Increased bioactivity of rat atrial extracts: relation to aging and blood pressure regulation

The purpose of this study was to evaluate the possible role of atrial factor(s) in the regulation of cardiovascular homeostasis and their relationship to aging. Rats were anesthetized and received jugular vein, carotid artery, and bilateral ureteral catheterization. After a half-hour equilibration period, the rats received 0.5 ml of atrial extract with a concentration of proANP (atrial natriuretic peptide) of 150 microg/ml prepared from either aged (18-20 month, "aged extract group", n = 12) or young (2-3 month, "young extract group", n = 12) rats. Mean arterial pressure (MAP) and renal function were monitored over five 20-minute periods. The atrial extract caused MAP to fall significantly in the aged extract group (p < .05) but MAP was unchanged in young extract group. There was a significant difference in MAP between the two groups (p < .05). Urine output increased significantly in both groups after extract infusion (p < .05 in both cases). Sodium and potassium excretion showed similar responses. However, the diuresis, natriuresis, and kaliuresis after extract infusion would have been expected to be relatively lower in the aged extract group compared to the young extract group considering the significantly lower MAP in the aged extract group. High performance gel permeation chromatography (HP-GPC) analysis of the atrial extract showed an increased quantity of a large molecular weight C-terminal peptide in atrial extracts from aged rats compared to young rat atria. Plasma levels of ANP and proANP 1-30 both increased significantly after extract infusion in both aged and young groups, and there was no significant difference in ANP concentration between the two groups. However, the concentration of proANP 1-30 was significantly increased in the aged group compared to the young group after extract infusion. These results suggest that changes in the structure or processing of proANP in aging may contribute to the different hemodynamic responses.