Abstract

Measurements of the specific activity of norepinephrine in coronary sinus blood were made in intact dogs from 20 minutes to 48 hours after the endogenous norepinephrine pool had been labeled with tritiated norepinephrine. The specific activity in blood was observed to decline sharply during augmented release of norepinephrine into the blood either by tyramine administration or nerve stimulation in all experiments up to 5 hours after labeling the neurotransmitter pool. Although the values in blood more nearly approximated the specific activity measured simultaneously in the heart during augmented release, they were still consistently greater than tissue values. At 24 and 48 hours after labeling the pool, no change in specific activity in blood was observed after administration of tyramine or nerve stimulation and the values in blood and tissue showed no differences. These findings are interpreted to indicate that exogenous norepinephrine initially mixes with a portion of the neurotransmitter store which is in more rapid exchange with the blood and only slowly equilibrates with the entire store. This is presented as further evidence for considering the neurotransmitter pool to be nonhomogeneous.