Alterations in Atrial Natriuretic Peptide (ANP) Secretion and Renal Effects in Aging

Aging is associated with hypertension and electrolyte disturbances. The purpose of this study was to determine the effect of aging upon secretion and renal actions of atrial natriuretic peptide (ANP). Rats were anesthetized and received tracheal, jugular vein, carotid artery, and bilateral uretheral catheterization. One set of young (2-3 mo) rats (Group 2, n = 9) and one set of old (18-21 mo) rats (Group 4, n = 7) received bilateral atrial appendectomies. Control young (Group 1, n = 8) and old (Group 3, n = 8) rats received a sham appendectomy. All rats were infused (iv) with 6% albumin in Krebs buffer, sufficient to increase blood volume by 15%. Finally, each rat was injected with ANP (1 microgram/kg). Sodium excretion rate (U(Na+)V) in response to volume expansion was significantly decreased in all groups compared to Group 1 (young control, p < .05). All groups demonstrated a striking increase in U(Na+)V with the ANP injection, but the response was greatest in young control rats when factored by body weight (p < .05). There were no significant differences in MAP between the groups, suggesting that the differences in U(Na+)V observed were not the result of hemodynamic factors. Isolated perfused atria from young (n = 9) and old (n = 8) rats were subjected to stretch and endothelin stimulation (50 nM). Atria from young rats showed a dramatic increase in ANP secretion in response to atrial stretch and a further marked increase in secretion in response to endothelin, whereas both of these responses were markedly attenuated in old rats (p < .05). These results suggested that the secretion and renal effects of ANP are impaired in aging. Changes in secretion and actions of ANP in aging could contribute to the development of hypertension or heart failure.