Hydralazine

Hydralazine

Hydralazine (apresoline) is a direct-acting smooth muscle relaxant used to treat hypertension by acting as a vasodilator primarily in arteries and arterioles. 

By relaxing vascular smooth muscle, vasodilators act to decrease peripheral resistance, thereby lowering blood pressure and decreasing afterload.^[1]

However, this only has a short term effect on blood pressure, as the system will reset to the previous, high blood pressure necessary to maintain pressure in the kidney necessary for natriuresis. The long term effect of antihypertensive drugs comes from their effects on the pressure natriuresis curve.

mode of action

Hydralazine increases guanosine monophosphate levels, decreasing the action of the second messenger IP₃, limiting calcium release from the sarcoplasmic reticulum of smooth muscle. This results in a vessel relaxation. It dilates arterioles more than veins.^[2]

It recently has been identified as a nitric oxide donor.^[3]

Activation of hypoxia-inducible factors has been suggested as a mechanism

clinical use

Hydralazine is not used as a primary drug for treating hypertension because it elicits a reflex sympathetic stimulation of the heart (the baroreceptor reflex). The sympathetic stimulation may increase heart rate and cardiac output, and in patients with coronary artery disease may cause angina pectoris or myocardial infarction.^[1] Hydralazine may also increase plasma renin concentration, resulting in fluid retention. 

In order to prevent these undesirable side-effects, hydralazine is usually prescribed in combination with a beta-blocker (e.g., propranolol) and adiuretic.

Hydralazine is used to treat severe hypertension, but again, it is not a first-line therapy for essential hypertension. However, hydralazine is the first-line therapy for hypertension in pregnancy, with methyldopa.