CAPTOPRIL is a sulfhydryl containing dipeptide surrogate of proline which abolishes the pressor action of Angiotensin – I but not that of Angiotensin – II: does not block AT1 or AT2 receptors.

Angiotensin Converting Enzyme (ACE) is a relatively nonspecific enzyme; splits off a dipeptidyl segment from several peptides including bradykinin, substance P, a natural stem cell regulating peptide, etc. In additional to Angiotensin-I.As such, captopril increases plasma kinin levels and potentiates the hypotensive action of exogenously administered bradykinin.

Pretreatment with B2 kinin receptor antagonists has shown that kinins do contribute to the acute vasodepressor action of Angiotensin Converting Enzyme (ACE) inhibitors, but they appear to have little role in the long-term hypotensive effects, probably because, firstly kinins play only a minor role, if at all, in BP regulation, and, secondly another enzyme ‘Kininase I’ (which also degrades bradykinin) is not inhibited by captopril.

Nevertheless, elevated kinins (and Prostaglandins (PGs) whose synthesis is enhanced by kinins) may be responsible for cough and angioedema induced by Angiotensin Converting Enzyme inhibitors in susceptible individuals.

Angiotensin Converting Enzymes (ACE) inhibitors interfere with degradation of substance P also. Rise in the level of stem cell regulator peptide caused by Angiotensin Converting Enzyme (ACE) inhibitors could, in part, be responsible for their cardioprotective effect in Congestive Heart Failure (CHF).

Captopril induced hypotension is a result f decrease in total peripheral resistance. The arterioles dilate and compliance of larger arteries is increased. Both systolic and diastolic BP fall. It has no effect on cardiac output. Cardiovascular reflexes are not interfered with and there is little dilatation of capacitance vessels. As such, postural hypotension is not a problem. Reflex sympathetic stimulation does not occur despite vasodilation, and ACE inhibitors can be safely used in patients with ischemic heart disease. The renal blood flow is not compromised even when BP falls substantially. This is due to greater dilation of renal vessels (Angiotensin-II markedly constrict them). Cerebral and coronary blood flow are also not compromised.

Adverse Effects

The adverse effects profile of all ACE inhibitors is similar. Captopril is well tolerated by most patients, especially if daily dose is kept below 150 mg.


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