ENDOTHELIN RECEPTOR ANTAGONISTS - EFFECT OF SERUM-ALBUMIN ON POTENCY AND COMPARISON OF PHARMACOLOGICAL CHARACTERISTICS

Endothelins (ETs) are 21-amino acid peptides that bind to membrane rec eptors to initiate pathophysiological effects. Two types of ET recepto rs, ETA and ETB, have been identified. Various ET receptor antagonists are being developed as therapeutic agents. This report examines the e ffects of bovine serum albumin (BSA) on the potency of ET receptor ant agonists and compares five ET receptor antagonists. Competition studie s show that in the absence of BSA, A-127722 and L-749329 inhibited FT- 1 binding to ETA receptor with the same IC50 value of 0.09 nM. Additio n of increasing concentrations of BSA incrementally decreased the pote ncy of the antagonists: in the presence of 5% BSA, the IC50 values inc reased to 4.3 and 820 nM, respectively. Similarly, addition of BSA dec reased the potency of antagonists in inhibiting ET-1-stimulated phosph atidylinositol hydrolysis. These results suggest that serum albumin ha s profound effects on the potencies of ET receptor antagonists. FR1393 17, PD-156707, L-749329, Ro-47-0203 and A-127722 were then selected fo r direct comparison under identical experimental conditions with 0.2% BSA. The potency of antagonists was assessed by binding studies for th e determination of IC50 and K-i values and by ET-1-stimulated phosphat idylinositol hydrolysis and arachidonic acid release for the determina tion of IC50 and pA(2) values. All five antagonists inhibited ET bindi ng and the biological effects exerted by ET in a competitive mode. The K-i values for A-127722, PD-156707, FR139317, Ro-47-0203 and L-749329 for the ETA receptor were 0.07, 0.38, 0.80, 3.67 and 33.6 nM, respect ively. A similar hierarchy was revealed by the functional assays. Our results suggest that the rank order of potency of the antagonists is A -127722 greater than or equal to PD-156707 greater than or equal to FR 139317 > Re-47-0203 > L-749329.