FGF23 and chronic kidney disease: Considering a recent publication implicating endogenous adenine in CKD progression and the well-established harmful effects of elevated serum phosphate levels in CKD, we tested whether these factors might directly affect cardiomyocytes independent of FGF23 signaling.27,44 In engineered heart tissue, adenine treatment did not impact contractile frequency or force, suggesting no direct cardiac effects.