Effects of L-Dopa and L-Tyrosine on Adrenergic Transmission in the Canine Saphenous Vein 1

Abstract

Abstract

The effects of L-dopa, and L-tyrosine on norepinephrine (NE) and dopamine (DA) content and overflow from adrenergically innervated canine saphenous veins were determined. L-Dopa treatment resulted in large increases in concentration of DA in tissue, and caused significant amounts of DA to overflow into the superfusate both under basal conditions and during electrical stimulation. The DA did not appear to be of vesicular origin, since DA concentrations in superfusate were not altered by removing Ca²⁺ from the medium. The increased amounts of DA in the junctional cleft did not appear to activate prejunctional dopaminergic or α-adrenergic receptors, since endogenous NE overflow in treated veins was not lower than that observed in untreated veins. Evoked tension responses were also not changed in L-dopa-treated veins indicating that DA did not activate postsynaptic α- or β-adrenergic receptors. Treatment with L-dopa, but not with L-tyrosine, caused small increases in tissue content of NE. Treatment with 57times; 10⁻⁵ M L-dopa or 5 × 10⁻⁴ M L-tyrosine attenuated the decrease in NE overflow which was found consistently in untreated veins. However, factors other than lack of precursor must have been responsible for the decrease in NE overflow, since release of NE (measured in the presence of blockers of neuronal and extraneuronal uptakes) did not decrease upon repeated nerve stimulations.