MMP2 and Paroxysmal atrial fibrillation: In particular, contrary to previous thoughts, MMP-2 has the capacity of myocardial intracellular expression, which can be actively up-regulated in response to several factors that have been involved in paroxysmal AF pathophysiology, including hypoxia, angiotensin II, endothelin-1 or pro-inflammatory cytokines [42,45,46].