Cellular mechanisms of the clearance function of type C receptors of atrial natriuretic factor

Clearance (C) receptors of atrial natriuretic factor (ANF) have an important role of removing ANF from the circulation. In the present study we investigated the cellular mechanisms of this function. ¹²⁵I-ANF_1-28 specifically bound to C-ANF receptors in cultured bovine vascular smooth muscle cells is internalized by a temperature-dependent process at a rate of 5% occupied receptors/min at 37°C. Internalized ¹²⁵I-ANF_1-28 is rapidly metabolized and released to the medium as [¹²⁵I]monoiodotyrosine, a process that is reversibly inhibited by NH₄Cl (10 mM). Retroendocytosis of receptor-ligand complex is detectable when intracellular accumulation of the ligand is maximized by previous incubation with NH₄Cl. In the presence of saturating concentrations of ¹²⁵I-ANF_1-28 when cells are warmed from 4 to 37°C, there is first a decrease and then an almost complete replenishment of surface C-ANF receptors, a phenomenon that is not altered by protein synthesis inhibition with cycloheximide. In the absence of temperature transition, the density of surface C-ANF receptors remains constant, and by 2 h of incubation the amount of ANF hydrolyzed far exceeds the total amount of C-ANF receptors. The total population of surface C-ANF receptors is internalized and recycled every hour, and these processes are constitutive since they also occur in the absence of ligand. Trypsinization and solubilization experiments further indicate that C-ANF receptors are rapidly internalized and recycled to the cell membrane. The results demonstrate that receptor-ligand internalization, extensive receptor recycling, and lysosomal hydrolysis of ANF are the cellular mechanisms by which C-ANF receptors exert their clearance function.