Free and conjugated catecholamines in dog pulmonary artery: presence and pharmacological action

Free and conjugated forms of norepinephrine (NE) and dopamine (DA) were detected in dog pulmonary artery, and small amounts were released during 2-Hz electrical stimulation. Hydrolysis of the conjugates by acid boiling and by enzymes indicated that they were primarily sulfates, but traces of glucuronide may also have been present. After isolation by column chromatography, free and conjugated amines were quantitated using liquid chromatography with electrochemical detection. Pulmonary artery tissue contained (in ng) 1,240 +/- 100 free NE, 15 +/- 5 conjugated NE, 47 +/- 6 free DA, and 2 +/- 0.3 conjugated DA/g tissue (means +/- SE). When strips of pulmonary artery were immersed in [3H]NE small amounts of [3H]NE sulfate were synthesized. The release of conjugated NE was not calcium dependent but release was attenuated somewhat by tetrodotoxin. Removal of endothelium did not change tissue content of conjugated or of free NE nor the amounts measured in superfusate. Small amounts of free DA were also detected in tissues and in superfusate during stimulation, but only when calcium was present in the superfusion fluid. Norepinephrine sulfate was not a potent agonist at either pre- or postsynaptic alpha-receptors; the agonist activity seen with high concentrations of norepinephrine sulfate seems likely to have been due at least in part to trace contamination by free NE.