RECEPTOR PROTEINS AND BIOLOGICAL EFFECTS OF C-TYPE NATRIURETIC PEPTIDES IN THE RENAL GLOMERULUS OF THE RAT

Binding studies on rat glomeruli using I-125-labeled Tyr(0)-C-type natriuretic peptide(1-22) [I-125-Tyr(0)-CNP-(1-22)] and I-125-labeled alpha-atrial natriuretic peptide (alpha-I-125-ANP), and the unlabeled ligands CNP-(1-22), alpha-ANP, and des-Gln(18), Ser(19),Gly(20), Leu(21),Gly(22)-ANP-(4-23)-NH2 (C-ANP) suggest that receptor-like sites that bind both alpha-ANP and C-ANP fall into two categories, one with high [dissociation constant (K-d) similar to 10(-9)M] and one with low (K-d similar to 10(-5)M) affinity for CNP-(1-22). Covalent attachment of I-125-Tyr(0)-CNP-(1-22) and alpha-I-125-ANP to these sites identifies two membrane proteins with corresponding properties. The first, which can be labeled by both radioligands, is a disulfide-bridged similar to 140-kDa protein that is reduced by dithiothreitol to similar to 67 kDa. This protein binds C-ANP and has K-d similar to 10(-10) M for CNP-(1-22). The second protein, which is labeled only by alpha-I-125-ANP, also binds C-ANP, but has K-d similar to 10(-5)M for CNP-(1-22). This similar to 77-kDa protein may also have a disulfide-bridged, high-molecular-mass form of similar to 140 kDa in the absence of dithiothreitol. Studies of glomerular function show that alpha-I-125-ANP is internalized whereas I-125-Tyr(0)-CNP-(1-22) is not. C-ANP abolishes the specific internalization of alpha-I-125-ANP. CNP-(1-22) inhibits internalization of 400 pM alpha-I-I25-ANP weakly, only similar to 60% being inhibited by 10 mu M CNP-(1-22), This implies that the similar to 77-kDa protein, with its low affinity for CNP-(1-22), mediates internalization. Furthermore, CNP-(1-22), as well as alpha-ANP and C-ANP, inhibits glomerular levels of adenosine 3',5'-cyclic monophosphate (cAMP), and CNP-(1-22) does so with a high affinity, which corresponds to its affinity for the similar to 67-kDa protein. The results suggest that the similar to 67-kDa receptor is distinct from the natriuretic peptide clearance receptor and may control cAMP levels.