Localization of receptors for natriuretic peptide and endothelin in the duct of the epididymis of the freshwater turtle

The presence of receptor subtypes for natriuretic peptides (NPs) and endoth elin (ET) in the epididymis of the freshwater turtle, Amyda japonica, was e xamined by quantitative in vitro autoradiography using iodinated mammalian- type atrial NP (I-125-ANP((1-28))), phylogenically conserved C-type NP (I-1 25-[Tyr(0)]-CNP(1-22)), and ET-1 (I-125-ET-1) as radiolabeled ligands. To c haracterize NP receptor (NPR) subtypes, we also performed an activation of particulate guanylyl cyclase (GC) in membranes of the epididymis by NPs. Sp ecific I-125-ANP((1-28)) and I-125-[Tyr(0)]-CNP(1-22) bindings were localiz ed in surrounding smooth muscle cell layer of the duct of the epididymis wi th an apparent dissociation constant (K-d) of 0.84 +/- 0.15 and 1.74 +/- 0. 39 nM and a maximal binding capacity (B-max) of 0.47 +/- 0.11 and 0.08 +/- 0.01 fmol/mm(2), respectively. Bindings of I-125-ANP((1-28)) and I-125-[Tyr (0)]-CNP(1-22) to these sites were also displaced by des[Gln(18),Ser(19),Gl y(20), Leu(21),Gly(22)]ANF((4-23)), specific ligand of the NP clearance rec eptor. Production of 3',5'-cyclic guanosine monophosphate by particulate GC in membranes of the epididymis was stimulated by ANP((1-28)), BNP(1-26), a nd CNP(1-22). Receptor subtypes for ET in the epididymis were characterized by competition with BQ 123 and BQ 788 as specific antagonists for ET recep tors, type A (ETA) and type B (ETB) subtypes, respectively. Specific I-125- ET-1 bindings were localized in the smooth muscle cell layer of the duct of the epididymis with K-d and B-max of 0.21 +/- 0.03 nM and 0.52 +/- 0.05 fm ol/mm(2), respectively. These specific bindings were potently inhibited in a dose-dependent manner by BQ 123, whereas BQ 788 (10 mu M) was not in comp eting for specific I-125-ET-1 bindings in this structure. Therefore, these results indicate that specific NP and ET receptors are localized in surroun ding smooth muscle cells of the duct of the epididymis of the freshwater tu rtle. It is also suggested that biological and clearance NPR-like subtypes coexist in these cells, and the predominant ET receptor subtype in this tis sue is the ETA-like receptor. The localization of specific receptors for NP s and ET in the epididymis may be involved in the control of the transport of sperm in the freshwater turtle. (C) 2000 Academic Press.