REGIONAL DIFFERENCES IN THE DENSITY AND SUBTYPE SPECIFICITY OF ENDOTHELIN RECEPTORS IN RABBIT URINARY-TRACT

We investigated the binding characteristics of endothelin (ET) recepto rs in rabbit ureter, bladder dome, bladder base, and urethra and compa red the observed receptor properties with those of cloned human ET(A) and ET(B) receptors expressed in Chinese hamster ovary K-1 (CHO) cells . Receptor binding experiments with [I-125]ET-1 revealed the presence of a single class of specific, saturable, high affinity [I-125]ET-1 bi nding sites in all of the regions of the studied urinary tract. The ra nk order of the densities (B-max values) of [I-125]ET-1 binding sites was: ureter much greater than bladder dome > bladder base = urethra. E T-1 and ET-2 inhibited [I-125]ET-1 binding to the membrane particulate s from the various regions of the urinary tract with single high affin ity constants. A selective ET(A) receptor antagonist, BQ 123, and sele ctive ET(B) agonists, ET-3 and sarafotoxin S6c (STXc), inhibited [I-12 5]ET-1 binding to bladder dome, bladder base, and urethra with high an d low affinity constants indicating the presence of both ET(A) and ET( B) receptor subtypes in these tissues. The subtype specificity of ET r eceptors in the rabbit tissues is confirmed with inhibition data obtai ned from similar binding studies in cloned human ET(A) and ET(B) recep tors. The proportions of high affinity binding sites for ET-3, represe nting ET(B) receptors, were approximately 25%, 27%, and 46% in bladder dome, bladder base, and urethra, respectively. Corresponding values f or STXc were approximately 17%, 28%, and 43% in bladder dome, bladder base, and urethra, respectively. In contrast to the findings for ET-3 and STXc, the proportions of high affinity binding sites for BQ 123, r epresenting ET(A) receptors, in bladder dome, bladder base, and urethr a were approximately 84%, 74%, and 60%, respectively. In ureter, these selective compounds inhibited [I-125]ET-1 binding with either a low ( ET-3 and STXc) or a high binding affinity (BQ 123), suggesting the pre sence of only a single receptor subtype (ET(A)) in this tissue. These data indicate that there are regional differences in the density and s ubtype specificity of ET receptors in the rabbit urinary tract.