3,5-Diiodo-l-thyronine and 3,5,3′-triiodo-l-thyronine both improve the cold tolerance of hypothyroid rats, but possibly via different mechanisms

 The effects of 3,5-diiodo-l-thyronine (3,5-T2, 2.5–10 μg/100 g BW) on cold tolerance, energy expenditure and oxidative capacity of four metabolically very active tissues (brown adipose tissue, skeletal muscle, liver and heart) were determined in hypothyroid, cold-exposed rats. Hypothyroid rats survived cold for only 3–4 days. 3,5-T2 improved survival dose dependently; with 10 μg/100 g BW the rats survived 3 weeks (limit of observation). This effect was paralleled by an increased energy expenditure of the whole animal for the entire 3 weeks. Similar effects were observed in hypothyroid rats treated with 3,3′,5-triiodo-l-thyronine (T3). 3,5-T2 stimulated the specific oxidative capacity (expressed as cytochrome oxidase activity per milligram protein) of all four tissues dose dependently. When the oxidative capacity was expressed as total activity (cytochrome oxidase activity times organ weight), the percentage increases were of the same order. T3 exerted similar effects, but the changes in total activity were much greater than in specific activity, indicating an effect on the tissue trophism. The effect of 3,5-T2 on cold tolerance thus mimics the effect of T3, but via different cellular mechanisms. T3 seems to act primarily on the trophism of the tissues, while 3,5-T2 may act directly on mitochondria without an effect on tissue trophism. Received: 24 October 1997 / Accepted: 2 April 1998     

Effect of ammonium on the expression of osmosensitive genes in Madin–Darby canine kidney cells

The cells of the kidney medulla are exposed routinely to high extracellular concentrations of various solutes including NaCl, urea and ammonium (NH₄⁺). Although it is well established that the expression of a variety of osmosensitive genes and proteins, which confer cytoprotection on renal medullary cells, is induced by high NaCl concentrations, the role of NH₄⁺ in these cellular responses is unclear. This study thus addressed the effect of NH₄⁺ on the expression of the betaine/GABA transporter (BGT-1), the sodium/ myo -inositol cotransporter (SMIT), aldose reductase (AR), and heat shock protein 70 (HSP70) in Madin–Darby canine kidney (MDCK) cells, using Northern and Western blot analyses and enzyme-linked immunosorbent assay (ELISA). The incidence of apoptosis was monitored by determining caspase-3 activity and annexin V binding. Addition of NH₄Cl (50 m m ; total osmolality 400 mosmol (kg H₂O)⁻¹ to the medium was more effective than equiosmolar NaCl in increasing BGT-1 and HSP70 mRNA abundance, but less effective in enhancing BGT-1 and HSP70 expression at the protein level. Qualitatively similar results were obtained for SMIT and AR mRNAs. Exposure to both isotonic and hypertonic, NH₄Cl-containing medium enhanced apoptosis compared with equiosmolar, NaCl-containing media. These results suggest that, in addition to NaCl, NH₄Cl may play a role in regulating the intracellular accumulation of organic osmolytes, the abundance of HSP70 and cell turnover in the renal medulla in vivo .     

Choline transport and its osmotic regulation in renal cells derived from the rabbit outer medullary thick ascending limb of Henle

 Organic osmolytes such as betaine and glycerophosphorylcholine (GPC) are of major importance concerning volume regulation of inner and outer medullary epithelial cells. Recently we demonstrated that the intracellular betaine content in rabbit kidney cells derived from the outer medullary thick ascending limb of Henle’s loop (TALH) is osmotically regulated by betaine synthesis. In this context it was our purpose to characterize the uptake of choline, a precursor of betaine and GPC. We found TALH cells to possess a specific choline transport system with a maximum velocity (V _max) of 71 ± 12 pmol ·μl^–1 cell water · min^–1 and an apparent affinity (K _m) of 155 ± 19 μmol · l^–1. The uptake of choline was sodium independent and not electrogenic, but it was significantly reduced by the removement of chloride from the incubation medium. After long-term adaptation of TALH cells to a hyperosmotic medium (600 mosmol · l^–1, osmolarity adjusted with NaCl or urea) a significant higher choline uptake rate was observed (V _max: 166 ± 9 (NaCl), 96 ± 12 (urea) pmol ·μl^–1 cell water · min^–1). Our results suggest that the uptake of choline is due to higher intracellular requirements of choline under hypertonic conditions. Finally, an increase in the V _max of the choline transport system may enable sufficient synthesis of betaine and GPC. Received: 7 April 1997 / Received after revision: 2 June 1997 / Accepted: 3 June 1997     

Cellular response to osmotic stress in the renal medulla

 Cells of the renal medulla, which are exposed under normal physiological conditions to widely fluctuating extracellular solute concentrations, respond to hypertonic stress by accumulating the organic osmolytes glycerophosphorylcholine (GPC), betaine, myo-inositol, sorbitol and free amino acids. Increased intracellular contents of these osmolytes are achieved by a combination of increased uptake (myo-inositol and betaine) and synthesis (sorbitol, possibly GPC), decreased degradation (GPC) and reduced osmolyte release. In the medulla of the concentrating kidney, accumulation of organic osmolytes, which do not perturb cell function even at high concentrations, allows the maintenance of ”normal” intracellular concentrations of inorganic electrolytes. Adaptation to decreasing extracellular solute concentrations, e.g. diuresis, is achieved primarily by activation of pathways allowing the efflux of organic osmolytes, and secondarily by inactivation of production (sorbitol) and uptake (betaine, myo-inositol) and stimulation of degradation (GPC). Apart from modulation of the osmolyte content, osmolality-dependent reorganization of the cytoskeleton and expression of specific stress proteins (heat shock proteins) may be further, as yet poorly characterized, components of the regulatory systems involved in the adaptation of medullary cells to osmotic stress.     

Role of G-proteins in the regulation of organic osmolyte efflux from isolated rat renal inner medullary collecting duct cells

 Hypotonic shock (change of osmolality from 600 mosmol to 300 mosmol by lowering NaCl concentration) increases the release of organic osmolytes from isolated inner medullary collecting duct (IMCD) cells in the following sequence: taurine > betaine > sorbitol > myo-inositol > glycerophosphorylcholine (GPC). The role of G-proteins in regulating the hypotonicity-induced efflux was analysed by exposing cells to various concentrations of a G-protein inhibitor, pertussis toxin (PTX; 20–200 ng/ml), and a G_iα-protein stimulator, mastoparan (10–50 μM). PTX diminished the hypotonic release of sorbitol and betaine by 43.2±9.5% and 32.2±7.8% (n = 5), respectively. Efflux of GPC, myo-inositol and taurine was not significantly altered. Mastoparan (10 μM) increased osmolyte release under isotonic conditions such that release of betaine was increased 3.8-fold and that of sorbitol 2.1-fold, while GPC, myo-inositol and taurine effluxes were only slightly augmented. Under hypotonic conditions, mastoparan stimulated betaine release (1.86±0.2-fold, n = 5) but not that of sorbitol. As tested in connection with sorbitol and betaine release, the effect of mastoparan was abolished by PTX, but not the A23187-evoked sorbitol release. Like mastoparan, arachidonic acid increased the release of sorbitol and betaine under isotonic conditions, but under hypotonic conditions it only increased the release of betaine. As to the role of intracellular Ca²⁺, hypotonic shock evoked an intracellular Ca²⁺ peak which could be prevented by PTX. Mastoparan increased intracellular Ca²⁺ under isotonic conditions, whether the extracellular Ca²⁺ concentration was low or high. The results indicate that G-proteins are involved in regulating sorbitol and betaine efflux from IMCD cells. The G-proteins regulating sorbitol release are probably involved in generating the proper intracellular Ca²⁺ signal. Betaine efflux, which is independent of intracellular Ca²⁺, might be regulated by a G-protein-stimulated release of arachidonic acid. Thus, probably several G-proteins are involved in controlling organic osmolyte efflux from IMCD cells. Received: 2 April 1996 / Received after revision: 30 June 1996 / Accepted 25 July 1996     

Solute composition and heat shock proteins in rat renal medulla

 The high content of heat shock proteins (HSPs) 25 and 72 in the hyperosmotic inner medulla of the concentrating kidney has been ascribed to the high NaCl and urea concentrations in this kidney zone. To assess the effects of variations in the composition of solutes in the renal medulla on the intrarenal distribution of HSPs, rats were fed either a high- or low-Na diet for 3 weeks. These diets result in greatly differing urine and inner medullary solute composition. Sodium dodecyl sulphate polyacrylamide gel electrophoresis and Western blot techniques were used to analyse HSP25 and HSP72 in the cortex, outer medulla and inner medulla. In addition, the amounts of organic osmolytes (sorbitol, myo-inositol, betaine and glycerophosphorylcholine) and urea in the tissue were determined by high-performance liquid chromatography. Intra- and extracellular electrolyte concentrations at the papillary tip were measured by electron microprobe analysis. In the high-Na group, urine osmolality was about 1000 mosmol/kg lower than in rats fed a low-Na diet, due to lower urea concentrations. The sum of urine sodium and potassium concentrations, however, did not differ between the two groups. Neither in the outer nor in the inner medulla was the sum of the concentrations of organic osmolytes affected by the dietary treatment. The sum of sodium, potassium and chloride concentrations did not differ between the two experimental groups, neither in the interstitial nor in the intracellular compartments. However, the urea content and the amounts of HSP25 and HSP72 were significantly lower in the inner medulla of the group of rats fed a high-Na diet. Our results suggest that urea participates in the regulation of the medullary levels of the HSPs and that both HSP25 and HSP72 are components of mechanisms protecting medullary cells against the deleterious effects of high urea concentrations. Received: 30 September 1996 / Received after revision: 30 December 1997 / Accepted: 21 January 1997     

The Saccharomyces cerevisiae aldose reductase is implied in the metabolism of methylglyoxal in response to stress conditions

The enzyme aldose reductase plays an important role in the osmo-protection mechanism of diverse organisms. Here, we show that yeast aldose reductase is encoded by the GRE3 gene. Expression of GRE3 is carbon-source independent and up-regulated by different stress conditions, such as NaCl, H2O2, 39 degrees C and carbon starvation. Measurements of enzyme activity and intracellular sorbitol in wild-type cells also indicate that yeast aldose reductase is stress-regulated. Overexpression of GRE3 increases methylglyoxal tolerance in Saccharomyces cerevisiae. Furthermore, high expression of GRE3 complements the deficiency of the glyoxalase system of a glo1delta mutant strain. Consistent with this, in vitro and in vivo assays of yeast aldose reductase activity indicate that methylglyoxal is an endogenous substrate of aldose reductase. Furthermore, addition of NaCl or H2O2 to exponential-phase cells triggers an initial transient increase in the intracellular level of methylglyoxal, which is dependent on the Gre3p and Glo1p function. These observations indicate that the metabolism of methylglyoxal is stimulated under stress conditions; and they support a methylglyoxal degradative pathway, in which this compound is metabolised by the action of aldose reductase.     

Sorbitol metabolism in inner medullary collecting duct cells of diabetic rats

Intracellular accumulation of sorbitol, generated fromd-glucose via the aldose reductase pathway, is thought to play an important role in diabetic complications such as lens cataracts and neuropathy. In order to elucidate the effect of diabetes on the renal inner medulla, another sorbitol-rich tissue, male Wistar rats were treated with a single dose of streptozotocin (60 mg/kg body weight, i.p.). Six wecks later total inner medullary tissue (IM) or isolated inner medullary collecting duct (IMCD) cells were prepared. In diabetic IM tissue, sorbitol content was 1.8-fold higher than in control IM tissue (134±17 vs. 74±22 mol/g tissue protein). Sorbitol production in both normal and diabetic IMCD cells was strongly dependent on extracellulard-glucose concentration. In normal cells, for example, sorbitol production was 90±9 mol sorbitol/g protein x h at 45 mMd-glucose compared to 13±1 mol/g protein x h at 5 mM. At identicald-glucose concentrations sorbitol synthesis in diabetic IMCD cells was, however, always significantly higher than in control cells (122% of control at 15 mM and 126% of control at 45 mM). In addition, aldose reductase activity in diabetic IM was found to be augmented. The maximal velocity was 4.2 times higher (97±22 U/g protein vs. 23±7 U/g protein) while theK _m of the enzyme remained unchanged. Membrane permeability for sorbitol or the response to changes in extracellular osmolarity was not significantly different in diabetic IMCD cells and normal cells with correspondingly high intracellular sorbitol concentrations. Similarly the kinetic parameters ofd-glucose uptake were not altered by streptozotocin treatment. These results suggest that increased medullary sorbitol content in diabetic rats is a result of increased sorbitol synthesis due to a higher extracellulard-glucose concentration and augmented aldose reductase activity in face of an unaltered sorbitol permeability of the plasma membrane.     

Influence of NaCl, urea, potassium and pH on HSP72 expression in MDCK cells

The renal inner medulla is characterised by elevated extracellular concentrations of NaCl, urea, potassium and hydrogen ions, an environment that may affect cell viability negatively. High amounts of HSP72, a stress protein allowing cells to resist harmful situations, are also observed in this region. The present study examined HSP72 induction by various medullary stress factors, individually or in combination, in MDCK cells, a renal epithelial cell line expressing characteristics of the medullary collecting duct. MDCK cells were incubated for 3 days in media containing elevated concentrations of NaCl, urea, potassium and hydrogen ions individually or in combination. HSP72 mRNA and protein expression were determined by Northern and Western blot analyses, respectively. HSP72 expression was enhanced moderately by addition of 50 mM NaCl to normal medium at pH 7.4 but enhanced strongly when added at pH 6.5. The latter degree of HSP72 induction was comparable to that observed when 150 mM NaCl was added at pH 7.4. In normal medium (pH 7.4) containing 300 mM urea, MDCK HSP72 expression was not different from controls. In contrast, urea-induced HSP72 expression was clearly evident when medium pH was lowered to 6.5. Potassium at 20 or 40 mM induced HSP72 only slightly. These results indicate that expression of HSP72 in renal epithelial cells is regulated synergistically by NaCl, urea and pH. Since HSP72 is only slightly induced by increased potassium, this probably reflects the changes in medium osmolality rather than a specific effect of potassium. The high medullary HSP72 content observed even in diuresis may be due to co-operative effects of medullary solutes on HSP72 expression.     

Mathematical model of renal regulation of urea excretion

A mathematical model of transport processes in the ascending limbs (a.l.), collecting ducts (c.d.), and surrounding interstitium-capillary plexus (i.c.p.) of the renal medulla has been developed and applied to an investigation of mechanisms which may be involved in renal tubular regulation of urea excretion. Solute and water balance equations for perfectly-mixed compartments in series describe the transport processes in the a.l., c.d., and i.c.p. Solute species considered are urea, salt (NaCl), and an idealised ‘nonreabsorbable solute’ which accounts for miscellaneous substances accumulated in the c.d. The membrane transport equations used in the model describe solute movement by passive diffusion, solvent drag, and active transport; and water movement by osmosis. Model parameters are evaluated from published experimental data. Digital computer solutions of the model equations indicate that virtually all experimental observations pertaining to the regulation of urea excretion can be explained by an increase in the solvent drag of urea from the cortical and medullary c.d. Therefore, the need to postulate an active transport of urea from the c.d. to explain the regulation of urea excretion is obviated.     

山梨醇增加糖尿病患者腰椎管狭窄症炎性反应因子的表达

目的探讨糖尿病腰椎管狭窄患者黄韧带增生肥厚的发生机制。方法 24例糖尿病和20例非糖尿病的腰椎管狭窄患者列为研究对象,观测黄韧带标本结构,D-Sorbitol/Xylitol试剂盒检测山梨醇水平。体外实验中使用小鼠成纤维细胞(NIH3T3)细胞系,用Western blot及q PCR分别检测高糖培养条件及醛糖还原酶抑制剂(ARI):依帕司他(EP)作用对细胞炎性反应因子及TGF-β表达水平的影响。结果糖尿病组较非糖尿病组的山梨醇水平更高、黄韧带平均厚度更大、标本弹力纤维降解、胶原纤维增生更为显著、免疫组化CD68阳性染色率更高(P0.01);体外实验中,NIH3T3细胞系在高糖培养与正常糖浓度培养相比山梨醇、促炎性细胞因子和TGF-β表达水平更高,而山梨醇、促炎性细胞因子和TGF-β增高的表达水平可被醛糖还原酶抑制剂所抑制并且呈剂量依赖(P0.05)。结论糖尿病腰椎管狭窄患者黄韧带中山梨醇水平显著增高,进而促进炎性反应因子及纤维化相关因子TGF-β表达增加,使得黄韧带炎性增生。     

Differential expression of heat shock protein 27 and 70 in renal papillary collecting duct and interstitial cells – implications for urea resistance

The adaptation of renal medullary cells to their hyperosmotic environment involves the accumulation of compatible organic osmolytes and the enhanced synthesis of heat shock proteins (HSP) 27 and 70. While the mechanisms leading to osmolyte accumulation are similar in papillary collecting duct (PCD) and papillary interstitial (PI) cells, the present data demonstrate that HSP27 and HSP70 are expressed differentially in these cells both in vivo and in vitro . HSP70 is abundant in PCD, but not expressed in PI cells in the papilla in situ , while HSP27 is expressed in both PCD and PI cells. These observations could be reproduced by non-permeant solutes in cultured cells. Osmotic stress strongly induced HSP70 in MDCK cells (as a model for PCD cells), but not in PI cells, while HSP27 was constitutively expressed in MDCK cells and was up-regulated in PI cells. Since prior hypertonic stress (NaCl addition) protects MDCK against subsequent exposure to high urea concentrations, this effect was also assessed in PI cells. In both cell lines, hypertonic pretreatment prior to urea exposure (400 m m ) strongly attenuated caspase-3 activation. Inhibition of HSP27 expression by antisense transfection diminished the protective effect of hypertonic preconditioning in PI cells, while attenuation of HSP70 expression in MDCK cells diminished the protective effect of hypertonic preconditioning in these cells. These observations indicate that PCD and PI cells employ cell-specific mechanisms for protection against high urea concentrations as present in the renal papilla during antidiuresis.     

Intracellular sorbitol content in isolated rat inner medullary collecting duct cells

In order to study the mechanisms involved in the regulation of renal inner medullary sorbitol content, collecting duct cells were isolated from rat inner medulla and the effect of extracellular osmolarity on sorbitol synthesis and sorbitol content was investigated. Cells isolated at 300 mosmol/l and incubated up to 24 h as primary cultures in 300 mosmol/l media or in media made 600 mosmol/l by the addition of 150 mM NaCl showed no difference in total synthesis. Intracellular sorbitol content was, however, 2.3-fold higher in the cells kept in the higher osmotic medium. Cells isolated at 600 mosmol/l released sorbitol about 8 times faster when transferred into hypoosmotic medium (300 mosmol/l) than when transferred into isoosmotic (600 mosmol/l) media. Cells exposed to hyperosmotic media (900 mosmol/l with NaCl) maintained a higher intracellular sorbitol content than cells incubated in isoosmotic media. Changes of intracellular sorbitol content could not be attributed entirely to cell lysis — as demonstrated by determination of cellular content of lactate and lactate dehydrogenase. The alteration in sorbitol membrane permeability was reversible and was only observed when poorly permeable solutes (such as NaCl and sucrose) were used for the experiments, changes in urea elicited no effect. It is proposed that rapid changes in membrane permeability to sorbitol play an important role in the adjustment of intracellular sorbitol concentration in inner medullary collecting duct cells to changes in extracellular osmolarity.Supported by grant DFG Gr 916/1-1     

Low NaCl intake elevates renal medullary endothelin-1 and endothelin A (ETA) receptor mRNA but not the sensitivity of renal Na+ excretion to ETA receptor blockade in rats

Aims: This study was performed to investigate the effects of NaCl intake on renal mRNA expression of pre‐pro‐endothelin‐1 (ET‐1), endothelin A (ET_A) and endothelin B (ET_B) receptors as well as on renal ET‐1 content in rats. We further tested for NaCl intake‐dependent differences in the contribution of the ET system to renal sodium handling. Methods: Male Sprague–Dawley rats with telemetric devices were randomized to 0.15%, 0.60% and 1.80% NaCl diets with or without losartan. Renal sodium balance and arterial pressure were monitored. Renal blood flow and fractional sodium excretion (FENa) were measured in response to acute infusion of ET_A and ET_B blockers into the inner stripe of the outer renal medulla. Results: Medullary pre‐pro‐ET‐1, ET_A and ET_B receptor mRNA was 50%, 81% and 33% higher in rats on 0.15% vs. 1.80% NaCl. Losartan reduced medullary gene expression in rats on 0.15% NaCl. Medullary ET‐1 content was 983 ± 88 and 479 ± 42 ng mg^?1 protein in rats on 0.15% and 1.80% NaCl (P < 0.001). Chronic ET_A receptor blocker treatment reduced arterial pressure by 8–10 mmHg in rats on 0.15% vs. 1.80% NaCl without affecting renal sodium balances. Acute medullary ET_A or ET_B receptor blockade did not alter medullary blood flow and FENa in animals on either diet. Conclusion: In rats renal medullary ET‐1 content and mRNA expression of three ET system components are inversely related to NaCl intake. Higher expression levels on low NaCl intake are AT₁ receptor dependent but are not associated with increased sensitivity of renal sodium handling to ET_A receptor blockade.     

Water removal and solute additions determining increases in renal medullary osmolality

Osmolality and solute concentrations of the mammalian renal medulla increase and decrease with changing urine osmolality. These changes are brought about by addition or removal of solute or water to or from the renal medullary tissue. In Munich-Wistar rats and Syrian hamsters, males and females, actual amounts of and the various solutes involved in these changes were determined. Kidneys were removed from animals killed in different stages of water diuresis and antidiuresis. The renal medulla was analyzed by a new method that permits determination of water and solutes on the same piece of tissue. Removal of water and addition of urea were the two most important factors in raising inner medullary osmolality. Papillary water content was inversely related to the papillary osmolality and was 50% lower in extreme antidiuresis compared with water diuresis. Rats had higher papillary water content than hamsters. In the outer medulla, water removal was significant in the hamsters but not in the rats. Addition of urea to the papillary tissue exceeded the osmotic equivalent of NaCl by a factor of 2.8 in both rats and hamsters. Females of both species showed greater changes than males in amounts of urea in the inner medulla.     

Effects of renal artery infusion of various hypertonic solutions on the renal blood flow and renal handling of PAH in the dog

Summary Effects of renal artery infusion of hypertonic solutions of glucose, mannitol, Na₂SO₄, NaCl and urea on renal blood flow (RBF) and renal handling of PAH were studied in anesthetized dogs. RBF was determined by continuous venous outflow recording and glomerular filtration rate as renal plasma flow times creatinine extraction ratio.Glucose solution increased, mannitol and Na₂SO₄ did not significantly alter while urea and NaCl depressed RBF. Only for NaCl RBF depression became greater with increasing solute loads applied. Spontaneous renal vasoconstriction and ureteral occlusion prevented RBF fall during urea and NaCl infusions. It is proposed that renal vascular reponse to local elevation of plasma osmolality is a resultant of two opposed effects: nonspecific vasodilatation as described for other vascular beds and vasoconstriction characteristic for renal vasculature and demonstrable only in normally functioning kidney.The extraction ratio of PAH (E _PAH) was not altered by hypertonic urea but decreased with the four remaining infusates. The net tubular transport of PAH fell with NaCl infusion but was not changed with glucose, Na₂SO₄, mannitol and urea. It is concluded thatE _PAH depression during NaCl infusion was due to inhibition of cellular PAH transport while that observed with glucose, Na₂SO₄ and mannitol reflected increased fraction of RBF perfusing nonsecretory renal medullary tissue.     

Expression of Na+/Cl–/betaine and Na+/myo-inositol transporters, aldose reductase and sorbitol dehydrogenase in macula densa cells of the kidney

 It has been suggested that macula densa cells may be exposed to hyperosmotic stress. Since chronic exposure to hypertonic stress causes the amount of intracellular organic osmolytes to increase, the expression of transporters and enzymes that participate in the intracellular accumulation of organic osmolytes was examined using non-radioactive in situ hybridization in the macula densa region of control rats and furosemide-treated animals. Both the sodium- and chloride-dependent betaine transporter (BGT) and sodium-dependent myo-inositol transporter (SMIT) were expressed preferentially in macula densa cells and for both mRNAs the signal intensity was visibly reduced by furosemide. The enzymes aldose reductase (which mediates the conversion of glucose to sorbitol) and sorbitol dehydrogenase (which converts sorbitol into fructose) were expressed not only in macula densa cells but also in the surrounding tubular cells, and the expression was insensitive to furosemide. Thus it remains unclear whether the expression of BGT and SMIT is related to a putative hypertonic juxtaglomerular region. Received: 27 April 1998 / Accepted: 8 July 1998     

Adenosine and kidney function

In this review we outline the unique effects of the autacoid adenosine in the kidney. Adenosine is present in the cytosol of renal cells and in the extracellular space of normoxic kidneys. Extracellular adenosine can derive from cellular adenosine release or extracellular breakdown of ATP, AMP, or cAMP. It is generated at enhanced rates when tubular NaCl reabsorption and thus transport work increase or when hypoxia is induced. Extracellular adenosine acts on adenosine receptor subtypes in the cell membranes to affect vascular and tubular functions. Adenosine lowers glomerular filtration rate (GFR) by constricting afferent arterioles, especially in superficial nephrons, and acts as a mediator of the tubuloglomerular feedback, i.e., a mechanism that coordinates GFR and tubular transport. In contrast, it leads to vasodilation in deep cortex and medulla. Moreover, adenosine tonically inhibits the renal release of renin and stimulates NaCl transport in the cortical proximal tubule but inhibits it in medullary segments including the medullary thick ascending limb. These differential effects of adenosine are subsequently analyzed in a more integrative way in the context of intrarenal metabolic regulation of kidney function, and potential pathophysiological consequences are outlined.     

Expression of aldose reductase, sorbitol dehydrogenase and Na+/myo-inositol and Na+/Cl–/betaine transporter mRNAs in individual cells of the kidney during changes in the diuretic state

 The effect of changes in medullary extracellular tonicity on mRNA expression for aldose reductase (AR), sorbitol dehydrogenase (SDH), Na⁺/Cl^–/betaine (BGT) and Na⁺/myo-inositol (SMIT) cotransporter in different kidney zones was studied using Northern blot analysis and non-radioactive in situ hybridization in four groups of rats: controls, acute diuresis (the loop diuretic furosemide was administered), chronic diuresis (5 days of diuresis), and antidiuresis [5 days of diuresis followed by 24 h deamino-Cys¹,d-Arg⁸ vasopressin (dDAVP)]. Acute administration of the loop diuretic furosemide significantly reduced AR, SMIT and BGT gene expression in the inner and outer medulla compared with controls. Administration of dDAVP to chronically diuretic rats raised the expression of these three mRNAs in the inner but not the outer medulla compared with the chronically diuretic rats. None of these alterations in medullary tonicity significantly changed SDH expression. The in situ hybridization studies showed AR, BGT and SMIT mRNAs to be expressed in both epithelial and non-epithelial cells of the outer and inner medulla. The various cell types (epithelial, endothelial and interstitial cells) differed in their expression pattern and intensity of AR, SDH, BGT and SMIT mRNA, but the inner medullary cells responded uniformly to a decrease in extracellular tonicity with a reduction, and to an increase with enhancement of their AR, BGT and SMIT expression. Received: 26 May 1998 / Accepted: 14 September 1998