The beta 2 receptor plays a role in the hypotensive effect of L-dopa

Abstract

Levodopa is a dopamine precursor exerting its cardiovascular effects by conversion to dopamine and by other mechanisms not elucidated yet. Since beta receptors are involved in levodopa Central Nervous System actions, we tried to find out whether adrenergic receptors are involved in the hypotensive effect of levodopa at peripheral level. Experiments were performed on urethane anesthetized (1 ml/100 g i.p.) and heparinized (300 U.I./kg i.v.) rats. Heart rate and mean carotid blood pressure were registered through a polygraph. Pulmonary ventilation was facilitated by a tracheal cannula. Drugs were infused into the femoral vein through a polyethylene catheter. Two series of the following drugs were assayed: a) Dopamine (3.12 to 25.0 micrograms/100 g i.v.) and levodopa (12.5 to 100 micrograms/100 g i.v.) as control groups. b) The aforementioned drugs and dosages were assayed 20 minutes after intravenous propranolol (50 micrograms/100 g), atenolol (100 micrograms/100 g) and ICI 118.55 (25 micrograms/100 g). Dopamine produced a dose-related hypotensive effect (p < 0.02) which was not modified by propranolol, atenolol or ICI 118.55. Levodopa produced a dose-related blood pressure fall which was not changed by atenolol pre-treatment. Contrariwise, propranolol and ICI 118.55 evoked a blood pressure raise with levodopa. Since propranolol and ICI 118.55 have in common the beta 2 receptor blockade, it is suggested that such receptor has a key role in the hypotensive response induced by levodopa.