[Insulin and arterial hypertension: the role of the kidney]

We have demonstrated that insulin stimulates sodium reabsorption in the distal nephron by stimulating the phosphatidylinositol 3-kinase (PI 3-kinase) pathway and that any stimulation of this enzyme (e.g. by EGF, by H2O2 or by exogenous PIP3, added apically) leads to a parallel increase in sodium reabsorption. We therefore suggest that hyperinsulinemia leads to hypertension through increased renal sodium reabsorption in the distal nephron.

[Sodium and water transport along the nephron]

Kidney is the most important organ to maintain the homeostasis. Glomeruli filtrate water and sodium as much as 150 l/day and 21 Eq/day, however, more than 99% of them reabsorbed along the nephron. Sodium reabsorption is the energy to reabsorb water and regulate electrolyte homeostasis, as the basic mechanism of electrolyte transport was driven by the electrochemical gradient across the cell membrane originated by the Na-K ATPase located in the basolateral membrane. On the apical membrane, there are various kinds of transporters, such as Na-H exchanger, Cl-bicarbonate exchanger, Na-amino acid cotransporter, Na-glucose cotransporter, Na-phosphate cotransporter, Na-Cl cotransporter, Na channels, K channels, and so on. Each segment has specific transporters and specific characteristics to water permeability. As a total, each nephron segment plays artistic roles to regulate water, sodium and other electrolytes transport.

Renal Dopaminergic System Activity in the Rat Remnant Kidney

BACKGROUND

Renal dopamine exerts natriuretic and diuretic effects by activating D1-like receptors. Uninephrectomy results in increased renal dopaminergic activity and dopamine-sensitive enhanced natriuresis.

METHODS

The present study evaluated renal adaptations in sodium handling and the role of dopamine in rats submitted to (3/4) nephrectomy: right nephrectomy and excision of both poles of the left kidney ((3/4)nx rats).

RESULTS

Two weeks after surgery the absolute urinary levels of dopamine were markedly reduced in (3/4)nx rats whereas the urinary dopamine excretion per % of residual nephrons was significantly increased in the remnant kidney of (3/4)nx rats. The V(max) values for renal aromatic L-amino acid decarboxylase, the enzyme responsible for the synthesis of renal dopamine, were decreased in (3/4)nx rats. Renal catechol-O-methyltransferase activity, the enzyme responsible for the methylation of dopamine, was increased in (3/4)nx rats whereas the renal activities of monoamine oxidases A and B did not differ between (3/4)nx and Sham animals. Volume expansion (5% body weight) resulted in similar natriuretic responses in (3/4)nx and Sham rats. During D1 antagonist administration (Sch-23390, 30 microg x h(-1) x kg(-1)) the natriuretic response to volume expansion was reduced in (3/4)nx rats more pronouncedly than in Sham animals.

CONCLUSION

The decrease in absolute renal dopamine output in (3/4)nx rats is related with reduced renal synthesis and enhanced O-methylation of the amine. However, this is accompanied in (3/4)nx rats by increased renal dopamine excretion per residual nephrons and dopamine-sensitive enhanced natriuresis.

Impact of Graft Mass on the Clinical Outcome of Kidney Transplants

The effect of nephronic mass reduction of kidney transplants has not been analyzed specifically in a large cohort. Transplant injuries in cadaver kidney graft may have led to an underestimation of the magnitude of this factor. The aim of this study was to analyze the consequences of kidney mass reduction on transplantation outcome. The weights of 1142 kidney grafts were collected prospectively immediately before grafting. Donors and recipients <15 yr of age, simultaneous kidney/pancreas grafts, and technical failures before day 7 were excluded from the analysis. The analysis was performed on Cockroft-calculated creatinine clearance and proteinuria in 964 patients for whom all of the necessary information was available. This study reports that the smallest kidneys transplanted into the largest recipients (donor kidney weight/recipient body weight [DKW/RBW] <2 g/kg, n = 88) increased their clearance by 2.38 ml/min every month for 6 mo (P < 0.0001) and by 0.27 ml/min thereafter (P < 0.0001). Conversely, creatinine clearance did not change for the largest kidneys transplanted into the smallest recipients (DKW/RBW ratios >/=4 g/kg). Next, using a Cox model analysis, it was shown that the risk of having a proteinuria >0.5 g/kg was significantly increased for the low DKW/RBW ratios <2 g/kg with 50% of patients having a proteinuria, compared with DKW/RBW ratios >/=4 g/kg (P < 0.001). In cadaver transplant recipients, graft mass has a rapid impact on graft filtration rate and proteinuria. Avoiding major kidney/recipient inadequacy should have a significant influence on long-term transplant function.

Feedback-mediated dynamics in two coupled nephrons

Previously, we developed a dynamic model for the tubuloglomerular feedback (TGF) system in a single, short-looped nephron of the mammalian kidney. In that model, a semi-linear hyperbolic partial differential equation was used to represent two fundamental processes of solute transport in the nephron's thick ascending limb (TAL): chloride advection by fluid flow along the TAL lumen and transepithelial chloride transport from the lumen to the interstitium. An empirical function and a time delay were used to relate glomerular filtration rate to the chloride concentration at the macula densa of the TAL. Analysis of the model equations indicated that stable limit-cycle oscillations (LCO) in nephron fluid flow and chloride concentration can emerge for sufficiently large feedback gain magnitude and time delay. In this study, the single-nephron model was extended to two nephrons, which were coupled through their filtration rates. Explicit analytical conditions were obtained for bifurcation loci corresponding to two special cases: (1) identical time delays but differing feedback gains, and (2) identical gains but differing delays. Similar to the case of a single nephron, our analysis indicates that stable LCO can emerge in coupled nephrons for sufficiently large gains and delays. However, these LCO may emerge at lower values of the feedback gain, relative to a single (i.e., uncoupled) nephron, or at shorter delays, provided the delays are sufficiently close. These results suggest that, in vivo, if two nephrons are sufficiently similar, then coupling will tend to increase the likelihood of LCO.

Preoperative renal volumes as a predictor of graft function in living donor transplantation

BACKGROUND

Nephron underdosing and donor kidney-recipient body size mismatch can lead to poor allograft function. The purpose of this study is to examine the relationship between donor kidney volume and posttransplantation graft function by using magnetic resonance imaging (MRI) to obtain renal volumes. Previous investigators used donor body surface area as a surrogate for kidney size or measured renal volume by using ultrasonography; both these techniques are inaccurate measures of renal volume. Intraoperative weights are more accurate, but provide information only after the transplantation is underway. More recently, MRI has been used preoperatively to screen living donors; these novel MRI techniques also provide information regarding renal size.

METHODS

We performed a retrospective analysis of 54 patients who underwent living donor transplantation at our institution from 2000 to 2002. All living donors underwent preoperative renovascular imaging using MRI, and renal volumes were obtained for each donor. A transplant kidney volume-recipient body weight (Vol/Wt) ratio was determined for each donor-recipient pair, and patients were divided into tertiles corresponding to 3 groups: high (>2.7), medium (2 to 2.7), and low (<2) "nephron dose" ratios.

RESULTS

Glomerular filtration rate (GFR) correlated with Vol/Wt ratio at 6 and 12 months (r = 0.46; P = 0.0005 and r = 0.41; P = 0.003). At 6 months, mean GFRs in the low, medium, and high groups were 52.4 +/- 2.8 (SEM), 64.5 +/- 6.2, and 82.0 +/- 4.4 mL/min, respectively (P < 0.0005). At 12 months, GFRs in the low, medium, and high groups were 51.6 +/- 3.6, 63.3 +/- 3.8, and 83.9 +/- 5.4 mL/min, respectively (P < 0.0001).

CONCLUSION

Transplantation of donor-recipient pairs with a Vol/Wt ratio less than 2 cm 3 /kg was associated with significantly worse graft function. Donor kidney volumes measured by means of preoperative MRI can be used to calculate Vol/Wt ratios before transplantation and identify patients at risk for a low GFR posttransplantation.

Characterization of basolateral chloride/bicarbonate exchange in macula densa cells

Functional and immunohistological studies were performed to identify basolateral chloride/bicarbonate exchange in macula densa cells. Using the isolated, perfused thick ascending limb with attached glomerulus preparation dissected from rabbit kidney, macula densa intracellular pH (pH(i)) was measured with fluorescence microscopy and BCECF. For these experiments, basolateral chloride was reduced, resulting in reversible macula densa cell alkalinization. Anion exchange activity was assessed by measuring the maximal net base efflux on readdition of bath chloride. Anion exchange activity required the presence of bicarbonate, was independent of changes in membrane potential, did not require the presence of sodium, and was inhibited by high concentrations of DIDS. Inhibition of macula densa anion exchange activity by basolateral DIDS increased luminal NaCl concentration-induced elevations in pH(i). Immunohistochemical studies using antibodies against AE2 demonstrated expression of AE2 along the basolateral membrane of macula densa cells of rabbit kidney. These results suggest that macula densa cells functionally and immunologically express a chloride/bicarbonate exchanger at the basolateral membrane. This transporter likely participates in the regulation of pH(i) and might be involved in macula densa signaling.

Receptor tyrosine kinases mediate epithelial Na(+) channel inhibition by epidermal growth factor

Epidermal growth factor (EGF) decreases Na(+) reabsorption across distal nephron epithelia. Activity of the epithelial Na(+) channel (ENaC) is limiting for Na(+) transport in this portion of the nephron. Abnormal ENaC activity and EGF signaling are both associated with polycystic kidney disease localized to the distal nephron. We tested here whether EGF and other ligands for receptor tyrosine kinases (RTK) decrease ENaC activity. EGF markedly and quickly decreased ENaC activity. The RTK inhibitor erbstatin blocked EGF actions on ENaC and when added alone increased channel activity, uncovering basal suppression by endogenous RTK. The protein tyrosine phosphatase inhibitor vanadate, similar to EGF, decreased ENaC activity. Growth factors and vanadate decreased ENaC activity by decreasing open probability. ENaC was not phosphorylated in response to EGF, indicating that intermediary proteins transduce the inhibitory signal from the EGF receptor (EGFR) to ENaC. We find that neither MAPK 1/2 nor c-Src is signaling intermediaries between EGFR and ENaC. Inhibition of ENaC paralleled decreases in plasma membrane phosphatidylinositol 4,5-bisphosphate levels [PtdIns(4,5)P(2)] and was abolished by clamping PtdIns(4,5)P(2). We conclude that EGF and other ligands for RTK decrease ENaC open probability by decreasing membrane PtdIns(4,5)P(2) levels.

Increased Endothelin Activity Mediates Augmented Distal Nephron Acidification Induced by Dietary Protein

The hypothesis that increased dietary protein augments distal nephron acidification and does so through an endothelin (ET-1)-dependent mechanism was tested. Munich-Wistar rats that ate minimum electrolyte diets of 50% (HiPro) and 20% (CON) casein-provided protein, the latter comparable to standard diet, were compared. HiPro versus CON had higher distal nephron net HCO(3) reabsorption by in vivo microperfusion (37.8 +/- 3.2 versus 16.6 +/- 1.5 pmol/mm per min; P < 0.001) as a result of higher H(+) secretion (41.3 +/- 4.0 versus 23.0 +/- 2.1 pmol/mm per min; P < 0.002) and lower HCO(3) secretion (-3.5 +/- 0.4 versus -6.4 +/- 0.8 pmol/mm per min; P < 0.001). Perfusion with H(+) inhibitors support that increased H(+) secretion was mediated by augmented Na(+)/H(+) exchange and H(+)-ATPase activity without augmented H(+),K(+)-ATPase activity. HiPro versus CON had higher levels of urine ET-1 excretion, renal cortical ET-1 addition to microdialysate in vivo, and renal cortical ET-1 mRNA, consistent with increased renal ET-1 production. Oral bosentan, an ET A/B receptor antagonist, decreased distal nephron net HCO(3) reabsorption (22.4 +/- 1.9 versus 37.8 +/- 3.2 pmol/mm per min; P < 0.001) as a result of lower H(+) secretion (28.4 +/- 2.4 versus 41.3 +/- 4.0 pmol/mm per min; P < 0.016) and higher HCO(3) secretion (-6.0 +/- 0.7 versus -3.5 +/- 0.4 pmol/mm per min; P < 0.006). The H(+) inhibitors had no additional effect in HiPro ingesting bosentan, supporting that ET mediated the increased distal nephron Na(+)/H(+) exchange and H(+)-ATPase activity in HiPro. Increased dietary protein augments distal nephron acidification that is mediated through an ET-sensitive increase in Na(+)/H(+) exchange and H(+)-ATPase activity.

Renal expression of osmotically responsive cation channel TRPV4 is restricted to water-impermeant nephron segments

TRPV4, a nonselective cation channel of the transient receptor potential (TRP) family, is gated by hypotonicity. Expression of TRPV4 mRNA has been detected in the circumventricular organs of the brain responsible for sensing systemic tonicity and in the kidney distal convoluted tubule (DCT), among other sites. No analysis of TRPV4 expression at the protein level has been undertaken and no systematic analysis of expression of this channel has been reported in the kidney. Via RNAse protection assay and immunoblotting, abundant expression of TRPV4 was detected in the cortex, medulla, and papilla. The expression pattern of TRPV4 was characterized in both rat and mouse kidney, which revealed similar patterns of immunoreactivity. TRPV4 expression was absent from the proximal tubule (PT) and descending thin limb (DTL), whereas the strongest expression was observed in the ascending thin limb (ATL). The thick ascending limb (TAL) was strongly positive as was the DCT and connecting tubule. Importantly, the water-permeant cells of the macula densa were unstained. Moderate TRPV4 expression was noted in all collecting duct portions and in papillary epithelium; intercalated cells (type A) exhibited a particularly strong signal. In all positive segments, TRPV4 expression was concentrated at the basolateral membrane. Therefore, TRPV4 is expressed in only those nephron segments that are constitutively (i.e., ATL, TAL, and DCT) or conditionally (i.e., collecting duct) water impermeant and where generation of a substantial transcellular osmotic gradient could be expected. TRPV4 expression is absent from nephron segments exhibiting constitutive water permeability and unregulated apical aquaporin expression (i.e., PT and DTL). These data, although circumstantial, are consistent with a role for TRPV4 in the response to anisotonicity in the mammalian kidney.

Acute regulation of the epithelial sodium channel gene by vasopressin and hyperosmolality

The amiloride-sensitive epithelial sodium channel (ENaC) plays a key role in sodium reabsorption in the collecting ducts. We examined ENaC mRNA distribution along the nephron and acute effects of vasopressin and hyperosmolality on ENaC mRNA expression. ENaCalpha, beta, and gamma mRNA expressions were observed in cortical, outer medullary and initial inner medullary collecting ducts (CCD, OMCD and ilMCD, respectively). ENaCalpha mRNA expression was also observed in medullary and cortical thick ascending limbs (MAL and CAL, respectively), while ENaCbeta and gamma mRNA expressions were not observed. Furthermore, ENaCalpha mRNA expression in MAL but not in collecting ducts was stimulated by acute exposure to arginine vasopressin (AVP), 8-(4-chlorophenylthio) (CPT)-cAMP and hyperosmolality. However, the physiological significance of these effects is not known, since ENaC protein is reported to be absent in MAL. These data suggest that ENaCalpha mRNA expression in MAL but not in collecting ducts is acutely regulated by AVP and hyperosmolality. The absence of stimulation of ENaCalpha mRNA expression in collecting ducts suggests the physiological significance of ENaCbeta and gamma mRNA for acute regulation by vasopressin. Determining the physiological significance of the acute effect of vasopressin in MAL will require further investigations.

ANG II-induced downregulation of RBF after a prolonged reduction of renal perfusion pressure is due to pre- and postglomerular constriction

Previous experiments from our laboratory showed that longer-lasting reductions in renal perfusion pressure (RPP) are associated with a gradual decrease in renal blood flow (RBF) that can be abolished by clamping plasma ANG II concentration ([ANG II]). The aim of the present study was to investigate the mechanisms behind the RBF downregulation in halothane-anesthetized Sprague-Dawley rats during a 30-min reduction in RPP to 88 mmHg. During the 30 min of reduced RPP we also measured glomerular filtration rate (GFR), proximal tubular pressure (Pprox), and proximal tubular flow rate (QLP). Early distal tubular fluid conductivity was measured as an estimate of early distal [NaCl] ([NaCl]ED), and changes in plasma renin concentration (PRC) over time were measured. During 30 min of reduced RPP, RBF decreased gradually from 6.5 ± 0.3 to 6.0 ± 0.3 ml/min after 5 min (NS) to 5.2 ± 0.2 ml/min after 30 min ( P < 0.05). This decrease occurred in parallel with a gradual increase in PRC from 38.2 ± 11.0 × 10-5to 87.1 ± 25.1 × 10-5Goldblatt units (GU)/ml after 5 min ( P < 0.05) to 158.5 ± 42.9 × 10-5GU/ml after 30 min ( P < 0.01). GFR, Pprox, and [NaCl]EDall decreased significantly after 5 min and remained low. Estimates of pre- and postglomerular resistances showed that the autoregulatory mechanisms initially dilated preglomerular vessels to maintain RBF and GFR. However, after 30 min of reduced RPP, both pre- and postglomerular resistance had increased. We conclude that the decrease in RBF over time is caused by increases in both pre- and postglomerular resistance due to rising plasma renin and ANG II concentrations.

Neuronal nitric oxide synthase inhibition sensitizes the tubuloglomerular feedback mechanism after volume expansion

BACKGROUND

In the kidney the neuronal isoform of nitric oxide synthase (nNOS) is located in the macula densa cells. These cells are known to be the sensor in the tubuloglomerular feedback. During volume expansion (VE), the tubuloglomerular feedback response is attenuated, allowing increased water and salt excretion. This study addressed the question whether inhibition of nNOS reestablishes the tubuloglomerular feedback response caused by acute extracellular VE.

METHODS

In rats, VE was achieved by infusion of isotonic saline solution at 50 mL/hour x kg body weight. When urine flow was stabilized, the tubuloglomerular feedback response was evaluated by measuring changes in proximal tubular stop-flow pressure (PSF) in response to various loop of Henle perfusion rates. The loop of Henle was perfused with artificial ultrafiltrate and with addition of 1 mol/L non-specific NOS inhibitor, Nomega-nitro-l-arginine methyl ester (L-NAME). In additional rats the nNOS inhibitor, 7-nitro indazole (7-NI), was given intraperitoneally. Single nephron glomerular filtration rate (SNGFR) was also measured. GFR was determined after VE and nNOS inhibition.

RESULTS

Acute VE decreased DeltaP(SF) and DeltaSNGFR while increasing the turning point, indicating decreased sensitivity of tubuloglomerular feedback response. After administration of L-NAME or 7-NI, DeltaP(SF) was maximally sensitized and the turning point and DeltaSNGFR were restored. GFR decreased after VE and nNOS inhibition compared to that after VE alone.

CONCLUSION

These results suggest that a functioning nitric oxide system, especially through the nNOS, is important in mediating normal renal responses and that increased production of and/or sensitivity to nitric oxide during sustained VE plays an important role in the adaptive mechanism of the tubuloglomerular feedback.

Gene expression of adenosine receptors along the nephron

BACKGROUND

In view of the multiple effects of adenosine on kidney function, this study aimed to determine the expression of adenosine receptors (AR) along the rat and mouse nephron.

METHODS

For this purpose, we semiquantified mRNA abundance for adenosine A1-, A2A-, A2B-, and A3 receptors by RNAse protection and by reverse transcription-polymerase chain reaction (RT-PCR) in the kidney zones and in the different nephron segments of mice and rats.

RESULTS

We found very similar expression patterns for rat and mice. For the kidney zones A1-AR mRNA and A2A-AR mRNA abundance displayed a marked difference, with an increase from cortex to the inner medulla. This was not seen for A2B receptors, which showed in general a rather weak expression. Along the nephron, A1-AR was strongly expressed in the thin limbs of Henle and in the collecting duct system and to a lesser extent in the medullary thick ascending limb. A2A-AR mRNA was clearly detected in glomeruli but not in other nephron segments. A2B-AR mRNA was strongly expressed in the cortical thick ascending limb of Henle and in the distal convoluted tubule. A3-AR mRNA was not found in any nephron segment.

CONCLUSION

Our data demonstrate a distinct mutual expression of the AR subtypes along the nephron. A1 receptors are expressed in medullary tubular structures, while A2B receptors are predominant in cortical tubular structures. A2A receptor expression in the kidney appears to be restricted to vascular cells.

Segment-specific expression of 2P domain potassium channel genes in human nephron

BACKGROUND

The 2P domain potassium (K2P) channels are a recently discovered ion channel superfamily. Structurally, K2P channels are distinguished by the presence of two pore forming loops within one channel subunit. Functionally, they are characterized by their ability to pass potassium across the physiologic voltage range. Thus, K2P channels are also called open rectifier, background, or leak potassium channels. Patch clamp studies of renal tubules have described several open rectifier potassium channels that have as yet eluded molecular identification. We sought to determine the segment-specific expression of transcripts for the 14 known K2P channel genes in human nephron to identify potential correlates of native leak channels.

METHODS

Human kidney samples were obtained from surgical cases and specific nephron segments were dissected. RNA was extracted and used as template for the generation of cDNA libraries. Real-time polymerase chain reaction (PCR) (TaqMan) was used to analyze gene expression.

RESULTS

We found significant (P < 0.05) expression of K2P10 in glomerulus, K2P5 in proximal tubule and K2P1 in cortical thick ascending limb of Henle's loop (cTAL) and in distal nephron segments. In addition, we repeatedly detected message for several other K2P channels with less abundance, including K2P3 and K2P6 in glomerulus, K2P10 in proximal tubule, K2P5 in thick ascending limb of Henle's loop, and K2P3, K2P5, and K2P13 in distal nephron segments.

CONCLUSION

K2P channels are expressed in specific segments of human kidney. These results provide a step toward assigning K2P channels to previously described native renal leaks.

Functional consequences at the single-nephron level of the lack of adenosine A1 receptors and tubuloglomerular feedback in mice

Mice deficient for adenosine A1 receptors (A1AR) lack tubuloglomerular feedback (TGF). In vivo micropuncture experiments were performed under anesthesia in A1AR-deficient and wild-type littermate mice to study the effects of chronic absence of A1AR on fluid and Na(+) reabsorption along the nephron, as well as the functional consequences at the single-nephron level of the lack TGF. Evidence is provided for an A1AR-mediated tonic inhibition of Na(+) reabsorption in a water-impermeable segment of the loop of Henle, possibly the thick ascending limb. In contrast, proximal tubular reabsorption of fluid, Na(+) and K(+) was unaffected by the chronic absence of A1AR. Experiments in which artificial tubular fluid was added to free-flowing late-proximal tubules demonstrated an essential role of A1AR/TGF in the stabilization of fluid and Na(+) delivery to the distal nephron. Further, the occurrence of spontaneous oscillations of hydrostatic pressure in proximal tubule ( P(PT)) at a frequency of about 32 mHz depended on intact A1AR/TGF. In comparison, the normal, stabilizing reduction in P(PT) following the initial rise in P(PT) during sustained small increases in proximal tubular flow rate does not require A1AR/TGF; TGF-independent mechanisms appear to compensate in this regard for a lack of TGF under physiological conditions and the lack of TGF is unmasked only when supraphysiological flow rates overwhelm TGF-independent compensation.

Is there an association between level of adult blood pressure and nephron number or renal filtration surface area?

BACKGROUND

Reductions in renal filtration surface area (FSA) have been linked to development of hypertension. This study investigated whether there are direct relationships, in the adult rat, between levels of blood pressure and nephron number or total renal FSA.

METHODS

F1 and F2 offspring were generated from a spontaneously hypertensive rat (SHR)/Wistar Kyoto (WKY) rat cross. Tail-cuff systolic blood pressure was measured twice weekly from 5 to 15 weeks of age and mean arterial blood pressure determined prior to sacrifice. At 15 weeks of age, the rats were perfusion-fixed and glomerular (and thereby nephron) number, glomerular size, total length, and surface area of glomerular capillaries and total renal FSA were determined using unbiased stereologic techniques.

RESULTS

In F1 offspring, blood pressure levels were midway between the SHR and WKY rats. Nephron number was significantly higher in the WKY rats compared to the SHR and F1 offspring. However, there was no difference in nephron number between the F1 rats and SHR and no difference in renal FSA between the three groups. In the F2 generation, where there is random segregation of the SHR and WKY genes, there was no significant correlation between either nephron number and adult blood pressure (r2= 0.16, P= 0.11) or total renal FSA and adult blood pressure (r2= 0.02, P= 0.58). There was a significant inverse correlation between nephron number and glomerular size (r2= 0.49, P= 0.0043).

CONCLUSION

There is not a direct corollary between nephron number or renal FSA and level of blood pressure in this rat model.

Modeling of basolateral ATP release induced by hypotonic treatment in A6 cells

ATP is released from the basolateral membrane of A6 epithelia in response to hypotonic treatment. This study addresses the problem of ATP diffusion through the permeable supports used to culture the cells. A theoretical analysis of a recently introduced experimental protocol is presented and a model of ATP diffusion through the compartments of the measuring system is proposed. The model provides the ATP profiles near the cell layer and in the measurement chamber. Comparison of results from computer simulations and experimental data showed that the permeable support introduces a marked delay for ATP diffusion, supporting the correlation of apparently time-separated events: the mobilization of Ca(2+) from internal stores and release of ATP from the cell. The model is consistent with experimental data obtained with the luciferin-luciferase pulse protocol and provides an indirect proof of related processes like the closure and opening of the lateral interspace that occur after imposing the hyposmotic shock. The influence of the pore structure of the permeable support in modulating the measured release rates revealed by computer simulation is experimentally validated for two types of Anopore filters.

Expression of Peroxisome Proliferator-Activated Receptor Isoform Proteins in the Rat Kidney

Peroxisome proliferator-activated receptors (PPARs) are nuclear hormone receptors mediating ligand-dependent transactivation. Among the 3 isoforms, PPAR- alpha is involved in lipid metabolism in the liver, while PPAR-gamma(-gamma1 and -gamma2) is involved in adipocyte differentiation. Recently, PPARs have been suggested to be involved in renal electrolyte metabolism as well as atherosclerosis. PPAR-alpha is known to regulate cytochrome P450 gene expression, and may possibly affect sodium retention in the kidney. Moreover, PPAR-gamma is involved in the transcription regulation of blood pressure regulatory genes, including thromboxane and angiotensin II type 1 receptors. In the kidney, although expression of PPARs has been reported, detailed immunohistochemical analyses have not been performed. We here generated isoform-specific anti-PPAR antibodies to localize their proteins in the kidney. Anti-PPAR antibodies were raised against synthetic peptides. Their isoform specificity was confirmed by immunoblot analyses, immunoprecipitations, and antibody supershift experiments by electrophoretic mobility shift assay. We therefore studied the protein expression of PPARs in the kidney of adult Sprague-Dawley rats using these antibodies. Immunoblot analyses demonstrated protein expression of PPAR-alpha and -gamma1, but not of -gamma2, in the kidney nuclear extracts. Immunohistochemical analyses demonstrated that both PPAR-alpha and -gamma1 proteins were widely expressed in the nuclei of mesangial and epithelial cells in glomeruli, proximal and distal tubules, the loop of Henle, medullary collecting ducts, and intima/media of renal vasculatures. PPAR-alpha and -gamma1 proteins are thus widely expressed along the nephron segments, and may affect gene expression at these segments. Further studies will be needed to identify additional target genes for PPARs along the nephron segments.

Cyclic GMP mediates influence of macula densa nitric oxide over tubuloglomerular feedback

BACKGROUND

Tubuloglomerular feedback (TGF) stabilizes nephron function by eliciting reciprocal changes in single-nephron glomerular filtration rate in response to changes in salt reaching the macula densa. Nitric oxide (NO) modulates TGF, making it less reactive. NO could come from NO synthase (NOS) in mesangium or microvessels (NOS III) or from neuronal NOS (NOS I) in the macula densa. Cyclic GMP is second messenger for many NO functions.

METHODS

Rat micropuncture was performed to confirm that macula densa NOS I makes the NO which modulates TGF and that cyclic GMP is a second messenger for this. The range of TGF was determined by measuring the single-nephron glomerular filtration rate while perfusing Henle's loop. The TGF slope was calculated from the response to flow perturbations in free-flowing nephrons using a noninvasive optical technique to measure flow. The NO-cyclic GMP axis was manipulated by tubular microperfusion of the antagonists S-methyl-thiocitrulline (SMTC) and 1H-[1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one (ODQ) and NO donor (spermine NONOate).

RESULTS

SMTC and ODQ each increased slope and range of TGF and tended to shift it leftward. NONOate reversed the effects of SMTC, but not of ODQ.

CONCLUSION

Cyclic GMP mediates desensitization of TGF by macula densa NOS I.

Development of an siRNA-based method for repressing specific genes in renal organ culture and its use to show that the Wt1 tumour suppressor is required for nephron differentiation

Wt1 is a tumour suppressor gene, mutation of which is a cause of Wilms' tumour, a childhood renal nephroblastoma. Wt1 is expressed in a rich pattern during renal development suggesting that it acts at three stages: determination of the kidney area, the differentiation of nephrons and maturation of glomeruli. Wt1-/- mice confirm that Wt1 is essential for the inception of kidney development; cells that ought to form kidneys die by apoptosis instead. Specific human WT1 mutations cause defects of glomerular maturation (Denys-Drash and Frasier syndromes), providing circumstantial evidence for action of Wt1 during glomerular maturation. There is, however, no genetic evidence for a function during nephron differentiation because this stage is never reached in Wt1-/- mice. We have therefore developed a novel technique, based on small interfering RNA (siRNA), to repress the expression of Wt1 and other specific genes at different stages of kidney development in culture. We find that early repression of Wt1 phenocopies the Wt1-/- mouse, but later repression prevents cells differentiating into nephrons and causes them instead to proliferate abnormally, possibly mimicking aspects of Wilms' tumour. In line with established hypotheses about genetic pathways that control kidney development, we find that repressing Pax2 using siRNAs represses Wt1 expression and blocks both bud growth and nephron differentiation, but that repressing Wnt4 blocks nephron differentiation without affecting Wt1 expression. As well as illuminating previously inaccessible aspects of Wt1 biology, our results suggest that siRNA in organ culture will be a powerful method for analyzing other developmental pathways and testing the effects of stage-specific loss of tumour suppressor genes.

Three-dimensional functional reconstruction of inner medullary thin limbs of Henle's loop

Digital three-dimensional (3-D) functional reconstructions of inner medullary nephrons were performed. Antibodies against aquaporins (AQP)-1 and -2 and the chloride channel ClC-K1 identified descending thin limbs (DTLs), collecting ducts (CDs), and ascending thin limbs (ATLs), respectively, through indirect immunofluorescence. Tubules were labeled in transverse sections and assembled into 3-D arrays, permitting individual tubule or combined surface representations to depths of 3.3 mm to be viewed in an interactive digital model. Surface representations of 75 tubules positioned near the central region of the inner medulla were reconstructed. In most DTL segments that form loops below 1 mm from the inner medullary base, AQP1 expression begins at the base, becomes intermittent for variable lengths, and continues nearly midway to the loop. The terminal DTL segment exhibiting undetectable AQP1 represents nearly 60% of the distance from the medullary base to the tip of the loop. AQP1 expression was entirely undetectable in shorter long-looped DTLs. ClC-K1 is expressed continuously along the terminal portion of all DTLs reconstructed here, beginning with a prebend region approximately 164 microm before the bend in all tubules and continuing through the entire ascent of the ATLs to the base of the inner medulla. CDs express AQP2 continuously and extensive branching patterns are illustrated. 3-D functional reconstruction of inner medullary nephrons is capable of showing axial distribution of membrane proteins in tubules of the inner medulla and can contribute to further development and refinement of models that attempt to elucidate the concentrating mechanism.