Angiotensin Receptor Blockers (ARBs)

Angiotensin II receptor blockers (ARBs) have beneficial
effects similar to those of ACE inhibitors, but angioedema
occurs less, and cough rarely occurs compared to ACE
inhibitors.

Mechanism of Action
Angiotensin I can be formed by nonrenin enzymes such as
cathepsin or tonin. Angiotensin I may be converted to
angiotensin II by trypsin, cathepsin, or the heart chymase,
but the exact contribution of these alternative pathways
to the formation of angiotensin II remains unclear.
Angiotensin II activates two subtypes of angiotensin II
receptors, AT1 and AT2, but only the AT1 receptor
mediates all the known clinical effects of angiotensin II
described above in Section I. AT1 receptors are present in
the heart, kidney, vascular smooth muscle cells, brain,
adrenal glands, platelets, the placenta, and in adipocytes.
The AT2 receptor affects the inhibition of cell growth,
promotion of cell differentiation, tissue repair, apoptosis,
and perhaps to a small degree, the production of
bradykinin, NO, and prostaglandins in the kidney. Other
effects may emerge with further research. The AT2
receptors have been cloned and are present at a low level
in the adrenal gland, heart, brain, kidney, and uterus.
ARBs were expected to have clinical effects that would
be equal to or superior to those observed with ACE
inhibitors. Clinical trials during the past five years have
indicated that these agents reduce microalbuminuria and
glomerulopathy and delay time to end-stage renal disease
in patients with type 2 diabetes. ACE inhibitors have been
shown to be useful in causing similar beneficial effects
in patients with type 1 diabetes, but evidence in type 2
diabetic nephropathy is not convincing. Direct comparison
of ARBs with ACE inhibitors in this subset of patients
has not been tested. Head-to-head trials of ARBs and ACE
inhibitors in diabetic nephropathy need to be conducted to
determine their respective place in renoprotection.
Dual blockade of the renin-angiotensin-aldosterone
system with both candesartan 16 mg once daily and lisinopril
20 mg daily has been shown to be better than either
treatment alone in reducing blood pressure and better
then candesartan alone in reducing microalbuminuria.

Available Angiotensin Receptor Blockers
Several selective AT1 receptor blockers have become
available. They have a high affinity for AT1 receptors
and negligible affinity for AT2 receptors. These include:
1. Candesartan — dosage 4–16 mg daily, maximum
32 mg once daily
2. Irbesartan — dosage 150–300 mg once daily
3. Eprosartan — dosage 300–400 mg twice daily
4. Losartan — dosage 25–100 mg once daily
5. Telmisartan — dosage 28–80 mg once daily
6. Valsartan — dosage 40–160 mg once daily


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