The drugs of this class (hydralazine and sodium nitroprusside) lower arterial blood pressure primarily by direct spasmolytic action on smooth musculature of arterioles, which leads to a reduction of resistance of peripheral vessels by causing dilation. Diastolic pressure is usually lowered more than the systolic pressure.
Hydralazine: Hydralazine, 1-hydrazinonaphthalazine (22.6.4), is synthesized by the oxidative chlorination of phthalide with simultaneous hydrolysis of product, which results in hydroxyphthalide (22.6.1), which upon reaction with hydrazine changes to phthalazone (22.6.2). This undergoes a reaction with phosphorous oxychloride, forming 1-chloroph-thalazine (22.6.3), in which substitution of the chlorine atom with hydrazine gives the desired hydralazine (22.6.4) [14-16].
Hydralazine exhibits an antihypertensive effect by directly relaxing smooth muscles of the vessels. It has an effect on arterial vessels while having a minimal effect on venous vessels. As a result, resistance of peripheral vessels decreases, and blood pressure is reduced (diastolic more than systolic).
It does not have a substantial effect on nonvascular smooth musculature or cardiac tissues. Homeostatic circulatory reflexes remain natural, and the resulting hypotension activates cardiovascular reflexes, which are expressed as an increase of heart work, power, and volume of cardiac output. Therefore, it is most effectively used in combination with /¡-blockers.
By itself, hydralazine is used for hypertension; however, it is not even the drug of choice for weak forms of this condition. Synonyms of this drug are apressin, hypatol, depressan, and others.
Sodium Nitroprusside, Na2Fe(CN)5NO (22.6.7): It is synthesized by successive reactions including the reaction of potassium ferrocyanide with nitric acid, which forms potassium nitroprusside (22.6.5), which is further transformed to copper nitroprusside (22.6.6), and reaction of this with sodium carbonate gives sodium nitroprusside (22.6.7).
2K4Fe(CN)6 + 4HNO2 + 2H2SO4 ^ K2Fe(CN)5NO + 2NO + 2(CN)2 + 2K2SO4 + 4H2O
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Your heart pumps blood throughout your body using a network of tubing called arteries and capillaries which return the blood back to your heart via your veins. Blood pressure is the force of the blood pushing against the walls of your arteries as your heart beats.Learn more...