AGE-RELATED ANP RELEASE IN RESPONSE TO ACUTE VS CHRONIC SODIUM LOADING

Atrial natriuretic peptide (ANP) levels were studied in young (2-week- old) and old (age 20 months) rats following acute (20 mL/kg bolus or n ormal saline i.v.) vs. chronic sodium loading (high-salt diet for 21 d ays). In both groups serum ANP rose significantly 1 h post i.v. loadin g. However, the increment was greater in the old animals. On chronic s odium load, in the young group, ANP rose significantly on days 3 and 7 , subsequently declining to baseline. In the old animals, no increase in ANP was noted throughout the chronic experiment. Twenty-four-hour u rinary sodium excretion was similar in the two groups following acute or chronic salt load. The results suggest that: (1) The old kidney req uires a greater ANP stimulus to excrete a given acute i.v. sodium load . (2) The relatively diminished elasticity of the old heart plays a ma jor role in the exaggerated ANP release in response to a bolus i.v. lo ad of saline. (3) The role of ANP in maintaining homeostatic renal sod ium excretion during short-term sodium oral loading is different in th e two age groups.