Abstract

Norepinephrine-C¹⁴ is synthesized in the cat spleen perfused with tyrothne-C¹⁴ and is released during nerve stimulation. There is a more rapid increase in the specific activity of released norepinephrine-C¹⁴ from spleens stimulated every 2 minutes than in that released from spleens stimulated every 10 minutes. After 3000 impulses delivered at 30 impulses/sec at intervals of either 2 minutes or 10 minutes, the specific activity of the released norepinephrine-C¹⁴ is greater than that remaining in the spleen, indicating selective release of newly synthesized norepinephrine. The ratio of the specific activity of norepinephrine-C¹⁴ released to that remaining in the spleen is higher with more frequent stimulation. In the presence of phenoxybenzamine the rate of increase of the specific activity of released norepinephrine-C¹⁴ is more rapid than in its absence; there is selective release of newly synthesized norepinephrine, but the ratio of specific activity of released to splenic norepinephrine-C¹⁴ is lower, indicating more rapid equilibration of norepinephrine stores as well as more rapid synthesis of the catecholamine. After perfusion for four hours the specific activity of the released norepinephrine-C¹⁴ is about 0.7 of the specific activity of norepinephrine totally derived from the perfused tyrosine-C¹⁴. These results suggest that newly synthesized norepinephrine plays a significant role in maintaining transmitter release and that this role varies with the interval between stimulation.