Mechanism of action of methyldopa in the rat. Role of 3-O-methylated metabolites.

We studied the effect of 3-O-methyl-methyldopa (OMMD), the 3-O-methylated metabolite of the antihypertensive drug methyldopa (alpha-methyldopa, AMD), on blood pressure in the spontaneously hypertensive rat. OMMD lowered blood pressure in a dose-related manner when given orally or intraperitoneally. Its action lasted longer than that of AMD, and daily oral administration produced a cumulative fall in blood pressure. Oral and intraperitoneal OMMD produced similar reductions of blood pressure and similar tissue OMMD levels. After intraperitoneal injection of different doses, levels of OMMD measured in brain, spinal cord, and plasma correlated with the magnitude of the antihypertensive effect. No AMD was detected in tissues after either route of administration, which suggests that the antihypertensive effect was not based on demethylation of OMMD to AMD. Peripheral inhibition of the enzyme, aromatic amino acid decarboxylase (AAAD), failed to suppress OMMD's effect on blood pressure; in contrast, central inhibition of AAAD did decrease OMMD's antihypertensive effect. These observations suggest that 3-O-methylated metabolites may participate in the antihypertensive effect of AMD.