The role of ETB clearing receptors has been studied in

The role of ETB clearing receptors has been studied in detail in endothelial cell–specific ETB knock-out mice. In these animals, clearance of an intravenous dna-pkcs of labeled ET-1 was reduced significantly compared with wild-type controls. Importantly, functioning ETB were retained on all other cell types such as epithelial cells.43, 48 Dynamic imaging of rats using positron emission tomography showed that after infusion of 18F ET-1, there was remarkably fast clearance of the radioligand from the circulation (plasma-half life (t1/2) = 0.43 min), with high levels of radioligand accumulated in the kidney, liver, and lung, which rapidly reached equilibrium, and this was maintained for at least 20 minutes. Infusion of BQ788 before injecting 18F ET-1 reduced the amount of radioligand visualized in the lung and kidney by 85% and 55%, respectively, consistent with blockade of ETB. However, infusion of BQ788 after 18F ET-1 did not displace the bound ligand. This finding is consistent with the internalization of the ligand-receptor complex to the lysosome where ET-1 is thought to be degraded, similar to other peptides, by cathepsin A. In support, cathepsin A knock-out mice showed reduced ET-1 degradation and significantly increased arterial blood pressure. Inactivation of ET-1 by kidney, liver, and lungs may be particularly important for ET-1 because it is structurally unusual compared with other vasoactive peptides, possessing two disulfide bridges that confer resistance to degradation by nonspecific peptidases. ET-1 promotes diuresis and natriuresis via ETB located on epithelial cells throughout the tubular epithelium, particularly the inner medullary collecting duct cells. Deletion of ETB, but not ETA, leads to salt-sensitive hypertension. In agreement, the effects of three doses of BQ-123 (0.1, 0.2, and 0.3 mg/kg) on renal hemodynamics, tubular function, and vasoactive hormones were measured in volunteers in a randomized, placebo-controlled, double-blind, dose-response study. The main effect was a dose-dependent increase in renal sodium excretion despite stimulation of the renin-angiotensin system as evidenced by an increase in angiotensin II levels, whereas there was little effect on atrial and brain natriuretic peptides or vasopressin. Goddard et al elegantly showed that ETA antagonism by BQ123 and angiotensin-converting enzyme (ACE) inhibition using enalapril were synergistic in reducing mean arterial pressure in volunteers. However, BQ-123 increased renal blood flow, increased urinary sodium excretion, and reduced renal vascular resistance only during ACE inhibition. These effects were abolished by ETB blockade using BQ788 and nitric oxide synthase inhibition, whereas cyclooxygenase inhibition had no effect. These results showed that synergism between ETA antagonism and ACE inhibition occurs via an ETB-mediated, nitric oxide–dependent, cyclooxygenase-independent mechanism. In patients with chronic kidney disease, TAK-044 beneficially reduced the mean arterial and systemic vascular resistance index and tended to increase renal plasma flow. TAK-044 had no effect on sodium or lithium clearance, or on the fractional excretion of sodium and lithium. Combining the results from a number of different studies has led to the proposal that antagonism of ETB may be undesirable in conditions such as chronic renal failure, and therefore ETA-selective antagonists might be superior to mixed ETA/ETB antagonists. This hypothesis was tested experimentally by comparing the action of BQ123 or BQ788 alone or in combination in hypertensive patients with chronic renal failure. Blocking the ETA alone significantly reduced blood pressure in these patients. The magnitude of change was significantly higher than when ETB also was blocked by BQ788. BQ788 alone caused the expected systemic and renal vasoconstriction, supporting the concept that ETB maintain tonic renal vasodilatation in patients. BQ-123 infused alone increased renal blood flow and renal vascular resistance and reduced proteinuria, consistent with a renoprotective action. This effect was lost when ETB were blocked by infusing both BQ788 and BQ123. There was no change in sodium excretion but this may have been the result of a comparatively small number of subjects.