The Action of Dopamine on Arterial Blood Pressure in the Rat

The mean arterial blood pressure in conscious normotensive Sprague‐Dawley rats was recorded by means of in‐dwelling arterial catheters. Dopamine hydrochloride (DA) was infused intravenously from 0.04 μg/min. up to 0.22 μg/min. The infusions always resulted in a hypertensive reaction. Intravenous injections of DA (1–50 μg/kg) or noradrenaline bitartrate monohydrate (NA) (0.02–0.5 μg/kg) were given before and after phenoxybenzamine (PBZ) (5 mg/kg); protriptyline (PTP) (10 mg/kg) and nialamide (100 mg/kg). The DA injections alone resulted in a pressor action. PBZ blocked or considerably diminished the pressor action of DA. PTP did not result in a clear‐cut augmentation of the blood pressure response to DA like that seen after the various equipotent NA doses tested. Furthermore, there was no prolongation of the duration of the pressor action of DA after PTP, though this was found after NA. Nialamide did not alter the magnitude of the pressor action of DA and NA. However, the hypertensive response of L‐DOPA (25 mg/kg) was markedly augmented by pretreatment with nialamide (10 mg/kg).