B-Type Natriuretic Peptide

THE NATRIURETIC PEPTIDE SYSTEM INCLUDES
atrial (ANP) and brain or B-type (BNP) natriuretic
peptide. Brain natriuretic peptide (BNP) is a 32 amino
acid cardiac natriuretic peptide that was originally isolated
from porcine brain tissue. The human BNP gene is located
on chromosome 1 and encodes the 108 amino acid
prohormone pro-BNP which circulates with the 32 amino
acid biologically active BNP and the N terminal 76 amino
acid component of the prohormone (NT-proBNP). These
three components can be measured by immunoassay.
BNP is released by cardiac myocytes of the ventricles in
response to myocardial wall stress. This is brought on by
increased transmural wall tension and elevations of end
diastolic pressure, ventricular volume expansion and
pressure overload. The secretion of this important peptide
serves to regulate sodium and water balance by the kidneys
and causes vasodilatation of arteries, which benefits the
failing heart.

Elevated BNP levels are detected in the blood of patients
with heart failure but levels are normal in patients with
shortness of breath caused by pulmonary disease. The
rapid measurement of levels of BNP in the blood has
proved useful in the diagnosis of heart failure.

More important, in emergency rooms the BNP test
rapidly clarifies whether severe shortness of breath is
caused by heart failure or by pulmonary disease. Heart
failure is readily diagnosed by the symptoms of severe
shortness of breath on minimal effort, shortness of breath
in bed, and swelling of the ankles caused by accumulation
of fluid. Confirmation is obtained on clinical examination
and from a chest x-ray (see the chapter Heart Failure).
Although NT-pro BNP levels are 2 to 10 times higher than
the BNP levels in patients with heart failure, NT-proBNP
is eliminated from the blood by the kidneys and is not
reliable in patients with coexisting renal insufficiency,
a condition that is often associated with heart failure.
Clearance of BNP is not dependent on kidney function.


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