To investigate the relation between renin content in each juxtaglomerular apparatus and reduction of plasma renin activity (PRA) with aging, the PRA and microdissected superficial or juxtamedullary single nephron renin content (SNRC) were determined in 5 young (3-6 mth) and 5 aged (13-18 mth) rats fed on a normal salt diet. The mean value of the PRA in the aged group was significantly lower than that of the young group. A highly significant correlation was found between the PRA and mean values of the superficial SNRC. The mean values of both the superficial and deep SNRC in the aged rats were significantly lower than those of the corresponding zones in the young rats. It is suggested that decreased synthesis of renin in each juxtaglomerular apparatus is an important factor in the decreased PRA observed with aging.
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