A decline in myocardial and hepatic norepinephrine turnover with age in Fischer 344 rats.

The purpose of the present study was to determine any age-related alterations in norepinephrine (NE) turnover rate in various tissues of the Fischer 344 rats. Female rats ages 3, 12, and 22 mo were given an intraperitoneal injection of alpha-methyl-p-tyrosine, a known inhibitor of tyrosine hydroxylase. NE turnover was determined mathematically by monitoring the exponential decline of tissues levels over 7 h. Cardiac NE turnover rates averaged 38.4, 31.4, and 25.2 ng X g-1 X h-1 for the 3-, 12-, and 22-mo-old age groups, respectively. A similar trend was found for the liver NE turnover rates (12.9, 9.2, and 6.8 ng X g-1 X h-1 for 3-, 12-, and 22-mo-old groups, respectively). It was concluded that the reduced catecholamine content seen in various tissues with advancing age can be explained, in part, by a diminished ability for catecholamine synthesis.