Effect of naturally occurring catecholamines on adrenergic neuroeffector interaction in the canine saphenous vein

Abstract

Experiments were performed to compare the effects of dopamine, epinine and norepinephrine on peripheral adrenergic neurotransmission. Helical strips of dog's saphenous veins were incubated in solutions containing 3H-norepinephrine and mounted for superfusion and determination of 3H-norepinephrine and metabolites in the superfusate. In control strips, but not in veins treated with cocaine, the catecholamines increased the efflux of 3H-norepinephrine and its deaminated metabolites, indicating intraneuronal displacement of transmitter. Equipotent doses (ED50 for activation of alpha receptors on the smooth muscle cells) of norepinephrine, but not of epinine and dopamine, reduced slightly the release of 3H-norepinephrine during electrical stimulation. In strips treated with cocaine, norepinephrine reduced the 3H-norepinephrine release evoked by nerve stimulation by 61.4, epinine by 44.1 and dopamine by 28.5%. The inhibitory effect of the three catecholamines was abolished by phentolamine. These experiments suggest that (1) prejunctional alpha receptors have a different affinity than postjunctional alpha receptors; (2) in the dog's saphenous vein alpha-receptor mediated feedback inhibition of norepinephrine release can be demonstrated best when neuronal uptake is inhibited. © 1979.