Effect of Age on Potassium-and Tyramine-Induced Release of Norepinephrine From Cardiac Synaptosomes in Male F344 Rats

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Abstract
Potassium (k+)-induced norepinephrine (NE) release was examined in preparations of cardiac synaptosomes and sliced atria from 6-, 24-, and 26-mo-old male F344 rats. Cardiac synaptosomes were prepared from rat hearts by collagenase digestion followed by homogenization in 0.32 m sucrose and centrifugation. The synaptosome preparations and the sliced atria were labeled with 3H-NE and then placed in a superfusion system. K+-induced net fractional release of NE from synaptosomes prepared from 24-and 26-mo-old rats (4.3% and 3.0%, respectively) was significantly reduced when compared to ne release from synaptosomes from 6-mo-old rats (5.2%). K+-induced NE release from sliced atria from 24-mo-old rats (4.7%) was also significantly reduced when compared to ne release from atria from 6-mo-old rats (6.3%). Perfusion of cardiac synaptosomes with buffer prepared without calcium (ca++ free, < 5 μM) reduced k+-induced release by 50% in all age groups studied. Perfusion with tyramine induced identical rates of NE release from cardiac synaptosomes prepared from 6-and 24-mo-old rats. These results confirm that depolarization-induced NE release from cardiac sympathetic nerves is reduced in the old male F344 rat

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