The role of venous capacity in fluid retention with endothelin A antagonism: Mathematical modelling of the RADAR...

Endothelin-1 (ET-1) receptor A (ETA) antagonists reduce proteinuria and prevent renal outcomes in chronic kidney disease (CKD) patients, but their utility has been limited because of associated fluid retention, resulting in increased heart failure risk. Understanding the mechanisms responsible for fluid retention could result in solutions that preserve renoprotective effects while mitigating fluid retention, but the complexity of the endothelin system has made identification of the underlying mechanisms challenging.

We utilized a previously developed mathematical model of ET-1 kinetics, ETA receptor antagonism, kidney function, haemodynamics, and sodium and water homeostasis to evaluate hypotheses for mechanisms of fluid retention with ETA antagonism. To do this, we simulated the RADAR clinical trial of atrasentan in patients with type 2 diabetes and CKD and evaluated the ability of the model to predict the observed decreases in haematocrit, urine albumin creatinine ratio (UACR), mean arterial pressure (MAP), and estimated glomerular filtration rate (eGFR).

An effect of ETA antagonism on venodilation and increased venous capacitance was found to be the critical mechanism necessary to reproduce the simultaneous decrease in both MAP and haematocrit observed in RADAR.

These findings indicate that fluid retention with ETA antagonism may not be caused by a direct antidiuretic effect within the kidney but is instead be an adaptive response to venodilation and increased venous capacity, which acutely tends to reduce cardiac filling pressure and cardiac output, and that fluid retention occurs in an attempt to maintain cardiac filling and cardiac output.