Asthma is a frequently occurring inflammatory lung disease that affects the whole bronchial tree including the small airways (<2mm). Since the introduction of the solution hydrofluoroalkane (HFA) technology it is possible to generate medication with small particles of approximately 1-2 μm, and therefore to reach the small airways. However, at this moment the investigators have no reliable instruments to identify the asthmatic subjects who particularly benefit from treatment with inhaled small particles. Recently the investigators research group investigated whether provocation with small and large particles AMP is able to identify responders and non-responders to treatment with small and large particles of inhaled corticosteroids. This provocation technique gave promising results but needs further optimization.

The aim of this study is to determine the optimal particle size of dry powder adenosine to assess small airway involvement in asthma. Secondary, to provide insight in the associations between the standard test, executed with nebulized AMP, and the new test, executed with dry powder Adenosine.

This is a cross-over study. The study population consists of 10 non-smoking asthma patients in the age of 18-65 years who have a PC20 metacholine value ≤4.9mg/ml.

Participating subjects perform 6 provocation tests in randomized order: one test with nebulized methacholine, one test with nebulized AMP, four times dry powder adenosine provocation test. (small particles with inhaled with a slow flow, small particles inhaled with a fast flow, large particles inhaled with a slow flow, large particles inhaled with a fast flow)

Inclusion Criteria:

• A doctor's diagnosis of asthma
• Age: 18-65 years
• PC20 AMP < 320 mg/ml
• Non-smoker
• Steroid naive or steroids have been stopped 4 weeks before entry into the baseline period

Exclusion Criteria:

• Recent exacerbation asthma (<2 months) or upper respiration tract infection (<2 weeks)
• Severe airway obstruction at baseline, FEV1pred< 50% or < 1.2L
• Diagnosis of COPD or any other pulmonary disease that could influence the study results as judged by the investigator
• Pregnancy or lactating women. (i.e. women of childbearing potential who do not use adequate anticonception as judged by the investigator).