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

Relationships of Chronic Kidney Disease and Thyroid Dysfunction in Non-Dialysis Patients: A Pilot Study

<b><i>Context:</i></b> Patients with chronic kidney disease (CKD) usually manifest with disorder of thyroid hormone; however, the correlation is unknown. <b><i>Objective:</i></b> The study was designed to explore the relationships between CKD and thyroid dysfunction. <b><i>Design, Setting, and Participants:</i></b> A total number of 905 non-dialysis participants were collected at Nanjing First Hospital from August 2009 to October 2012 according to the case records system. Patients were grouped via the estimated glomerular filtration rate (eGFR) according to the KDIGO guideline. Levels of thyroid hormone and biomarkers in different CKD groups were compared by ANOVA. Prevalence of different thyroid diseases was calculated by χ² test. <b><i>Results:</i></b> We found that FT3 or T3 became more prevalent with increasing eGFR with the lowest level in CKD5 (<i>p</i> <i>&#x3c;</i> 0.01). No significant differences were found between groups in FT4, T4, or TSH (<i>p</i> &#x3e; 0.05). Frequency of euthyroid sick syndrome (ESS) in CKD groups was high, especially in CKD stage 5 (69.1%, <i>p</i> &#x3c; 0.01). eGFR had positive correlation with T3 and FT3 (<i>r</i> = 0.239, <i>p</i> = 0.0001; <i>r</i> = 0.292, <i>p</i> = 0.0001). ESS had correlations with prealbumin, β2-microglobin, eGFR, and C-reactive protein (<i>r</i> = 0.095, <i>p</i> = 0.004; <i>r</i> = –0.12, <i>p</i> = 0.001; <i>r</i> = 0.091,<i> p</i> = 0.007; <i>r</i> = –0.096, <i>p</i> = 0.008; <i>r</i> = 0.154, <i>p</i> = 0.001). After adjustment for prealbumin, uric acid, HbA1c, age, gender, eGFR, and β2-microglobin, binary regression revealed that hemoglobin, C-reactive protein, and albumin were independent influence factors of ESS (<i>p</i> = 0.016, <i>r</i> = 1.014; <i>p</i> = 0.023, <i>r</i> = 1.007; <i>p</i> = 0.029, <i>r</i> = 0.996). <b><i>Conclusion:</i></b> CKD patients have a high morbidity of ESS, mainly low T3 syndrome. Anemia, inflammation, and malnutrition may contribute to ESS in CKD.

The thyroid and the kidney: a complex interplay in health and disease

Thyroid hormones may directly affect the kidney and altered kidney function may also contribute to thyroid disorders. The renal manifestations of thyroid disorders are based on hemodynamic alterations or/and to direct effects of thyroid hormones. The renin-angiotensin system plays a crucial role in the cross-talk between the thyroid and the kidney. Hypothyroidism may be accompanied by an increase of serum creatinine and reduction of glomerular filtration rate (GFR), whereas hyperthyroidism may increase GFR. Treatment of thyroid disorders may lead to normalization of GFR. Primary and subclinical hypothyroidism and low triiodothyronine (T3) syndrome are common features in patients with chronic kidney disease (CKD). In addition low levels of thyroid hormones may predict a higher risk of cardiovascular and overall mortality in patients with end-stage renal disease. The causal nature of this correlation remains uncertain. In this review, special emphasis is given to the thyroid pathophysiology, its impact on kidney function and CKD and the interpretation of laboratorial findings of thyroid dysfunction in CKD.

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.

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 upregulates Na,K-ATPase alpha and beta mRNA in primary cultures of proximal tubule cells

In vivo studies have demonstrated that thyroid hormone regulates the activity of Na,K-ATPase in the mammalian kidney. However, it is still unclear whether upregulation of Na,K-ATPase by thyroid hormone is mediated through the direct action on renal tubule cells or through other mediators, such as an increase in glomerular filtration rate. Using primary cultures of rabbit renal proximal tubule cells, studies were undertaken to elucidate this problem. We found that Na,K-ATPase activity was increased by 26 +/- 8%, 30 +/- 9%, 39 +/- 9% after 24-h treatment with T3 of 10(-11), 10(-9), 10(-7) M, respectively. We further demonstrated that 24-h incubation of T3 (10(-7) M) enhanced alpha- and beta-protein abundance by 44 +/- 29% and 31 +/- 16%, and alpha- and beta-mRNA levels by 84 +/- 27% and 65 +/- 11%, respectively. The time course studies revealed that the significant increase in Na,K-ATPase activity, alpha- and beta-protein and mRNA abundance didn't appear until 24-h of T3 treatment. Our data indicate that thyroid hormone directly upregulates Na,K-ATPase in proximal tubule cells via a pretranslational mechanism.

Ion content and cell volume in isolated collecting duct cells: effect of hypotonicity

On isolated inner medullary collecting duct (IMCD) cells of the rat kidney the capability of osmoregulatory adaptation was investigated in vitro. IMCD cells were isolated by differential centrifugation at 600 mOsm (268 mM NaCl) and subsequently exposed to hypotonic buffers (300 mOsm, 118 mM NaCl). The alterations of ion content and cell volume following this change in extracellular osmolarity were studied by electron probe microanalysis and determination of intracellular water. After swelling within 40 seconds to 152 +/- 15% of control (P < 0.001; N = 9) cell volume was restored after 15 minutes. This regulatory volume decrease (RVD) was observed irrespective whether extracellular osmolarity was changed by using NaCl or mannitol as the major osmolyte. During RVD the cells lost sodium (48 +/- 11%) and chloride (14 +/- 5%), and the potassium content remained nearly unchanged. Correspondingly, sodium and chloride concentrations were progressively lowered, whereas the potassium concentration changed only transiently. RVD was diminished by 10(-4) M NPPB, 10(-3) M SITS and in the absence of HCO3-. Twenty millimoles of ouabain or 5 mM barium also inhibited RVD with little additive effect. A total of 10(-3) M amiloride and 10(-4) M bumetanide showed no effect on the hypoosmotic volume response. The experiments show that in isolated IMCD cells exposed to hypotonic conditions, rapid reversible changes in cell volume and sustained alterations in cell inorganic ion content occur, and thereby transmembrane sodium and potassium gradients are maintained. Since the loss in inorganic electrolytes does not account for RVD, the major part of volume regulation seems to occur via changes in organic osmolytes.(ABSTRACT TRUNCATED AT 250 WORDS)

Thyroid hormone induction of Na(+)-K(+)-ATPase and its mRNAs in a rat liver cell line

The expression of mRNAs encoding the alpha- and beta-subunits of Na(+)-K(+)-ATPase (Na(+)-K+ pump) was examined in a rat liver cell line, Clone 9, in various thyroidal states. Northern blot analysis of total RNA isolated from cells incubated in hypothyroid serum-containing medium revealed the expression of mRNAs encoding Na(+)-K(+)-ATPase alpha 1-(mRNA alpha 1) and beta- (mRNA beta) subunits; mRNAs encoding the alpha 2- and alpha 3-subunits were undetectable. There was a discrepancy in the abundance of mRNA alpha 1 relative to mRNA beta such that mRNA alpha 1 exceeded the sum of the multiple mRNA beta bands by approximately 35-fold. 3,3',5-Triiodothyronine (T3) produced a coordinate augmentation of mRNA alpha 1 and mRNA beta contents that was demonstrable within 2 h and preceded the stimulation of Na(+)-K(+)-ATPase activity. After incubation of cells with T3 for 48 h, Na(+)-K(+)-ATPase activity was stimulated by 1.32-fold, whereas mRNA alpha 1 and mRNA beta abundances were increased 1.46- and 2.87-fold, respectively. Treatment of cells for 6 h with 10 micrograms/ml cycloheximide, a concentration sufficient to inhibit protein synthesis by 95%, elicited a 3.5- and 5.1-fold increase in mRNA alpha 1 and mRNA beta content, respectively. Cycloheximide abrogated the stimulatory effect of T3 on mRNA beta abundance, whereas the T3-induced increase in mRNA alpha 1 content was not prevented.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular genetics of Na,K-ATPase

Researchers in the past few years have successfully used molecular-genetic approaches to determine the primary structures of several P-type ATPases. The amino-acid sequences of distinct members of this class of ion-transport ATPases (Na,K-, H,K-, and Ca-ATPases) have been deduced by cDNA cloning and sequencing. The Na,K-ATPase belongs to a multiple gene family, the principal diversity apparently resulting from distinct catalytic alpha isoforms. Computer analyses of the hydrophobicity and potential secondary structure of the alpha subunits and primary sequence comparisons with homologs from various species as well as other P-type ATPases have identified common structural features. This has provided the molecular foundation for the design of models and hypotheses aimed at understanding the relationship between structure and function. Development of a hypothetical transmembrane organization for the alpha subunit and application of site-specific mutagenesis techniques have allowed significant progress to be made toward identifying amino acids involved in cardiac glycoside resistance and possibly binding. However, the complex structural and functional features of this protein indicate that extensive research is necessary before a clear understanding of the molecular basis of active cation transport is achieved. This is complicated further by the paucity of information regarding the structural and functional contributions of the beta subunit. Until such information is obtained, the proposed model and functional hypotheses should be considered judiciously. Considerable progress also has been made in characterizing the regulatory complexity involved in expression of multiple alpha-isoform and beta-subunit genes in various tissues and cells during development and in response to hormones and cations. The regulatory mechanisms appear to function at several molecular levels, involving transcriptional, posttranscriptional, translational, and posttranslational processes in a tissue- or cell-specific manner. However, much research is needed to precisely define the contributions of each of these mechanisms. Recent isolation of the genes for these subunits provides the framework for future advances in this area. Continued application of biochemical, biophysical, and molecular genetic techniques is required to provide a detailed understanding of the mechanisms involved in cation transport of this biologically and pharmacologically important enzyme.

Atrial natriuretic peptide has no direct effect on proximal tubule sodium and water reabsorption

Infusion of ANP has been shown to increase the urinary excretion of sodium and water. However it is still controversial in which tubular segment sodium reabsorption is inhibited. To clarify this problem we have performed in vivo and in vitro studies to examine the direct effect of ANP on rat proximal tubules. The in vivo effect of ANP has been tested by using the micropuncture technique and in particular the shrinking droplet method that allows each investigated tubule to serve as its own control. Addition of either low (10(-9) M) or high (2 x 10(-6) M) concentrations of ANP to the luminal perfusate resulted in no significant change in isotonic fluid reabsorption (Jv). The same holds when the proximal tubules were perfused on both the tubular and peritubular side, with modified Ringer solution containing 10(-9) M ANP. To examine possible in vitro effects of ANP we prepared highly purified proximal tubule suspension derived from rat renal cortex and monitored oxygen consumption (QO2) that is tightly coupled to sodium transport in this segment. Synthetic ANP, either at low (10(-9) M) or at high (10(-6) M) concentrations, did not affect basal rate of tubular respiration. Moreover the peptide hormone (10(-9) M) did not inhibit nystatin stimulated and ouabain sensitive QO2. These results indicate that the enhancement of renal sodium excretion induced by ANP is not related to a direct inhibition of sodium transport in the proximal tubule.

Model of gentamicin-induced nephrotoxicity and its amelioration by calcium and thyroxine

The exact mechanism of gentamicin-induced acute renal failure is presently unknown; various mechanisms have been proposed but there is no proposed commonality between them. In animals, dietary calcium loading and L-thyroxine administration have been shown to ameliorate toxicity, with again no common process. A mechanism of competitive displacement of calcium and other cations from anionic phospholipids at the plasma and organelle membrane level, resulting in a decrease in Na+ -K+ ATPase, adenylate cyclase, mitochondrial function and ATP production, protein synthesis, solute reabsorption and overall cellular function is proposed. A further proposal is dietary calcium loading and thyroxine (which increases intracellular calcium) reverse gentamicin-induced acute renal failure by increasing the calcium and solute flux, thereby competitively inhibiting the primary lesion: anionic phospholipid binding.

Effects of thyroid hormone on Na+-K+ transport in resting and stimulated rat skeletal muscle

The effects of hypothyroidism and 3,5,3'-triiodothyronine (T3) treatment on passive Na+-K+ fluxes and Na+-K+ pump concentration were investigated in isolated rat muscle. Within 12 h after a single dose of T3 (20 micrograms/100 g body wt), K+ efflux had increased by 21% in soleus and by 20% in extensor digitorum longus muscle. In the presence of ouabain, even larger effects were observed. These changes were associated with a 12% rise in amiloride-suppressible Na+ influx but no significant increase in [3H]ouabain binding site concentration. After 3 days of T3 treatment, the stimulating effect on K+ efflux and Na+ influx in soleus reached a plateau approximately 80 and 40% above control levels, respectively, whereas the maximum increase in [3H]ouabain binding site concentration (103%) was only fully developed after 8 days. Hypothyroidism decreased 86Rb efflux by 30%. The efflux of K+ and the influx of Na+ per contraction (both approximately 7 nmol/g wet wt) as well as the net loss of K+ induced by electrical stimulation were unaffected by T3 treatment. The rise in resting K+ efflux after 12-24 h of T3 treatment could be partly blocked by dantrolene or trifluoroperazine, indicating that an increase in the cytoplasmic Ca2+ concentration may contribute to the early rise in K+ efflux. It is concluded that the early rise in the resting passive leaks of Na+ and K+ induced by T3 is a major driving force for Na+-K+ pump synthesis.

Absence of increased electroneutral Na+-H+ exchange in renal cortical brush-border membranes from hyperthyroid rats

The fluorescence quenching of acridine orange was used to compare Na+-H+ exchange and ion conductances in renal cortical brush-border membrane vesicles (BBMV) isolated from euthyroid and hyperthyroid rats. In BBMV from euthyroid animals, Na+-H+ exchange was entirely electroneutral. In BBMV from hyperthyroid rats, the total rates of Na+-H+ exchange were about 30% higher than in BBMV from euthyroid animals. However, the electroneutral exchange in these membranes was similar to that in BBMV from euthyroid rats; the observed increase in exchange was due to electrically coupled Na+ and H+ movements through conductive pathways in the membranes. Ion conductances in isolated BBMV were tested with outwardly directed K+ gradients in the presence of carbonyl cyanide m-chlorophenylhydrazone (K+ conductance) or valinomycin (H+ conductance). The K+ conductance was negligible and similar in BBMV from both groups of rats. A significant H+ conductance was present in both kinds of membrane preparations and was by 37% higher in BBMV from hyperthyroid animals. Therefore, our experiments failed to demonstrate an increased electroneutral Na+-H+ exchange in BBMV from hyperthyroid rats. Instead, a finding of a significant electrically BBMV from hyperthyroid rats. Instead, a finding of a significant electrically coupled Na+-H+ antiport in the presence of increased H+ conductance in BBMV from hyperthyroid rats indicates that these membranes may also have increased Na+ conductance.

Thyroid hormone coordinately regulates Na+-K+-ATPase alpha- and beta-subunit mRNA levels in kidney

Synthesis of the sodium pump, Na+-K+-ATPase, is regulated by thyroid hormone in responsive tissues. The purpose of this study was to determine if triiodothyronine (T3) regulates the concentration of the mRNAs coding for the two enzyme subunits, alpha and beta, and the time course of the response. A single dose of T3 (250 micrograms/100 g body wt) was administered to hypothyroid rats that were killed at various times after injection. In the kidney cortexes of the T3-injected animals, as well as hypothyroid and euthyroid rats, alpha- and beta-mRNA concentrations were measured by dot blot using cDNAs corresponding to the two mRNAs; alpha-subunit abundance was measured by Western blot using antibodies to the enzyme, and Na+-K+-ATPase activity was measured enzymatically. alpha- and beta-mRNAs increased coordinately, after a 6-h time lag to 1.6-fold over hypothyroid levels by 12 h after T3. alpha-Subunit abundance increased significantly by 48 h and to 1.4-fold over hypothyroid by 72 h after T3. Na+-K+-ATPase activity increased with the same time course as the increase in alpha-subunit abundance to 1.3-fold over hypothyroid by 72 h after T3. We conclude that T3 regulates Na+-K+-ATPase synthesis and activity by coordinately increasing the mRNAs of both the alpha- and beta-subunits of the enzyme.

Lack of stimulation of kidney Na-K-ATPase by thyroid hormones in long-term thyroidectomized rabbits

The effects of long-term thyroidectomy and of subsequent triiodothyronine administration on kidney Na-K-ATPase were studied at the level of single nephron segments and were compared to the short-term effects previously reported. After 8-11 weeks, thyroidectomy resulted in a marked decrease in Na-K-ATPase activity in all the segments of the rabbit nephron, the proximal tubule, the thick ascending limb of Henle's loop, the distal convoluted tubule and the collecting tubule. Within this delay, thyroidectomy also decreased the ouabain-insensitive Mg-ATPase activity, the basal and hormone-stimulated adenylate-cyclase activity, and the volume of tubular epithelium in all the segments where these parameters were measured. Administration of 50 micrograms/kg body weight triiodothyronine to 8-11 weeks thyroidectomized rabbits did not restore Na-K-ATPase activity in any nephron segment within 48 h. These observations are different from those reported in animals thyroidectomized only 1 week before study since, within this latter delay, thyroidectomy altered specifically Na-K-ATPase activity, this action was observed on the proximal and collecting tubules exclusively and, triiodothyronine administration corrected Na-K-ATPase alterations after 48 h. Results of the present study indicate that in the long term, thyroidectomy has a wide spectrum of renal effects which involves the whole nephron and most cellular functions. The tubular involution induced by long-term thyroidectomy is probably responsible for the inability of kidney cells to quickly increase their Na-K-ATPase activity in response to hormonal stimulation.

Kinetics of triiodothyronine action on Na-K-ATPase in single segments of rabbit nephron

This study was initiated to define the dose- and time-dependence of triiodothyronine (T3) action on Na-K-ATPase in single microdissected nephron segments. For this purpose, the activity and the number of catalytic sites of Na-K-ATPase, as determined by the specific binding of 3H-ouabain, were measured following a single injection of T3 to rabbits thyroidectomized since 8-12 days. Triiodothyronine restored both the activity and the number of catalytic sites of Na-K-ATPase in a dose-dependent manner in all nephron segments where the enzyme was decreased following thyroidectomy, i.e., the proximal and the collecting tubule. At a dose of 50 micrograms/kg bw, T3 restored Na-K-ATPase activity and 3H-ouabain binding with the same kinetics. However, the kinetics depended on the nephron segments: in the proximal tubule, Na-K-ATPase stimulation occurred after a 12 h period of latency and was completed within 24 h whereas in the collecting tubule, the stimulation was biphasic with a first increase within the first 3 h and a second increase concomitantly to that observed in the proximal tubule. These results indicate that thyroid hormones regulate Na-K-ATPase activity by altering the number of catalytic sites of the enzyme. This control depends on two different mechanisms which differ by their time-dependence.

Potassium transport in the rabbit renal proximal tubule: effects of barium, ouabain, valinomycin, and other ionophores

Potassium fluxes in a suspension of rabbit proximal tubules were monitored using a potassium-sensitive extracellular electrode. Ouabain (10(-4) M) and barium (5 mM) were used to selectively quantitate the potassium efflux pathway (105 +/- 5 nmol K+ X mg protein-1 X min-1) and the sodium pump-related potassium influx (108 +/- 7), respectively. These equal and opposite fluxes suggest that potassium accumulation in the cell occurs mainly through the sodium pump and that potassium efflux occurs mainly through barium-sensitive potassium channels. Thus the activity of the sodium pump (Na,K-ATPase) in the basolateral membrane of the proximal tubule is balanced by the efflux of potassium, presumably across the basolateral membrane, which has a high potassium permeability. In addition, the effect of valinomycin and other ionophores was examined on potassium fluxes and several metabolic parameters [oxygen consumption (QO2), ATP content]. The addition of valinomycin to the tubules produced a net efflux of potassium which was quantitatively equivalent to the efflux produced by the addition of ouabain. The valinomycin-induced efflux was mainly due to the activity of valinomycin as a mitochondrial uncoupler, which indirectly inhibited the sodium pump by allowing a rapid reduction of the intracellular ATP. Amphotericin, nystatin, and monensin all produced large net releases of intracellular potassium. The action of the ionophores could be localized to the plasma or mitochondrial membrane and classified into three groups, as follows: those which demonstrated full mitochondrial uncoupler activity (FCCP, valinomycin), those which had no uncoupler activity (amphotericin B, nystatin); and those which displayed partial uncoupler activity (monensin, nigericin).

The effects of thyroid hormones on 3H-ouabain binding site concentration, Na,K-contents and 86Rb-efflux in rat skeletal muscle

Using a recently developed method based upon vanadate facilitated 3H-ouabain binding, the total concentration of 3H-ouabain binding sites was determined in biopsies of rat skeletal muscles containing varying proportions of slow-twitch fibres. In extensor digitorum longus, diaphragm, gastrocnemius and soleus muscles from mature (12-week-old) hyperthyroid rats the values obtained were respectively 2.6, 3.5, 5.1 and 9.8 times higher than those found in the same muscles from hypothyroid animals. This indicates that the effect of thyroid hormones is more pronounced on slow-twitch than on fast-twitch fibres. The changes in 3H-ouabain binding site concentration with thyroid status could not be accounted for by differences in affinity or the rate of 3H-ouabain binding. In young (4-5 week old) rats, where the K-content and the 3H-ouabain binding site concentration in muscle had been reduced by K-depletion, T3-pretreatment produced an even larger relative increase in the 3H-ouabain binding site concentration than in age-matched controls, but no increase in K-content. Therefore, the downregulation of 3H-ouabain binding sites seen during K-depletion cannot be attributed to a decreased response to thyroid hormones. In normal rats the marked stimulating effect of thyroid hormone on the synthesis of 3H-ouabain binding sites was not associated with any significant change in K-content, but clearly preceded by a significant (P less than 0.001) rise in the efflux of 86Rb.

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.

The use of micropuncture, isolated tubule, and vesicle technique in the study of the action of thyroid hormones on the proximal tubule function

In hypothyroid rats (TX), the isotonic fluid reabsorption (Jv), that is closely linked to the transepithelial sodium transport (JNa), is impaired. The administration of physiological doses (10 micrograms/kg body weight per day) of tri-iodothyronine (T3) doubles Jv in three days (TX+T3). This phenomenon could be explained by several mechanisms: a direct stimulation of Na-K-ATPase, an increase in the Na+ entry step, changes in the permeability properties of the luminal and/or basal lateral membranes. Using a kinetic microassay, Na-K-ATPase activity was measured in early (S1) and late (S2) proximal tubules segments isolated from control, TX, and TX+3T3 animals. In TX rats the enzyme activity was lower (70%) in both segments versus control rats, it remained unchanged after 3 days, and it increased after 7 days of T3 substitution. The Na+ permeability of brush border membrane (BBM) vesicles isolated from TX and TX+T3 rats was identical. However the valuation of the K+ membrane permeability by in vivo perfusion of the lumen and peritubular space of proximal tubules of TX rats, with perfusate containing the K+ ionophore valinomycin (1 microgram/ml), induced a significant increase in Jv that accounted for 40% of that elicited by T3. Taken together, the in vivo and in vitro experiments suggest that the early effect on Jv of physiological doses of T3 cannot be explained by a direct action of T3 either on the Na+ entry step across the BBM or on the Na+ exit step (i.e., the Na-K-ATPase), but rather by an increase in K+ permeability of proximal tubular cell membranes.(ABSTRACT TRUNCATED AT 250 WORDS)