Binding sites of atrial natriuretic peptide in human renal tissue-quantification by in vitro receptor autoradiography

Identification of renal natriuretic peptide receptor subpopulations by use of the non-peptide antagonist, HS-142-1

1. The renal actions of natriuretic peptides are dictated by the distribution of guanylyl cyclase-linked (NPRA and NPRB) and non-guanylyl cyclase-linked (NPRC) receptors. Natriuretic peptide receptors have previously been distinguished on the basis of their differential affinity for peptide fragments and analogues; however, most of the available ligands are not fully selective. We have used the specific guanylyl cyclase-linked receptor antagonist, HS-142-1, to investigate the differential distribution of natriuretic peptide receptor subtypes in the human, bovine and rat kidney. 2. Specific, high affinity 3-([125I]-iodotyrosyl)-rat-ANP-(1-28)([125I]-rANP1-28) binding sites were identified in all three species, localized to glomeruli, inner medulla, intrarenal arteries and regions in the outer medulla corresponding to vasa recta bundles. Binding sites were also identified in the smooth muscle lining of the hilar region in the bovine and rat kidney. 3. In the rat, [125I]-rANP1-28 binding was inhibited by unlabelled peptide sequences with a rank order of potency (rANP1-28 > pCNP1-22 > C-ANP4-23). The glomeruli exhibited a heterogeneous population of binding sites, C-ANP4-23 and pCNP1-22 producing a significantly better fit to a two component inhibition curve compared to the single component curve for rANP1-28. 4. Competitive inhibition experiments with the receptor selective ligands, C-ANP4-23 and HS-142-1, suggested that, like the rat, human and bovine glomeruli possessed a heterogeneous population of binding sites, whilst those in the inner medulla and intrarenal arteries of all three species represented a homogeneous population. Rat glomeruli exhibited a high proportion (>80%) of the NPRc receptor subtype whereas in human and bovine glomeruli this receptor represented less than 20% of the total population, the majority of binding sites being HS-142-1-sensitive.5. C-ANP4-23 exhibited a significantly higher inhibitory potency for binding sites in rat glomeruli compared to those in human and bovine kidney whilst HS-142-1 was significantly more potent in the rat and bovine kidney compared to man. No evidence was found to suggest the presence of a renal NPRBreceptor subtype.6. The relative density, affinity and proportion of natriuretic receptor subtypes in the kidney exhibit significant species differences. HS-142-1 may be a valuable tool in further elucidating the localization and function of these receptors, but heterogeneity between species should be considered when selecting experimental models.

Atrial natriuretic factor (ANF) binding sites in frog kidney and adrenal

Atrial natriuretic factor (ANF) binding sites were localized and quantified in kidney and adrenal of the frog Rana temporaria by quantitative in vitro autoradiography. [125I]-rat ANF(99-126) binding was present in kidney glomeruli and in the outer layer of interrenal tissue in the adrenal gland. ANF binding exhibited positive cooperativity with a half-maximal binding concentration (EC50) of 102 +/- 16 pM in glomeruli and 93 +/- 19 pM in interrenal tissue (n = 8). The corresponding maximal binding capacities (Bmax) were 1.33 +/- 0.16 and 1.21 +/- 0.36 fmol/mm2. [125I]-Rat ANF(99-126) binding was competitively displaced by unlabeled ANF analogues with an intact disulfide bridge showing a lower affinity than the iodinated ligand. The presence of ANF binding in glomeruli and steroidogenic interrenal cells suggests physiological functions of ANF for the osmomineral regulation in the frog by influencing glomerular filtration rate and adrenal steroid secretion.

Localization and quantification of [125I]-endothelin binding sites in human fetal and adult kidneys--relevance to renal ontogeny and pathophysiology

Endothelins, 21 amino acid peptides, produced by endothelial cells are potent vasoconstrictors and mitogens. According to experimental studies in animals, endothelins seem to be involved in the regulation of renal hemodynamics. In order to gain insight into its potential effects in man, a quantitative analysis of its binding sites was performed in human kidneys. Because of the proliferative action of endothelin in cell culture we also compared binding sites in fetal and adult kidneys. Binding sites for [125I]-endothelin-1,2,3 were visualized by in-vitro autoradiography and quantified by densitometry. In both adult and fetal tissue, specific binding sites occurred in the cortex, medulla, and renal vessels. Unlabeled endothelins and sarafotoxin, a peptide with a high sequence homology to endothelins, inhibited [125I]-endothelin-1 binding with IC50 in the 9.8 to 0.023 nM range, whereas unrelated peptides (angiotensin II, atrial natriuretic peptide) and the calcium antagonist nitrendipine failed to compete for [125I]-endothelin-1 binding sites. Linear Scatchard analysis revealed that the number of binding sites (expressed per tissue equivalent: TE) were consistently higher in fetal than in adult kidneys, while affinities did not differ significantly in cortex, medulla, and vessels (fetal/adult: cortex KD 43.4 +/- 19.6/55.9 +/- 16.7 nM; BMax 13.5 +/- 7.8/2.7 +/- 1.3 fmol/mg TE; medulla KD 26.3 +/- 10.9/34.6 +/- 7.4 nM; BMax 10.1 +/- 0.9/3.7 +/- 1.1 fmol/mg TE; vessels KD 41.1 +/- 22.9/23.7 +/- 8.1 nM; BMax 12.9 +/- 3.9/4.1 +/- 1.2 fmol/mg TE). Medullary capillaries and veins showed strong binding in human and rat kidneys which may be important for the pathophysiology of acute renal failure. Human adult and fetal glomeruli had only a few binding sites. This contrasts to findings in the rat kidney in which glomeruli have a high concentration of endothelin binding sites; although this does not role out an influence per se, it does point out the need to subject the assumption of a relevant glomerular effect of endothelin in man to closer scrutiny. The diffuse and strong binding in fetal kidney may indicate a role for endothelin in the process of renal maturation.

Binding sites for atrial natriuretic peptide in the kidney and aorta of the hagfish (Myxine glutinosa)

1. In the atlantic hagfish (Myxine glutinosa) binding sites for atrial natriuretic peptide (ANP) were visualized by autoradiography in contractile structures of the renal system (glomeruli, neck segment, and archinephric duct) and in the aorta. 2. Since the location of binding sites is comparable to that in higher vertebrates including man, these results suggest that ANP may act as a hormone already in cylcostomata.