The effects of L-dopa and α-methyldopa on reflexes and sympathetic nerve function

Abstract

L-dopa (10 mg/kg, i.v.) and β-methyldopa (100 mg/kg i.v.) were compared for their relative central and peripheral effects on reflexes and symphathetic nerve function. In MAO-inhibited dogs, L-dopa alone depressed reflex responses to both bilateral carotid artery occlusion (BCO) and vertival tilting, decreased blood pressure and inhibited responses to stimulation of the lumbar sympathetic chain. Pretreatment with MK 486, an extra-cerebral decarboxylase inhibitor, prevented the effects of L-dopa on the reflex response to vertical tilting and on nerve stimulation, but not on blood pressure or responses to BCO. Pretreatment with RO 4–4602, an inhibitor of both cerebral and extra-cerebral decarboxylase, prevented all the actions of L-dopa. α-Methyldopa lowered blood pressure and reduced responses to BCO but not to vertical tilting or nerve stimulation. These results indicate that, although L-dopa impaired peripheral sympathetic nerve function in MAO-inhibited dogs, the hypotension was caused predominantly by a central mechanism. Hypotension after α-methyldopa also appeared due to a central mode of action. A central component of action was associated with an ability to selectivity depress the bilateral carotid occlusion reflex, whereas the peripheral action of L-dopa was associated with suppression of reflex responses to postural changes.