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

Abstract

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 A₁ receptor (A₁R) antagonists as treatment for decompensated heart failure. A₁R antagonists induce an eukaliuretic natriuresis and diuresis by blocking A₁R-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 A₁R in the pre-glomerular arteriole, so blocking A₁R allows natriuresis to proceed while GFR remains constant or increases. The influence of A₁R over vascular resistance in the kidney is augmented by angiotensin II while A₁R activation directly suppresses renin secretion. These interactions could modulate the overall impact of A₁R blockade on kidney function in patients taking angiotensin II blockers. A₁R 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 A₁R blockade could protect against renal ischaemia, A₁R blockade may act on non-resident cells to exacerbate reperfusion injury, were ischaemia to occur. Despite these uncertainties, the available data on A₁R antagonist therapy in patients with decompensated heart failure are promising and warrant confirmation in further studies.

INTRODUCTION

Concomitant renal dysfunction is one of the strongest risk factors for mortality in ambulatory heart failure patients [1–3]. In patients hospitalised for decompensated heart failure, worsening of renal function further predicts an adverse outcome [4]. Whereas the mechanism underlying impaired renal function may vary, therapeutic approaches that improve kidney function in these settings have a chance to improve clinical outcome. Especially in patients with worsening of renal function, volume overload, and refractoriness to classical diuretics like loop diuretics and thiazides, effective therapy of cardiorenal dysfunction is very difficult, indicating the need to better understand renal dysfunction in this setting and to develop new therapeutic strategies. As adenosine A₁ receptor (A₁R) antagonists make their way into the clinical arena as therapeutic agents in heart failure, it is appropriate to review the cardiorenal physiology of adenosine.

In addition to serving as the precursor to ATP, adenosine is an autocoid that binds to cell surface receptors in kidney, heart, brain, retina, and skeletal muscle to mediate various aspects of organ function. Most of these actions of adenosine revolve around local adjustments that match blood flow with energy consumption. According to this paradigm, the interstitial concentration of adenosine rises when neighboring cells are in negative energy balance. Vasodilatory adenosine A₂ receptors (A₂R) are activated, in turn, which adjusts blood flow to meet demand. Since the half-life and diffusion distance of adenosine are short, this is a fast and efficient means to match supply and demand over short distances within an organ. The role of adenosine in the kidney is, at the same time, analogous, yet dissimilar to, and more complicated than, its role in other organs.

In this review, we first describe the basis for a more complicated role of adenosine in the kidney and outline its differential effects on the renal cortical and medullary vascular structures, its role in tubuloglomerular feedback (TGF), the regulation of renin secretion and in transport processes in the tubular and collecting duct system. These issues are subsequently discussed with regard to a potential role of A₁R as a new target in the treatment of patients with acute decompensated heart failure or cardiorenal failure. The reader is referred to relatively recent reviews on the role of adenosine in kidney function in general [5] and fluid retention in particular [6;7].

Differential vascular actions of adenosine in the kidney cortex and medulla

While blood flows to other organs to meet a demand for energy, blood that flows to the kidney generates a demand for energy. This occurs because glomerular filtration rate (GFR) is closely tied to the renal blood flow and nearly all of the glomerular filtrate must be reabsorbed, which requires energy. Hence, the way to recover from negative energy balance in the kidney is not necessarily to increase its blood flow, but to lower the filtration fraction, since this will lessen the number of sodium ions that must be transported per oxygen delivered. From the physical forces that drive glomerular filtration, it is apparent that the simplest way to lower the filtration fraction is to reduce the glomerular capillary pressure. As outlined below, the renal adenosine system employs vasoconstrictor A₁R on the afferent arteriole and vasodilatory A₂R on the efferent arteriole. Activating either population of glomerular adenosine receptors will suffice to reduce the filtration fraction, whereas nuances related to their relative activities will determine the impact of endogenous adenosine on single nephron GFR and glomerular blood flow.

In comparison, blood flow in the renal medulla is nutritive. It derives from the postglomerular circulation of deep nephrons and due to the way the kidney concentrates the urine, blood flow and O₂ supply are low in this area although active NaCl reabsorption in the medullary thick ascending limb is essential for function. This asks for a vasodilator to prevent hypoxic injury in the renal medulla. As outlined in the following, adenosine lowers GFR but is a vasodilator in the renal medulla.

Adenosine lowers GFR via activation of A₁R

Intravenous infusion of adenosine in healthy awake volunteers was found to lower GFR by ~25% while blood pressure and renal blood flow were unchanged [8;9]. Direct infusion of adenosine into the renal artery of volunteers also reduced GFR [10], indicating a primary renal site of action based on the short plasma half life of adenosine of a few seconds. Furthermore, oral application of the A₁R antagonist FK-453 to healthy male subjects increased GFR by ~20% without significantly altering effective renal plasma flow or mean arterial blood pressure [11], indicating that endogenous adenosine through activation of A₁R elicits a tonic suppression of GFR.

Continuous adenosine infusion into the renal artery of rats or dogs reduced single nephron GFR (SNGFR) in superficial nephrons to a larger extent than whole kidney GFR, indicating that deep-cortical vasodilation (see below) counteracts superficial vasoconstriction [12–14]. The adenosine-induced fall in SNGFR in superficial nephrons was the result of afferent arteriolar vasoconstriction with a parallel fall of the hydrostatic pressure in glomerular capillaries and in postglomerular star vessels [12;14](Figure 1). Direct videometric assessment of pre- and postglomerular arteries using the “split-hydronephrotic” rat kidney technique indicated adenosine-induced constriction of afferent arterioles via high affinity A₁R and dilation via activation of both high affinity A_2AR and low affinity A_2bR [15]. In general, activation of A₁R lead to constriction mainly of afferent arterioles near the glomerulus whereas A₂R activation lead to dilation mainly of postglomerular arteries [16–18].

Fig. 1. Differential effects of adenosine (ADO) on renal haemodynamics and transport – basis for a therapeutical effect of A₁R antagonism in heart failure.

Partial oxygen pressure (pO₂) is lower in renal medulla than cortex exposing the medulla to a greater risk for hypoxic damage. The line plots illustrate the relationships between the given parameters. Small circles on these lines indicate ambient physiological conditions. (1) In every nephron segment, an increase in reabsorption or transport of sodium (T_Na) increases extracellular ADO. (2) ADO via A₁R mediates TGF and constricts the afferent arteriole to lower GFR. (3) In the proximal tubule, ADO via A₁R stimulates T_Na and thus lowers the Na⁺ load to segments residing in the semi-hypoxic medulla. (4) In contrast, ADO via A₁R inhibits T_Na in the medulla including medullary thick ascending limb (mTAL). (5) In addition, ADO via A₂R enhances medullary blood flow (MBF), which increases O₂ delivery and further limits O₂ consuming transport in the medulla. (6) Heart failure can be associated with increased plasma concentrations of ADO and angiotensin II, and endothelial dysfunction can impair nitric oxide formation, all of which may enhance the A₁R-mediated lowering of GFR. (7) A₁R antagonism induces natriursesis and diuresis by inhibiting proximal reabsorption and preserving or increasing GFR. (8) A₁R antagonism can enhance T_Na in mTAL. This is prevented by co-administration of loop diuretics, and diuresis and natriuresis are potentiated.

Factors affecting adenosine-induced cortical vasoconstriction

Many factors and conditions affect the renal vasoconstrictor response to adenosine, which can be of clinical relevance. One prominent factor is the renin-angiotensin system. Whereas suppression of the system by high salt intake [19;20], angiotensin II AT1 receptor antagonists [21–24], or inhibitors of angiotensin I converting enzyme [17] reduces or blocks the renal vasoconstrictive action of adenosine, activation of the renin-angiotensin system potentiates adenosine-induced vasoconstriction and lowering of GFR [13;19;23]. The assumed interaction of angiotensin II and adenosine in preglomerular vessels was confirmed in dogs [25;26], and further studies revealed a mutual dependency and cooperation of adenosine and angiotensin II in producing afferent arteriolar constriction [27–29]. A significant increase in the sensitivity of the kidney to adenosine-induced vasoconstriction is also induced by inhibiting the synthesis of vasodilators like nitric oxide (NO) [30;31] or prostaglandins [32;33]. Table 1 summarizes further interactions between different conditions, drugs and hormones and the renal vasoconstriction mediated by adenosine.

Table 1.

Parallel response of adenosine (ADO)-induced renal vasoconstriction and tubuloglomerular feedback (TGF) activity to a number of different experimental conditions, drugs and hormones

	ADO-induced vasoconstriction	TGF activity	References
			ADO	TGF
Conditions
low salt diet, volume depletion or hemorrhage	potentiation	↑	[13;19]	[133–136]
high salt diet or volume expansion	attenuation	↓	[13;19]	[137]
Drugs/ Hormones
theophylline or ADO A1 antagonists	inhibition	↓	[138]	[23;43;46;47;141]
calcium antagonists	inhibition	↓	[139]	[142–144]
angiotensin II blockade	inhibition	↓	[22;23;26]	[145]
NOS inhibition	potentiation	↑	[30]	[146–150]
ADO re-uptake ↓	potentiation	↑	[20;140]	[23;43;151]
angiotensin II	potentiation	↑	[25;27]	[152–157]

NOS, nitric oxide synthase;

Adenosine induces medullary vasodilation via activation of A₂R

After an initial adenosine-induced vasoconstriction in all cortical zones in response to intrarenal continuous adenosine infusion in rats, the superficial cortical vasoconstriction persisted while a deep cortical vasodilation was observed [34;35]. Moreover, experiments in isolated-perfused rabbit afferent arterioles indicated that A₁R-mediated afferent arteriolar constriction dominates in superficial nephrons whereas deep cortical nephrons, which supply the blood flow to the renal medulla, can respond to adenosine with A₂R-mediated vasodilation [36–40]. In accordance, renal interstitial infusion of the A₂R agonist CGS-21680C in rats increased medullary blood flow [41]. Moreover, infusion of the selective A₂R antagonist DMPX (but not the A₁R antagonist DPCPX) into the renal medulla of rats decreased medullary blood flow [42], indicating that endogenous adenosine dilates medullary vessels and sustains medullary blood flow via activation of A₂R (Figure 1). Furthermore, by washing out the high osmolality in the medullary interstitium, the A₂R-mediated rise in medullary blood flow lowers medullary transport activity [42].

In summary, adenosine can induce both, vasoconstriction and vasodilation in the kidney (Figure 1). Adenosine-induced vasoconstriction via A₁R activation is predominant in the outer cortex by increasing the resistance of afferent arterioles, which lowers GFR and thus renal transport work. In the deep cortex and medulla, adenosine-induced vasodilation via A₂R activation is associated with an increase of medullary blood flow and thus increased medullary oxygenation.

Adenosine is a mediator of tubuloglomerular feedback (TGF) via activation of A₁R

The way the mammalian kidney developed to fulfill its functions in body homeostasis includes a rather high GFR (~180 l/day in human). About 99% of the filtered fluid and NaCl needs subsequently to be reabsorbed along the nephron, such that urinary excretion closely matches intake. As a consequence, GFR is a critical determinant of renal transport work, and GFR and reabsorption have to be closely coordinated to prevent renal loss or retention of fluid and NaCl. The importance of the symmetry between nephron filtration and reabsorption can be appreciated from the fact that a disparity of as little as 5 % between filtered load and the reabsorption rate would lead to a net loss of about one third of the total extracellular fluid volume within 1 day, a situation which inevitably would lead to vascular collapse. A mechanism that helps to coordinate GFR with the tubular transport activity or capacity and which contributes to the GFR lowering effect of adenosine in kidney cortex under physiological conditions, is the TGF. In this mechanism, the tubular NaCl load at the end of the thick ascending limb (TAL)(where about 85% of the filtered NaCl has been reabsorbed and 15% is left in the tubular fluid) is sensed by specialized tubular cells, the macula densa. These cells generate a signal to affect primarily afferent arteriolar tone, such that an inverse relationship is established between this tubular NaCl load and SNGFR of the same nephron. In doing so, the TGF helps to stabilize the NaCl load to further distal segments, which, under systemic neuro-humoral control, are the sites of fine regulation of NaCl and fluid balance.

Evidence for a role of adenosine and A₁R in the TGF mechanism derived from studies showing that the reduction in SNGFR or glomerular capillary pressure in response to increasing the NaCl concentration at the macula densa is inhibited by unselective adenosine receptor blockers like theophylline or PSPX [43–45], as well as by selective A₁R antagonists like DPCPX, KW-3902, CVT-124, or FK838 [44;46–50]. Moreover, gene-targeted mice lacking A₁R also lack this TGF response [51–53], and additional experiments in this mouse model demonstrated the role of A₁R - and thus TGF-mediated control of GFR in stabilizing the Na⁺ delivery to the distal tubule [53]. Under certain conditions, adenosine-induced vasodilation of the efferent arteriole may potentiate its vasoconstrictive effect on the afferent arteriole with regard to TGF-induced reduction of GFR [54;55]. Importantly, however, an intact TGF response requires local concentrations of adenosine to fluctuate in dependence of the NaCl concentration in the tubular fluid at the macula densa, indicating that adenosine serves as a mediator of TGF [47].

The concept that adenosine may be a mediator of TGF was first proposed by Osswald and colleagues in 1980 [43]. A current model is illustrated in Figure 2. Changes in luminal concentrations of Na⁺, K⁺ and Cl⁻ alter NaCl uptake by macula densa cells via the furosemide-sensitive Na-K-2Cl cotransporter in the luminal membrane. This triggers basolateral ATP release [56;57] as well as transport-dependent hydrolysis by basolateral Na⁺-K⁺-ATPase [58] of ATP to AMP, the latter also being released. Plasma membrane-bound ecto-nucleoside triphosphate diphosphohydrolase-1/cd39 converts ATP and ADP to AMP [59] and ecto-5′-nucleotidase/cd73 converts extracellular AMP to adenosine [47;60–62]. Part of the extracellular adenosine involved in the TGF response is generated independent of ecto-5′-nucleotidase and may reflect direct adenosine release from macula densa cells [61]. Extracellular adenosine binds to adenosine A₁ receptors at the surface of extraglomerular mesangial cells [63–66] and increases cytosolic Ca²⁺ concentrations [65]. Because of the intensive coupling by gap junctions between extraglomerular mesangial cells and smooth muscle cells of glomerular arterioles, intracellular Ca²⁺ transients could be transmitted to these target structures inducing afferent arteriolar constriction [67;68]. Thus, a model is envisioned in which both ATP and adenosine would be considered mediators of TGF since both are released or generated, respectively, in dependence of the NaCl concentration at the macula densa and both are part of a signalling cascade, in which adenosine via A₁R activation triggers the final effects of the TGF response, i.e., preglomerular vasoconstriction. Under physiological conditions, adenosine-induced afferent arteriolar constriction may primarily derive from tonic activation of the TGF mechanism (Table 1 shows the parallel response of adenosine-induced vasoconstriction and TGF activity to a number of different experimental conditions, drugs and hormones). The TGF induces oscillations (2–3/min) in SNGFR and thus in the NaCl load to the tubule including the medullary TAL, where NaCl is reabsorbed by furosemide-sensitive Na-K-2Cl cotransporter. Since the latter segment has to work under relative hypoxic conditions, the phases of reduced NaCl load may help to preserve the integrity of the renal medulla.

Fig. 2. Proposed mechanism of adenosine acting as a mediator of the tubuloglomerular feedback.

Left panel: Schematic drawing showing the macula densa (MD) segment located at the vascular pole with the afferent arteriole (AA) entering and the efferent arteriole (EA) leaving the glomerulus. EGM, extraglomerular mesangium; BM, glomerular basement membrane; EP, epithelial podocytes with foot processes (F); B, BS, Bowman’s capsule and space, respectively; PT, proximal tubule. (Adapted from Kriz, Nonnenmacher and Kaissling). Right panel: Schematic enlargement of area in red rectangle. Numbers in circles refer to the following sequence of events. (1), Increase in concentration-dependent uptake of Na⁺, K⁺ and Cl⁻ via the furosemide-sensitive Na⁺-K⁺-2Cl⁻-cotransporter (NKCC2); (2)(3), transport-related, intra- and/or extracellular generation of adenosine (ADO); (4), extracellular ADO activates adenosine A₁ receptors triggering an increase in cytosolic Ca²⁺ in extraglomerular mesangium cells (MC); (5), the intensive coupling between extraglomerular MC, granular cells containing renin, and smooth muscle cells of the afferent arteriole (VSMC) by gap junctions allows propagation of the increased Ca²⁺ signal resulting in afferent arteriolar vasoconstriction and inhibition of renin release. Factors such as nitric oxide, arachidonic acid breakdown products or angiotensin (Ang) II modulate the described cascade. NOS I, neuronal nitric oxide synthase; COX-2, cyclooxygenase-2 (adapted from [5;132]).

Adenosine inhibits renin secretion via A₁R activation

The renin-angiotensin-aldosterone system plays a central role in electrolyte homeostasis and in the regulation of blood pressure. The level of activity of this system is determined primarily by the rate at which the kidneys secrete renin into the circulation. Major stimuli that control renal renin release include the renal perfusion pressure, sympathetic nerve activation, and the NaCl concentration sensed by the macula densa in the early distal tubule. The role of adenosine in the control of renin secretion has recently been reviewed [5]. In 1970 Tagawa and Vander reported that adenosine infusion into the renal artery of salt-depleted dogs leads to a sustained inhibition of renal renin secretion into the venous blood [69]. This acute inhibitory effect of adenosine on renin release was subsequently confirmed in various species including humans [10]. The adenosine-induced inhibition of renin release is mediated by A₁R as specific agonists and antagonists for this receptor type can mimic or block the inhibitory actions of exogenous adenosine on renin secretion, respectively. Most notably, a single application of the A₁R antagonist FK-453 was found to increase plasma renin concentrations in humans [11] indicating a tonic inhibition of renin secretion by A₁R activation. In accordance, mice lacking A₁R have increased renal mRNA expression and content of renin [70] as well as greater plasma renin activity [52;71] compared with wild-type mice.

Jackson suggested that increases in intracellular cAMP in renin secreting cells cause efflux of cAMP, which activates the extracellular cAMP-adenosine pathway, i.e. cAMP is converted to adenosine in the extracellular space. The generated adenosine by acting on A₁R on the renin secreting cells then acts as a negative-feedback control on renin release [72]. Besides a role for adenosine derived from renin secreting cells, further studies indicated that high NaCl concentrations in the tubular lumen enhance adenosine generation in a macula densa-dependent way and that the adenosine generated inhibits renin release via activation of A₁R [73–76](Figure 2). In contrast to A₁R stimulation, activation of A₂R can lead to an increase of renin secretion [77;78]. Whether this stimulatory effect of adenosine A₂R activation is of physiological significance remains to be determined. Indirect evidence was provided by the fact that the unselective adenosine receptor antagonist caffeine reduced plasma renin concentration in mice lacking the A₁R [71].

Differential effects of adenosine on fluid and electrolyte transport

Adenosine affects fluid and electrolyte transport in the kidney indirectly through effects on renal blood flow, GFR and renin release, as well as by direct effects on the tubular and collecting duct system as reviewed recently [5].

Adenosine increases transport in proximal tubule via activation of A₁R

In vitro studies showed that endogenously formed adenosine stimulates proximal tubular reabsorption of fluid, Na⁺, HCO₃⁻, and phosphate by activation of A₁R [79–82]. Importantly, in vivo studies in rats and humans demonstrated that systemic application of selective A₁R antagonists (such as CVT-124, DPCPX, KW-3902, or FK453) elicits diuresis and natriuresis predominantly due to inhibition of reabsorption in the proximal tubule [11;83–87], indicating a tonic stimulation of proximal tubular reabsorption via A₁R activation (Figure 1). Mechanisms involved in A₁R-mediated increases in proximal tubular reabsorption were proposed to include increases of intracellular Ca²⁺ [88], reductions of intracellular cAMP levels [89], and activation of the Na⁺-H⁺-exchanger NHE3 [88], which is known to play a major role for Na⁺ reabsorption in proximal tubule [90].

In general, natriuretics that act proximal to the aldosterone-sensitive distal nephron stimulate K⁺ secretion in the latter segment and thus increase renal K⁺ excretion. The finding that A₁R antagonists do not increase renal K⁺ excretion suggests that A₁R may also affect transport mechanisms in the distal nephron. As a consequence, selective A₁R antagonists are being developed as eukaliuretic natriuretics in Na⁺ retaining states such as heart failure [91–93](see below). Similar to selective A₁R blockade, systemic application of the unselective adenosine receptor antagonists theophylline or caffeine is known to induce natriuretic and diuretic responses. Experiments in knockout mice demonstrated that intact A₁R are necessary for both caffeine- and theophylline-induced inhibition of renal Na⁺ and fluid reabsorption. These findings strongly suggest that A₁R blockade mediates the natriuresis and diuresis in response to these compounds [94].

Adenosine inhibits transport in medullary TAL via activation of A₁R

A tonic, adenosine-mediated stimulation of cortical proximal tubular reabsorption reduces the workload delivered to the medullary parts of the proximal tubule and the TAL, which pass through a kidney region in which blood flow and oxygen supply is much lower than in the cortex. In contrast to the proximal tubule, adenosine via activation of A₁R can inhibit NaCl reabsorption in medullary TAL [95–97]. Medullary TAL is a site of adenosine release and adenosine release in this segment is transport-dependent [98;99] and enhanced significantly during hypoxic conditions [98]. Furthermore, studies using pharmacological inhibition [42] or gene-knockout [53] are consistent with a tonic activation of Na⁺ reabsorption in medullary TAL by A₁R activation (Figure 1). This is relevant since the renal medulla has a low partial oxygen pressure [100] and the described inhibitory effects of adenosine on transport work together with its A₂R-mediated renal medullary vasodilation (see above) may serve to maintain metabolic balance in the renal medulla.

Effects of adenosine on transport in distal convolution and cortical collecting duct

As mentioned above, the natriuretic but eukaliuretic effect of A₁R inhibitors suggests an additional site of action in the aldosterone-sensitive distal nephron, but the exact site of action and the involved mechanisms are unclear. There is evidence in the distal nephron that adenosine released during cell swelling may bind to A₁R and activates Cl⁻ channels in the apical membrane to restore cell volume [101;102]. One may speculate that Na⁺ transport-induced cell swelling in these segments activates A₁R to co-activate Cl⁻ and K⁺ secretion. In addition, A₁R activation can stimulate Mg²⁺ and Ca²⁺ uptake in the cortical collecting duct in vitro [103–105], which is mediated by members of the transient receptor potential cation channel family, namely TRPM6 [106] and TRPV5 [107], respectively. Whether this interaction is of clinical relevance (e.g. during pharmacological inhibition of A₁R) is unclear.

Adenosine can inhibit effects of vasopressin in inner medullary collecting duct

Extracellular adenosine may serve to feedback inhibit vasopressin-induced cAMP-mediated stimulation of Na⁺ and fluid reabsorption in the inner medullary collecting duct [108;109] and decrease vasopressin-stimulated electrogenic Cl⁻ secretion through activation of A₁R [110]. Jackson and colleagues provided evidence for the extracellular cAMP-adenosine pathway as an important source of extracellular adenosine under these conditions [111]. Again, the clinical relevance of this interaction is unclear.

To summarize, the in vitro and in vivo studies on kidney fluid and electrolyte transport indicate that under physiological conditions, endogenous adenosine by activation of A₁R stimulates NaCl reabsorption in proximal tubule, which is a tubular segment with relatively high basal oxygen supply. In contrast, adenosine inhibits NaCl reabsorption in medullary TAL and IMCD, i.e., nephron segments with relatively low oxygen delivery (Figure 1). In accordance, interstitial infusion of adenosine in rat kidney decreased partial pressure of O₂ in the cortex but increased it in the medulla, consistent with an important regulatory and protective role of adenosine in renal medullary O₂ balance [112].

Potential role of A₁R antagonists in the treatment of heart failure

As outlined above, effective therapy of cardiorenal dysfunction is very difficult, indicating the need to better understand renal dysfunction in this setting and to develop new therapeutic strategies. The mainstay of therapy for the management of fluid overload in both systemic volume overload and acute pulmonary oedema decompensated heart failure includes conventional diuretics such as loop and thiazide diuretics. However, enhanced solute excretion is often achieved at the expense of glomerular filtration. The latter may arise as a consequence of volume depletion and in the case of loop diuretics also by immediate direct preload reduction due to increased venous compliance, which by inducing systemic changes, such as activation of the sympathetic nervous system, are likely to mediate the adverse effects on renal haemodynamics [113;114].

A three-fold increase in circulating levels of adenosine has been reported in patients with chronic heart failure compared with controls (~200 vs. 60 nM)[115]. Studies in dogs with volume overload heart failure showed that myocardial adenosine release is elevated three-fold above normal dogs [116]. Whether elevated adenosine release in the failing myocardium increases circulating adenosine to an extent, which affects afferent arteriolar tone and thus GFR, is unclear. However, heart failure-associated activation of the renin-angiotensin-system and/or impairment of the local formation of NO (endothelial dysfunction) or prostaglandins can sensitize the renal vasculature to A₁R-mediated vasoconstrictor and GFR-lowering effects of adenosine. In addition, impaired renal perfusion and hypoxia enhance adenosine formation within the kidney [117]. As a consequence, the normally homeostatic adenosine system may become maladaptive and overshoots with regard to the down-regulation of GFR (Figure 1). On the other hand, the renin-angiotensin system is pharmacologically blocked in most patients with heart failure, which may attenuate a sensitizing influence of angiotensin II (see below). Fluid retention is further potentiated by stimulation of NaCl and fluid reabsorption in the proximal tubule, a mechanism also mediated by A₁R activation (see above). Based on this concept, pharmacological blockade of A₁R could improve kidney function and fluid retention in heart failure. Since adenosine, by activation of A₁R, mediates TGF, the expected TGF-induced reduction in GFR in response to inhibition of proximal reabsorption by A₁R antagonists should be blunted. In accordance, a study in rats showed that A₁R antagonism with KW-3902 prevented the GFR-lowering effect of the proximal diuretic benzolamide, a carbonic anhydrase inhibitor [84].

Animal studies

In a pig model of systolic dysfunction and induction of chronic heart failure by pacer-induced tachycardia, Lucas et al. observed that acute application of the selective A₁R antagonist BG9719 lowered pulmonary capillary wedge pressure and pulmonary vascular resistance in the absence of significant changes in mean arterial blood pressure, heart rate or cardiac output compared with vehicle controls [118]. Whereas basal creatinine clearance (Ccr) was not affected in this heart failure model, the compound increased Ccr and urinary flow rate and sodium excretion compared with vehicle control. Similar effects were described by Jackson et al. in aged, lean SHHF/Mcc-fa(cp) rats, a rodent model of hypertensive dilated cardiomyopathy in response to the same compound [114]. The rats were pretreated for 72 h before experiments with the loop diuretic furosemide to mimic the clinical setting of chronic diuretic therapy and were given 1% NaCl as drinking water to reduce dehydration/sodium depletion. Under these conditions, acute application of BG-9719 increased GFR and urinary flow rate and sodium excretion. In comparison, acute application of furosemide decreased renal blood flow and GFR and increased fractional potassium excretion. Neither drug altered afterload; however, furosemide, but not BG9719, decreased preload. Neither drug altered systolic function (+dP/dt(max)); however, furosemide, but not BG9719, attenuated diastolic function (decreased −dP/dt(max), increased tau). The authors concluded that in the setting of left ventricular dysfunction, chronic salt loading and prior loop diuretic treatment, selective A₁R antagonists are effective diuretic/natriuretic agents with a favorable renal haemodynamic/cardiac performance profile [114].

Human studies

Gottlieb et al. determined the acute effects of furosemide and BG9719 on renal function in 12 patients with New York Heart Association (NYHA) functional class II to IV [91]. Mean GFR in the placebo group was 84 ml/min/1.73m². Whereas furosemide lowered GFR to 63 ml/min/1.73m², the latter was not affected by BG9719 (82 ml/min/1.73m²). Sodium excretion increased from 8 mEq following placebo administration to 37 mEq following BG9719 administration. In the six patients in whom it was measured, sodium excretion was 104 mEq following furosemide administration. The data showed that natriuresis is effectively induced by both furosemide and the A₁R antagonist in patients with chronic heart failure, but only furosemide decreased GFR. In a follow-up study, Gottlieb et al. studied the effects of BG9719 in 63 patients catergorized as NYHA functional class II, III, or IV, which despite receiving standard therapy, including furosemide (at least 80 mg daily) and ACE inhibitors, remained oedematous [93]. This was a randomised, double-blind, ascending-dose, cross-over study, evaluating 3 doses of BG9719. Patients received 7-h infusions of placebo or 1 of 3 doses of BG9719 to yield serum concentrations of 0.1, 0.75, or 2.5 ug/ml. It was observed that BG9719 tripled urine output without inducing a decrease in GFR or potassium loss. In these same patients, furosemide increased urine output 8-fold and increased potassium excretion while reducing GFR. Importantly, when BG9719 was given in addition to furosemide, urinary sodium excretion additionally increased and GFR was not altered compared with placebo. These results indicate that A₁R antagonism might preserve renal function while simultaneously promoting natriuresis during acute treatment of heart failure [93]. Similar results were more recently reported in a randomized, double-blind, placebo-controlled, two-way crossover study by Dittrich et al. using the A₁R antagonist KW-3902 in 32 outpatients with congestive heart failure and impaired renal function (median GFR of 50 mL/min) [119]. Baseline GFR and renal plasma flow were assessed 3 hours before and over 8 hours following the intravenous administration of furosemide along with KW-3902 (30mg) or placebo. After a washout period of 3 to 8 days (median 6 days), the crossover portion of the study was performed. KW-3902 increased GFR by 32% and renal plasma flow by 48% compared with placebo. Notably, subjects who initially received KW-3902 had a statistically significant 10 mL/min increase in GFR when they returned for the crossover phase compared with the previous baseline. Thus, the increase in GFR persisted for several days longer than predicted by pharmacokinetics. These findings suggest that KW-3902 reset the complex network that determines kidney function in these patients and provided first evidence for potential longer-term benefits of using A₁R antagonists [119]. Along these lines, a recent randomised, double-blind, placebo-controlled study by Greenberg et al. assessed the pharmacokinetics and clinical effects of the selective A₁R antagonist, BG9928, given orally for 10 days to 50 patients with heart failure and systolic dysfunction who were receiving standard therapy [120]. BG9928 over the dose range of 3 to 225 mg/day was well tolerated and increased sodium excretion compared with placebo without causing kaliuresis or reducing renal function (assessed from adjusted creatinine clearance). These effects were maintained over the 10 day period. Moreover, BG9928 at doses of 15, 75, or 225 mg reduced body weight at the end of the study compared with placebo [120]. Natriuresis associated with preservation or improvement of GFR in response to A₁R antagonism and its combination with furosemide can be of clinical relevance because in patients with heart failure, worsening renal function is associated with a prolonged duration of hospitalisation, higher in-hospital costs, and an increased risk of mortality [121;122].

Perspectives

The above described acute and short-term human studies employing A₁R antagonists in patients with heart failure yielded promising results. However, considering the above outlined complexity of adenosine actions, further clinical trials with more patients are needed to show that A₁R antagonism induces no adverse clinical consequences. Whereas the presented animal and human studies were acute or short-term treatments, it remains to be determined whether longer term application of A₁R antagonism has beneficial effects.

If, as the classical teaching goes, angiotensin II and adenosine require each other to affect afferent arteriolar resistance (see above), then patients will respond quite differently to A₁R blockade if they are already on ACE inhibitors or angiotensin AT1 receptor blockers. Nearly all patients with chronic heart failure are on the maximum tolerated doses of these compounds already. The only exceptions are those patients who are precluded based on renal failure or hyperkalaemia. The fact that patients pretreated with these compounds respond to A₁R blockade with greater values of GFR relative to loop diuretic treatment [93] indicates that the mutual dependency and cooperation of adenosine and angiotensin II in producing afferent arteriolar constriction is not absolute, in the clinical setting.

Some concerns relate to the fact that, like loop diuretics, acute and chronic application of A₁R antagonists can increase renin release (see above). A transient increase in plasma renin activity was also observed in response to the A₁R antagonist BG9719 in a heart failure model in the pig [118]. A chronic activation of the renin-angiotensin-aldosterone system could be of relevance due to its prominent role in cardiac remodelling and the progression of heart failure [123]. However, there seems to be no immediate effect of the renin released acutely in response to A₁R antagonists on blood pressure, which should be very sensitive to meaningful sudden increases in renin. On the other hand, if A₁R blockade leads to later elevation of renin, that could be a normal physiologic response to a prior reduction in total body salt. However, if A₁R blockade fails to increase GFR or reduce total body salt and renin goes up anyway, that rise in renin would be “pathologic”. When more data on A₁R-renin interactions become available from heart failure trials, they will require interpretation with the foregoing nuances in mind.

Are there potential dangers to the kidney of A₁R blockade? There is potential for A₁R blockers to worsen reperfusion injury after acute renal ischaemia even though these blockers could lessen the prior probability of developing ischaemia to begin with. The worsening of ischaemia-reperfusion injury during genetic or pharmacological blockade of A₁R is reflected by a heightened immune response to ischaemia-reperfusion inducing inflammation and necrosis [124]. The worsening may also relate to the fact that antagonism of A₁R can impair TGF-induced oscillations and enhance reabsorption in the medullary TAL (see above), which both could affect the integrity of the semi-hypoxic medulla, although A₂R-mediated medullary vasodilation should still be intact. Anyway, to attenuate medullary hypoxia, A₁R antagonists may be combined with a loop diuretic, which will inhibit transport in medullary TAL (Figure 1). This combination therapy, as shown above, inhibits reabsorption in different nephron segments and induces additive diuretic effects while preserving GFR in heart failure patients (see above). The problem of exacerbating reperfusion injury apparently does not extend to toxic acute renal failure, against which A₁R blockers reportedly afford prevention (for review see reference [5].)

Paradoxically, A₁R agonists have also been under development as cardioprotective therapy in heart failure. Potential beneficial effects could include inhibition of the neuro-humoral axes involved in myocardial hypertrophy and remodelling by activation of cardial A₁R [125]. In fact, the selective A₁R agonist, 2-chloroadenosinesoine, attenuated cardiac hypertrophy, pulmonary oedema, and systolic dysfunction in a murine left ventricular pressure-overload model [126], and activation of A₁R can contribute to ischaemic pre-conditioning and confers a resistance to further ischaemic insult [127]. However, the prediction of the net effect of A₁R agonism on the neuron-humoral axes does not appear as straight forward as expected, since acute application of the A₁R agonist, SDZ-WAG 994, in heart failure patients actually increased plasma norepinephrine concentrations [128]. Notably, in contrast to A₁R antagonists, no clinical studies have been published on the use of A₁R agonists in heart failure within the last 3 years. Moreover, because the negative inotropic effects of A₁R agonists are enhanced in failing cardiomyocytes [129], it is possible that blockade of A₁R could actually have beneficial effects on heart performance in the setting of left ventricular dysfunction.

Further studies in more patients are needed to more fully establish the acute effects of A₁R antagonists on renal and cardiac function in patients with heart failure and cardiorenal failure. It will also be important to further assess whether the salutary effects observed in single-dose trials with A₁R antagonists are durable. A₁R blockade is a unique opportunity to lower renal vascular resistance independent of all other organs. The inability to do this is a major drawback of all vasodilator heart failure therapies that have been tried so far. This puts A₁R blockade in a category all by itself. The importance of this is evident from the Guyton model, which predicts that dilating any organ other than the kidney will merely lead to salt retention and, as effective arterial filling declines, will actually reduce kidney function [130;131].