br A major physiological action of ET
A major physiological action of ET-1 is to function as one of the most powerful vasoconstrictors of human blood vessels. As such, ET-1 plays a major role in regulating vascular function in all organ systems, including the kidney (Fig. 1). As in other vessels, ET-1 is thought to be released from endothelial cells lining intrarenal vessels throughout the cortex and medulla. In the human vasculature, including that of the kidney, under normal physiological conditions release of ET-1 from endothelial cells causes sustained vasoconstriction via ETA that predominate on the underlying smooth muscle. Under pathophysiological conditions in which ET-1 is overproduced, vascular cells also may undergo proliferation and contribute to vascular remodeling and the development of renal fibrosis. Figure 1 shows the ratio of the densities of the two receptor subtypes measured by radioligand binding assays with the ETA subtype representing greater than 90% of ET receptors in the smooth muscle layer of all renal vessels studied. This includes the large conduit vessels, the arcuate arteries, and veins at the corticomedullary junction, as well as small intrarenal vessels such as the afferent and efferent vessels of the glomerulus.23, 24, 25, 26, 27 In a detailed study using human isolated main stem renal LX7101 HCL and veins in organ baths, ET-1 was, as expected, a potent vasoconstrictor, with the concentration producing half-maximal response (EC50) values of 4 and 1 nmol/L, respectively. In renal artery, ET-3 and the ETB agonist sarafotoxin 6c showed little or no activity up to 300 nmol/L. In veins, some but not all samples responded to ET-3, but this peptide was much less potent than ET-1, consistent with an ETA- mediated action. Interestingly, S6c concentration-related contractions were found in some individuals and, although more potent than ET-1, the maximum response was 30% to 60% of that obtained with ET-1. Crucially, however, the ETA antagonist BQ123 fully reversed the ET-1 contractions in both arteries and veins without reducing the maximum agonist response, consistent with a competitive antagonist. Therefore, in renal vessels the endogenous peptides ET-1 and ET-3 appear to mediate vasoconstriction via the ETA, indicating that ETB-mediated responses in human renal vessels are of little importance. The pharmacology of isolated renal arteries and veins is similar to vessels obtained from other human vascular beds, with ETA antagonists fully reversing an ET-1 response. This is critical to understanding the importance of selectivity for the two subtypes. Sarafotoxin S6c–induced constrictor responses have been used previously as evidence of significant ETB constrictor responses in human vessels. However, it is not an endogenous ligand and ET-1 responses are fully reversed using ET antagonists. Bohm et al performed key experimental medicine studies that showed in volunteers in vivo that BQ123 inhibited the ET-1–mediated increase in renal vascular resistance whereas BQ788 (ETB antagonist) potentiated the ET-1 effect, implying a constrictor role for ETA and that ETB clears ET-1 from the plasma. Kaasjager et al also concluded that the systemic and renal vasoconstrictor effects of ET-1 in human beings are mediated by the ETA. A further unusual feature of ET-1 compared with other vasoconstrictors is that the constrictor response is sustained over a considerable period of time, lasting for several hours or in some cases several days. Contractions compared with many other vasoconstrictors are slow to wash out, which is consistent with a slow dissociation rate for ET-1 and may contribute to sustained hypertension and/or ET-induced vasospasm associated with pathophysiological conditions such as chronic kidney disease. Importantly, ET antagonists are able to relax ETA- mediated vasoconstriction in vessels preconstricted with ET-1 and this may reflect rapid internalization of the ligand receptor complex for recycling to the membrane (Fig. 1). In contrast, binding of ET-1 to ETB in vivo often is not displaced by ETB antagonists, which is in agreement with ETB being internalized by a different pathway and degraded in the lysosome.