Localization and Characterization of Binding Sites for Circulating and Cerebroventricular Atrial Natriuretic Factor in Rat Choroid Plexus

Abstract

In vitro autoradiographic studies showed that high-affinity atrial natriuretic factor (ANF) binding sites are present on rat choroid plexus (Kd = 83.8 pM, B_max = 22.9 fmol/mg protein). Guanylate cyclase-coupled receptors (ANF-Ri) represent 30% and non-guanylate cyclase-coupled ANF receptors (ANF-R2) represent 70% of the total ANF receptors present in this tissue. To provide detailed cellular localization of the binding sites, the technique of electron-microscopic autoradiography was applied using ^l25I-ANF (Ser 99-Tyr 126) as an in vivo ligand. In order to identify possible binding sites at the basolateral and the apical aspects of the choroid plexus, the ligand was injected into the carotid artery or into the lateral cerebral ventricles, respectively. Light-microscopic autoradiography demonstrated that ANF binds specifically to choroid plexus regardless of its route of administration. Electron-microscopic autoradiography showed that silver grains were localized primarily on epithelial cells of the choroid plexus (96–99%) and marginally on endothelial and pial cells. In choroidal epithelial cells, ultrastructural analysis of silver grain distribution revealed that, at 2 min after intracarotid or intracerebroventricular ¹²⁵I-ANF injection, lysosomes were the most distinctly labeled organelle (highest relative specific radioactivity). HPLC Chromatographic analysis disclosed that 96–99% of choroid plexus-bound ANF was already degraded 2 min after injection and that at least 63–66% of the degradation took place at the plasma membrane. These results indicate that ANF binding sites are present on both aspects of choroidal epithelium, and suggest that ANF is very quickly degraded in choroid plexus by membrane-associated as well as lysosome-associated processes.

Choroidal readaptation to gravity in rats after spaceflight and head-down tilt

“…Indeed, the apical membrane in choroid plexus is characterized by the presence of molecules such as Na ϩ -K ϩ -adenosinetriphosphatase (25), ANP receptors (18), and G proteins (24), which are involved in fluid transport and/or regulatory processes. Considering the important role played by this membrane domain in these cells (see Refs.…”

Atrial natriuretic factor receptor subtypes in the rat central nervous system.

“…- 17 We therefore conducted in vitro autoradiographic and cross-linking studies to explore the presence and anatomical location of ANF-C receptors in the brain and related structures, as well as the relative proportion of ANF-C sites to all ANF specific binding sites in a given tissue.…”