Norepinephrine Content, Release, and Disposition in Isolated Dog Coronary Artery

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Abstract
The content of norepinephrine [NE], its release and subsequent disposition was studied in unlabeled and [3H]NE-labeled strips of canine coronary artery using standard superfusion techniques. Endogenous norepinephrine was isolated from superfusate and vessel by column chromatography then measured using high-pressure liquid chromatography with electrochemical detection. Release of NE was studied in both unlabeled and radiolabeled preparations. The relative importance of neuronal extraneuronal dispositions of norepinephrine was determined by quantitation of the efflux of the 3H metabolites of norepinephrine, known to arise by neuronal and extraneuronal metabolism, during basal conditions and during increased efflux of NE by electric stimulation. Similar amounts of endogenous NE were present in segments of left (0.36 .mu.g NE/g) and right (0.42 .mu.g NE/g) coronary arteries. Levels were not significantly different after 15 min of continuous electric stimulation (0.37 and 0.41 .mu.g NE/g in right and left arteries, respectively). This was apparent even though the fractional loss with electric stimulation seemed high; .apprx. 3 times greater from the coronary artery than from a segment of the peripheral vasculature. Of the quantity of NE released during electrical stimulation, 37% overflows in the superfusate, 43% is taken up in the sympathetic neurons and 20% is metabolized extraneuronally. Of that entering neurons .apprx. 1/2 is metabolized to 3,4-dihydroxyphenylglycol and the remainder is stored in the vesicles. By comparison with the saphenous vein from the same animal, the amount of NE in the vessels is less and a greater proportion of that release reenters the neurons and presumably helps to maintain the stores of transmitter.