1. In the presence of calcium, both high- [dissociation equilibrium constant (Kd) 5.8 × 10−11 mol/l] and low-(Kd 1.2 × 10−8 mol/l) affinity binding of atrial natriuretic peptide was detected on plasma membranes prepared from sodium-depleted rats.
2. In contrast, under calcium-free conditions only high-affinity binding (Kd 6.9 × 10−11 mol/l) was detected.
3. The requirement for calcium for low-affinity binding suggests a different mechanism of action than that for the high-affinity receptor.