Adenosine Monophosphate Inhalation Challenge

AMP (or 5'-adenylic acid) is a ubiquitous nucleic acid used for many processes in the human body. AMP is a component of ribonucleic acid, which is used for the translation of DNA into proteins. AMP is also a precursor of adenosine triphosphate, which powers living cells. Inhalation of AMP induces bronchoconstriction in asthmatics.

Inhaled AMP is rapidly converted to adenosine and has been thought to elicit bronchospasm by binding to and activating adenosine receptors on primed mast cells. This binding causes cells to degranulate, releasing mediators such as prostanoids and eicosanoids that cause smooth muscle constriction and mucosal edema (Joos et al. 2003). AMP is also thought to act on A2b receptors in vascular beds and neurosecretory cells to induce mucosal edema directly (Polosa and Holgate 1997).

By targeting inflammatory cells, it has been proposed that AMP may be a more sensitive marker of inflamed airways in asthma as compared to direct agents such as methacholine and histamine. AMP responsiveness is closely associated with measurements of airway inflammation such as sputum eosinophils and eosinophil products including ECP (Polosa et al. 2000). Adenosine has also been proposed to involve C fibers and parasympathetic reflexes (Figure 10.2).

AMP challenges deliver nebulized solution by tidal breathing or by using a modified dosimeter method. Doubling concentrations of AMP are inhaled until the required fall in FEV1 is reached or until the highest concentration of 800 mg/mL is administered (Polosa and Holgate 1997). The response to AMP is determined by measuring the provocative concentration of inhaled AMP causing the FEV1 to decrease by 20%. The exact cutoff point between normal and abnormal PC20 AMP remains controversial; however, the most commonly designated cutoff is 160 mg/mL.

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