Increased urinary excretion of aquaporin 2 in patients with liver cirrhosis

Abstract

Background and aim: Water retention is a major clinical problem in patients with liver cirrhosis. Recent research suggests that renal aquaporins may be pathophysiologically involved in this condition. The aim of the present cross sectional study of patients with liver cirrhosis was to determine if 24 hour urinary excretion of renal aquaporin 2 (AQP2) differed from that of healthy control subjects and if such excretion was related to the severity of liver disease and to the patient's water balance.

Results: Twenty four hour urinary excretion of AQP2 and free water clearance were measured in 33 stable cirrhosis patients on usual medication and in eight healthy subjects. AQP2 excretion, quantitated by immunoblotting, was eight times higher in cirrhosis patients than in controls (0.167 (0.270) U/day v 0.021 (0.017); p<0.05). Stratification according to clinical manifestations (Child- Pugh classes) revealed that it increased with the clinical severity of cirrhosis (class A 0.04 (0.04); class B 0.09 (0.16); class C 0.31 (0.35); p<0.05) but was not related to liver function, as measured by galactose elimination capacity. Excretion correlated inversely with free water clearance (rho=-0.57, p<0.01). It was higher in patients with oesophagogastric varices but not in those with ascites. Plasma vasopressin concentrations were not related to AQP2 excretion and there was no relation to dose or type of diuretic treatment.

Conclusions: Urinary AQP2 excretion was increased in patients with cirrhosis. Moreover, urinary AQP2 excretion increased with severity of cirrhosis in parallel with impairment of free water clearance. This suggests a functional association between increased AQP2 excretion and increased renal reabsorption of water in cirrhosis.

Figure 1

Immunoblots and standard curve for determination of aquaporin 2 (AQP2) expression levels in urine. (A) Immunoblot of membrane fractions of inner medulla prepared from rat and human kidneys. The immunoblot was exposed to affinity purified anti-AQP2, diluted 1:2000, revealing 29 and 35–50 kDa bands for both rat and human kidneys. (B) Preabsorption control performed with anti-AQP2 previously incubated with immunising peptide. (C) Dot blot of serial peptide dilution, which was exposed to affinity purified anti- AQP2, diluted 1:800. (D) Standard curve obtained from serial peptide dilution including the best logarithmic equation of the relationship between AQP2 and signal intensity.

Figure 2

Twenty four hour urinary excretion of aquaporin 2 (AQP2) in control subjects and in patients with cirrhosis stratified according to Child-Pugh class (Child A, Child B, and Child C).

Figure 3

Relation between 24 hour spontaneous free water clearance (ml/min) and 24 hour urinary excretion of aquaporin 2 (U/day) in cirrhosis patients (rho=−0.57, p<0.01). Both were calculated for the same 24 hour period, and the abscissa is given as natural log.

Figure 4

Daily dose of (A) furosemide and (B) aldosterone antagonist and 24 hour urinary excretion of aquaporin 2 (AQP2) in cirrhosis patients. No relation was found between dose or type of diuretic and AQP2 excretion.