P2Y2 receptor activation inhibits the expression of the sodium-chloride cotransporter NCC in distal convoluted tubule cells.

Luminal nucleotide stimulation is known to reduce Na(+) transport in the distal nephron. Previous studies suggest that this mechanism may involve the thiazide-sensitive Na(+)-Cl(-) cotransporter (NCC), which plays an essential role in NaCl reabsorption in the cells lining the distal convoluted tubule (DCT). Here we show that stimulation of mouse DCT (mDCT) cells with ATP or UTP promoted Ca(2+) transients and decreased the expression of NCC at both mRNA and protein levels. Specific siRNA-mediated silencing of P2Y2 receptors almost completely abolished ATP/UTP-induced Ca(2+) transients and significantly reduced ATP/UTP-induced decrease of NCC expression. To test whether local variations in the intracellular Ca(2+) concentration ([Ca(2+)]i) may control NCC transcription, we overexpressed the Ca(2+)-binding protein parvalbumin selectively in the cytosol or in the nucleus of mDCT cells. The decrease in NCC mRNA upon nucleotide stimulation was abolished in cells overexpressing cytosolic PV but not in cells overexpressing either a nuclear-targeted PV or a mutated PV unable to bind Ca(2+). Using a firefly luciferase reporter gene strategy, we observed that the activity of NCC promoter region from -1 to -2,200 bp was not regulated by changes in [Ca(2+)]i. In contrast, high cytosolic calcium level induced instability of NCC mRNA. We conclude that in mDCT cells: (1) P2Y2 receptor is essential for the intracellular Ca(2+) signaling induced by ATP/UTP stimulation; (2) P2Y2-mediated increase of cytoplasmic Ca(2+) concentration down-regulates the expression of NCC; (3) the decrease of NCC expression occurs, at least in part, via destabilization of its mRNA.

Adenosine formed by 5'-nucleotidase mediates tubuloglomerular feedback.

Nephron function is stabilized by tubuloglomerular feedback (TGF). TGF operates within the juxtaglomerular apparatus, sensing changes in tubular flow and eliciting compensatory changes in single nephron GFR (SNGFR). The mediator(s) of TGF remains unconfirmed. One theory is that ATP consumed in active transport by the macula densa leads to formation of adenosine, which causes glomerular vasoconstriction. We performed micropuncture in rats to test this hypothesis. Adenosine activity was manipulated by microperfusing nephrons with adenosine A1 receptor blocker, A1-agonist, or 5'-nucleotidase inhibitor. Effects on TGF were characterized by changes in TGF efficiency (the compensation for small perturbations in tubular flow) and by changes in the maximum range over which TGF can cause SNGFR to change. These data were further applied to generate TGF profiles [SNGFR versus late proximal flow (V(LP))]. TGF efficiency was significantly reduced by blocking A1-receptors. TGF efficiency, TGF range, and the slope of the TGF profile (DeltaSNGFR/DeltaV(LP)) were all significantly reduced by blocking 5'-nucleotidase. When adenosine activity was clamped by combining 5'-nucleotidase inhibitor with A1-agonist to determine whether TGF requires adenosine to be present or to fluctuate, the TGF slope was reduced by 83%, indicating that adenosine activity must fluctuate for normal TGF to occur and that adenosine is a mediator of TGF.

Ornithine decarboxylase, kidney size, and the tubular hypothesis of glomerular hyperfiltration in experimental diabetes.

In early diabetes, the kidney grows and the glomerular filtration rate (GFR) increases. This growth is linked to ornithine decarboxylase (ODC). The study of hyperfiltration has focused on microvascular abnormalities, but hyperfiltration may actually result from a prior increase in capacity for proximal reabsorption which reduces the signal for tubuloglomerular feedback (TGF). Experiments were performed in Wistar rats after 1 week of streptozotocin diabetes. Kidney weight, ODC activity, and GFR were correlated in diabetic and control rats given difluoromethylornithine (DFMO; Marion Merrell Dow, Cincinnati, Ohio, USA) to inhibit ODC. We assessed proximal reabsorption by micropuncture, using TGF as a tool for manipulating single-nephron GFR (SNGFR), then plotting proximal reabsorption versus SNGFR. ODC activity was elevated 15-fold in diabetic kidneys and normalized by DFMO, which also attenuated hyperfiltration and hypertrophy. Micropuncture data revealed an overall increase in proximal reabsorption in diabetic rats too great to be accounted for by glomerulotubular balance. DFMO prevented the overall increase in proximal reabsorption. These data confirm that ODC is required for the full effect of diabetes on kidney size and proximal reabsorption in early streptozotocin diabetes and are consistent with the hypothesis that diabetic hyperfiltration results from normal physiologic actions of TGF operating in a larger kidney, independent of any primary malfunction of the glomerular microvasculature.

Agmatine, a bioactive metabolite of arginine. Production, degradation, and functional effects in the kidney of the rat.

Until recently, conversion of arginine to agmatine by arginine decarboxylase (ADC) was considered important only in plants and bacteria. In the following, we demonstrate ADC activity in the membrane-enriched fraction of brain, liver, and kidney cortex and medulla by radiochemical assay. Diamine oxidase, an enzyme shown here to metabolize agmatine, was localized by immunohistochemistry in kidney glomeruli and other nonrenal cells. Production of labeled agmatine, citrulline, and ornithine from [3H]arginine was demonstrated and endogenous agmatine levels (10(-6)M) in plasma ultrafiltrate and kidney were measured by HPLC. Microperfusion of agmatine into renal interstitium and into the urinary space of surface glomeruli of Wistar-Frömter rats produced reversible increases in nephron filtration rate (SNGFR) and absolute proximal reabsorption (APR). Renal denervation did not alter SNGFR effects but prevented APR changes. Yohimbine (an alpha 2 antagonist) microperfusion into the urinary space produced opposite effects to that of agmatine. Microperfusion of urinary space with BU-224 (microM), a synthetic imidazoline2 (I2) agonist, duplicated agmatine effects on SNGFR but not APR whereas an I1 agonist had no effect. Agmatine effects on SNGFR and APR are not only dissociable but appear to be mediated by different mechanisms. The production and degradation of this biologically active substance derived from arginine constitutes a novel endogenous regulatory system in the kidney.

Sodium reabsorption in thick ascending limb of Henle's loop: effect of potassium channel blockade in vivo.

1. Based on previous in vitro studies, inhibition of K(+) recycling in thick ascending limb (TAL) is expected to lower Na(+) reabsorption through (i) reducing the luminal availability of K(+) to reload the Na(+)-2Cl(-)-K(+) cotransporter and (ii) diminishing the lumen positive transepithelial potential difference which drives paracellular cation transport. 2. This issue was investigated in anaesthetized rats employing microperfusion of Henle's loop downstream from late proximal tubular site with K(+)-free artificial tubular fluid in nephrons with superficial glomeruli. 3. The unselective K(+) channel blocker Cs(+) (5 - 40 mM) dose-dependently increased early distal tubular delivery of fluid and Na(+) with a maximum increase of approximately 20 and 185%, respectively, indicating predominant effects on water-impermeable TAL. 4. The modest inhibition of Na(+) reabsorption in response to the 15 mM of Cs(+) but not the enhanced inhibition by 20 mM Cs(+) was prevented by luminal K(+) supplementation. Furthermore, pretreatment with 20 mM Cs(+) did not attenuate the inhibitory effect of furosemide (100 microM) on Na(+)-2Cl(-)-K(+) cotransport. 5. Neither inhibitors of large (charybdotoxin 1 microM) nor low (glibenclamide 250 microM; U37883A 100 microM) conductance K(+) channels altered loop of Henle fluid or Na(+) reabsorption. 6. The intermediate conductance K(+) channel blockers verapamil and quinine (100 microM) modestly increased early distal tubular Na(+) but not fluid delivery, indicating a role for this K(+) channel in Na(+) reabsorption in TAL. As observed for equieffective concentrations of Cs(+) (15 mM), Na(+) reabsorption was preserved by K(+) supplementation. 7. The results indicate that modest inhibition of K(+) channels lowers the luminal availability of K(+) and thus transcellular Na(+) reabsorption in TAL. More complete inhibition lowers paracellular Na(+) transport probably by reducing or even abolishing the lumen positive transepithelial potential difference. Under the latter conditions, transcellular Na(+) transport may be restored by paracellular K(+) backleak.

Eukaliuric diuresis and natriuresis in response to the KATP channel blocker U37883A: micropuncture studies on the tubular site of action.

1. Systemic application of U37883A, a blocker of ATP sensitive potassium (KATP) channels, elicits diuresis and natriuresis without significantly altering urinary potassium excretion. 2. To elucidate tubular sites of action upstream to the distal nephron, micropuncture experiments were performed in nephrons with superficial glomeruli of anaesthetized Munich-Wistar-Frömter rats during systemic application of U37883A (1, 5 or 15 mg kg-1 i.v.). 3. The observed eukaliuric diuresis and natriuresis in response to U37883A at 15 mg kg-1 was accompanied by an increase in early distal tubular flow rate (VED) from 10 - 18 nl min(-1) reflecting a reduction in fractional reabsorption of fluid up to this site (FR-fluid) of 13%. The latter proposed an effect on water-permeable segments such as the proximal tubule which could fully account for the observed reduction in fractional reabsorption of Na+ up to the early distal tubule (FR-Na+) of 8% and the increase in early distal tubular Na+ concentration ([Na+]ED) from 35 - 51 mM whereas [K+]ED was left unaltered. 4. In comparison, furosemide (3 mg kg-1 i.v.), which acts in the water-impermeable thick ascending limb, elicited diuresis, natriuresis and kaliuresis which were associated with a fall in FR-Na+ of 10% with no change in FR-fluid, and a rise in [Na+]ED from 42 - 117 mM and [K+]ED from 1.2 - 5.7 mM with no change in VED. 5. Direct late proximal tubular fluid collections confirmed a significant inhibition of fluid reabsorption in proximal convoluted tubule in response to systemic application of U37883A. 6. These findings suggest that the diuretic and natriuretic effect upstream to the distal tubule in response to systemic application of U37883A involves actions on water-permeable segments such as the proximal convoluted tubule.

Activities of nitric oxide in normal physiology and uremia.

Nitric oxide (NO) generated from arginine exerts a variety of renal and extrarenal physiological and pathophysiological effects. NO is generated by two types of nitric oxide synthases: acutely responsive, constitutive NOS and slower, more persistent inducible NOS (iNOS). The latter is transcriptionally dependent, often stimulated by cytokines. NO regulates glomerular ultrafiltration, tubular reabsorption, and intrarenal renin secretion; many of these renal effects are mediated by interactions with angiotensin II and adrenergic (alpha 2) activity. Decreased NO activity also enhances tubuloglomerular feedback activity, which could contribute to renal vasoconstriction, NaCl retention, and elevated blood pressure. Loss of renal function could influence NO activity via: (1) endothelial dysfunction; (2) decreased arginine synthesis by kidney; (3) responses to arginine analogs that act as NOS inhibitors; (4) increased cytokine activity; and (5) altered oxidation:reduction status of cells, etc. For example, platelet dysfunction in uremia may be caused by cytokine-induced iNOS activation. Moreover, acutely responsive, constitutive NOS activity may be depressed in progressive loss of renal function. Decreased NO activity might contribute to baroreceptor dysfunction observed in hypertension and progressive renal disease. Studies of the impact of uremia suggest that iNOS may be chronically stimulated by cytokines, whereas acutely responsive, constitutive NOS activity may be concurrently depressed.

Resetting protects efficiency of tubuloglomerular feedback.

Tubuloglomerular feedback (TGF) may effect long-term protection of total body salt and water or may govern minute-to-minute autoregulation of renal function. The task for which TGF is best suited depends on the orientation of ambient tubular flow relative to the inflection point of the TGF curve and on the tendency of TGF to reset in response to prolonged stimulation. Current data suggest that the TGF curve is coupled closely to ambient flow in individual nephrons such that the system is capable of compensating both negative and positive perturbations in tubular flow. This coupling is mediated by events within the juxtaglomerular apparatus that cause the TGF curve to reset laterally in response to sustained shifts in tubular flow. This resetting of TGF occurs within 30 to 60 minutes of an applied stimulus, suggesting that TGF is better suited to mediate dynamic autoregulation than to account for sustained vasoconstriction during proximal tubular injury.

Luminal signal in tubuloglomerular feedback: what about potassium?

Evidence suggests that a minimal luminal [K+] is required to elicit a full tubuloglomerular feedback (TGF) response, consistent with transmission of the TGF signal across the macula densa (MD) via the Na+-2Cl(-)-K+ cotransporter. Furthermore, it appears that luminal [K+] at the MD is close to the K+ affinity of the Na+-2Cl(-)-K+ cotransporter and changes in response to altering late proximal tubular flow rate (VLP), that is, a maneuver that induces a TGF response. These findings suggest that luminal [K+] (besides [Cl-]) could be rate limiting in TGF. In the thick ascending limb of Henle's loop (TALH), most of the luminal K+ is derived from recycling across the apical tubular membrane. Because changing VLP causes relatively greater alterations in the absolute Na+ and Cl- delivery to Henle's loop than in K+ load, the parallel changes of VLP and luminal [K+] at the MD, despite significant alteration in K+-dependent reabsorption of Na+ and Cl- via the Na+-2Cl(-)-K+ cotransporter, imply a transport-dependent adaptation of K+ recycling in TALH.

Dipyridamole prevents diabetes-induced alterations of kidney function in rats.

One possible factor for glomerular hyperfiltration in early diabetes could be a deficiency of renal vasoconstrictive mechanisms. Dipyridamole (DIP) inhibits cellular uptake of adenosine thereby increasing interstitial adenosine concentration. The effect of DIP on tubuloglomerular feedback (TGF) and on urinary protein excretion (UPE), glomerular filtration rate (GFR), and kidney weight was studied in early diabetes in rats. One day after onset of streptozotocin (STZ)-induced insulin-dependent diabetes mellitus (IDDM) daily treatment with DIP (50 mg/100 g twice a day via a gastric tube) was started in one group (STZ-DIP) and with vehicle alone in another group (STZ). Rats were housed in metabolic cages for 24 h to measure UPE 7, 14, and 21 days after STZ-injection. Non-diabetic animals, also receiving vehicle, served as controls (CON). While 7, 14, and 21 days after STZ-injection UPE was enhanced by 88, 123, and 153% in the STZ-group (n = 5) as compared to the CON-group (n = 6), the increase in UPE in the STZ-DIP-group (n = 5) was reduced by 82, 66, and 60%, respectively. Subsequently these diabetic rats were prepared for clearance and micropuncture study. Weight-matched (wm) non-diabetic rats served as controls (CONwm). TGF activity was assessed as the difference between stop flow pressures (delta SFP) in the early proximal tubule at 0 and 50 nl/min perfusion rates of Henle's loop. delta SFP was 8.8 +/- 0.7 mmHg (mean +/- SEM) in the CONwm-group (n = 5).(ABSTRACT TRUNCATED AT 250 WORDS)

Kidney function in mice: thiobutabarbital versus alpha-chloralose anesthesia.

Mice that lack or over-express a gene of interest are important tools for unraveling gene function. The determination of single nephron function by micropuncture or precise determination of glomerular filtration rate (GFR) by inulin clearance method require experiments under anesthesia. A good anesthetic protocol should allow for reasonable and stable glomerular and tubular function. The aim of this study was to compare the commonly used thiobutabarbital (TBB) versus alpha-chloralose (CHL) anesthesia with regard to absolute levels and the stability of blood pressure, heart rate, and kidney function. Male CD1 mice were anesthetized with TBB (100 mg/kg body weight i.p.) or CHL (120 mg/kg body weight i.p.), plus ketamine (100 mg/kg body weight i.m.) given to every mouse for analgesia. After preparation for clearance experiments, two 30-min urine collections were performed at periods 1 and 2 (P1 and P2). It was observed that heart rate and mean arterial blood pressure did not differ between TBB ( n=9) vs. CHL ( n=9) and were stable through P1 and P2. In CHL, GFR as well as fractional excretion of fluid, Na(+) and K(+) were stable from P1 to P2 (P1: 190+/-15 microl/min, 1.6+/-0.2%, 0.7+/-0.1%, 35+/-5%; percent change in P2: 1+/-6, 26+/-10, 29+/-15, 6+/-10 respectively). In TBB, GFR was significantly greater vs. CHL in P1 and did not significantly change in P2 (246+/-8 microl/min, p<0.05; percent change: -6.5+/-4). Fractional excretion of fluid, Na(+) and K(+) were not significantly different vs. CHL in P1, but significantly increased in P2 (P1: 1.5+/-0.2%, 1.1+/-0.2%, 31+/-3%; percent change in P2: 122+/-23, 128+/-21 and 29+/-6 respectively; each p<0.05 vs. P1). In conclusion, mice under both anesthetic regimens present reasonable and stable blood pressure and reasonable kidney function, but kidney reabsorption is more stable under CHL than under TBB anesthesia, which may facilitate study of the response in kidney function to acute interventions.

Intratubular application of sodium azide inhibits loop of Henle reabsorption and tubuloglomerular feedback response in anesthetized rats.

Sodium azide (NaN3, AZ) is a potent inhibitor and uncoupler of oxidative phosphorylation as well as a nitrovasodilator after being converted to nitric oxide (NO). We studied the effect of intratubular application of AZ on loop of Henle reabsorption and tubuloglomerular feedback (TGF) employing renal micropuncture experiments in nephrons with superficial glomeruli of anesthetized Munich-Wistar-Fromter rats. During perfusion of Henle's loop downstream from an obstructing wax block, AZ (3x10(-5) mol/l and 3x10(-4) mol/l) concentration-dependently increased early distal tubular flow rate and sodium and potassium ion concentration (V(ED), [Na+]ED, [K+]ED). In comparison, application of furosemide (10(-4) mol/l), the action of which is restricted to the water-impermeable thick ascending limb of Henle's loop (TALH) and the macula densa, similarly increased [Na+]ED and [K+]ED, but did not affect V(ED). The effect of AZ on loop of Henle reabsorption appeared to be predominantly localized upstream to the TALH since (1) AZ significantly inhibited net fluid reabsorption (the latter being completely abolished at 3x10(-4) mol/l), (2) the effect of AZ on [Na+]ED and [K+]ED could be mimicked by perfusing the Henle's loop at a flow rate that caused a comparable increase in V(ED) (reflecting a comparable load to TALH), and (3) the effects of AZ and furosemide were additive. In spite of the increase in [Na+]ED and [K+]ED, intratubular application of AZ caused a concentration-dependent inhibition of TGF response, the latter being assessed as the fall in early proximal tubular stop flow pressure during perfusion of Henle's loop at increasing flow rate. Like AZ and furosemide, the NO donor sodium nitroprusside (10(-4) mol/l) blunted the TGF response, but in contrast to furosemide or AZ, it caused a minor decrease in V(ED), without changing [Na+]ED or [K+]ED. The inhibitory effect of AZ on TGF was abolished by the NO scavenger carboxy PTIO. In summary, AZ inhibits both reabsorption in the water-permeable segment of Henle's loop and the TGF response. The effect on reabsorption may be linked to metabolic inhibition rather than NO release, whereas the blunted TGF response appears to involve conversion to NO.

Potassium diet as a determinant for the renal response to systemic potassium channel modulation in anesthetized rats.

The role of potassium intake in the response of kidney function and plasma renin activity (PRA) to systemic application of U37883A (4-morpholinecarboximidine-N-1-adamantyl-N'-cyclohexyl-hydro chloride), a putative blocker of ATP-sensitive potassium channels (K(ATP)), and P1075 (N-cyano-N'-(1,1-dimethylpropyl)-N"-pyridylguanidine), an opener of K(ATP) channels, was studied in the anesthetized rat. It was found that under normal potassium diet (0.7% K), U37883A (15 mg/kg, i.v.) increased urinary flow rate (UV) and sodium excretion (UNaV), decreased urinary potassium excretion (UKV), and significantly diminished heart rate (HR) without affecting mean arterial blood pressure (MAP) or glomerular filtration rate (GFR). P1075 (10 microg/kg, i.v.) lowered UV, UNaV and UKV, at least in part due to the fall in MAP and GFR. PRA was diminished by U37883A and increased by P1075. Variation in potassium diet (0.04 or 2% K) left the response in MAP, HR or GFR to both potassium channel modulators essentially unchanged. The reduction in renal excretion rates to P1075 also appeared unaffected, further supporting a predominant role of the change in MAP and GFR in this response. Variation in potassium diet, however, elicited the following alterations: (1) under both low and high potassium diet U37883A did no longer cause a significant natriuresis; (2) U37883A elicited a significant kaliuresis under high potassium diet, whereas potassium excretion remained essentially unchanged on very low levels under low potassium diet; (3) the increase in PRA to P1075 was blunted under low potassium diet. Additional experiments provided evidence that P1075 releases renin from freshly isolated juxtaglomerular cells of rats on normal but not on low potassium diet. In summary, systemic potassium channel modulation employing U37883A or P1075, respectively, exerts distinct effects on blood pressure and heart rate independent of potassium diet. In contrast, potassium diet appears to be a determinant for the concomitant responses in plasma renin activity and renal sodium and potassium excretion.

Eukaliuric natriuresis and diuresis in response to disprocynium24: studies on the tubular site of action.

We previously described that 1,1'-diisopropyl-2,4'-cyanine (disprocynium24, DP24) exerts an eukaliuric diuresis and natriuresis in the anesthetized rat. The purpose of the present study was to localize the tubular site of action of DP24. Employing micropuncture experiments in anesthetized rats, we first tested the effect of systemic application of DP24 (300 microg/kg + 300 microg/kg h, i.v.) on whole kidney excretion rates as well as on fluid, sodium and potassium ion delivery to the early distal tubule (V(ED), Na+(ED), K+(ED)). It was found that the eukaliuric diuresis and natriuresis in response to DP24 was accompanied by a substantial increase in V(ED) and Na+(ED), suggesting a predominant tubular site of action upstream to the early distal tubule, most likely in the proximal tubule. DP24 caused a comparable fractional, although minor absolute increase in K+(ED) as compared to Na+(ED). Second, application of DP24 into the first surface loop of the proximal tubule significantly increased V(ED) and Na+(ED) at a concentration of about 10(-7) M, indicating that DP24 may act from the intratubular site. Third, microperfusion of tubular segments revealed that effects of DP24 on the proximal convoluted tubule and the loop of Henle accounted for about 70 and 30%, respectively, of its diuretic and natriuretic action upstream to the early distal tubule. With regard to the loop of Henle, the quantitative effect of DP24 on fluid and Na+ reabsorption proposed a predominant effect on the straight part of the proximal tubule rather than the thick ascending limb. Intratubular DP24 did not affect reabsorption in the distal tubule. In summary, the present findings indicate that: (1) the diuretic and natriuretic effect of DP24 resides predominantly in the proximal tubule, and (2) DP24 may act from the intratubular site. Since DP24 increased V(ED) and Na+(ED) without apparently affecting sodium or potassium ion transport in the distal tubule, the mechanism of the eukaliuric response remains unclear.

Acute renal response to the non-peptide vasopressin V2-receptor antagonist SR 121463B in anesthetized rats.

Vasopressin V2-receptor antagonists are promising agents for the use in water-retaining diseases. Potential renal mechanisms of action include effects on water permeability in the collecting duct as well as on electrolyte transport in the thick ascending limb of Henle's loop (TALH). To elucidate sites of action upstream of the distal tubule, e.g., in TALH, micropuncture experiments were performed in anesthetized rats during application of the V2-receptor antagonist SR 121463B. As compared to vehicle-treated rats, SR 121463B (0.3 mg/kg i.v.) did not affect mean arterial blood pressure (means +/- SEM, n=10 rats per group: 108+/-4 mmHg vs. 107+/-4 mmHg), whole kidney GFR (1.1+/-0.1 ml/min vs. 1.1+/-0.1 ml/min), or whole kidney fractional reabsorption (FR) of potassium (66+/-5% vs. 68+/-4%). The drug, however, reduced whole kidney FR of fluid (92+/-1% vs. 99+/-1%), increased urinary flow rate (84+/-7 microl/min vs. 8+/-1 microl/min) and electrolyte-free-water clearance (72+/-8 microl/min vs. 2+/-1 microl/min), and reduced urinary osmolality (148+/-11 mosmol/kg vs. 1,200+/-185 mosmol/kg). This pronounced diuretic response was associated with a minor reduction in whole kidney FR of sodium (99.6+/-0.1% vs. 99.9+/-0.1%) and chloride (98.3+/-0.2% vs. 98.9+/-0.1%). As compared to vehicle application, SR 121463B did not significantly alter single nephron GFR (39+/-2 nl/min vs. 39+/-1 nl/min, n=22 and 23 nephrons, respectively) or the FR up to the early distal tubule of fluid (76+/-2% vs. 76+/-1%), sodium (92+/-1% vs. 93+/-1%), potassium (91+/-1% vs. 90+/-1%) or chloride (90+/-1% vs. 91+/-1%). Together these data indicate a predominant aquaretic effect of SR 121463B which is located downstream of the early distal tubule. This response is compatible with blockade of vasopressin V2-receptors in the collecting duct and, as directly demonstrated by immunohistochemistry, subsequent retrieval of aquaporin-2 from apical plasma membrane, which inhibits water permeability and transport.

Disprocynium24 induces a dopamine-independent, eukaliuric diuresis and natriuresis in the anaesthetized rat.

In the anaesthetized rat, intravenous administration of the isocyanine 1,1'-diisopropyl-2,4'-cyanine (disprocynium24) at doses up to 600 microg/kg resulted in marked diuresis and natriuresis without affecting urinary potassium excretion. Fractional sodium excretion was increased over 10-fold indicating a high ceiling-diuretic action. The effects of disprocynium24 on renal function were accompanied by a dose-dependent reduction in heart rate (HR) and mean arterial blood pressure (MAP). Acute administration of 600 microg/kg disprocynium24 decreased MAP by 25% and, in addition, caused a fall in glomerular filtration rate (GFR). Since i) disprocynium24 has been shown to interfere with urinary dopamine excretion (UDAV) and ii) dopamine has been implicated with the regulation of renal sodium excretion, we hypothesized that the effects of disprocynium24 might be mediated by its effects on renal dopamine handling. The following findings, however, argue against this hypothesis. First, administration of disprocynium24 in single doses up to 600 microg/kg caused a diuresis and natriuresis, but did not significantly affect U(DA)V. Second, neither the systemic nor the renal response to disprocynium24 were markedly altered by pretreatment with the dopamine D1- or D2-receptor blockers SCH23390 (10 microg x kg(-1) x min[-1]) or S(-)sulpiride (15 microg x kg(-1) x min[-1]), respectively.

Tubuloglomerular feedback responses of the downstream efferent resistance: unmasking a role for adenosine?

This Commentary aims to integrate or interrelate the available in vivo data with the in vitro study by Ren and co-workers, which comes to the somewhat surprising conclusion that tubuloglomerular feedback activation vasodilates the efferent arteriole by an adenosine-dependent mechanism.

The role of the BK channel in potassium homeostasis and flow-induced renal potassium excretion.

The kidney is the major regulator of potassium homeostasis. In addition to the ROMK channels, large conductance Ca(2+)-activated K(+) (BK) channels are expressed in the apical membrane of the aldosterone sensitive distal nephron where they could contribute to renal K(+) secretion. We studied flow-induced K(+) secretion in BK channel alpha-subunit knockout (BK(-/-)) mice by acute pharmacologic blockade of vasopressin V(2) receptors, which caused similar diuresis in wild-type and knockout mice. However, wild-type mice, unlike the BK(-/-), had a concomitant increase in urinary K(+) excretion and a significant correlation between urinary flow rate and K(+) excretion. Both genotypes excreted similar urinary amounts of K(+) irrespective of K(+) diet. This was associated, however, with higher plasma aldosterone and stronger expression of ROMK in the apical membrane of the aldosterone-sensitive portions of the distal nephron in the knockout than in the wild-type under control diet and even more so with the high-K(+) diet. High-K(+) intake significantly increased the renal expression of the BK channel in the wild-type mouse. Finally, despite the higher plasma K(+) and aldosterone levels, BK(-/-) mice restrict urinary K(+) excretion when placed on a low-K(+) diet to the same extent as the wild-type. These studies suggest a role of the BK channel alpha-subunit in flow-induced K(+) secretion and in K(+) homeostasis. Higher aldosterone and an upregulation of ROMK may compensate for the absence of functional BK channels.

Primary kidney growth and its consequences at the onset of diabetes mellitus.

Diabetes mellitus is a primary contributor to progressive kidney dysfunction leading to end-stage renal disease (ESRD). In the early phase of diabetes, prior to the onset of further complications, both kidney size and glomerular filtration rate (GFR) increase. Glomerular hyperfiltration is considered a risk factor for downstream complications and progression to ESRD. Abnormalities in vascular control have been purported to account for the glomerular hyperfiltration in early diabetes. In this review we discuss a tubulo-centric concept in which tubular growth and subsequent hyper-reabsorption contribute to the onset of glomerular hyperfiltration that demarks the early stage of diabetes. Kidney growth, in this concept, is no longer relegated to a compensatory response to hyperfiltration, but rather plays a primary and active role in its genesis and progression. As such, components of kidney growth, such as the polyamines, may provide a means of early detection of diabetic kidney dysfunction and more effective therapeutic intervention.