The Effect of Age on the Tyramine-Sensitive Intraneuronal Pool of Norepinephrine in Rat Heart

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Abstract
The effect of age on the amount of norepinephrine (NE) released by tyramine (TYR) was investigated in hearts from Fischer 344 rats, 6, 12, and 24 months of age. Isolated hearts were perfused through an aortic cannula with Krebs-Ringer solution containing 1.6 × 10−4M TYR. α-Methyl-p-tyrosine (7.5 × 10−5M) and fusaric acid (10−5M) were added to inhibit de novo synthesis of NE. The effluent from the heart was collected continuously throughout 4 h of perfusion with TYR. The content of NE in the perfusion effluent was measured by electrochemical (EC) detection methods after alumina extraction and high-performance liquid chromatography (HPLC) separation. In addition, the amount of NE remaining in the heart after TYR perfusion was measured with HPLC/EC methodology. The amount of NE released from hearts was not significantly different among the ages [6 months, 612.5 ± 68.2 ng; 12 months, 640.2 ± 53.0 ng; 24 months, 593.8 ± 38.2 ng; p > 0.05, analysis of variance (ANOVA)]. These amounts represent more than 93% of total cardiac NE at all ages (6 months, 97.6 ± 0.5%; 12 months, 94.8 ± 1.3%; 24 months, 93.7 ± 1.7%). The t½ for the decline in NE in hearts was also similar at the three ages (6 months, 45.0 ± 4.9 min; 12 months, 53.6 ± 5.2 min; 24 months, 60.4 ± 7.8 min; p > 0.05, ANOVA). These results indicate that the amount of NE released by TYR does not change with increasing age.

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