Inhibition of adenosine-1 receptor-mediated preglomerular vasoconstriction in AT1A receptor-deficient mice

Inhibition of ROMK blocks macula densa tubuloglomerular feedback yet causes renal vasoconstriction in anesthetized rats

The Na⁺-K⁺-2Cl^- cotransporter (NKCC2) on the loop of Henle is the site of action of furosemide. Because outer medullary potassium channel (ROMK) inhibitors prevent reabsorption by NKCC2, we tested the hypothesis that ROMK inhibition with a novel selective ROMK inhibitor (compound C) blocks tubuloglomerular feedback (TGF) and reduces vascular resistance. Loop perfusion of either ROMK inhibitor or furosemide caused dose-dependent blunting of TGF, but the response to furosemide was 10-fold more sensitive (IC₅₀ = 10^-6 M for furosemide and IC₅₀ = 10^-5 M for compound C). During systemic infusion, both diuretics inhibited TGF, but ROMK inhibitor was 10-fold more sensitive (compound C: 63% inhibition; furosemide: 32% inhibition). Despite blockade of TGF, 1 h of constant systemic infusion of both diuretics reduced the glomerular filtration rate (GFR) and renal blood flow (RBF) by 40-60% and increased renal vascular resistance (RVR) by 100-200%. Neither diuretic altered blood pressure or hematocrit. Proximal tubule hydrostatic pressures (P_PT) increased transiently with both diuretics (compound C: 56% increase; furosemide: 70% increase) but returned to baseline. ROMK inhibitor caused more natriuresis (3,400 vs. 1,600% increase) and calciuresis (1,200 vs. 800% increase) but less kaliuresis (33 vs. 167% increase) than furosemide. In conclusion, blockade of ROMK or Na⁺-K⁺-2Cl^- transport inhibits TGF yet increases renal vascular resistance. The renal vasoconstriction was independent of volume depletion, blood pressure, TGF, or P_PT.

0.9% saline is neither normal nor physiological

The purpose of this review is to objectively evaluate the biochemical and pathophysiological properties of 0.9% saline (henceforth: saline) and to discuss the impact of saline infusion, specifically on systemic acid-base balance and renal hemodynamics. Studies have shown that electrolyte balance, including effects of saline infusion on serum electrolytes, is often poorly understood among practicing physicians and inappropriate saline prescribing can cause increased morbidity and mortality. Large-volume (>2 L) saline infusion in healthy adults induces hyperchloremia which is associated with metabolic acidosis, hyperkalemia, and negative protein balance. Saline overload (80 ml/kg) in rodents can cause intestinal edema and contractile dysfunction associated with activation of sodium-proton exchanger (NHE) and decrease in myosin light chain phosphorylation. Saline infusion can also adversely affect renal hemodynamics. Microperfusion experiments and real-time imaging studies have demonstrated a reduction in renal perfusion and an expansion in kidney volume, compromising O2 delivery to the renal parenchyma following saline infusion. Clinically, saline infusion for patients post abdominal and cardiovascular surgery is associated with a greater number of adverse effects including more frequent blood product transfusion and bicarbonate therapy, reduced gastric blood flow, delayed recovery of gut function, impaired cardiac contractility in response to inotropes, prolonged hospital stay, and possibly increased mortality. In critically ill patients, saline infusion, compared to balanced fluid infusions, increases the occurrence of acute kidney injury. In summary, saline is a highly acidic fluid. With the exception of saline infusion for patients with hypochloremic metabolic alkalosis and volume depletion due to vomiting or upper gastrointestinal suction, indiscriminate use, especially for acutely ill patients, may cause unnecessary complications and should be avoided. More education regarding saline-related effects and adequate electrolyte management is needed.

P2 receptors in renal autoregulation

Autoregulation of renal blood flow and glomerular filtration rate is an essential function of the renal microcirculation. While the existence of this phenomenon has been known for many years, the exact mechanisms that underlie this regulatory system remain poorly understood. The work of many investigators has provided insights into many aspects of the autoregulatory mechanism, but many critical components remain elusive. This review is intended to update the reader on the role of P2 purinoceptors as a postulated mechanism responsible for renal autoregulatory resistance adjustments. It will summarize recent advances in normal function and it will touch on more recent ideas regarding autoregulatory insufficiency in hypertension and inflammation. Current thoughts on the nature of the mechanosensor responsible for myogenic behavior will be also be discussed as well as current thoughts on the mechanisms involved in ATP release to the extracellular fluid space.

Renal autoregulation in health and disease

Intrarenal autoregulatory mechanisms maintain renal blood flow (RBF) and glomerular filtration rate (GFR) independent of renal perfusion pressure (RPP) over a defined range (80-180 mmHg). Such autoregulation is mediated largely by the myogenic and the macula densa-tubuloglomerular feedback (MD-TGF) responses that regulate preglomerular vasomotor tone primarily of the afferent arteriole. Differences in response times allow separation of these mechanisms in the time and frequency domains. Mechanotransduction initiating the myogenic response requires a sensing mechanism activated by stretch of vascular smooth muscle cells (VSMCs) and coupled to intracellular signaling pathways eliciting plasma membrane depolarization and a rise in cytosolic free calcium concentration ([Ca(2+)]i). Proposed mechanosensors include epithelial sodium channels (ENaC), integrins, and/or transient receptor potential (TRP) channels. Increased [Ca(2+)]i occurs predominantly by Ca(2+) influx through L-type voltage-operated Ca(2+) channels (VOCC). Increased [Ca(2+)]i activates inositol trisphosphate receptors (IP3R) and ryanodine receptors (RyR) to mobilize Ca(2+) from sarcoplasmic reticular stores. Myogenic vasoconstriction is sustained by increased Ca(2+) sensitivity, mediated by protein kinase C and Rho/Rho-kinase that favors a positive balance between myosin light-chain kinase and phosphatase. Increased RPP activates MD-TGF by transducing a signal of epithelial MD salt reabsorption to adjust afferent arteriolar vasoconstriction. A combination of vascular and tubular mechanisms, novel to the kidney, provides for high autoregulatory efficiency that maintains RBF and GFR, stabilizes sodium excretion, and buffers transmission of RPP to sensitive glomerular capillaries, thereby protecting against hypertensive barotrauma. A unique aspect of the myogenic response in the renal vasculature is modulation of its strength and speed by the MD-TGF and by a connecting tubule glomerular feedback (CT-GF) mechanism. Reactive oxygen species and nitric oxide are modulators of myogenic and MD-TGF mechanisms. Attenuated renal autoregulation contributes to renal damage in many, but not all, models of renal, diabetic, and hypertensive diseases. This review provides a summary of our current knowledge regarding underlying mechanisms enabling renal autoregulation in health and disease and methods used for its study.

Elucidating mechanisms underlying altered renal autoregulation in diabetes

Previous studies have reported that high-salt intake paradoxically activates tubuloglomerular feedback (TGF) in type 1 diabetes. Using Zucker lean (ZL) and diabetic fatty (ZDF) rats on normal and high-salt diets, renal hemodynamics and the renin-angiotensin system (RAS) were characterized. On normal salt diet, glomerular filtration rate (GFR) was higher in ZDF than ZL rats. Autoregulation of GFR was less efficient and lithium clearance was lower in ZDF rats than ZL rats. Salt load reduced GFR in ZDF rats with restoration of lithium clearance and partial improvement in autoregulatory index (AI). The administration of 8-cyclopentyl-1,3-dipropylxanthine, a selective adenosine-1 receptor antagonist to ZDF rats on a high-salt diet abolished the improvement of AI in GFR. However, this effect was seen by neither (Cx40)GAP27 nor (Cx37,43)GAP27, which inhibits connexin (Cx) 40 or Cx37. Renal ANG II was higher in ZDF than ZL rats on normal salt diet, but the difference was eliminated by a salt load. The present data provide the first demonstration for a salt paradox in type 2 diabetes and implicate that in addition to Cx alterations, an enhanced proximal reabsorption attenuates TGF, underlying glomerular hyperfiltration and RAS activation. These data suggest that a high-salt diet standardizes distal delivery in diabetes, suppressing the RAS, and improving GFR autoregulation and hyperfiltration through adenosine.

Tubular reabsorption and diabetes-induced glomerular hyperfiltration

Elevated glomerular filtration rate (GFR) is a common observation in early diabetes mellitus and closely correlates with the progression of diabetic nephropathy. Hyperfiltration has been explained to be the result of a reduced load of sodium and chloride passing macula densa, secondarily to an increased proximal reabsorption of glucose and sodium by the sodium-glucose co-transporters. This results in an inactivation of the tubuloglomerular feedback (TGF), leading to a reduced afferent arteriolar vasoconstriction and subsequently an increase in GFR. This hypothesis has recently been questioned due to the observation that adenosine A(1)-receptor knockout mice, previously shown to lack a functional TGF mechanism, still display a pronounced hyperfiltration when diabetes is induced. Leyssac demonstrated in the 1960s (Acta Physiol Scand58, 1963:236) that GFR and proximal reabsorption can work independently of each other. Furthermore, by the use of micropuncture technique a reduced hydrostatic pressure in Bowman's space or in the proximal tubule of diabetic rats has been observed. A reduced pressure in Bowman's space will increase the pressure gradient over the filtration barrier and can contribute to the development of diabetic hyperfiltration. When inhibiting proximal reabsorption with a carbonic anhydrase inhibitor, GFR decreases and proximal tubular pressure increases. Measuring intratubular pressure allows a sufficient time resolution to reveal that net filtration pressure decreases before TGF is activated which highlights the importance of intratubular pressure as a regulator of GFR. Taken together, these results imply that the reduced intratubular pressure observed in diabetes might be crucial for the development of glomerular hyperfiltration.

ATP, P2 receptors and the renal microcirculation

Purinoceptors are rapidly becoming recognised as important regulators of tissue and organ function. Renal expression of P2 receptors is broad and diverse, as reflected by the fact that P2 receptors have been identified in virtually every major tubular/vascular element. While P2 receptor expression by these renal structures is recognised, the physiological functions that they serve remains to be clarified. Renal vascular P2 receptor expression is complex and poorly understood. Evidence suggests that different complements of P2 receptors are expressed by individual renal vascular segments. This unique distribution has given rise to the postulate that P2 receptors are important for renal vascular function, including regulation of preglomerular resistance and autoregulatory behaviour. More recent studies have also uncovered evidence that hypertension reduces renal vascular reactivity to P2 receptor stimulation in concert with compromised autoregulatory capability. This review will consolidate findings related to the role of P2 receptors in regulating renal microvascular function and will present areas of controversy related to the respective roles of ATP and adenosine in autoregulatory resistance adjustments.

Tubuloglomerular feedback: mechanistic insights from gene-manipulated mice

Tubuloglomerular feedback (TGF) describes a causal and direct relationship between tubular NaCl concentration at the end of the ascending limb of the loop of Henle and afferent arteriolar tone. The use of genetically altered mice has led to an expansion of our understanding of the mechanisms underlying the functional coupling of epithelial, mesangial, and vascular cells in TGF. Studies in mice with deletions of the A or B isoform of NKCC2 (Na,K,2Cl cotransporter) and of ROMK indicate that NaCl uptake is required for response initiation. A role for transcellular salt transport is suggested by the inhibitory effect of ouabain in mutant mice with an ouabain-sensitive alpha1 Na,K-ATPase. No effect on TGF was observed in NHE2- and H/K-ATPase-deficient mice. TGF responses are abolished in A1 adenosine receptor-deficient mice, and studies in mice with null mutations in NTPDase1 or ecto-5'-nucleotidase indicate that adenosine involved in TGF is mainly derived from dephosphorylation of released ATP. Angiotensin II is a required cofactor for the elicitation of TGF responses, as AT1 receptor or angiotensin-converting enzyme deficiencies reduce TGF responses, mostly by reducing adenosine effectiveness. Overall, the evidence from these studies in genetically altered mice indicates that transcellular NaCl transport induces the generation of adenosine that, in conjunction with angiotensin II, elicits afferent arteriolar constriction.

Effect of adenosine on membrane potential and Ca2+ in juxtaglomerular cells. Comparison with angiotensin II

BACKGROUND

Renin is mainly secreted from the juxtaglomerular cells (JGC) in the kidney situated in the afferent arteriole close to the vessel pole. Angiotensin II (ANG II) and adenosine inhibit renin secretion and synergistically constrict the afferent arteriole. ANG II depolarises JGC and increases the cytoplasmic free Ca2+ concentration [Ca2+]i. The responses of JGC to adenosine are less known.

METHODS

Effects of adenosine on membrane potential and [Ca2+]i were studied in afferent arterioles from NaCl-depleted rats and mice.

RESULT

Stimulation of A1 adenosine receptors (A1AR) by adenosine (10 microM) or cyclohexyladenosine (1 microM) increased the spiking frequency of JGC, slightly depolarised the cells and, in < or =50% of the cases, increased [Ca2+]i. These effects were much smaller than those of ANG II (3 nM). Simultaneous application of cyclohexyladenosine and ANG II gave only additive effects on [Ca2+]i; in addition, responses to ANG II in JGC from A1AR knockout mice were similar to those from control mice.

CONCLUSION

The small changes in membrane potential and [Ca2+]i in response to A1AR stimulation as compared to those of ANG II may suggest that these 2 tissue hormones use different signal transduction mechanisms to affect JGC function, including the inhibition of renin release.

Adenosine and kidney function: potential implications in patients with heart failure

Therapy of heart failure is more difficult when renal function is impaired. Here, we outline the effects on kidney function of the autacoid, adenosine, which forms the basis for adenosine A(1) receptor (A(1)R) antagonists as treatment for decompensated heart failure. A(1)R antagonists induce a eukaliuretic natriuresis and diuresis by blocking A(1)R-mediated NaCl reabsorption in the proximal tubule and the collecting duct. Normally, suppressing proximal reabsorption will lower glomerular filtration rate (GFR) through the tubuloglomerular feedback mechanism (TGF). But the TGF response, itself, is mediated by A(1)R in the preglomerular arteriole, so blocking A(1)R allows natriuresis to proceed while GFR remains constant or increases. The influence of A(1)R over vascular resistance in the kidney is augmented by angiotensin II while A(1)R activation directly suppresses renin secretion. These interactions could modulate the overall impact of A(1)R blockade on kidney function in patients taking angiotensin II blockers. A(1)R blockers may increase the energy utilized for transport in the semi-hypoxic medullary thick ascending limb, an effect that could be prevented with loop diuretics. Finally, while the vasodilatory effect of A(1)R blockade could protect against renal ischaemia, A(1)R blockade may act on non-resident cells to exacerbate reperfusion injury, where ischaemia to occur. Despite these uncertainties, the available data on A(1)R antagonist therapy in patients with decompensated heart failure are promising and warrant confirmation in further studies.

Renal interstitial adenosine is increased in angiotensin II-induced hypertensive rats

Since marked renal vasoconstriction is observed in angiotensin II (ANG II)-mediated hypertensive rats, we studied the possible interaction between ANG II and adenosine in this model. ANG II was infused into male Wistar rats through osmotic minipumps (435 ng·kg−1·min−1) for 14 days. In sham and ANG II groups, renal tissue and interstitial adenosine were measured; both increased to a similar twofold extent in the ANG II-treated rats (31.40 ± 4 vs. 62.0 ± 8.4 nM, sham vs. ANG II, interstitial adenosine; P< 0.001). The latter decreased by 47% with the specific blockade of 5′-nucleotidase. Glomerular hemodynamics demonstrated marked renal vasoconstriction in the angiotensin-treated group, which was reverted by an adenosine A1-receptor antagonist (8-cyclopentyl-1,3-dipropylxanthine, 10 μg·kg−1·min−1). 5′-Nucleotidase and adenosine deaminase (ADA) activities were measured in the cytosolic and membrane fractions. Only the membrane ADA activity decreased from 1,202 ± 80 to 900 ± 50 mU/mg protein in the ANG II-treated rats ( P< 0.05), as well as in their protein and mRNA expression. Despite the adenosine elevation, A1and A2breceptor protein did not change; in contrast, downregulation was observed in A2areceptor and upregulation in A3receptor. A similar pattern was found in the cortex and in the medulla; mRNA significantly decreased only in the A3receptor in both segments. These results suggest that the elevation of renal tissue and interstitial adenosine contributes to the renal vasoconstriction observed in the ANG II-induced hypertension and that it is mediated by a decrease in the activity and expression of ADA, increased production of adenosine, and an induced imbalance in adenosine receptors.

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.

Adenosine protects against angiotensin II-induced apoptosis in rat cardiocyte cultures

Adenosine has been found to be cardioprotective during episodes of cardiac ischemia/reperfusion through activation of the A1 and possibly A1 receptors. Therefore, we have investigated whether activation of these receptors can protect also against apoptotic death induced by angiotensin II (Ang II) in neonatal rat cardiomyocyte cultures. Exposure to Ang II (10 nM) resulted in a 3-fold increase in programmed cell death (p < 0.05). Pretreatment with the A1 adenosine receptor agonist 2-chloro-N6-cyclopentyladenosine (CCPA, 1 microM), abolished the effects of Ang II on programmed cardiomyocyte death. Moreover, exposure of cells to the A1 adenosine receptor antagonist 8-cyclopentyl- 1,3-dipropylxanthine (CPX) before pretreatment with CCPA, prevented the protective effect of the latter. Pretreatment with the A3 adenosine receptor agonist N6-(3-iodobenzyl) adenosine-5'-N-methyluronamide (IB-MECA, 0.1 microM), led to a partial decrease in apoptotic rate induced by Ang II. Exposure of myocytes to Ang II caused an immediate increase in the concentration of intracellular free Ca2+ that lasted 40-60 sec. Pretreatment of cells with CCPA or IB-MECA did not block Ang II-induced Ca2+ elevation. In conclusion, activation of adenosine A1 receptors can protect the cardiac cells from apoptosis induced by Ang II, while activation of the adenosine A3 receptors confers partial cardioprotection.

The extracellular cyclic AMP-adenosine pathway in renal physiology

Many cell types in the kidney express adenosine receptors, and adenosine has multiple effects on renal function. Although adenosine is produced within the kidney by several biochemical reactions, recent studies support a novel mechanism for renal adenosine production, the extracellular cAMP-adenosine pathway. This extracellular cAMP-adenosine pathway is initiated by efflux of cAMP from cells following activation of adenylyl cyclase. Extracellular cAMP is then converted to adenosine by the serial actions of ecto-phosphodiesterase and ecto-5'-nucleotidase. When extracellular cAMP is converted to adenosine near the biophase of cAMP production and efflux, this local extracellular cAMP-adenosine pathway permits tight coupling of the site of adenosine production to the site of adenosine receptors. cAMP in renal compartments may also be formed by tissues/organs remote from the kidney. For example, stimulation of hepatic adenylyl cyclase by the pancreatic hormone glucagon increases circulating cAMP, which is filtered at the glomerulus and concentrated in the tubular lumen as water is extracted from the ultrafiltrate. Conversion of hepatic-derived cAMP to adenosine in the kidney completes a pancreatohepatorenal cAMP-adenosine pathway that may serve as an endocrine link between the pancreas, liver, and kidney.

Physiological role for P2X1 receptors in renal microvascular autoregulatory behavior

This study tests the hypothesis that P2X1 receptors mediate pressure-induced afferent arteriolar autoregulatory responses. Afferent arterioles from rats and P2X1 KO mice were examined using the juxtamedullary nephron technique. Arteriolar diameter was measured in response to step increases in renal perfusion pressure (RPP). Autoregulatory adjustments in diameter were measured before and during P2X receptor blockade with NF279 or A1 receptor blockade with 1,3-dipropyl-8-cyclopentylxanthine (DPCPX). Acute papillectomy or furosemide perfusion was performed to interrupt distal tubular fluid flow past the macula densa, thus minimizing tubuloglomerular feedback-dependent influences on afferent arteriolar function. Under control conditions, arteriolar diameter decreased by 17% and 29% at RPP of 130 and 160 mmHg, respectively. Blockade of P2X1 receptors with NF279 blocked pressure-mediated vasoconstriction, reflecting an attenuated autoregulatory response. The A1 receptor blocker DPCPX did not alter autoregulatory behavior or the response to ATP. Deletion of P2X1 receptors in KO mice significantly blunted autoregulatory responses induced by an increase in RPP, and this response was not further impaired by papillectomy or furosemide. WT control mice exhibited typical RPP-dependent vasoconstriction that was significantly attenuated by papillectomy. These data provide compelling new evidence indicating that tubuloglomerular feedback signals are coupled to autoregulatory preglomerular vasoconstriction through ATP-mediated activation of P2X1 receptors.

AT1b Receptor Predominantly Mediates Contractions in Major Mouse Blood Vessels

In rodents, angiotensin (Ang) II type-1 (AT1) receptors exist as two pharmacologically identical subtypes: AT1a and AT1b. Recent studies have utilized mouse models with specific subtype receptor deletions to differentiate the functional difference between AT1 subtypes. However, little information is available on AT1 subtype expression in mouse vasculature. Therefore, in this study, AT1a-/- mice and wild-type littermates (AT1a+/+) were used to examine AT1 subtype expression and its functional relevance in mouse arterial vessels. Using RT-PCR and restriction enzyme digestion, we showed that AT1b accounts for most of the total AT1 mRNA in mouse abdominal aorta and femoral artery. In contrast, AT1a is the predominant subtype in kidney. To study the functional role of AT1 subtypes, we measured the in vitro contractility in vessels from AT1a-/- and AT1a+/+ mice. The Ang II concentration response curves in abdominal aorta and femoral artery were comparable between the two mouse strains. Furthermore, the Ang II response in AT1a-/- mouse vessels was completely antagonized by losartan, an AT1 antagonist. These results demonstrate that AT1b receptor is a major mediator for Ang II contractile response in mouse vessels, such as abdominal aorta and femoral artery.

Paracrine factors in tubuloglomerular feedback: adenosine, ATP, and nitric oxide

The tubuloglomerular feedback response, the change in afferent arteriolar tone caused by a change in NaCl concentration at the macula densa, is likely initiated by the generation of a vasoactive mediator within the confines of the juxtaglomerular apparatus. Substantial progress has been made in identifying the nature of this mediator and the factors that modulate its effect on vascular tone. In support of earlier studies using P1 purinergic antagonists, the application of the knockout technique has shown that adenosine 1 receptors are absolutely required for eliciting TGF responses. The background level of angiotensin II appears to be an important cofactor determining the efficiency of A1AR-induced vasoconstriction, probably through a synergistic interaction at the level of the G protein-dependent transduction mechanism. The source of the adenosine is still unclear, but it is conceivable that adenosine is generated extracellularly from released ATP through a cascade of ecto-nucleotidases. There is also evidence that ATP may activate P2 receptors in preglomerular vessels, which may contribute to autoregulation of renal vascular resistance. Nitric oxide (NO), generated by the neuronal isoform of nitric oxide synthase in macula densa cells, reduces the constrictor effect of adenosine, but the regulation of NO release and its exact role in states of TGF-induced hyperfiltration are still unclear.

Macula densa cell signaling

Macula densa cells are renal sensor elements that detect changes in distal tubular fluid composition and transmit signals to the glomerular vascular elements. This tubuloglomerular feedback mechanism plays an important role in regulating glomerular filtration rate and blood flow. Macula densa cells detect changes in luminal sodium chloride concentration through a complex series of ion transport-related intracellular events. NaCl entry via a Na:K:2Cl cotransporter and Cl exit through a basolateral channel lead to cell depolarization and increases in cytosolic calcium. Na/H exchange (NHE2) results in cell alkalization, whereas intracellular [Na] is regulated by an apically located H(Na)-K ATPase and not by the traditional basolateral Na:K ATPase. Communication from macula densa cells to the glomerular vascular elements involves ATP release across the macula densa basolateral membrane through a maxi-anion channel. The adaptation of multi-photon microscopy is providing new insights into macula densa-glomerular signaling.

Expression of adenosine receptors in the preglomerular microcirculation

The purpose of this study was to systematically investigate the abundance of each of the adenosine receptor subtypes in the preglomerular microcirculation vs. other vascular segments and vs. the renal cortex and medulla. Rat preglomerular microvessels (PGMVs) were isolated by iron oxide loading followed by magnetic separation. For comparison, mesenteric microvessels, segments of the aorta (thoracic, middle abdominal, and lower abdominal), renal cortex, and renal medulla were obtained by dissection. Adenosine receptor protein and mRNA expression were examined by Western blotting, Northern blotting, and RT-PCR. Our results indicate that compared with other vascular segments and renal tissues, A1 and A2B receptor protein and mRNA are abundantly expressed in the preglomerular microcirculation, whereas A2A and A3 receptor protein and mRNA are barely detectable or undetectable in PGMVs. We conclude that, relative to other vascular and renal tissues, A1 and A2B receptors are well expressed in PGMVs, whereas A2A and A3 receptors are notably deficient. Thus A1 and A2B receptors, but not A2A or A3 receptors, may importantly regulate the preglomerular microcirculation.

Blood pressure and NaCl-sensitive hypertension are influenced by angiotensin-converting enzyme gene expression in transgenic mice

ACE plays an important role in the regulation of arterial pressure; however, a linear relationship between ACE expression and arterial pressure has not been demonstrated. The present study employed telemetric monitoring in female transgenic mice to determine the influence of partial and complete deletion of the ACE gene on basal arterial pressure and arterial pressure responses to a high-NaCl diet. On the basal NaCl diet, 24-hour mean arterial pressure was significantly correlated with the number of functional copies of the ACE gene; ie, arterial pressure was lowest in 0-copy (80 +/- 1 mm Hg), intermediate in 1-copy (100 +/- 1 mm Hg), and highest in 2-copy (113 +/- 1 mm Hg) ACE mice. The high-NaCl diet significantly increased mean arterial pressure in 0-copy (99 +/- 1 mm Hg) and 1-copy (108 +/- 1 mm Hg) mice but not in 2-copy mice (114 +/- 1 mm Hg). These results demonstrate a copy-dependent relationship between ACE gene expression and both basal arterial pressure and arterial pressure responses to a high-NaCl diet, suggesting that either partial or complete reduction in the ACE gene can alter arterial pressure.

Abolished tubuloglomerular feedback and increased plasma renin in adenosine A1 receptor-deficient mice

The hypothesis that adenosine acting on adenosine A1 receptors (A1R) regulates several renal functions and mediates tubuloglomerular feedback (TGF) was examined using A1R knockout mice. We anesthetized knockout, wild-type, and heterozygous mice and measured glomerular filtration rate, TGF response using the stop-flow pressure (P(sf)) technique, and plasma renin concentration. The A1R knockout mice had an increased blood pressure compared with wild-type and heterozygote mice. Glomerular filtration rate was similar in all genotypes. Proximal tubular P(sf) was decreased from 36.7 +/- 1.2 to 25.3 +/- 1.6 mmHg in the A1R+/+ mice and from 38.1 +/- 1.0 to 27.4 +/- 1.1 mmHg in A1R+/- mice in response to an increase in tubular flow rate from 0 to 35 nl/min. This response was abolished in the homozygous A1R-/- mice (from 39.1 +/- 4.1 to 39.2 +/- 4.5 mmHg). Plasma renin activity was significantly greater in the A1R knockout mice [74.2 +/- 14.3 milli-Goldblatt units (mGU)/ml] mice compared with the wild-type and A1R+/- mice (36.3 +/- 8.5 and 34.1 +/- 9.6 mGU/ml), respectively. The results demonstrate that adenosine acting on A1R is required for TGF and modulates renin release.

Role of the extracellular cAMP-adenosine pathway in renal physiology

Adenosine exerts physiologically significant receptor-mediated effects on renal function. For example, adenosine participates in the regulation of preglomerular and postglomerular vascular resistances, glomerular filtration rate, renin release, epithelial transport, intrarenal inflammation, and growth of mesangial and vascular smooth muscle cells. It is important, therefore, to understand the mechanisms that generate extracellular adenosine within the kidney. In addition to three "classic" pathways of adenosine biosynthesis, contemporary studies are revealing a novel mechanism for renal adenosine production termed the "extracellular cAMP-adenosine pathway." The extracellular cAMP-adenosine pathway is defined as the egress of cAMP from cells during activation of adenylyl cyclase, followed by the extracellular conversion of cAMP to adenosine by the serial actions of ecto-phosphodiesterase and ecto-5'-nucleotidase. This mechanism of extracellular adenosine production may provide hormonal control of adenosine levels in the cell-surface biophase in which adenosine receptors reside. Tight coupling of the site of adenosine production to the site of adenosine receptors would permit a low-capacity mechanism of adenosine biosynthesis to have a large impact on adenosine receptor activation. The purposes of this review are to summarize the physiological roles of adenosine in the kidney; to describe the classic pathways of renal adenosine biosynthesis; to review the evidence for the existence of the extracellular cAMP-adenosine pathway; and to describe possible physiological roles of the extracellular cAMP-adenosine pathway, with particular emphasis on the kidney.

Mediation of tubuloglomerular feedback by adenosine: evidence from mice lacking adenosine 1 receptors

Adenosine is a determinant of metabolic control of organ function increasing oxygen supply through the A2 class of adenosine receptors and reducing oxygen demand through A1 adenosine receptors (A1AR). In the kidney, activation of A1AR in afferent glomerular arterioles has been suggested to contribute to tubuloglomerular feedback (TGF), the vasoconstriction elicited by elevations in [NaCl] in the macula densa region of the nephron. To further elucidate the role of A1AR in TGF, we have generated mice in which the entire A1AR coding sequence was deleted by homologous recombination. Homozygous A1AR mutants that do not express A1AR mRNA transcripts and do not respond to A1AR agonists are viable and without gross anatomical abnormalities. Plasma and urinary electrolytes were not different between genotypes. Likewise, arterial blood pressure, heart rates, and glomerular filtration rates were indistinguishable between A1AR(+/+), A1AR(+/-), and A1AR(-/-) mice. TGF responses to an increase in loop of Henle flow rate from 0 to 30 nl/min, whether determined as change of stop flow pressure or early proximal flow rate, were completely abolished in A1AR(-/-) mice (stop flow pressure response, -6.8 +/- 0.55 mmHg and -0.4 +/- 0.2 in A1AR(+/+) and A1AR(-/-) mice; early proximal flow rate response, -3.4 +/- 0.4 nl/min and +0.02 +/- 0.3 nl/min in A1AR(+/+) and A1AR(-/-) mice). Absence of TGF responses in A1AR-deficient mice suggests that adenosine is a required constituent of the juxtaglomerular signaling pathway. A1AR null mutant mice are a promising tool to study the functional role of A1AR in different target tissues.