kidney disease

Thyroid hormones (TH) are essential for an adequate growth and development of the kidney. Conversely, the kidney is not only an organ for metabolism and elimination of TH, but also a target organ of some of the iodothyronines' actions. Thyroid dysfunction causes remarkable changes in glomerular and tubular functions and electrolyte and water homeostasis. Hypothyroidism is accompanied by a decrease in glomerular filtration, hyponatremia, and an alteration of the ability for water excretion. Excessive levels of TH generate an increase in glomerular filtration rate and renal plasma flow. Renal disease, in turn, leads to significant changes in thyroid function. The association of different types of glomerulopathies with both hyper- and hypofunction of the thyroid has been reported. Less frequently, tubulointerstitial disease has been associated with functional thyroid disorders. Nephrotic syndrome is accompanied by changes in the concentrations of TH due primarily to loss of protein in the urine. Acute kidney injury and chronic kidney disease are accompanied by notable effects on the hypothalamus-pituitary-thyroid axis. The secretion of pituitary thyrotropin (TSH) is impaired in uremia. Contrary to other non-thyroidal chronic disease, in uraemic patients it is not unusual to observe the sick euthyroid syndrome with low serum triodothyronine (T(3)) without elevation of reverse T(3) (rT(3)). Some authors have reported associations between thyroid cancer and kidney tumors and each of these organs can develop metastases into the other. Finally, data from recent research suggest that TH, especially T(3), can be considered as a marker for survival in patients with kidney disease.

Interactions between thyroid and kidney function in pathological conditions of these organ systems: a review

Thyroidal status affects kidney function already in the embryonic stage. Thyroid hormones influence general tissue growth as well as tubular functions, electrolyte handling and neural input. Hyper- and hypo-functioning of the thyroid influences mature kidney function indirectly by affecting the cardiovascular system and the renal blood flow, and directly by affecting glomerular filtration, electrolyte pumps, the secretory and absorptive capacity of the tubuli, and the structure of the kidney. Hyperthyroidism accelerates several physiologic processes, a fact which is reflected in the decreased systemic vascular resistance, increased cardiac output (CO), increased renal blood flow (RBF), hypertrophic and hyperplastic tubuli, and increased glomerular filtration rate (GFR). Renal failure can progress due to glomerulosclerosis, proteinuria and oxidative stress. Hypothyroidism has a more negative influence on kidney function. Peripheral vascular resistance is increased with intrarenal vasoconstriction, and CO is decreased, causing decreased RBF. The influence on the different tubular functions is modest, although the transport capacity is below normal. The GFR is decreased up to 40% in hypothyroid humans. Despite the negative influences on glomerular and tubular kidney function, a hypothyroid state has been described as beneficial in kidney disease. Kidney disease is associated with decreased thyroid hormone concentrations caused by central effects and by changes in peripheral hormone metabolism and thyroid hormone binding proteins. Geriatric cats form an animal model of disease because both hyperthyroidism and chronic kidney disease (CKD) have high prevalence among them, and the link between thyroid and kidney affects the evaluation of clinical wellbeing and the possible treatment options.

Short- and long-term follow-up of glomerular and tubular renal markers of kidney function in hyperthyroid cats after treatment with radioiodine

Hyperthyroidism can mask co-existing chronic kidney disease (CKD). Previous studies showed that post-treatment renal azotemia can be predicted by pre-treatment assessment of glomerular filtration rate (GFR). We hypothesized that treatment of hyperthyroidism may have different effects on glomerular and tubular function and these changes might be predicted by additional pre-treatment variables than GFR. Serum total T4 (TT4), creatinine and blood urea nitrogen (BUN), blood pressure (BP), body weight (BW), GFR, urine specific gravity (USG), urinary protein/creatinine ratio (UPC) and retinol binding protein/creatinine ratio (uRBP/c) were evaluated before and 1, 4, 12 and 24 weeks post-treatment with radioiodine ((131)I) in 21 non-azotemic hyperthyroid cats. Cats were divided 24 weeks post-treatment into group A (normal kidney function, n=16) and group B (impaired kidney function, n=5). Serum TT4, GFR, UPC and uRBP/c decreased significantly after treatment for the complete group and group A (P<0.05), although GFR and uRBP/c did not change in group B. Serum creatinine and BW increased significantly from 1 week after treatment (P<0.05). There was no change in BUN, USG or BP. Pre-treatment serum TT4, GFR and USG differed significantly between group A and B (P<0.05). GFR at 4 weeks after treatment and maximum decrease in GFR could be partially predicted by a formula using pre-treatment GFR, serum TT4, serum creatinine, BUN and/or USG. Significant changes in kidney function occur within 4 weeks post-treatment and none thereafter. Pre-treatment measurement of GFR, USG and serum TT4 can have possible predictive value regarding the development of post-treatment renal azotemia.

Effect of thyroid hormone on the postnatal renal expression of NHE8

We previously demonstrated that there are developmental changes in proximal tubule Na(+)/H(+) exchanger (NHE) activity. There is a maturational increase in postnatal brush-border membrane (BBM) vesicle NHE3 protein abundance and decrease in NHE8 protein abundance. The purpose of this study was to determine whether thyroid hormone plays a role in the rat renal maturational isoform switch from NHE8 to NHE3 and whether thyroid hormone regulates NHE8. Administration of thyroid hormone to neonatal rats, before the normal postnatal increase in serum thyroid hormone levels at 3 wk of age, resulted in a premature increase in NHE3/beta-actin BBM protein abundance and mRNA abundance. Thyroid hormone also caused a premature decrease in BBM NHE8/beta-actin protein abundance, whereas there was no change in mRNA expression (standardized to 28s). Rats made hypothyroid from birth were studied at 28 days, after the normal maturational increase in thyroid hormone. In these hypothyroid adult rats, the maturational increase in BBM NHE3 protein abundance and NHE3 mRNA expression was prevented. In contrast, the developmental decrease in BBM NHE8 protein abundance was prevented in hypothyroid adults, but mRNA expression was unchanged in hypothyroid rats. To determine whether the effect of thyroid hormone was due to a direct epithelial effect, we studied normal rat kidney cells in culture. We recently showed that this cell line expresses NHE8, but does not express NHE3. Thyroid hormone caused a decrease in surface expression of NHE8, determined by biotinylation, but total cellular abundance remained unchanged. NHE8 activity, measured as the sodium-dependent rate of intracellular pH recovery from an acid load, was less with thyroid treatment than control. In conclusion, thyroid hormone plays a potential role in the developmental isoform change from NHE8 to NHE3 and decreases NHE8 activity.

Thyroid Hormone-Induced Alterations in Membrane Structure-Function Relationships: Studies on Kinetic Properties of Rat Kidney Microsomal Na+,K+-ATPase and Lipid/Phospholipid Profiles

The effects of thyroidectomy (Tx) and subsequent treatment with 3,5,3'-triiodothyronine (T(3)) or combined replacement therapy (T(R)) with T(3 )and thyroxine (T(4)) on the substrate and temperature kinetics properties of Na+,K+-ATPase and lipid/phospholipid makeup of rat kidney microsomes were examined. Enzyme activity was somewhat high in the hypothyroid (Tx) animals and increased significantly following T(3) treatment, while T(R) treatment caused a decrease. In the Tx and T(3) groups enzyme activity resolved in two kinetic components, while in the T(R) group the enzyme showed allosteric behavior up to 0.5 mM: ATP concentration. The K(m) and V(max) values of both the components decreased in Tx animals without affecting the catalytic efficiency. T(3) treatment caused a significant increase in the V(max) of both the components, with a significant increase in the catalytic efficiency, while the K(m) values were not upregulated. The T(R) regimen lowered the K(m) and V(max) of component II but improved the catalytic efficiency. Thyroid status-dependent changes were also noted in the temperature kinetics of the enzyme. Regression analysis revealed that changes in the substrate and temperature kinetics parameters correlated with specific phospholipid components.

Thyroid hormone modulates renin and ANG II receptor expression in fetal sheep

Fetal renin-angiotensin system (RAS) activity is developmentally regulated, increasing in late gestation toward term. At the same time, fetal hemodynamic parameters change, with blood pressure increasing and heart rate decreasing. During this period, fetal plasma thyroid hormone concentrations also increase significantly. In this study we utilized the technique of thyroidectomy (TX), which removes thyroid hormone from the circulation, to investigate the importance of thyroid hormone on the developmental changes in the RAS (in plasma, kidney, heart, and lung) and hemodynamic regulation in fetal sheep. TX was performed at 120 days of gestational age (dGA), and control fetuses were sham operated. Immediately before necropsy (∼137 dGA), fetuses were infused with isoproterenol and the hemodynamic responses were noted. TX significantly decreased plasma thyroid hormone concentrations and renal renin mRNA and renal active renin levels but did not change fetal plasma active renin levels. TX decreased both angiotensin II receptor subtype 1 (AT1) mRNA and protein levels in kidney and lung but not in the left ventricle. TX also was associated with increased ANG II receptor subtype 2 (AT2) mRNA and protein at the 44-kDa band in kidney, whereas AT2 protein was decreased at the 78-kDa level in kidney and lung tissue only. TX fetuses had significantly lower basal mean arterial blood pressures (MAP) and heart rates than controls. Isoproterenol infusion decreased MAP in TX fetuses. These findings support the hypothesis that thyroid hormone is important in modulating maturation of RAS and cardiovascular function in the late-gestation fetal sheep.

Hypothyroidism induces expression of the peptide transporter PEPT2

The kidney is a target organ for thyroid hormone action and a variety of renal transport processes are altered in response to impaired thyroid functions. To investigate the effect of thyroid hormone on the expression of the renal proximal tubular high-affinity-type H+-peptide cotransporter (PEPT2) in rats, hypothyroidism was induced in animals by administration of methimazole (0.05%) via drinking water. After 7 weeks of treatment, hypothyroidism was confirmed by determining serum free T3 and free T4 concentrations. Northern blotting was used to examine the expression of PEPT2 mRNA in kidney tissues from hypothyroid rats compared to control rats. Hypothyroidism resulted in an increased level of total renal PEPT2 mRNA (121.1±3.3% vs. control 100±2.8%; p=0.008). The mRNA results were confirmed by immuno-blotting, which demonstrated significantly increased protein levels (162% vs. control 100%; p<0.01). Immunohistochemistry also revealed increased PEPT2 protein levels in the proximal tubules of treated compared to non-treated rats. In summary, PEPT2 is the first proximal tubule transporter protein that shows increased expression in states of hypothyreosis. As PEPT2 reabsorbs filtered di- and tripeptides and peptide-like drugs, the present findings may have important implications in nutritional amino acid homeostasis and for drug dynamics in states of altered thyroid function.

Thyroid hormone modulates rabbit proximal straight tubule paracellular permeability

Proximal straight tubules (PST) from both neonatal and hypothyroid adult rabbits have a lower rate of passive volume absorption when perfused with a high-chloride solution simulating late proximal tubular fluid than adult rabbit PST. We hypothesized that the maturational increase in serum thyroid hormone levels mediates the developmental changes in PST paracellular permeability. Neonatal tubules had lower chloride permeability, higher transepithelial resistance, but comparable mannitol permeability compared with adult PST. The present in vitro microperfusion study directly examined whether thyroid hormone affects passive solute flux and whether thyroid hormone could explain the developmental changes in PST paracellular permeability. Passive chloride transport was 62.1 +/- 4.5, 23.1 +/- 7.7, and 111.6 +/- 5.6 pmol.mm(-1).min(-1) in PST from euthyroid, hypothyroid, and hypothyroid animals that received thyroid treatment, respectively (control different from hypothyroid and thyroid treatment at P < 0.05). This was due to a thyroid hormone-mediated change in chloride permeability (P(Cl)). Mannitol permeability was 3.65 + 1.03, -0.19 + 0.72, and 3.60 + 1.12 x 10(-6) cm/s in PST from euthyroid animals, hypothyroid animals, and hypothyroid rabbits that received thyroid replacement, respectively (P < 0.05 hypothyroid vs. euthyroid and thyroid replacement). We demonstrate that PST from hypothyroid animals have a higher passive P(Na)/P(Cl) and P(HCO3)/P(Cl) than euthyroid controls. Finally, we examined whether these changes in permeability were paralleled by a change in PST paracellular resistance. Resistance was measured by current injection and cable analysis. The resistance in PST from hypothyroid rabbits was 6.3 +/- 0.8 Omega.cm(2), which was not different from control of 4.8 +/- 0.7 Omega.cm(2), or 7.0 +/- 0.7 Omega.cm(2) in hypothyroid animals that received thyroid replacement. Therefore, the maturational increase in thyroid hormone levels does not fully explain the developmental changes in the paracellular pathway.

Renal expression of sodium transporters and aquaporin-2 in hypothyroid rats

Hypothyroidism is associated with significant abnormalities in the renal handling of salt and water. To address the involvement of tubular transport proteins in these abnormalities, rats were rendered pharmacologically hypothyroid and the abundance of major tubular transport proteins was assessed by immunoblot and immunohistochemistry. Hypothyroidism resulted in a marked reduction in kidney size and creatinine clearance along with decreased or unchanged total kidney abundance of the transport proteins. Whereas the proximal tubular type 3 Na/H exchanger (NHE3) and type 2 Na-phosphate cotransporter (NaPi2) stood out by their disproportionately reduced abundance, the bumetanide-sensitive type 2 Na-K-2Cl cotransporter (NKCC2) and aquaporin-2 (AQP2) were unaltered in their total kidney abundance despite a markedly lower kidney mass. The latter proteins in fact showed enhanced immunostaining. Decreased NHE3 and NaPi2 expression was most likely due to a combination of triiodo-l-thyronine (T(3)) deficiency along with a reduced glomerular filtration rate. The increased abundance of NKCC2 and AQP2 may have been caused by an increased action of vasopressin since urinary excretion of this hormone was elevated. On the other hand, the thiazide-sensitive Na-Cl cotransporter; the alpha-, beta-, and gamma-subunits of the amiloride-sensitive epithelial Na channel; and the alpha(1)-subunit of Na-K-ATPase showed a moderate decrease in total kidney abundance that was largely proportional to the smaller kidney mass. Although the observed expression of transporters was associated with a balanced renal sodium handling, altered transporter abundance may become functionally relevant if the hypothyroid kidney is challenged by an additional destabilization of the milieu interieur that has previously been shown to result in an inadequate natriuresis and clinical symptoms.

Urinary concentrating defect in hypothyroid rats: role of sodium, potassium, 2-chloride co-transporter, and aquaporins

Hypothyroidism is associated with impaired urinary concentrating ability in humans and animals. The purpose of this study was to examine protein expression of renal sodium chloride and urea transporters and aquaporins in hypothyroid rats (HT) with diminished urinary concentration as compared with euthyroid controls (CTL) and hypothyroid rats replaced with L-thyroxine (HT+T). Hypothyroidism was induced by aminotriazole administration. Body weight, water intake, urine output, solute and urea excretion, serum and urine osmolality, serum creatinine, 24-h creatinine clearance, and fractional excretion of sodium were comparable among the three groups. However, with 36 h of water deprivation, HT rats demonstrated significantly greater urine flow rates and decreased urine and medullary osmolality as compared with CTL and HT+T rats at comparable plasma vasopressin concentrations. Western blot analyses revealed decreased renal protein abundance of transporters, including Na-K-2Cl, Na-K-ATPase, and NHE3, in HT rats as compared with CTL and HT+T rats. Protein abundance of renal AQP1 and urea transporters UTA(1) and UTA(2) did not differ significantly among study groups. There was however a significant decrease in protein abundance of AQP2, AQP3, and AQP4 in HT rats as compared with CTL and HT+T rats. These findings demonstrate a decrease in the medullary osmotic gradient secondary to impaired countercurrent multiplication and downregulation of aquaporins 2, 3, and 4 as contributors to the urinary concentrating defect in the hypothyroid rat.

Molecular basis of renal handling of calcium in response to thyroid hormone status of rat

We investigated the effect of thyroid hormone status on renal handing of Ca2+. Further, like kinetics of Ca2+ transport across brush-border membrane (BBM) and basolateral membrane (BLM) of renal epithelial cells was carried out. FE(Ca) was decreased in hyperthyroid (Hyper-T) rats and increased in hypothyroid (Hypo-T) rats as compared to euthyroid (Eu-T) rats. Ca2+ uptake into renal brush-border membrane vesicles (BBMV) was increased in Hyper-T rats and decreased in Hypo-T rats as compared to Eu-T rats. K(m) was lower in Hyper-T rats and higher in Hypo-T rats as compared to Eu-T rats whereas, V(max) remained unaltered. The transition temperature for calcium uptake varied inversely with the thyroid hormone status. Renal BBM of Hyper-T rats showed decreased anisotropy and polarisation of DPH as compared to EU-T rats whereas these values were increased in Hypo-T rats. Thus, the altered BBM fluidity appears to modulate Ca2+ transport across BBM. Na+/Ca2+ exchange activity of renal cells was increased in Hyper-T and decreased in Hypo-T rats as compared to Eu-T rats. V(max) for Na+/Ca2+ exchange was increased in Hyper-T rats and deceased in Hypo-T rats as compared to Eu-T rats, whereas, [Na+](0.5) was similar in all three groups. The c-AMP levels of renal cortex of Hyper-T rats was increased and that of Hypo-T rats decreased as compared to Eu-T rats. Thus, thyroid hormones increased Ca2+ reabsorption in the kidney of rat. Thyroid hormone-mediated modulation of BBM fluidity appears to stimulate Ca2+ uptake into renal BBMV. Thyroid hormones possibly activated the Na+/Ca2+ exchanger through cAMP-dependent pathway.

Maturation of proximal straight tubule NaCl transport: role of thyroid hormone

We have recently demonstrated that the rates of both active and passive proximal straight tubule (PST) NaCl transport in neonatal rabbits were less than in adults. In this segment NaCl entry across the apical membrane is via parallel Na(+)/H(+) and Cl(-)/OH(-) exchangers, which increases in activity with maturation. The present in vitro microperfusion study examined whether thyroid hormone plays a role in the maturational increase in PST NaCl transport. Neonatal and adult PST were perfused with a high-chloride-low bicarbonate solution without organic solutes, simulating late proximal tubule fluid. Thyroid hormone-treated neonates had a higher rate of PST total and passive NaCl transport. In 8-wk-old animals that were hypothyroid since birth, the maturational increase in total and passive NaCl transport was prevented. Thyroid treatment for 4 days in hypothyroid 8-wk-old rabbits increased the rate of both total and passive NaCl transport. The maturational increases in both Na(+)/H(+) and Cl(-)/OH(-) exchange activities were blunted in 8-wk-old hypothyroid animals and increased to control levels with thyroid treatment. This study demonstrates that thyroid hormone is a factor responsible for the maturational increase in both active and passive PST NaCl transport.

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

Thyroid hormone stimulates renal proximal tubule NaCl and NaHCO3 absorption in part by activating the apical membrane Na/H exchanger NHE3. We used a renal epithelial cell line, the opossum kidney (OK) cell, to define the mechanism by which 3,5,3'-triiodothyronine (T3) increases NHE3 activity. T3 stimulated NHE3 activity, an effect that was blocked by inhibition of cellular transcription or translation. The increase in activity was associated with increases in steady-state cell surface and total cellular NHE3 protein and NHE3 transcript abundance. T3 stimulated transcription of the NHE3 gene and had no effect on NHE3 transcript stability. The transcriptional activity of the 5'-flanking region of the rat NHE3 gene was stimulated by T3 when expressed in OK cells. When heterologously expressed rat NHE3 transcript levels were clamped constant with a constitutive promoter in OK cells, T3 has no effect on rat NHE3 protein abundance, suggesting the absence of regulation of NHE3 protein stability or translation. These studies demonstrate that T3 stimulates NHE3 activity by activating NHE3 gene transcription and increasing NHE3 transcript and protein abundance.

Effects of thyroid hormones on urinary and renal kallikreins

The effects of thyroid hormones on the urinary excretion of kallikrein and on renal kallikrein were studied in rats. Total and active urinary kallikrein was decreased after thyroidectomy, but renal kallikrein content remained unchanged. Diuresis increased, and kidney weight and plasma aldosterone concentration decreased. Treatment with 3,5,3'-triiodo-L-thyronine restored the urinary kallikrein in thyroidectomized rats to normal and increased it in intact rats. It also produced increases in kidney weight and plasma aldosterone and a decrease in diuresis. The effect of thyroid hormones on the urinary kallikrein response to mineralocorticoids was also tested. Deoxycorticosterone acetate increased urinary kallikrein more in normal than in thyroidectomized rats. These results suggest that thyroidectomy decreases renal kallikrein synthesis and lowers the turnover rate of the enzyme, changes not detectable by a single measurement of the renal kallikrein content but reflected by an alteration in the urinary excretion of the enzyme. Thyroid hormones participate in the control of urinary kallikrein. This effect, however, is probably indirect and may be mediated by mineralocorticoids since thyroid function affects both the plasma level of aldosterone, which is known to influence renal kallikrein, and the kallikrein response to exogenous mineralocorticoids.

Abnormal taste preference for saccharin in hypothyroid rats

Taste preferences for saccharin in concentrations ranging from 0.16 mM to 50 mM were determined in rats made hypothyroid with radioactive iodine and in their littermate controls. Hypothyroid rats demonstrated taste preferences for saccharin which were similar to those of controls only at very low (0.016 mM) or very high (49.0 mM) saccharin concentrations. At these concentrations of tastant, the preferences for tastant and water were similar to one another. At a concentration of 5.1 mM, preferences were also very similar in both groups but were very high. At intermediate saccharin concentrations of 1.1 and 3.0 mM, hypothyroid animals showed significantly lower percent preferences for the sweet tastant than did controls, mean +/- SEM (62.48 +/- 5.97 vs. 82.92 +/- 4.60, p = 0.0002) for the 1.1 mM concentration and (74.98 +/- 5.12 vs. 89.40 +/- 2.54, p = 0.0029) for the 3.0 mM concentration. These changes in taste preference for saccharin in hypothyroid rats were similar in direction and magnitude to those previously published by this laboratory using sucrose as the tastant. Thus, hypothyroid rats demonstrate abnormalities in taste preference for both the nonnutritive sweetener, sodium saccharin, as well as for the nutritive sweetener, sucrose.

Short-term hypothyroidism and vasopressin gene expression in the rat

Hypothyroidism is associated with abnormalities in renal water handling, which include a delay in excretion of an acute water load, decreased urinary concentrating ability, and increased urine volume. In the present study, we investigated the role of vasopressin in aminotriazole-induced hypothyroidism by measuring vasopressin concentration in the plasma and pituitary along with vasopressin mRNA levels in the hypothalamus. After 5 weeks of aminotriazole treatment, L-thyroxine levels were significantly lower in the experimental animals (122 +/- 8 v 26 +/- 1 nmol/L [9.5 +/- 0.6 v 2.0 +/- 0.1 micrograms/dL]; P less than 0.001). Serum sodium (148 +/- 0.5 v 144 +/- 1.2 mmol/L [mEq/L]; P less than 0.01), and plasma osmolality (311 +/- 2.5 v 304 +/- 1.8 mmol/kg [mOsm/kg] H2O; P less than 0.05) were also lower in the experimental animals. There were no differences in plasma (1.9 +/- 0.4 v 1.5 +/- 0.2 pg/mL) or pituitary (1.5 +/- 0.4 v 1.5 +/- 0.2 microgram/pituitary) vasopressin levels. In addition, steady-state vasopressin mRNA levels were not different between the two groups (1,286 +/- 210 v 1,093 +/- 138 pg/hypothalamus). One week of L-thyroxine replacement resulted in significant increases in serum thyroxine levels without changes in the other variables measured. These results indicate that short-term hypothyroidism, which has been shown to exert substantial effects on renal function, causes only a modest central alteration in the plasma vasopressin-osmolality relationship, which occurs in the absence of detectable changes in vasopressin synthesis.

The protective mechanism of thyroidectomy in a rat model of chronic renal failure

Selective thyroidectomy (Tx) has been shown to attenuate proteinuria and disease progression in models of chronic renal failure (CRF). In this investigation, four groups of Munich-Wistar rats were studied to determine if glomerular dynamics or another renal metabolic consequence of Tx was responsible for the protective effect as measured by 24-hour protein excretion (UPROT). The groups were TxT4, thyroxine-replaced Tx rats with five-sixths nephrectomy; Tx, Tx rats not receiving replacement thyroxine with five-sixths nephrectomy; TxI, Tx rats not receiving replacement thyroxine with five-sixths nephrectomy that were given continuous intraperitoneal isoproterenol to restore systemic and renal hemodynamics; and TxT4(C), two-kidney Tx rats receiving replacement thyroxine that served as normal controls. Five-sixths nephrectomy was carried out 2 weeks after Tx, and experiments were carried out 1 week later. Serum T4 was profoundly reduced and there was failure to gain weight in Tx and TxI rats, despite similar protein intakes in all groups. Cardiac output was reduced in Tx, but was similar in TxI to levels in TxT4 rats. Whole-kidney glomerular filtration rate was lower in Tx, at 0.145 +/- 0.052 mL/min (P less than 0.05), but similar in TxI (0.305 +/- 0.147 mL/min) to that in TxT4 rats (0.317 +/- 0.135 mL/min). Twenty-four-hour urinary protein, which was 129 +/- 57 mg in TxT4, was reduced in Tx to 9 +/- 3 mg (P less than 0.01) but restored in TxI to 89 +/- 30 mg, a level similar to that in TxT4.(ABSTRACT TRUNCATED AT 250 WORDS)

Renal concentrating ability in obesity. Effect of modified fasting and the supplementation of T3, sodium chloride and carbohydrate

The renal concentrating ability (RCA) was studied in 30 obese subjects before and after modified fasting (MF) and T3 supplementation, and during hypocaloric-carbohydrate refeeding. We also studied the effect of sodium supplementation on the RCA during MF. Modified fasting induced a low T3-high rT3 state ("sick euthyroid"). During T3-supplementation plasma T3 levels increased but were in the normal range for normal weight controls. Plasma sodium, potassium, and calcium remained within the normal range during all study periods. After MF (14 days) the mean maximal urinary osmolality was significantly lower compared to prefast values both after dehydration alone (706 +/- 12 mosm/kg H2O v 975 +/- 14, P less than 0.001) and after dehydration plus sc vasopressin administration (676 +/- 19 v 899 +/- 17, P less than 0.001). After 14 days MF followed by 14 days MF + T3-supplementation plasma urea, urinary urea excretion, and the creatinine clearance were significantly greater than after MF alone as was the RCA (764 +/- 15 v 652 +/- 25, P less than 0.002). Sodium chloride supplementation increased RCA (P less than 0.02) but no additive effect of T3 and sodium chloride supplementation was observed. Severe dietary salt restriction induced a significant decline in RCA (P less than 0.005). Refeeding with carbohydrate increased plasma T3 from 79.9 +/- 7.7 to 97 +/- 7.5 ng/100 mL (NS) and decreased plasma rT3 from 0.33 +/- 0.02 to 0.27 +/- 0.02 ng/mL, (P less than 0.02); no significant change in RCA was observed.(ABSTRACT TRUNCATED AT 250 WORDS)

Response of the medullary thick ascending limb to hypothyroidism in the rat

Hypothyroidism in the rat is associated with a decrease in kidney size and weight. We have shown previously that this decrease involves all nephron segments in the cortex and outer medulla and is especially pronounced in the medullary thick ascending limb (mTAL). Therefore, the present study was undertaken for examination of the effect of hypothyroidism on the ultrastructure of the rat mTAL. Hypothyroidism was induced by feeding aminotriazole (ATZ), 0.5 g/kg of food, for 4 weeks. A second group of animals received ATZ plus a daily injection of L-thyroxine (T4). A third group was fed a normal diet only and served as control animals. The kidneys were preserved by in vivo perfusion with 3% glutaraldehyde, and tissue from the inner and outer stripe of the outer medulla was processed for electron microscopy. Morphometric analysis revealed a significant decrease in the cross-sectional area of the mTAL in both the inner and outer stripe of the outer medulla in hypothyroid animals. No changes were observed in the surface density of either the apical or basolateral plasma membranes following ATZ treatment. However, when calculated per millimeter tubule, there was a significant decrease in the surface area (SL) of both the apical and basolateral plasma membranes of the mTAL of hypothyroid animals in comparison with control animals. Simultaneous treatment with T4 prevented the ATZ-induced reduction in both tubule cross-sectional area and in the SL of the plasma membrane. These findings suggest that the observed changes in SL of the plasma membrane of the mTAL are due to a regulatory role of thyroid hormone in membrane proliferation as well as in cell growth in general.

Thyroid hormones increase Na+-H+ exchange activity in renal brush border membranes

Na+-H+ exchange activity, i.e., amiloride-sensitive Na+ and H+ flux, in renal proximal tubule brush border (luminal) membrane vesicles was increased in the hyperthyroid rat and decreased in the hypothyroid rat, relative to the euthyroid animal. A positive correlation was found between Na+-H+ exchange activity and serum concentrations of thyroxine (T4) and triiodothyronine (T3). The thyroid status of the animal did not alter amiloride-insensitive Na+ uptake. The rate of passive pH gradient dissipation was higher in membrane vesicles from hyperthyroid rats compared to the rate in vesicles from hypothyroid animals, a result which would tend to limit the increase in Na+ uptake in vesicles from hyperthyroid animals. Na+-dependent phosphate uptake was increased in membrane vesicles from hyperthyroid rats; Na+-dependent D-glucose and L-proline uptakes were not changed by the thyroid status of the animal. The effect of thyroid hormones in increasing the uptake of Na+ in the brush border membrane vesicle is consistent with the action of the hormones in enhancing renal Na+ reabsorption. Further, the regulation of transtubular Na+ flux has now been shown to be concomitant with modulation of the entry of Na+ into the tubular cell across its luminal membrane, mediated by the exchange reaction, and with the previously reported control of the pumping of Na+ out of the cell across its basolateral membrane, mediated by the Na+,K+-ATPase.

Short term effect of low doses of tri-iodothyronine on proximal tubular membrane Na-K-ATPase and potassium permeability in thyroidectomized rats

Tri-iodothyronine (T3), even when administered for short time and at low doses, induces a large increase in the isotonic fluid reabsorption (Jv) in proximal tubules of thyroidectomized rats (TX). In order to investigate the role of the Na-K-ATPase in this process, we measured the Na-K-ATPase activity in early proximal convoluted tubules (S1) and proximal straight tubules (S2) microdissected from TX rats and rats treated with low doses of T3 (10 micrograms/kg body wt), either for 3 days (TX + 3T3) or for 7 days (TX + 7T3). In both segments no changes in Na-K-ATPase activity were found in TX + 3T3 rats versus TX rats, while an increase was registered in TX + 7T3 rats. Using micropuncture techniques, Jv measured on the same tubular segments increased by 68% in TX + 3T3 rats versus TX. Thus, no correlation between Jv and Na-K-ATPase activity measured in vitro could be detected after short term treatment of TX rats with T3. Na-K-ATPase activity in vivo is also regulated by the potassium permeability of the membrane, which might be altered by tri-iodothyronine. This hypothesis was tested by perfusing intraluminally and peritubularly proximal tubules of TX rats with the K ionophore, valinomycin (1 microgram/ml). In the dual perfusion experiments valinomycin elicited 40% of the action induced on Jv by 3 days treatment with T3. On the other hand, no further increase in Jv was recorded when valinomycin was applied in TX rats pretreated with T3.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of volume reabsorption by thyroid hormones in the proximal tubule of rat: minor role of luminal sodium permeability

In order to investigate whether changes in luminal membrane sodium permeability can explain the increase in isotonic fluid reabsorption (Jv) found in proximal tubules of thyroidectomized rats (TX) treated with tri-iodothyronine (T3), experiments were carried out on TX rats and TX rats treated for 3 days (TX + T3) with physiological doses (10 micrograms/kg body wt) of T3. Two sets of experiments were performed: 1) in vivo, using the micropuncture technique for the measurements of Jv; 2) in vitro, using isolated brush border membrane vesicles for the direct measurement of Na+ permeability. In micropuncture studies a 65% increase in Jv of TX rats was observed after treatment with T3. Luminal perfusion of proximal tubules of TX rats with Amphotericin B (10 micrograms/ml), to increase luminal sodium permeability, enhanced Jv only by 15%. Brush border membrane vesicles isolated from TX and TX + T3 rats showed the same sodium permeability in uptake or efflux experiments. These results were confirmed by the fact that sodium gradient dependent histidine transport into brush border membrane vesicles did not change after T3 treatment. Finally, measuring the amiloride sensitive sodium uptake, it was also found that Na+-H+ exchange was also only slightly affected by T3. These micropuncture and vesicle data indicate that the large effect of T3 on the trans-cellular sodium transport and volume reabsorption in the proximal tubule, cannot be explained by an action of T3 on the sodium entry step across the brush border membrane.

Effects of thyroid status on renal calcium and magnesium handling

Renal calcium and magnesium handling was studied in rats with chronic thyroid hormone deficiency or excess, hyperthyroidism. Mean kidney weight of the thyroid deficient rats was 42% of age matched, euthyroid and hyperthyroid animals and glomerular filtration rate was 71% of normal. Fractional sodium excretion was consistently elevated in thyroid deficient rats (0.26%) as compared to euthyroid (0.07%) and hyperthyroid animals (0.07%). Urinary calcium excretion (0.39%) was also elevated and parallel to sodium excretion in thyroid deficiency. Despite this renal leak of sodium and calcium, thyroid deficient animals conserved magnesium much more efficiently than either euthyroid or hyperthyroid rats (5.7% vs 17.4% respectively). Plasma magnesium concentration was elevated by acute MgCl2 infusions to determine the reabsorptive capacity of magnesium. Thyroid deficient rats reabsorbed 15-30% more of the filtered magnesium at any given plasma concentration. Although these effects on electrolyte reabsorption are modest compared to the hemodynamic alterations, the data suggest that thyroid hormone has a direct effect on the tubule which if chronically absent results in subtle sodium and calcium wasting and renal retention of magnesium. Administration of thyroid hormone to euthyroid or thyroid deficient rats twenty-four hours prior to experimentation had no effect on calcium and magnesium handling.

Kidney structure in hypothyroidism

Hypothyroidism is associated with changes in renal function and a decrease in kidney weight. The present study was carried out for examination of the effect of the antithyroid drug aminotriazole (ATZ) on kidney structure and especially for determination of whether hypothyroidism affects the whole nephron or only a specific segment of it. Female Sprague-Dawley rats received ATZ in their diet, 0.5 g/kg food for 4 weeks. Control rats received a normal diet. Additional groups of animals receiving ATZ in the diet were given daily injections of L-thyroxine (T4) for determination of whether ATZ-induced changes could be prevented by thyroid hormone. After collection of physiologic data the kidneys were fixed by in vivo perfusion with 3% glutaraldehyde in preparation for light-microscopic morphometry. Following ATZ treatment there was a significant decrease in serum triiodothyronine (T3) and in GFR. Kidney weights were decreased, mainly because of a reduction of cortical tissue. Morphometry showed no changes in the relative volumes of the various compartments of the kidney, indicating that the decrease in weight involved all segments of the nephron. Direct measurements of tubular diameters revealed a decrease in the peritubular diameters in both proximal tubules and the thick ascending limb and a decreased cell height in the thick ascending limb. All the ATZ-induced changes could be prevented by simultaneous treatment with T4, suggesting that the changes were caused by the antithyroid effect of ATZ and were not a nonspecific toxic effect.

Effect of experimental hyperthyroidism on the elemental content of rat hepatocytes

X-ray microanalysis has been used to determine elemental content in the cytoplasm of hepatocytes of hyperthyroid rats in comparison with euthyroid controls. No significant differences were found for any examined element (sodium, phosphorus, sulfur, chlorine, potassium, magnesium and calcium). In contrast to earlier reports from another laboratory, these data indicate that thyroid hormones do not substantially affect elemental content in rat hepatocytes.

X-ray microanalysis of cardiocytes in the Snell dwarf mouse

X-ray microanalysis has been used to determine elemental content in the nucleus, myofibrillar cytoplasm and mitochondrially enriched cytoplasm of cardiocytes in Snell dwarf mice in comparison with phenotypically normal mice from the same strain. It was found that there was significantly lower chlorine concentration in all three subcellular locations and significantly lower sodium concentration in the nucleus of dwarf mouse cardiocytes. In both normal and dwarf mice, statistically significant subcellular compartmentalization was found for phosphorus, sulfur, and potassium.

Renal tubular sodium reabsorption and oxygen consumption in the hypothyroid dog

The degree to which an insufficiency of thyroid hormone (TH) affects renal tubular sodium reabsorption (RNa) and oxygen consumption (QO2) in vivo, was investigated in a mature mammalian kidney. The results suggest that TH influences renal QO2 as a result of alterations in renal hemodynamics and the renal tubular handling of sodium. Changes in GFR played a major role in the regulation of RNa and QO2. This is suggested by the comparison of hypothyroid animals with and without renal compensatory hypertrophy. In the latter group a twofold lower GFR was associated with parallel changes in RNa and QO2. However, in going from euthyroid (E) to hypothyroid (H) conditions, the decrease in RNa of 31 +/- 4% was greater than the reduction in GFR of 25 +/- 3% (p less than .05). Thus, a decrease in RNa of 6 +- 2% under hypothyroid conditions could not be attributed to the changes in GFR. The normal renal stoichiometry between RNa and QO2 in hypothyroid animals indicates that the above changes in RNa were accompanied by comparable changes in QO2. The decrease in RNa in hypothyroid animals was not associated with a significant change in kidney weight (E:48.2 +/- 4.0gm., n = 5 and H:45.4 +/- 1.4gm, n = 5; pN.S.) The results suggest that an adaptive response to changes in GFR is the predominant cause of the decrease in RNa and renal QO2 under hypothyroid conditions. A modest reduction in renal tubular sodium reabsorptive capacity in hypothyroid animals is probably secondary to a direct effect of TH on active sodium transport.

Changes in glomerulotubular dimensions, single nephron glomerular filtration rates and the renin-angiotensin system in hypothyroid rats

Glomerular diameters (GD) and lengths of attached proximal convoluted tubules (TPL) were measured in nephrons dissected from the superficial (S), intermediate and juxtamedullary (JM) cortex (7-15 each) of acid-macerated kidneys of weight-matched (E) euthyroid and (H) hypothyroid (2-6 months after radioiodine treatment or thyroidectomy) male Sherman-Wistar rats. Incoordination of growth in H rats was evident in a more marked retardation in kidney than in total body growth. A similar incoordination of microstructural growth was evident in maintenance or GD within normal limits with respect to body weight while attached TPL fell 23% on the average below control values relative to body weight. These changes affected the total nephron population uniformly. As a result, GD/TPL in all nephrons increased significantly (p less than 0.01), by 27% in S and by 29% in JM nephrons. The glomerulotubular dimensional imbalance was associated with a marked and uniformly distributed reduction in single nephron glomerular filtration rate (ferrocyanide method), by 36% in S and JM nephrons. Plasma renin activity fell within normal limits while plasma renin substrate was decreased to 56% of control values. These findings are construed as evidence that growth retardation in hypothyroid rats affects the parenchyma of the kidney (and perhaps other viscera) more than the vasculature.

Effect of triiodothyronine on renal growth and renal sodium reabsorption in hypothyroid rats

Experiments were carried out to compare temporal changes in the paraaminohippuric acid clearance (CPAH), renal sodium reabsorption (RNa+), ribonucleic acid (RNA), and deoxyribonucleic acid (DNA) content in hypothyroid rats after a single injection of triiodothyronine (T3) (50 micrograms/100 g body wt). CPAH and RNa+ showed no changes at 24 and 48 h. At 72 h, however, significant increases of 41% and 42% (per g kidney wet wt) were observed in CPAH and RNa+, respectively. The cortex in T3-treated hypothyroid rats showed a significant increase in the protein/DNA and RNA/DNA ratios at 24 h and progressive increases to a level of 24%, and 37%, respectively, at 48 h. No changes in DNA content were observed at either time-points. The results show that the increases in RNA/DNA and protein/DNA ratios upon T3 treatment preceded the increases in CPAH and RNa+, suggesting a direct effect of T3 on renal cortical growth, rather than a secondary response to a primary increase in renal functions.