ALPL and atrial fibrillation: Histologically, calcified human and animal discs show increased levels of calcium, inorganic phosphate, TNAP, and collagen type X. The mechanism of calcification differs by disc region: in AF and EP, it represents heterotopic calcification regulated by pyrophosphate metabolism (PPi), while NP calcification is dystrophic-characterized by amorphous calcium crystal deposition following cell necrosis, independent of PPi signaling [31,37].