Localization and quantification of atrial natriuretic factor binding sites in the kidney of Xenopus laevis

Atrial natriuretic factor (ANF) binding sites in the skin, the bladder, and the kidney of the anuran amphibian Xenopus laevis were localized and quantified using quantitative in vitro autoradiography. Specific binding of 125I-rANF occurred only in the glomeruli and in the adrenal tissue of the kidney. The association of 125I-rANF binding was much higher than the dissociation and there was no steady state between the ligand and the binding sites. Scatchard and Hill's analyses of saturation experiments showed 125I-rANF to bind to heterogeneous sites with positive cooperativity. The effective concentrations, where 50% of maximal binding occurs (EC50), in glomeruli and in adrenal tissue were 75.7 +/- 8.5 and 74.7 +/- 12.1 pM (n = 8), respectively. The corresponding maximum binding capacities (Bmax) were 0.847 +/- 0.131 fM/mm2 in glomeruli and 1.161 +/- 0.179 fM/mm2 in adrenal tissue. Displacement studies have demonstrated the same affinity of these 125I-rANF binding sites to unlabeled rANF, hANF, and rAtriopeptin II, while 125I-labeled rANF had a much higher affinity. The N-terminal ANF fragment (99-109) and the C-terminal rANF fragment (116-126) had only weak displacing effects, whereas unrelated peptides did not alter the binding of 125I-rANF. The osmotic stress of acclimation to 1.5% salt water increased renal but not adrenal ANF binding.