Attenuation of Postganglionic Sympathetic Nerve Activity by L-Dopa

In anesthetized dogs and cats, intravenous L-dopa reduced the increase in mean blood pressure in response to bilateral carotid occlusion by 77.5% ( P <0.01). In the presence of a dopa decarboxylase inhibitor (Ro 4·4602), this effect was absent. Transmission across the superior cervical sympathetic ganglion in the cat was unaltered by the administration of L-dopa. Carotid artery infusion in the dog of 20% of the effective intravenous dose of L-dopa or dopamine failed to inhibit the response to bilateral carotid occlusion. The pressor response to intravenous tyramine in the dog was potentiated ( P <0.05) by L-dopa. In the dog, the increase in femoral vascular resistance to lumbar sympathetic nerve stimulation was attenuated ( P <0.05) by L-dopa while the response to intra-arterial norepinephrine was unchanged. Thus, inhibition of postganglionic sympathetic nerve activity is a possible mechanism by which L-dopa impaired the carotid sinus reflex.