Natriuretic effect of an adenosine-1 receptor antagonist in cirrhotic patients with ascites

"Adenosine an old player with new possibilities in kidney diseases": Preclinical evidences and clinical perspectives

Renal injury might originate from multiple factors like ischemia reperfusion (I/R), drug toxicity, cystic fibrosis, radio contrast agent etc. The four adenosine receptor subtypes have been identified and found to show diverse physiological and pathological roles in kidney diseases. The activation of A₁ adenosine receptor (A₁) protects against acute kidney injury by improving renal hemodynamic alterations, decreasing tubular necrosis and its inhibition might facilitate removal of toxin or drug metabolite in chronic kidney disease models. Furthermore, recent findings revealed that A_2A receptor subtype activation regulates macrophage phenotype in experimental models of nephritis. Interestingly the emerging role of adenosine kinase inhibitors in kidney diseases has been discussed which act by increasing adenosine availability at target sites and thereby promote A_2A receptor stimulation. In addition, the least explored adenosine receptor subtype A₃ inhibition was observed to exert anti- oxidant, immunosuppressive and anti-fibrotic effects, but more studies are required to confirm its benefits in other renal injury models. The clinical studies targeting A₁ receptor in patients with pre-existing kidney disease have yielded disappointing results, perhaps owing to the origin of unexpected neurological complications during the course of trial. Importantly, conducting well designed clinical trials and testing adenosine modulators with lesser brain penetrability could clear the way for clinical approval of these agents for patients with renal functional impairments.

Adenosine, type 1 receptors: role in proximal tubule Na+ reabsorption

Adenosine type 1 receptor (A1 -AR) antagonists induce diuresis and natriuresis in experimental animals and humans. Much of this effect is due to inhibition of A1 -ARs in the proximal tubule, which is responsible for 60-70% of the reabsorption of filtered Na(+) and fluid. Intratubular application of receptor antagonists indicates that A1 -AR mediates a portion of Na(+) uptake in PT and PT cells, via multiple transport systems, including Na(+) /H(+) exchanger-3 (NHE3), Na(+) /PO4(-) co-transporter and Na(+) -dependent glucose transporter, SGLT. Renal microperfusion and recollection studies have shown that fluid reabsorption is reduced by A1 -AR antagonists and is lower in A1 -AR KO mice, compared to WT mice. Absolute proximal reabsorption (APR) measured by free-flow micropuncture is equivocal, with studies that show either lower APR or similar APR in A1 -AR KO mice, compared to WT mice. Inhibition of A1 -ARs lowers elevated blood pressure in models of salt-sensitive hypertension, partially due to their effects in the proximal tubule.

Renal effects of the novel selective adenosine A1 receptor blocker SLV329 in experimental liver cirrhosis in rats

Liver cirrhosis is often complicated by an impaired renal excretion of water and sodium. Diuretics tend to further deteriorate renal function. It is unknown whether chronic selective adenosine A(1) receptor blockade, via inhibition of the hepatorenal reflex and the tubuloglomerular feedback, might exert diuretic and natriuretic effects without a reduction of the glomerular filtration rate. In healthy animals intravenous treatment with the novel A(1) receptor antagonist SLV329 resulted in a strong dose-dependent diuretic (up to 3.4-fold) and natriuretic (up to 13.5-fold) effect without affecting creatinine clearance. Male Wistar rats with thioacetamide-induced liver cirrhosis received SLV329, vehicle or furosemide for 12 weeks. The creatinine clearance of cirrhotic animals decreased significantly (-36.5%, p<0.05), especially in those receiving furosemide (-41.9%, p<0.01). SLV329 was able to prevent this decline of creatinine clearance. Mortality was significantly lower in cirrhotic animals treated with SLV329 in comparison to animals treated with furosemide (17% vs. 54%, p<0.05). SLV329 did not relevantly influence the degree of liver fibrosis, kidney histology or expression of hepatic or renal adenosine receptors. In conclusion, chronic treatment with SLV329 prevented the decrease of creatinine clearance in a rat model of liver cirrhosis. Further studies will have to establish whether adenosine A(1) receptor antagonists are clinically beneficial at different stages of liver cirrhosis.

Renal and circulatory dysfunction in cirrhosis: current management and future perspectives

Chronic liver diseases are amongst the top leading causes of death in Europe as well as in other areas of the world. Chronic liver diseases are characterized by unrelenting progression of liver inflammation and fibrosis over a prolonged period of time, usually more than 20 years, which may eventually lead to cirrhosis. Advanced cirrhosis leads to a complex syndrome of chronic liver failure which involves many different organs besides the liver, including the brain, heart and systemic circulation, adrenal glands, lungs, and kidneys. The high morbidity and mortality secondary to chronic liver failure is due to complications related to the dysfunction of these organs, either alone or, more frequently, in combination. Understanding the mechanisms leading to organ dysfunction is crucial to the development of strategies for treatment and prevention of complications of cirrhosis. This article reviews our current knowledge, as well as future perspectives, on the management of circulatory and renal dysfunction in chronic liver failure.

Afferent mechanisms of sodium retention in cirrhosis and hepatorenal syndrome

Cirrhosis induces extra-cellular fluid volume expansion, which when the disease is advanced can be severe and poorly responsive to therapy. Prevention and/or effective therapy for cirrhotic edema requires understanding the stimulus that initiates and maintains sodium retention. Despite much study, this stimulus remains unknown. Work over the last several years has shown that signals originating in the liver can influence a variety of systemic functions, including extra-cellular fluid volume control. We review work on the afferent mechanisms triggering sodium retention in cirrhosis and suggest that the data are most consistent with the existence of a sensor in the hepatic circulation that contributes to normal extra-cellular fluid volume control (that is, a 'volume' sensor) and that in cirrhosis, the sensor is pathologically activated by the hepatic circulatory abnormalities caused by the disease. Detailed analysis of the hepatic circulation in normal conditions and cirrhosis is needed.

Treatment of hepatorenal syndrome

Hepatorenal syndrome (HRS) is a type of renal failure that occurs in patients with advanced cirrhosis. It is a result of splanchnic arterial vasodilation, renal vasoconstriction, reduced effective arterial volume, and potentially reduced cardiac output. Often, HRS is a fatal complication, and the only definitive treatment currently available is liver or liver-kidney transplantation. A number of other treatment modalities have been tested for the management of HRS, but most evidence is derived from small noncontrolled studies. The primary role of these treatment options is to provide a bridge to liver transplantation. Treatment may also provide acute reversal of renal failure and some symptomatic relief, but relapse is a common occurrence. The best therapeutic options appear to be those that reverse portal hypertension, splanchnic vasodilation, and/or renal vasoconstriction. Vasopressin analogs, particularly terlipressin, have emerged as the preferred pharmacologic therapies for management of HRS. Albumin is an appropriate adjunctive therapy to terlipressin and can be used to prevent HRS in patients with spontaneous bacterial peritonitis. Transjugular intrahepatic portosystemic shunt may provide a surgical option for qualified patients with HRS. Octreotide is ineffective as monotherapy but may be used as adjunctive therapy to other vasoactive agents. Dopamine agonists, endothelin antagonists, natriuretic peptides, and nitric oxide synthase inhibitors have not been effective for reversing HRS. Artificial hepatic support therapies have demonstrated the ability to improve laboratory abnormalities in patients with HRS, but their effect on clinical outcomes has not been determined. The role of renal replacement therapies or the newer artificial hepatic support therapies need further evaluation before they can be routinely recommended.

Intrahepatic adenosine-mediated activation of hepatorenal reflex is via A1 receptors in rats

Previous studies have shown that intrahepatic adenosine is involved in activation of the hepatorenal reflex that regulates renal sodium and water excretion. The present study aims to determine which subtype of adenosine receptors is implicated in the process. Mean arterial pressure, portal venous pressure and flow, and renal arterial flow were monitored in pentobarbital anesthetized rats. Urine was collected from the bladder. Intraportal administration of 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), a selective adenosine A1 receptor antagonist, increased urine flow by 24%, 89%, and 143% at the dose of 0.01, 0.03, and 0.1 mg x kg(-1), respectively; in contrast, DPCPX, when administered intravenously at the same doses, only increased urine flow by 0%, 18%, and 36%. The increases in urine flow induced by intraportal administration of DPCPX were abolished in rats with liver denervation. Intrahepatic infusion of adenosine significantly decreased urine flow and this response was abolished by intraportal administration of DPCPX. Neither intraportal nor intravenous administration of 3,7-dimethyl-1-propargylxanthine, a selective adenosine A2 receptor antagonist, showed significant influence on urine flow. Systemic arterial pressure, renal blood flow and glomerular filtration rate were unaltered by the administration of any of the drugs. In conclusion, intrahepatic adenosine A1 receptors are responsible for the adenosine-mediated hepatorenal reflex that regulates renal water and sodium excretion.

Cyclic AMP-phosphodiesterases inhibitor improves sodium excretion in rats with cirrhosis and ascites

BACKGROUND

The mechanisms responsible for renal dysfunction and sodium retention in cirrhosis remain unclear. Cyclic AMP (cAMP) regulates sodium reabsorption in the proximal nephron. This study investigates the role of cAMP metabolism in renal dysfunction in cirrhosis.

METHODS

Renal function was studied by the clearance technique in anesthetized control and cirrhotic rats with or without ascites. cAMP phosphodiesterase (PDE) activity was measured in the renal cortex in vitro. Moroever, the effects on renal function of the intravenous administration of cAMP and rolipram, a powerful and specific cAMP-PDE4 inhibitor, were evaluated.

RESULTS

In control and in non-ascitic cirrhotic rats, cAMP administration significantly increased sodium and phosphate excretions, but did not change these excretions in cirrhotic rats with ascites. cAMP-PDE activity was higher in ascitic than in control rats (P < 0.05). Rolipram infusion significantly increased sodium and phosphate excretion only in cirrhotic rats with ascites.

CONCLUSION

These results suggest that increased renal cAMP-PDE activity is responsible for resistance to the natriuretic effects of cAMP in cirrhosis and plays a role in the development of ascites.

Adenosine type 1 receptor antagonists in fluid retaining disorders

Adenosine is a vasoactive hormone whose action is mediated through at least four receptors. The most prevalent receptors are type 1, which promote vasoconstriction, and type 2, comprised of 2 subtypes (a,b) that promote vasodilation. In the kidney, type 1 receptors located on preglomerular vessels and in the tubule are involved in the regulation of glomerular filtration. Whole body fluid balance is strongly dependent on the ability of the kidney to maintain stable glomerular filtration. Several antagonists to adenosine type 1 receptors have been developed. These agents generate excess fluid (diuresis) and sodium (natriuresis) excretion in control animals and animal models of fluid retention, as well as in normal and oedematous humans. In both animals and humans, these effects are generally achieved without major changes in glomerular filtration. Animal studies have confirmed the location of adenosine type 1 receptors in relevant tissue sites in the kidney. More highly selective antagonists for adenosine type 1 receptors are regularly developed, improving their use in fluid retaining disorders. Clinical trials with these agents have commenced for the treatment of hypertension, renal failure and congestive heart failure, all disorders that include varying levels of fluid retention. The clinical trial results have been mixed. The early results with congestive heart failure suggest great promise for these agents, whereas trials in hypertension and renal failure have been equivocal.

Hepatorenal syndrome. Definition, pathophysiology, and intervention

Hepatorenal syndrome is a well characterized entity in which vasodilation of splanchnic vessels and intense constriction of the renal cortical vasculature occur in concert. The condition is often fatal unless orthotopic liver transplantation (OLT) is performed. Many extracorporeal blood purification techniques exist which can be offered to patients awaiting OLT. Continuous hemofiltration, with or without other modalities such as therapeutic plasma exchange and hemoperfusion, may be helpful in improving the level of consciousness of these patients. Unfortunately, mortality and hepatic regeneration do not appear to be affected by such interventions. The development of a hybrid bioartifical liver support system and pharmacologic manipulation of the hemodynamic perturbations that occur in HRS provide particularly appealing prospects as a means of providing a bridge to liver transplantation in the future.

Pharmacology of diuretics

The diuretics in our therapeutic armamentarium have predictable effects based on their nephron sites of action. All but spironolactone must reach the lumen or urinary side of the nephron to exert their effects. Thus, in settings of decreased renal function, doses must be increased to deliver more diuretic into the urine. In other edematous disorders, such as congestive heart failure (CHF) and cirrhosis, adequate amounts of diuretic reach the site of action if renal function is satisfactory. Diminished response in these conditions is caused by a decrease in the sensitivity of the nephron to the diuretic, the mechanism of which is unknown. Rather than using large single doses of diuretic in CHF and cirrhosis, multiple doses and/or combinations of diuretics should be used. Therefore, thiazide diuretics coupled with loop diuretics are most logical because they affect different nephron sites and the thiazide counteracts distal nephron hypertrophy that may occur with loop diuretics alone. Ample studies have shown that such combinations can result in a truly synergistic response. Using pharmacokinetics and pharmacodynamics of diuretics, we can design therapeutic regimens in which satisfactory control of fluid and electrolyte homeostasis can be achieved in the vast majority of patients.