Atrial natriuretic peptide receptors in guinea pig placentas

The multiple physiologic effects of atrial natriuretic peptide are mediated through specific atrial natriuretic peptide plasma membrane receptors. We have attempted to determine whether atrial natriuretic peptide receptors are present in guinea pig placentas and subplacentas at approximately 45 and 65 days' gestation. A microsomal plasma fraction was prepared from each component. Atrial natriuretic peptide receptors were detected by in vitro radioligand techniques with alpha-human atrial natriuretic peptide labeled with iodine 125. In placentas we identified one set of specific binding sites for atrial natriuretic peptide at both gestational ages. Although the maximal concentration of the receptors did not change with advancing gestation, the dissociation constant was higher at 65 than at 45 days' gestation. In subplacentas two sets of binding sites were identified, one of low capacity-high affinity and the other of low affinity-high capacity. The maximal concentration of the atrial natriuretic peptide receptors did not change as a function of gestation. However, the dissociation constant, for both the high- and low-affinity sites, significantly decreased with advancing gestation.