SMAD2 and chronic obstructive pulmonary disease: Muscarinic receptor antagonism (by drugs such as solifenacin succinate) has been utilised as a therapeutic mechanism in COPD [48], whilst, in contrast, it has been suggested that choline can attenuate cardiac fibrosis by agonist action on M3 muscarinic receptors, regulating TGF-β1/Smad2/3 in in vitro and in vivo models of cardiac fibrosis [49].