Understanding the Natriuretic Peptides

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US Cardiology, 2006;3(2):1-3

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What is a Natriuretic Peptide?

Three types of natriuretic peptides (NPs) have been identified in the human body: A, or atrial type (ANP), identified in the late 1980s; B, or brain type (BNP), identified in 1988; and C type (CNP), identified in 1990.These three hormones have a common structure, as shown in Figure 1. ANP and BNP, when released from the atria and the ventricles of the heart, are known to have vasodilatory effects and promote urinary sodium loss (natriuresis), as well as urinary water loss (diuresis). CNP is released from the endothelium, and as far as is known today, this molecule has no systemic effects.

As with many hormones in the body, both ANP and BNP are initially synthesized as prohormones. In response to atrial and ventricular wall stretch, such as that seen in congestive heart failure as a result of pressure and/or volume overload, the prohormone is cleaved to generate the active hormone (e.g. BNP) and an N-terminal (NT) biologically inactive fragment (i.e. NTproBNP), and both are released into the blood stream in a 1:1 ratio (see Figure 2).

Although the plasma levels of all four molecules (ANP, NTproANP, BNP, and NTproBNP) reflect volume status and myocardial pressure, only BNP and NTproBNP are used diagnostically. This is most likely due to the fact that ANP has a very short in vivo half-life of two to five minutes, and is present in the plasma at low concentrations; for this reason, the rest of this article will focus on BNP and NTproBNP.The in vivo half-life of BNP is 20 minutes, while that of NTproBNP is 60 to 120 minutes. There is also a significant difference in the in vitro stability of both molecules: four to eight hours at room temperature versus 48 hours, respectively. Thus, although the two molecules are released from the heart in a 1:1 fashion, the circulating concentrations start to diverge very quickly, resulting in higher plasma concentrations of NTproBNP compared to BNP.

What is Congestive Heart Failure?

Heart failure (HF) is a condition that occurs when the heart cannot deliver a sufficient oxygenated blood supply to the rest of the body to meet its demands. Congestive HF (CHF) is a common form of HF that results in excessive fluid retention and often leads to congested lungs and swelling of the lower extremities. Patients suffering from CHF typically present to the emergency department with dyspnea (shortness of breath), and may also have evidence of edema. These symptoms can mimic a number of other conditions, such as asthma, an acute coronary syndrome, chronic obstructive pulmonary disease, or a pulmonary embolism.The treatments that should be initiated vary dramatically for these different conditions, and thus diagnostic tools, such as the (NP) assays that can help distinguish between these conditions are very important.

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References
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