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Atrial natriuretic peptide in aging rats : evidence for altered processing, secretion and receptor binding Kao, Jonathan


The recently discovered atrial natriuretic peptide (ANP) has potent diuretic, natriuretic and hypotensive effects, and is believed to be involved in the maintenance of sodium homeostasis in both normal and pathological conditions. The mammalian aging process is associated with a host of abnormalities that include, among others, a compromised ability to regulate sodium homeostasis. There are reports that demonstrate a positive correlation between plasma ANP levels and age in man; accordingly, the aim of this study was to examine whether age-related sodium imbalance is associated with disturbances in the homeostasis of ANP. Specifically, the intracellular storage, processing and secretion of ANP from the atrium was studied and associated with circulating ANP concentrations and ANP receptor binding kinetics. Studies were conducted with four groups of male Wistar rats designated as 1-, 3-, 10-, and 20-month-old. 24-hour renal clearances were conducted to assess age-related changes in renal functions. GFR and UNaV increased steadily from 1 to 10 months of age and decreased in the 20-month-old, while fractional excretion of water (FEH₂O) and sodium (FENa) declined initially (from 1 to 10 months) and then rose in the 20-month-old group. Circulating ANP levels in the rats was significantly correlated with the increase in age (N = 147, r = 0.59, p < 0.0005). Atria of the animals were isolated and superfused with a modified Langendorff apparatus. The spontaneous release of ANP increased from 1 to 3 months, and steadily decreased after 3 months. The results indicate that ANP secretion increases with maturation and thereafter declines with advancing age. ANP concentrations in the right and left atria were also quantified. The results revealed that atrial ANP content increased from 1 to 3 months and decreased progressively with age. There was a positive correlation between the rate of ANP release and atrial ANP content (N= 42, r=0.50, p<0.01), suggesting that the release of ANP from the right atrium was associated with the atrial content. The concurrence of a reduction in ANP secretion but with elevation in plasma ANP concentration in the aged (20-month-old) rats, suggests that there may be an impairment in renal clearance of ANP. It was established that the main molecular species present in the atrium was γ-ANP and that this was unaffected by age as assessed by reverse-phase high performance liquid chromatography (RP-HPLC) coupled with radioimmunoassay. The molecular forms of ANP secreted by the atrium consisted of predominantly α-ANP, with a smaller amounts of γ-ANP. γ-ANP constituted only 1% of the total secreted ANP in the 1-, 3-, or 10-month-old rats, but up to 8% was detected in 20-month-old rats. Although both α-ANP and γ-ANP were present in the circulation, the ratio of γ-ANP/α-ANP increased significantly with age. The concentration of γ-ANP in the plasma of the 20-month-old rats was two- to three-fold higher than in the two younger groups (1- and 3-month-old). These data imply that the post-transcriptional processing of prohormone γ-ANP to active α-ANP is altered with age. Radio-ligand binding experiments were carried out using glomerular ANP receptors to determine whether the age-related alterations in plasma ANP levels has an effect on the binding of ANP to its target tissues. Both the receptor density (Bmax) and the equilibrium dissociation constant (kd increased from 1 to 3 months but decreased from 3 to 20 months. Collectively, these results suggest that: 1) Aging affects atrial ANP content and consequently influences the release of ANP from the isolated atria. 2) The processing of prohormone γ-ANP to active α-ANP is modified with age. 3) Plasma levels of ANP increase with age, which may result in down-regulation of ANP receptor density and increased efficacy in receptor binding affinity. These may represent the compensatory responses.

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