Multiple types of binding sites for atrial natriuretic peptide in rat olfactory bulb membranes and synaptosomes

The binding of atrial natriuretic peptide (ANP) to rat olfactory bulb membranes and synaptosomes was examined. [¹²⁵I]ANP (rat, 99-126) bound specifically to a single class of binding site on olfactory bulb membrane preparation with dissociation constant (K_d) of 106 pM and maximum binding capacity (B_max) of 13.6 fmol/mg protein. Comparable results were obtained when the binding was characterized using displacement and kinetic experiments. The ring deleted analog of ANP, C-ANP (rat, 4-23) displaced [¹²⁵I]ANP only minimally from its binding site in the membrane preparation. Saturation, displacement and blocking experiments on [¹²⁵I]ANP binding to rat olfactory bulb synaptosomes revealed the presence of two distinct binding site. Simultaneous analysis of homogeneous and heterogeneous displacement curves and blocking experiments revealed the quantitative characteristics of these receptors to be: K_d1 = 44 pM, B_max1 = 42 fmol/mg protein and K_d2 = 1050 pM, B_max2 = 173 fmol/mg protein, for the high and low affinity binding sites, respectively. Kinetic experiments further confirmed the differences between the receptors present in the membranes and the synaptosomes preparations. The demonstration of multiple ANP binding sites in olfactory bulb synaptosomes but not membrane preparations raises the possibility of a particular function of ANP in nerve terminals.