Specific endothelin binding sites in renal medullary collecting duct cells: lack of interaction with ANP binding and cGMP signalling

The diverse biological actions of endothelins (ET) appear to be mediated by specific cell-surface receptors. Autoradiography and membrane binding studies have shown abundant ET binding sites in the kidney. However, their expression in specific types of renal cells is unclear. We studied the binding of 125 I-labelled endothelin-1 in freshly isolated cell suspensions from canine inner medullary collecting duct. Competition binding experiments revealed the presence of specific high-affinity binding sites: unlabelled ET-1 and ET-2 competed with the radioligand with an IC 50 of 135 and 83 pM, respectively, while the IC 50 of ET-3 and big ET-1 were 2 and 4 orders of magnitude higher, indicating the presence of ET A -type receptor. Angiotensin II, vasopressin, and atrial natriuretic peptide (ANP) did not compete for ET binding even at a concentration of 10 −6 M. Saturation binding experiments showed a single class of binding sites of high density (B max = 56.7 ± 10.3 fmol/10 6 cells) and high affinity (K d = 69.8 ± 10 pM). In contrast, ANP receptors in the same cell preparations appeared as two classes of binding sites with widely different affinity and density. The high-affinity ANP site (K d = 311 ± 48 pM) was compatible with ANP-B (guanylate cyclase-coupled) receptor. ET-1 did not compete for this receptor. ET-1 (10 −7 M) did not alter ANP-induced cGMP generation in these cells (3.8-fold increase at 10 −7 M ANP), nor basal levels of cGMP. The expression by the distal tubular epithelium of specific ET-1 binding sites strongly suggests the presence of a functional receptor, which may mediate the inhibition of Na + transport in these cells. The mechanism and the transduction pathway of this effect appear to be different and independent from those of ANP.