Methyldopa is changed into α-methylnorepinephrine, which replaces norepinephrine in adrenergic nerve granules. This α-methylnorepinephrine, interacts with presynaptic central α2-adrenergic receptors, on release by nerve stimulation. On interaction it causes a decrease in sympathetic outflow and total peripheral resistance resulting in reduction of arterial pressure.
Methyldopa also blocks dopa decarboxylase resulting in the reduction of the stores of norepinephrine in the sympathetic nervous system leading to a decrease in blood pressure.
This causes a decrease in renal vascular resistance, which is probably due to weak vasoconstrictor effect of α-methylnorepinephrine in renal vessels than norepinephrine.
It does not affect the blood flow to the important organs.
Pharmacokinetics:
Its onset of action is 120 minutes and duration of action is 17-25 hours.
Therapeutic uses:
It can be used for mild to moderately serious hypertension. It is especially useful for hypertensive patients, who are pregnant or have renal insufficiency.
Adverse effects:
Sedation, lassitude, Hemolytic anemia, orthostatic hypotension, rebound hypotension on sudden withdrawal, depression
Contraindications:
It is contraindicated in
1. pheochromocytoma
2. during the administration of MAO inhibitors
3. acute hepatic disease
Dosage:
It is given orally in the dose of 1-2 gm in divided doses.
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