CHARACTERISTICS OF ANP-BINDING SITES IN THE ADRENAL CAPSULES OF TERM-PREGNANT RATS

Significant increases of circulatory volume and plasma aldosterone lev els are observed in pregnancy. We investigated whether a decrease of a trial natriuretic peptide (ANP) receptors in the adrenal zona glomerul osa (ZG) could explain the marked elevation of plasma aldosterone occu ring during pregnancy. I-125-ANP binding was measured in competition e xperiments using rANP(1-28), ANP(4-23), a truncated analog which has h igh specificity for the ANP-C receptor, or a combination of both. West ern blot experiments were also performed with an investigation into th e effect of ANP on aldosterone secretion in adrenal capsule suspension s. I-125-ANP binding on adrenal ZG membranes was displaced by ANP(1-28 ) with an affinity (K-d) of 313 +/- 39 and 323 +/- 60 pM (NS) for preg nant and non-pregnant rats, respectively. The density of sites (B-max) decreased slightly but not significantly during pregnancy. Displaceme nt experiments with ANP(4-23) demonstrated a B-max of 137 and 134 fmol /mg of proteins (NS) for pregnant and non pregnant rats, respectively. Studies were performed to block the ANP-C site. Displacing the remain ing I-125-ANP binding with ANP(1-28) led to an affinity constant and r eceptor density which were not significantly different between the two groups of rats. The results obtained with Western blots showed a sing le band of 123 kDa with no significant variations in ANP-R(1) receptor s in the ZG during gestation. The sensitivity of potassium-, ACTH- or angiotensin II-stimulated aldosterone secretion to ANP was not altered by gestation. These data show that the apparent hyperaldosteronism fo und in normal term-pregnant rats is not the consequence of modificatio ns in the affinity, number and properties of ANP receptor types or in the sensitivity of the aldosterone response to ANP.